194
CONSTRUCTION, LAYING AND
INSTALLATION TECHNIQUES FOR
EXTRUDED AND SELF CONTAINED
FLUID FILLED CABLE SYSTEMS
Working Group
21.17
October 2001
STUDY COMMITTEE 21: HV INSULATED CABLES
CONSTRUCTION, LAYING AND INSTALLATION TECHNIQUES
FOR EXTRUDED AND
SELF CONTAINED FLUID FILLED CABLE SYSTEMS
TECHNICAL BROCHURE
Picture 1 : Direct burial
WG21-17
August 2001
Page 1 / 145
MEMBERSHIP LIST OF WG 21 – 17
J.P.M. ANTONISSEN (The Netherlands),
P. ARGAUT (France),
R. AWAD (Canada),
B. DRUGGE replaced by F. RÜTER (Sweden),
T. FAGERENG (Norway),
M. GENOVESI replaced by F. MAGNANI (Italy),
A. GILLE (Belgium),
P. HUDSON (United Kingdom) (Secretary),
R. JOHNSTON (Australia),
T. KARASAKI replaced by G. KATSUTA and after by T. SASAKI (Japan),
K. LAGERSTEDT (Denmark),
H.S. LEE (South Korea),
Y. MAUGAIN1 (France) (Convenor),
M. PORTILLO (Spain),
T. J. RODENBAUGH (United States),
R. SAMICO (Brazil),
R. SCHROTH (Germany).
1
EDF – RTE, 34, 40 rue Henri Régnault F-92400 COURBEVOIE - France
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LAYING AND INSTALLATION TECHNIQUES USED
FOR EXTRUDED AND SELF CONTAINED FLUID FILLED CABLES
TECHNICAL BROCHURE
WORKING GROUP 21-17
TABLE OF CONTENTS
1. INTRODUCTION
9
1.1 Terms of reference
9
1.2 Scope of work
1.2.1
What is the difference between “construction techniques” and “installation techniques” ?
1.2.2
What is an innovative construction technique ?
1.2.3
How is it possible for a newcomer in the cable world to design an underground link ?
2. DESCRIPTION OF THE CABLE SYSTEM
10
11
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14
2.1 Description of the cable
14
2.2 Main cable systems configurations
2.2.1
Meshed underground network
2.2.2
Siphon
2.2.3
Substation entrance
2.2.4
Power generator output
2.2.5
Auxiliary supply
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2.3 Cable
2.3.1
Extruded -dielectric cables :
2.3.1.1 Cable description
2.3.2
Cables with lapped insulation
2.3.2.1 Cable Description :
2.3.2.2 Self-Contained Fluid Filled Cable : SCFF
2.3.2.3 Impregnated Paper Characteristics :
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2.4 Accessories
2.4.1
General
2.4.2
Accessory types
2.4.2.1 Types of joints
2.4.2.2 Types of terminations
2.4.3
Compatibility of the accessory with the cable
2.4.3.1 Number of cable cores
2.4.3.2 Cable constructional details
2.4.3.3 Conductor area and diameter
2.4.3.4 Operating temperature of the cable conductor and sheath
2.4.3.5 Compatibility of the accessory with the type of cable insulation and semi-conducting screens
2.4.3.6 Cable electrical design stresses to be withstood by the accessory
2.4.3.7 Mechanical forces and movements generated by the cable on the accessory
2.4.3.8 Short circuit forces
2.4.4
Compatibility of the accessory performance with that of the cable system
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2.4.4.1 Circuit performance parameters
2.4.4.2 Circuit life required
2.4.4.3 Metallic screen bonding requirements
2.4.4.4 Earth fault requirements
2.4.5
Compatibility of the accessory with the cable system design and operating conditions
2.4.5.1 Type of cable installation design
2.4.5.2 Standard dimensions for cable termination
2.4.5.3 Types of accessory installations
2.4.5.4 Jointing limitations in restricted installation locations
2.4.5.5 Mechanical forces applied to the accessory
2.4.5.6 Climatic conditions
2.4.5.7 Type of accessory outer protection required
2.4.5.8 Situations requiring special accessory protection
2.4.5.9 Quality Assurance scheme for accessory installation
2.4.5.10
Training of Personnel
2.4.5.11
Assembly instructions
2.4.5.12
Special assembly tools
2.4.5.13
Preparation of the assembly environment
2.4.6
Compatibility of the accessory with specified after laying tests
2.4.6.1 Voltage test on main insulation
2.4.6.2 Partial discharge detection
2.4.6.3 Voltage withstand test on the cable over sheath and joint protection
2.4.6.4 Current balance test on the cable sheath and screening wires
2.4.7
Maintenance requirements of the accessory
2.4.7.1 Monitoring of fluid insulation
2.4.7.2 Voltage withstand tests on the over sheath and joint protection
2.4.7.3 Shelf life of accessories for emergency spares
2.4.7.4 Availability of accessory kits for emergency spares
2.4.8
Economics of accessory selection
2.4.8.1 Cost of the accessory complete with all components
2.4.8.2 Cost of guarantee and insurance
2.4.8.3 Cost of assembly time
2.4.8.4 Cost of preparing the installation environment for the accessory
2.4.8.5 Cost of safe working conditions
2.4.8.6 Cost of special jointing tools
2.4.8.7 Cost of training
2.4.8.8 Comparative cost of cable and accessories
2.4.8.9 Cost of verification of accessory performance
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3. CONSTRUCTION TECHNIQUES
31
3.1 Definition of the main technical terms
31
3.2 Description of traditional techniques
3.2.1
Ducts
3.2.1.1 Description of the technique
3.2.1.2 Limits of the technique
3.2.1.3 Adaptation of the technique to the cable system design
3.2.2
Direct burial
3.2.2.1 Description of the technique
3.2.2.2 Limits of the technique
3.2.3
Tunnels
3.2.3.1 Description of the technique
3.2.3.2 Limits of the technique
3.2.3.3 Adaptation of the technique to the cable system design
3.2.4
Troughs
3.2.4.1 Description of the technique
3.2.4.2 Existing installation techniques
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3.2.4.3 Installation methods
3.2.4.4 Limits of the technique for buried troughs
3.2.4.5 Limits of the technique for surface troughs
3.3 Description of innovative techniques
3.3.1
Bridges
3.3.1.1 Description of the technique
3.3.1.2 Limits of the technique
3.3.2
Shafts
3.3.2.1 Description of the technique
3.3.2.2 Limits of the technique
3.3.3
Horizontal drilling
3.3.3.1 Description of the technique
3.3.3.2 Limits of the technique
3.3.3.3 Adaptation of the technique to the cable system design
3.3.4
Pipe jacking
3.3.4.1 Description of the technique
3.3.4.2 Limits of the technique
3.3.4.3 Adaptation of the technique to the cable system design
3.3.5
Microtunnels
3.3.5.1 Description of the technique
3.3.5.2 Limits of the technique
3.3.5.3 Adaptation of the technique to the cable system design
3.3.6
Mechanical laying
3.3.6.1 Description of the technique
3.3.6.2 Limits of the technique
3.3.7
Embedding
3.3.7.1 Description of the technique
3.3.7.2 Limits of the technique
3.3.8
Use of existing structures
3.3.8.1 Description of the technique
3.3.8.2 Limits of the technique
3.3.8.3 Adaptation of the technique to the cable system design
4. CABLE INSTALLATION DESIGN AND LAYING TECHNIQUES
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4.1 Cable installation design
4.1.1
Installation design in air
4.1.1.1 Rigid systems
4.1.1.2 Flexible systems (Western approach)
4.1.1.3 Flexible systems (Japanese approach)
4.1.1.4 Cable in ducts
4.1.2
Installation design for buried cables
4.1.2.1 Backfill
4.1.2.2 Cooling systems
4.1.3
Transition between different installation types
4.1.3.1 Transition between ducts and manholes (open air)
4.1.3.2 Transition between flexible and rigid systems (open air)
4.1.3.3 Transition between flexible and rigid systems (buried)
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4.2 Cable laying and installation techniques
4.2.1
Cable pulling calculations
4.2.1.1 Clearance in ducts
4.2.1.2 Pulling tension
4.2.1.3 Side wall pressure
4.2.2
Installation Methods
4.2.2.1 Introduction
4.2.2.2 Nose pulling
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4.2.2.3 Synchronised power drive rollers
4.2.2.4 Caterpillar or hauling machine
4.2.2.5 Bond Pulling
4.2.2.6 Mechanical laying
4.2.2.7 Other installation methods in tunnel
4.2.3
Installation process
4.2.3.1 Transportation of cable to site
4.2.3.2 Cable Bending Radius
4.2.3.3 Cable Temperature
4.2.3.4 Pulling Length
4.2.3.5 Route Profile
4.2.3.6 Obstacles
4.2.3.7 Setting Up
4.2.3.8 Installation of Cable
4.2.3.9 Final Installation Stages
4.2.3.10
Site Quality Assurance
4.2.3.11
After Laying Tests
4.2.4
Adaptation of the Cable System Design to the Technique/Environment
4.2.4.1 Adaptation of the Cable System Design to the Technique
4.2.4.2 Adaptation of the Cable System Design to the Environment
5. EXTERNAL ASPECTS
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5.1 Location (Urban vs. Rural)
110
5.2 Right of way
110
5.3 Magnetic fields
5.3.1
Flat arrangement
5.3.2
Trefoil arrangement
5.3.3
Vertical arrangement
5.3.4
Comparison between overhead lines and buried links
5.3.5
Conclusion
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5.4 Existing services
120
5.5 Legal aspects
122
5.6 Safety aspects
5.6.1
Protection of the link from external damage
5.6.2
Protection of the environment from a system fault
5.6.3
Protection of the workers
5.6.4
Protection of the public
5.6.5
Safety of the different laying techniques
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5.7 Environment
125
6. DESIGN OF A LINK
127
6.1 Methodology
127
6.2 Study cases
133
7. GLOSSARY
139
8. BIBLIOGRAPHY
142
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LIST OF FIGURES
Figure 1 : Percentage of use for the different techniques
Figure 2 : Percentage of use for the different techniques
Figure 3 : Percentage of techniques used
Figure 4 : Meshed underground network
Figure 5 : Siphon, an underground cable between 2 overhead lines
Figure 6 : Underground substation entrance
Figure 7 : Power generator output
Figure 8 : Auxiliary transformer supply
Figure 9 : Tunnel boring methods
Figure 10 : Shield machine
Figure 11 : Cooling system in tunnel
Figure 12 : Filled troughs
Figure 13 : Unfilled troughs
Figure 14 : Unfilled troughs in air
Figure 15 : Mechanical Laying
Figure 16 : Maximum external cable diameter in terms of internal pipe diameter and clearance
Figure 17 : Cable cleated with movement in a vertical plan
Figure 18 : Plan view of cables installed with movement in a horizontal plan
Figure 19 : Horizontal snaking
Figure 20 : Vertical snaking
Figure 21 : Shape of bend part
Figure 22 : Horizontal bend
Figure 23 : Vertical bend (pulling up)
Figure 24 : Vertical bend (pulling down)
Figure 25 : Upward slope
Figure 26 : Downward slope
Figure 27 : Cable installation in tunnel
Figure 28 : Magnetic belt pulling machine
Figure 29 : Flat arrangement, 1 circuit
Figure 30 : Brms profiles with various s
Figure 31 : Brms profiles with various d
Figure 32 : Flat arrangement, 2 circuits
Figure 33 : Brms profiles for two cable system configurations with various h
Figure 34 : Brms profiles for two cable system configurations with various g
Figure 35 : Trefoil arrangement, 1 circuit
Figure 36 : Brms profiles for both flat and trefoil formations with various s flat and s trefoil
Figure 37 : Brms profiles with various d for both flat and trefoil formations
Figure 38 : Trefoil arrangement, 2 circuits
Figure 39 : Brms profiles for two cable system configurations with various h
Figure 40 : Brms profiles for two cable system configurations with various g
Figure 41 : Vertical arrangement, 1 circuit
Figure 42 : Brms profiles for flat, trefoil and vertical formations with various s flat, s trefoil and s vertical
Figure 43 : Vertical arrangement, 2 circuits
Figure 44 : Brms profiles for two cable system configurations with fixed h, d, g and s = s t = s v = 0.3m
Figure 45 : Stage 1
Figure 46 : Stage 2
Figure 47 : Stage 3
Figure 48 : Stage 4
Figure 49 : Stage 5
Figure 50 : Possible routes
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page
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LIST OF TABLES
Table 1 : Horizontal drilling references
Table 2 : Pipe jacking figures
Table 3 : Horizontal snaking calculations
Table 4 : Vertical snaking calculations
Table 5 : Vertical cable installation at shafts
Table 6 : Offset calculations
Table 7 : Route cost
page
52
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86
89
138
LIST OF PICTURES
Picture 1 : Direct burial
Picture 2 : 400 kV XLPE cable
Picture 3 : PVC ducts – double circuit
Picture 4 : Direct burial
Picture 5 : Open cut gallery
Picture 6 : Cables in trough
Picture 7 : Unfilled troughs in air
Picture 8 : Dedicated tunnel for cables
Picture 9 : Pipe Jacking
Picture 10 : Microtunnelling
Picture 11 : Mechanical laying
Picture 12 : Embedding
Picture 13 : ROV machine
Picture 14 : Snaking in a tunnel
Picture 15 : Cable pulling in duct
Picture 16 : Cable installation in tunnel
Picture 17 : Cable laying Locomotive undergoing trials.
Picture 18 : Cable reel
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1
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1. INTRODUCTION
1.1
Terms of reference
GENERAL
The following terms of reference had been established by S. SIN (France), P. COUNESON
(Belgium), W-D. SCHUPPE (Germany) and S.G. SWINGLER (United Kingdom).
They were accepted by SC 21 on August 1996 meeting.
INITIAL TITLE OF THE WORKING GROUP
The name of this new group is " Laying and Installation Techniques for High Voltage Cable Systems ".
INITIAL TERMS OF REFERENCE
To review existing and innovative methods for HV cable installation. The review should include cable
installed in trenches, ducts and tunnels.
To compare the relative merits of the installation methods and to give recommendations for their
application.
Starting from the studies of the previous working group 21-01, it is anticipated that the method of
working will be :
- Remind existing practices for cable installation and identify the factors responsible for the choice
of a particular practice.
- Review possible innovations, improvements and alternatives in the light of increasing economic and
environmental pressures.
- Give recommendations for the application of new installation technologies to high voltage cable
systems.
In reviewing the achievements of WG 21-01 and the existing information available in their reports, the
Task Force noted the need for a document summarising methods for design calculations. The work
required is to :
- Review the calculations and parameters necessary to perform design calculations for cable
installation (including for example, on the one hand, pulling tension during installation, and on the
other hand requirements for installations in tunnels, ducts, manholes and towers).
- Compare theoretical productions with the results of engineering trials.
- Recommend simplified methods for the calculation of design parameters for cable laying.
The Task Force evaluated the work required and the skills necessary for its rapid and effective
completion. The results are necessary for the main task of the proposed working group in order to
evaluate the optimum installation techniques taking into account network conditions, regulation, cables
WG 21-17 Technical Brochure
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type, etc. It is, however, unlikely that the main working group would have the necessary skills and
resources to complete this task. It is therefore recommended that the Working Group establishes a
subsidiary Task Force to advise and report on methods of calculation within timescales acceptable to
the main Working Group.
REVISIONS
1. A first revision was accepted in 1997 by the CIGRE Study Committee 21 on the limitation of the
terms of reference.
It impacts the type of cable studied. The scope of work was limited to land extruded cables as
submarine ones are studied in other Working Groups and as technical brochures are published on these
items. Nevertheless, an extension to LP SCFF (Low Pressure Self Contained Fluid Filled cable) has
been asked.
2. A second one was decided in 1999 by adding the review of the link safety with respect to the
environment.
It has been decided that this Group will focus on what is under the soil, the upper part being treated by
the Group 21-19 "Technical and environmental issues regarding integration of underground cable
systems".
3. A third one was asked in 2000 by the SC on the term “Laying”.
This word is usually understood all around the world more as the pulling than the civil works prior to the
pulling. As an example, we can refer to the concept “After laying test” which is well known by the
cable industry. As so, it was considered that the word “Construction” should be added for a better
comprehension in the title of the Working Group and in some chapters of the Technical brochure to
explain the civil works that are necessary to build an underground link.
The name of the group is now " Construction, Laying and Installation Techniques for High Voltage
Cable Systems ".
MEMBERSHIP
The membership of the Working Group should largely be made up of representatives from utilities with
significant experience of cable installation. The subsidiary Task Force will require representatives from
cable manufacturers and construction companies.
TIME SCHEDULE
The Working Group should start their work before the end of 1996 and produce a final report in
advance of the Study Committee 21 meeting in September 2000.
RECOMMENDATIONS
The WG will review existing and innovative methods for HV cable installation and give
recommendations for their optimum implementation. The final report of the WG will be available in
advance of the 2000 meeting of Study Committee 21.
1.2
Scope of work
The group was composed of permanent and corresponding members, but all members were asked to
contribute, either by writing part of the document or by checking it. In its final version, the technical
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brochure represents a comprehensive state of art shared by the technical cable systems community
throughout the world.
None of the members were implicated in the writing of the terms of reference. As so, the first task
was to go through them to be sure to have a good understanding.
Two questionnaires were prepared and sent in December 1997 to Utilities (46 replies from 22
countries) and Cable manufacturers (27 replies from 16 countries) in order to collect the domestic
practice of the different countries.
The first one dealt with laying and installation techniques and the second one with design calculations.
Based on the replies and the technical knowledge of the writers, the technical brochure was then
established.
Two task forces were then created, one with utilities and the other with cable manufacturers. A person
from each task force was included in the other to guarantee an homogeneous work.
As milestones of the Working Group's work, two technical papers were published :
- a first paper in Jicable (June 99, paper A4.4) which was also published in REE special report n°4
(1999), in REE (May 2000),
- a second one in CIGRE session 2000 (paper 21-202),
These two papers are a summary of the main results obtained from the completed questionnaires.
In addition, a session concerning "Trends in high voltage cable laying and installation techniques" was
chaired in the 1999 ICC-CIGRE Colloquium.
Throughout the life of the Working Group life, there was continuing discussion about :
- What is the difference between “construction techniques” and “installation techniques” ?
- What is an innovative construction technique ?
- How is it possible for a newcomer in the cable world to design an underground link ?
Finally, the twelve existing construction techniques (traditional and innovative) are reported and
explained. A hypothetical case study is presented in Chapter 6.2 in order to demonstrate the way a
comparative evaluation could be carried out. Cable engineers should apply the methodology to their
actual projects at the earliest possible stage. Estimated installation cost and anticipated environmental
constraints should be used in order to compare these techniques and choose the optimal ones.
Installation cost depends on many factors such as location, local regulations, etc… and will greatly vary
from one project to another.
1.2.1 What is the difference between “construction techniques” and “installation
techniques” ?
At the beginning of the Working Group's work, the difference was not very clear with the both words
being used to define the same processes in a number of countries..
Throughout this brochure, the terms have to be considered as follows: The term “construction
techniques ” is considered as relating to the techniques used to create the cable route, mainly covering
the civil works such as trenching. Likewise the term “installation techniques” is considered to relate to
the cable system design and cable installation methods.
Cable design issues associated with the laying and installation techniques have also been considered
under the general subject of "Installation Techniques".
The cable installation was then the rest : the pulling and backfilling, the fixing when laid in open air.
1.2.2 What is an innovative construction technique ?
Twelve different existing laying techniques were identified : they are detailed in the corresponding
sections. Among them, only three are commonly used , (i.e. mentioned by more than 50% of the
companies which replied to questionnaire N° 1 concerning utilities). These are : trenches (direct burial),
ducts and tunnels.
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To be more precise, the Working Group detailed the analysis on 2 voltage ranges : 60-170 kV,
corresponding to HV and 220-500 kV corresponding to EHV. Therefore, 24 laying techniques can be
considered, 12 for HV and 12 for EHV.
HV extruded cable systems
80
70
60
50
%
40
30
20
Mechanical
laying
Existing
structures
Embedding
Troughs
Shafts
Pipe jacking
Horizontal
drilling
Microtunnels
Bridges
Tunnels
Trenches
0
Ducts
10
Figure 1 : Percentage of use for the different techniques
EHV extruded cable systems
35
30
25
20
%
15
10
Mechanical
laying
Existing
structures
Embedding
Microtunnels
Pipe jacking
Horizontal
drilling
Bridges
Troughs
Shafts
Trenches
Tunnels
0
Ducts
5
Figure 2 : Percentage of use for the different techniques
6 out of 46 companies have already used 50% of the different techniques (among the 24) and only 1
out of 46 has used 90% of them.
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Techniques used among 24 available
16
12
10
8
6
4
90-100 %
80-90 %
70-80 %
60-70 %
50-60 %
40-50 %
30-40 %
20-30 %
0
10-20 %
2
0-10 %
Number of utilities
14
Figure 3 : Percentage of techniques used
To conclude, it was evident that a technique used by more than 50 % of the companies may be
considered as a traditional technique and the others may be considered as innovative even though they
are already in use in other countries.
1.2.3 How is it possible for a newcomer in the cable world to design an underground link
?
This lead to a lot of discussions among the members of the Working Group, however all agreed on the
principle that : a reliable link is based on a reliable cable design and manufacture, a reliable cable
system design and reliable construction and installation techniques.
It therefore appears necessary to not only give the description of the different techniques, but also to
give guidance on the overall design process. For this, it was decided that the best approach would be to
define the process from the beginning to allow a complete understanding of what is needed to ensure a
reliable project..
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2. DESCRIPTION OF THE CABLE SYSTEM
The purpose of this chapter is to give a quick overview on the different cable system components, to
draw attention to the fact that the cable design is usually dependant upon the construction and
installation techniques.
An understanding of the cost can be obtained by taking into consideration the cost of the different
components and the cost of their installation. The optimum costs can be developed by selecting
different solutions depending upon each of the cable system sections along the route. This will be
developed in chapter 6.
2.1
Description of the cable
Underground power transmission lines in the voltage range 60 kV and above make use of one of the
following types of cable systems :
• Extruded-dielectric insulated cables.
• Self-contained medium or low pressure Fluid-filled cables (SCFF).
Where cables interconnect with other circuits, the transition is achieved through a termination. The
length of a continuous section of cable is often limited by the size or weight of the cable reel that can
be transported to the installation site, sometimes by the safe pulling tension that can be applied to the
cable, or by the maximum induced voltage on the metallic screen of the cable. The lengths are then
connected in joint-bays. This is achieved through joints (or splices).
Joints and terminations are the main components of equipment called cable accessories.
2.2
Main cable systems configurations
Various configurations such as single circuit, double circuit and triple circuit lines with different
arrangements of transformer and generator connections are in use.
Many types of connections comprising overhead lines, underground cables or both are possible and can
be found. The length of such transmission lines and cables can vary significantly.
For load reasons, one circuit can consist of several cable systems. Note that in the subsequent figures
each cable can consist of several cable systems
Main configurations, given below, are representative of the most common practical situations.
2.2.1 Meshed underground network
Some parts of a HV network may be entirely underground as can be often seen in large towns where
urbanisation prevents the construction of overhead lines. Cables connect the busbars in the system, as
indicated in Figure 4.
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Substation 2
Substation 1
Underground
cables
Substation 3
Figure 4 : Meshed underground network
2.2.2 Siphon
A siphon is an underground cable connected between two overhead lines. It is assumed that no
switching device is located between line and cable. This configuration allows a HV/EHV link to pass
through areas too wide for an overhead line span such as rivers or small lakes. The configuration may
also permit the transmission line to pass through or near a protected site or an urbanised area.
Overhead line
Overhead line
Underground cable
Figure 5 : Siphon, an underground cable between 2 overhead lines
2.2.3 Substation entrance
An underground cable is often used as the interface between an overhead line and a substation,
especially when it is a gas insulated station. This configuration allows the design of more compact
stations, in particularly when there is a large number of incoming overhead lines.
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Substation
Overhead line
Underground
cable
Figure 6 : Underground substation entrance
2.2.4 Power generator output
An underground cable may be used to carry power from an inaccessible generator to a busbar. In this
case, there is not room enough to put a breaker between the generator and the cable. In many hydro
power stations the generator is located inside a mountain. In order to save space the generator is
connected directly to the step-up transformer, without usage of a circuit breaker. The secondary side
of the transformer is connected to an outdoor substation via cable which may have a length up to
several kilometres. The substation (air insulated or gas insulated) is connected to one or more overhead
lines.
Overhead
line
Underground cable
Generator
Busbar
Figure 7 : Power generator output
2.2.5 Auxiliary supply
In this configuration, a cable is connected between a high power busbar and the auxiliary transformer
of a power unit. The cable is usually short.
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Transformer
Generator
Auxiliary
transformer
Busbar
Overhead
line
Underground
cable
Figure 8 : Auxiliary transformer supply
2.3
Cable
Although the present work is focusing on construction and installation techniques of extruded and selfcontained fluid-filled cables, it seems useful to give a brief overall view of the different types of cables
in service at the present time. These cables belong to two main families:
• cables with extruded insulation : extruded – dielectric cables
• cables with lapped insulation : SCFF, HPFF, HPGF. but SCFF are only considered here.
In this document, only extruded and SCFF cables are considered.
2.3.1 Extruded -dielectric cables :
Extruded-dielectric cables, also known as soliddielectric cables, have been introduced for medium
voltage cables in the fifties. The first high voltage
cables with extruded insulation on 110 kV systems
where installed in the 1960'. Insulation materials are
either Ethylene-Propylene Rubber (EPR), low/highdensity polyethylene (LDPE/HDPE) or crosslinked
polyethylene (XLPE). EPR, XLPE, LDPE and HDPE
have been in use for many years. XLPE becomes a
predominant choice for high voltage cables up to 500
kV level.
Maximum conductor temperature in normal operation
is depending on the insulation material: 70 °C for
LDPE, 80°C for HDPE and 90°C for EPR and XLPE.
Picture 2 : 400 kV XLPE cable
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2.3.1.1 Cable description
The conductor is in most cases stranded copper or aluminium, sometimes solid aluminium. For sizes
generally equal to or larger than 1200 mm² for copper and 1600 mm² for aluminium, the conductor is
segmented to reduce the ac/dc resistance ratio.
A semi-conducting bedding tape is sometimes wrapped over the conductor before extrusion. This
prevents the inner semi-conducting layer from entering the strand interstices during the extrusion
process and, in turn, facilitates removal for splicing and terminating.
The inner semi-conducting layer is extruded over the conductor or semi-conducting bedding tape. Its
purpose is to provide a smooth interface between the conductor and the insulation, and an uniform
electric field. It avoids the presence of air between metallic and insulation materials (no partial
discharge) and constitutes a thermal barrier in short-circuit conditions.
The insulation and outer semi-conducting layer are the other parts of the dielectric which are
preferably applied by triple extrusion process. Indeed, the simultaneous extrusion of the semiconducting layers and the insulation through a common (triple) cross-head is the best solution to
eliminate protrusions at the interfaces which are sources of high voltage stress points.
A metallic screen made with copper or aluminium wires and/or a metallic sheath carries the
capacitive current and the fault current of a specified magnitude and duration before reaching a
specified temperature.
A metallic sheath is normally applied to prevent the ingress of moisture. Its design must take into
account thermal and mechanical considerations. Since extruded dielectric materials have significantly
higher coefficients of expansion than metals, the radial volumetric expansion can be quite large. The
sheath must remain in good contact with the outer semi-conducting layer during heating and cooling.
A jacket or outer covering or oversheath (made of PE or PVC) prevents the corrosion of the metallic
sheath and isolates it from the ground. It is also required to protect the cable during handling and pulling
operations.
2.3.2 Cables with lapped insulation
Impregnated paper cables, widely used from the beginning of the last century, made possible
underground power transmission up to highest voltages. Many grids are still fitted out to a large extent
with these cables, even if they are replaced by extruded-dielectric cables to an ever-increasing extent.
2.3.2.1 Cable Description :
A conductor screen, containing carbon black or acetylene black, or a metallised paper, is lapped around
the conductor to provide a smooth interface between conductor and insulation and an uniform electric
field.
The insulation consists of either a pure cellulose material, a high-quality kraft paper or, more recently, a
laminated paper-polypropylene. Many individual crossed layers of tape (width 10 to 30 mm, thickness
0,06 to 0,15 mm) are helically applied to the thickness required for the rated voltage. According to
different methods, the cable is first dried in a tank and then impregnated with a degassed and dried
impregnating compound.
An insulation shield has the same function as the conductor shield on the outer side of the insulation.
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Every type of cable is subjected to load variations. The temperature cycling during operation leads to
thermal expansion and contraction of both the conductors and insulation materials. Small cavities may
appear in the insulation under the metallic sheath. The service life may be reduced by partial discharge
under high electric field. Therefore, a pressurising fluid with high-quality dielectric characteristics is
used to impregnate the insulation and fill the cable core. It increases dielectric strength, suppresses
ionisation in the insulation and delays moisture ingress in case of sheath leaking.
2.3.2.2 Self-Contained Fluid Filled Cable : SCFF
A self-contained fluid filled cable is internally pressurised with low viscosity dielectric fluid. Each
individual phase is contained within a hermetically sealed metallic sheath, typically extruded lead or
corrugated aluminium.
A central hollow core in the conductor provides a passage for dielectric fluid. The oil pressure
necessary to prevent from ionisation is 1 to 3 bar, but recent developments allow operation until 15 bar
high pressure.
For three core cables, the phases are generally contained within a common sealed metallic sheath,
again typically extruded lead or aluminium. Ducts located between the phase conductors provide for
passage of the dielectric fluid.
2.3.2.3 Impregnated Paper Characteristics :
Both kraft-paper and laminated paper-polypropylene insulations have normal operating temperatures
limits of 85°C, and allowable maximum emergency operating temperatures of 105°C.
The hydraulic system design must take into account the cable route and elevation differences to ensure
that all parts of the cable route are maintained at a pressure above atmospheric under all operating
conditions. In addition, the design must ensure that the pressure limits are not exceeded.
To achieve this, it is normal for longer routes to be divided into a number of hydraulically separate
sections by using stop joints which maintain electrical continuity but isolate adjacent cable sections
hydraulically.
In some applications, the cable is impregnated with a special non-draining compound.
2.4
Accessories
As a general note, we only discuss extruded cable in this section with no reference whatsoever to
SCFF accessory design issues.
2.4.1 General
The reliability and performance of a cable circuit is dependent in equal measures on the designs of the
cable and accessory and on the skill and experience of the person who is assembling the accessory.
The cable insulation is extruded or lapped in the factory under controlled process conditions using
selected materials of high quality. It is equally important that the same quality measures are employed
for the manufacture of the accessories in the factory and for their assembly on site onto the specially
prepared cable.
It is essential to select the design of accessory to be compatible with the particular cable type and the
particular service application. Compatibility should be validated and be supported by appropriate tests,
or service experience. In particular the performance of the accessory is dependent on the quality, skill
and training of the jointing personnel in the installation conditions and on the use of the specialised tools
required for a particular accessory.
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The itemised sub-headings below form the basis of the information that is needed by the manufacturer
and installer of the cable and accessories. For many applications the cable manufacturer also
manufactures, supplies and installs the accessories as part of the complete cable circuit, thus the
information is immediately available in-house. In the event that the user purchases the accessories
separately from the cable, then the following items form the basis of the questions that should be asked
to the manufacturers of the cable and accessories to ensure that the accessories are suitable. As the
design of the cable can depend upon the construction and installation technique, the
accessory design or selection must be made accordingly.
2.4.2 Accessory types
2.4.2.1 Types of joints
A joint is the insulated and fully protected connection between two or sometimes more cables. It is also
termed “splice”. The following types exist :
•
•
•
•
Straight joint,
Transition joint,
Screen interruption joint,
Y branch joint.
For SCFF systems, an additional joint is required to isolate adjacent hydraulic sections of the cable
route to ensure the system hydraulic pressure limits are not exceeded.
This is referred as a stop joint.
The design requirements common to each type of joint are :
• A high current connection between conductors,
• Joint insulation which meets the same performance standard as the cable,
• A high current connection to permit the flow of short circuit current between the two cable sheaths
or screen wires,
• A metallic joint shell or screen wire connection electrically insulated from earth potential to match
the insulating integrity of the cable oversheath,
• Protection of the joint and cable insulation against the ingress of water,
• Protection of the joint metalwork against corrosion,
• A tough protective sleeve against mechanical aggressions,
• A heat dissipating device to ensure that the joint is not a hot spot along the power link.
2.4.2.2 Types of terminations
A termination is the connection between a cable and other electrical equipment. It is also termed
pothead. The following types exist :
•
•
•
•
•
Metal enclosed GIS termination, (GIS: Gas Insulated Switchgear)
Oil immersed transformer termination,
Outdoor termination,
Indoor termination,
Temporary termination.
The design requirements common to each type of termination are :
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• A high current connection between the cable conductor and an external busbar,
• Insulation which meets the same performance standard as the cable,
• A high current connection to permit the flow of short circuit current from the cable metallic sheath
or screen wires via a bonding lead to the system earth,
• A connection to the cable metallic sheath or earth wires which is electrically insulated from earth
potential to match the insulating integrity of the cable oversheath,
• Protection of the cable insulation against the ingress of water and the ingress of pressurised
dielectric fluid from adjacent metalclad busbar trunking,
• Protection of metalwork against corrosion,
• Provision of support to the cable,
• Ability to withstand cable thermomechanical loads and external forces such as wind, ice and busbar
loading.
2.4.3 Compatibility of the accessory with the cable
2.4.3.1 Number of cable cores
The user should determine whether the cable construction is of single, three core or triplex construction
(i.e. three single core cables twisted together). The design of the accessory and the method of
assembly is dependent upon the number of cable cores; however it is unusual for three core extruded
cables to be employed above 60 kV.
2.4.3.2 Cable constructional details
For satisfactory service performance it is most important that the correct size of accessory is selected
to suit the particular cable. The outer diameter of the cable insulation, its tolerance and shape are
particularly important in the selection of an accessory employing a premoulded component, such as an
elastomeric stress cone or an elastomeric joint moulding. Such components are designed to fit a
specific range of diameters of prepared cable insulation, (that is with the insulation screen removed and
the insulation smoothed and shaped). The components must not be used outside this range. The
minimum diameter is determined by the need to achieve sufficient pressure to eliminate voids at the
interface with the cable insulation. The maximum diameter is determined by such considerations as a)
preventing damage by over stretching during assembly and b) limiting the maximum pressure at the
interface such that compression set of the cable insulation and moulded insulation is minimised.
The diameter and tolerance of the conductor and of its compaction (the radio of the effective cross
sectional area of the metal to the total area occupied) are needed in selecting a connector that will
exhibit stable conductivity and high mechanical strength.
The diameters and tolerances of the cable metallic barrier and over sheath are needed to ensure that
accessory metallic flanges and other components can be passed back over the cable during assembly.
The following dimensional and constructional details should be obtained by the user to ensure
compatibility of the accessory with the cable :
The detailed cable construction should be obtained from the cable manufacturer, which includes the
following information as a minimum requirement. Diameters, maximum and minimum tolerances,
eccentricity dimensions, construction and material need to be obtained for each of the following cable
components :
• conductor and special features (e.g. water blocking), if any
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•
•
•
•
•
•
conductor screen
insulation (ovality and eccentricity dimensions are required)
insulation screen
screen wires, if any
longitudinal water blocking, if any
metallic barrier, if any, for example whether an extruded sheath, a welded sheath, or a laminated foil
barrier. Also whether of cylindrical or corrugated form
• over sheath
• armour, if any
• special features (e.g. presence of optical fibre or pilot wires).
2.4.3.3 Conductor area and diameter
The user should ensure that the accessory has been designed and tested for the particular cable
conductor size. The electrical performance of an accessory design can become critical on large
conductor cables because of the high cable insulation screen stress.
The user should ensure that the conductor connections in the complete kit of components are supplied
to suit the particular conductor construction. The conductor connection must be capable of carrying the
same current as the cable conductor and must be capable of withstanding the cable longitudinal
thermomechanical forces, depending on the installation design, these being proportional to the
cross sectional area.
2.4.3.4 Operating temperature of the cable conductor and sheath
The operating temperature of the cable conductor and sheath under continuous, short term overload
and short circuit current loading have to be taken into account properly.
The materials of the accessory must be capable of operating satisfactorily at the operating
temperatures specified for the cable. IEC 61443 Standard may be taken as a reference. The short term
overload temperatures depend upon the type of cable and application. The temperature of the
conductor under short circuit is typically taken as 250°C for XLPE and 160°C for paper insulated
cable. The permitted short circuit temperature of the cable extruded metallic sheath or screen wires is
determined by the type of metallic sheath and thermoplastic over sheath, this temperature usually being
significantly less than that of the cable insulation.
2.4.3.5 Compatibility of the accessory with the type of cable insulation and semi-conducting screens
•
Physical compatibility with the extruded cable
The insulation of the polymeric cable must be identified by the user. There are significant differences
between the electrical and mechanical characteristics of extruded insulation. The usual insulants for
extruded polymeric cables in the voltage class of 60 kV and above being XLPE (crosslinked
polyethylene), LDPE (low density polyethylene), HDPE (high density polyethylene) and EPR (ethylene
propylene rubber).
•
Chemical compatibility with the extruded cable
The type of insulating liquid or lubricant used in joints and terminations should be identified to ensure
that these do not affect the properties of the polymeric insulation and semi-conducting screens
employed in the cable and accessories. For example a) hydrocarbon liquids at elevated temperature
can cause swelling of XLPE and EPR insulation and reduction of the conducting properties of screens
and b) silicone liquids can have an effect on silicone rubber components.
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•
Compatibility with the paper insulated lapped cable
In the case of transition joints between polymeric cable and paper insulated cable it is important to
establish whether the cable is of the internally or externally pressurised type and whether the fluid
dielectric is a gas or a liquid; these details will determine the performance requirements of the barrier
plate that segregates the two cables. In the case of mass impregnated non pressurised cables it is
important to determine the type of impregnating compound and whether it is of the liquid type or of the
non draining type; these details will determine the chemical suitability of the materials employed within
the joint to segregate the impregnating fluid from the insulation of the polymeric cable and joint.
Penetration of a hydrocarbon impregnating fluid into the polymeric cable can result in swelling and
modification of the electrical characteristics of the semi-conducting screens and insulation of both the
cable and accessory components, thereby reducing their electrical performance. Loss of the
impregnating fluid into the polymeric cable can result in eventual electrical failure of the paper cable.
2.4.3.6 Cable electrical design stresses to be withstood by the accessory
The user is advised to obtain the magnitude of the cable stresses at the conductor and insulation
screens, or obtain the dimensions of the cable, thereby permitting the stresses to be calculated. The unit
of stress is kV/mm calculated at U0 voltage. There are significant differences in the magnitude of the
electrical design stress employed in cables, these being dependent upon the type and thickness of
insulation, the conductor size, the system voltage and the lightning impulse voltage. It is essential that
the accessory has been designed and tested to operate at the particular cable design stress.
The stress at the cable insulation screen is of particular significance because this normally determines
the maximum design stress in the accessory. The insulation screen stress is usually of higher magnitude
in those cables designed for high system voltages and large conductor diameters.
2.4.3.7 Mechanical forces and movements generated by the cable on the accessory
The magnitude of the forces and movements generated by the cable on the accessory depends upon
the cable materials, the method of cable manufacture and the type of cable installation design (i.e. rigid
or flexible installation).
The following mechanical strains are dependent on the cable construction :
• insulation retraction (shrink back) (extruded insulation),
• insulation radial thermal expansion,
• oversheath retraction (shrink back).
The following forces are dependent upon the cable construction, current loading, operating temperature,
method and type of cable constraint and accessory design :
• conductor thermomechanical thrust and retraction,
• sheath thermomechanical thrust and retraction.
2.4.3.8 Short circuit forces
Electromagnetic forces are present during a short circuit between the individual conducting components
of the accessory and between the adjacent cables and the accessory. The following information is
applicable :
• method of restraint of the accessory and cable,
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• method of restraint and the spacing of adjacent cables.
2.4.4 Compatibility of the accessory performance with that of the cable system
2.4.4.1 Circuit performance parameters
The current rating and optimum circuit economics are dictated by the cable conductor size, cable
material costs and the method of installation. To achieve the optimum economical solution it is
important that the accessory design is not allowed to limit the performance of the cable. The accessory
must therefore match the following cable performance :
• Rated voltages(Nominal system voltage U and maximum Um),
• Current rating (Current magnitude),
• Continuous, cyclic and short time overload (Current magnitude, time and temperature),
• Short circuit rating, “ phase to earth ” and “ phase to phase ” (Current magnitude, asymmetry, time
and temperature),
• Basic impulse level (Withstand voltages for lightning impulse and switching surge), (Flash over
voltage for the system insulation co-ordination of outdoor terminations, if specified ).
2.4.4.2 Circuit life required
The accessory should match the design life specified for the particular cable circuit. This is typically
requested to be from 20 to 40 years, however some cable circuits are installed as temporary links, for
example in an overhead line circuit. Such accessories may be designed to be suitable for quick
assembly with a reduction in performance and service life.
2.4.4.3 Metallic screen bonding requirements
The following information is required on a) the type of bonding leads, (concentric or single conductors)
and their conductor size and overall dimensions and b) the type of cable bonding scheme, for example
solidly earthed or specially bonded metallic screens.
- Magnitude of induced sheath or screen wire voltage under normal and short circuit current,
- Magnitude of circulating sheath or screen wire current under normal loading,
- Magnitude of short circuit current,
- Magnitude of specified over sheath lightning withstand voltage and dc withstand voltage.
It is important that the accessory design incorporates means of connecting the cable screen wires,
metallic tapes or sheath and joint shell to the insulation screen.
2.4.4.4 Earth fault requirements
Some Utilities require that short circuit currents be returned within the cable system. The user should
ensure that the accessory is also able to withstand this current.
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2.4.5 Compatibility of the accessory with the cable system design and operating
conditions
The user is advised to ensure that accessory design is a) compatible with the particular cable
installation design, as this determines the mechanical loading applied, b) capable of being assembled in
the site environmental conditions and c) capable of a satisfactory service performance under adverse
climatic conditions.
2.4.5.1 Type of cable installation design
• Rigidly constrained (cable laid direct in the ground or close cleated)
• Flexible unconstrained (cable horizontally snaked or vertically waved)
• Semi-flexible (cable constrained, but permitted to exhibit a controlled deflection, for example at a
bridge crossing or adjacent to gas immersed switch gear)
• Unfilled duct.
2.4.5.2 Standard dimensions for cable termination
The user is advised to ensure the following dimensional compliance :
• Outdoor and indoor termination :
Harmonisation with existing equipment of the overall height of the off-going bus bar connector and of
the bottom metalwork fixing arrangements to the support structure.
• GIS and transformer termination :
Harmonisation of the cable termination with both the design of the metal clad switch gear (internal
diameter, overall length, off-going bus bar connector, bottom metalwork sealing arrangements and
pressure) and the design of the support structure (fixing arrangements for the particular cable
constraint selected).
2.4.5.3 Types of accessory installations
•
•
•
•
•
•
•
Buried in the ground (laid direct)
Jointing chamber,
Tunnel,
Above ground,
Bridge,
Tower,
Shaft.
2.4.5.4 Jointing limitations in restricted installation locations
• Space limitations,
• Time limitations (for example arising from road or rail traffic influences),
• Tolerance limitations of assembly personnel (for example arising from extremes of temperature,
humidity, vibration, noise and induced voltage).
2.4.5.5 Mechanical forces applied to the accessory
• Thermomechanical forces
• Earthquake,
• Vibration,
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•
•
•
•
•
•
•
Off-going bus bar at terminations,
Wind loading on bus bars at terminations,
Ice loading on bus bars at terminations,
Short circuit loading on bus bars at terminations,
GIS pressure,
Angle of installation of terminations,
Hydraulic or pneumatic pressure forces at transition joints.
2.4.5.6 Climatic conditions
Accessories require to be suitable for the extremes of climatic conditions expected both in service and
during assembly. Some types of accessories are required to be assembled under controlled
environmental conditions.
•
•
•
•
•
•
•
Altitude (reduction of electrical strength of air),
Air pollution (reduction of electrical strength of outdoor insulator surface),
Precipitation (reduction in electrical strength of air and outdoor insulator surface),
Salt fog (reduction in electrical strength of outdoor insulator surface),
Moisture condensation (reduction in electrical strength of insulator surface),
Temperature,
Atmospheric humidity.
2.4.5.7 Type of accessory outer protection required
The accessory protection is required to provide corrosion protection and, for a specially bonded cable
circuit, insulation from ground.
•
•
•
•
Joint box (laid direct in the ground or in air),
Pedestal insulator (in air),
Moulded sheet insulation (in air, to protect personnel against electric shock),
Metallic fences or screens (in air, to protect personnel against electric shock).
2.4.5.8 Situations requiring special accessory protection
• Submerged under water,
• Fire risk,
• Termite infestation.
2.4.5.9 Quality Assurance scheme for accessory installation
Assembly of the accessories onto cable with extruded insulation is the most vulnerable part of a project
involving the manufacture. Accessories and cables are manufactured and tested under controlled
factory conditions, whereas the in-service performance of the accessory is dependent upon the training,
skill and reliability of the personnel, who are often required to work under adverse site conditions.
For many project applications one company will manufacture the cable and accessories and undertake
to complete the installation of the circuit. In other applications the installer may complete the circuit
using cable and accessories supplied by different manufacturers. In some applications the installer may
only assemble the accessories. For each application the requirements of the QA system are equally
rigorous :
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•
Quality Assurance approval for installation
The user should ensure that the installer provides evidence of an approved quality assurance system for
installation to an internationally recognised standard.
•
Quality Plan
The installer is required to produce a Quality Plan for each project, this includes the project time
schedule together with the requirements for suitably qualified personnel, training, on-site storage of
components and accessories, tools, testing equipment, constructing materials, assembly instructions,
preparation of the jointing environment and records of the assembly work. It is important that the
records of assembly are traceable to the location of each accessory in the cable circuit. If purchasing
separately, the user is advised to ensure that, for the purposes of traceability, the quality systems of the
cable manufacturer, accessory manufacturer and installer are compatible.
2.4.5.10 Training of Personnel
When selecting the designs of accessories the user should ensure that training courses are available for
the jointing and supervisory personnel. It is strongly advised that personnel receive training on the
particular designs of accessories and cable.
Examples of the elements of a training course for assembly personnel are :
• General training at specific system voltages with the standard range of accessories required by the
user
• Repeat training after a defined period for those personnel who have completed general training
• Specified training on a new accessory or cable design for those personnel who have completed
general training.
At the end of the training course the proficiency of the assembly personnel is normally assessed, for
example, by a verbal or written examination, by a practical test and preferably by performing on the
assembled accessories an electrical partial discharge test and voltage withstand test.
Proficiency is recognised at the completion of training by the issue of a certificate, which should be
checked by the user as part of the quality plan for a specific project. In many instances a kit of general
jointing tools and a set of general assembly instructions is also issued to the personnel following
satisfactory completion of training.
2.4.5.11 Assembly instructions
The accessory manufacturer is required to supply a complete set of assembly instructions together with
drawings of the particular accessory.
The instructions should also include lists of the specified assembly tools, the specified consumable
materials and the health and safety precautions. Recommendations for the preparation of the assembly
environment should also be given.
It is important that the user studies the instructions before work begins to ensure that the workplace is
correctly prepared and that all the tools and consumable materials are available.
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2.4.5.12 Special assembly tools
Most designs of accessories, particularly those operating at higher system voltages, require special tools
which are purchased or hired from the accessory manufacturer. The user should ensure that full
instructions are provided and that the personnel are trained in their use. These tools may take the form,
for example, of a) hydraulic compression presses or welding equipment for connecting the conductors,
b) cutting equipment to remove the insulation screen and to shape the cable insulation c) assembly
machines which stretch and position pre moulded elastomeric components, d) taping machines that
apply tape and e) heated mould tools and mobile extruders for field moulded joints.
2.4.5.13 Preparation of the assembly environment
It is strongly recommended that the assembly area for both joints and termination to be enclosed within
a tent or temporary building, with the objective of providing a clean and dry environment. The enclosure
should be a) well lit to facilitate accurate preparation of the cable insulation, b) provided with a sound
floor and c) lined with sealed materials to facilitate cleanliness. In extremes of climate it is good
practice to provide control of temperature and humidity to ensure a) consistent performance of the
personnel and b) consistent properties of the polymeric materials.
•
Joint assembly :
• An appropriately sized joint bay or chamber.
• The provision of a temporary and/or permanent support for the completed joint.
•
Termination assembly :
• A permanent support structure.
• A temporary weatherproof structure during assembly.
• Means of lifting the cable and insulator into position.
2.4.6 Compatibility of the accessory with specified after laying tests
When the installation of the cable and accessories has been completed it is standard practice to
perform electrical tests to demonstrate that the assembly of the accessories is of satisfactory quality
and that mechanical damage to the cable and accessories has not occurred during installation.
The following tests can be performed. It is important to ensure that the accessory design is suitable for
the particular test :
2.4.6.1 Voltage test on main insulation
DC tests have been traditionally applied to transmission circuits, however their use on cable with
extruded polymeric insulation is not recommended. Experience has shown that the dc voltage test is not
always sufficiently sensitive to detect damaged cable insulation or incorrectly assembled accessories
and hence prevent them from entering service. In particular the electrical stress distribution under dc
voltage in an accessory is usually significantly different from that under ac voltage in normal service.
The application of an ac voltage is now under evaluation as an after laying test, either by the application
of service voltage from the transmission system or by the application of test voltage from mobile test
equipment.
2.4.6.2 Partial discharge detection
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Partial discharge detection techniques are at present being developed for some cable and accessory
applications to check for the absence of damage to the cable during installation and incorrect assembly
of the accessories. Methods are not yet available for this to be done in a simple manner as a routine
commissioning test on normal cable circuits.
2.4.6.3 Voltage withstand test on the cable over sheath and joint protection
It is usual for specially bonded cable systems, including their accessories, to be subjected to an after
laying test comprised of the application of a dc withstand voltage applied to the metallic sheath or
screen wires.
2.4.6.4 Current balance test on the cable sheath and screening wires
This test is performed on cross bonded cable systems at or adjacent to accessory positions to confirm
that a) the bonding connections of the accessory are correct and b) the cable lengths and spacing are
symmetrical, such that the magnitude of residual circulating current is of an acceptably low magnitude.
2.4.7 Maintenance requirements of the accessory
The user should ensure that adequate maintenance tests and checks have been recommended by the
cable and accessory suppliers, for example :
2.4.7.1 Monitoring of fluid insulation
Liquid and gas levels : some types of termination, straight joints and transition joints are filled with
insulating liquid or gas and may require to be regularly inspected or monitored in service to ensure that
neither the liquid or gas have escaped.
2.4.7.2 Voltage withstand tests on the over sheath and joint protection
These tests are similar to the after laying tests, but are usually performed at reduced voltage levels.
2.4.7.3 Shelf life of accessories for emergency spares
The user should ensure that information is provided on the shelf life of the components in an accessory
for long term storage as these may vary according to the type of material, the way they are packed and
the appropriate temperature and humidity conditions of storage.
2.4.7.4 Availability of accessory kits for emergency spares
The user is recommended to obtain either a sufficient stock of spare accessories or to have an
agreement with the manufacturer to supply accessories at short notice. The design of an accessory for
emergency use may be different from that installed.
2.4.8 Economics of accessory selection
A comparison of the relative costs of different designs of accessory kits should not be undertaken
without giving due consideration to the total costs of installation and assembly. The following are the
main items of cost :
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2.4.8.1 Cost of the accessory complete with all components
The accessory design should be checked to ensure that it is a complete kit and will be supplied with all
the components and assembly instructions for the particular application. Some components that may not
necessarily be supplied by all accessory manufacturers are for example a) conductor connections and
anti-corrosion protection for joints and b) bus bar take-off connectors and support metalwork for
termination.
2.4.8.2 Cost of guarantee and insurance
At the higher system voltages it is more usual for the cable and accessories to be supplied, installed and
guaranteed as a “ turn-key ” project. Under such circumstances the guarantee will usually extend to a
specified number of years in service. If the user decides to divide the supply and installation of
accessories between companies, it is recommended that the cost of financial self insurance be
considered, because the responsibility for an accessory failure in service can be difficult to apportion
between the accessory manufacturer, the cable manufacturer and the installer.
2.4.8.3 Cost of assembly time
The jointing time required to assemble accessories can differ dependent on their design. Similarly the
time required to assemble the anti-corrosion protection and the final mechanical support to the
accessory can be the over-riding factors in determining the jointing time.
2.4.8.4 Cost of preparing the installation environment for the accessory
Accessories require the provision of a weatherproof enclosure together with the environmental
conditions necessary for jointing (e.g. good lighting, cleanliness and, when necessary, air conditioning.
The supply of electricity and gas may be required).
2.4.8.5 Cost of safe working conditions
In addition to the cost of constructing the installation environment to comply with the regulations for
safe working practices , the provision may be required for temporary and permanent protection to a)
the installer's personnel from electric shock during assembly and b) the user's personnel when the
accessory is in service.
2.4.8.6 Cost of special jointing tools
There may be significant differences in purchase cost and hiring charges of the tools required for
different accessories.
2.4.8.7 Cost of training
Qualified jointers who are trained to assemble the particular accessory should always be employed.
The user should decide whether it will be more cost effective to a) employ qualified and experienced
personnel to assemble the accessories, or b) employ qualified and experienced personnel to install the
cable and assemble the accessories as part of a turn-key contract, or c) incur the on-going costs of
training and regular repeat training for his own personnel.
2.4.8.8 Comparative cost of cable and accessories
The design of the cable can influence the cost of the accessory design. Thus a reduction in the cost of
the cable construction may result in an increase in the cost of the accessories. Similarly an increase
in the cost of installation by laying longer lengths of cable may achieve a reduction in overall
costs by requiring fewer joints.
2.4.8.9 Cost of verification of accessory performance
If a type test report is not available for the particular cable and accessory in combination then the user
is advised to allow for the cost of performing a type approval test. This cost may be born by the
supplier, in the case of a turn-key project, but this is less usually so in the case of separately supplied
cable and accessories.
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3. CONSTRUCTION TECHNIQUES
Twelve high voltage cable construction techniques have been identified and are reported in this section.
Four are considered traditional while eight are labeled innovative. They are being used to a varying
degree by different companies around the world.
3.1
Definition of the main technical terms
See the glossary, page 139.
3.2
Description of traditional techniques
Among the twelve identified techniques, four are categorized as traditional. This is mainly because
most or all companies around the world consistently use one or more of them in laying high voltage
cables. They have been successfully used for many decades due to their simplicity, relatively low cost
and the availability of materials and equipment as well as qualified entrepreneurs to execute the
necessary work.
3.2.1 Ducts
3.2.1.1 Description of the technique
Ducts are normally used jointly with manholes in a system that is
favored in urban areas of major cities for its convenience. It offers the
possibility of carrying out the civil work independently from the
electrical work. Also, the flexibility of cable maintenance or
replacement with minimum disturbance to local traffic and economic
activities are considered advantageous. In less congested areas, joint
bays would replace manholes to reduce cost.
Picture 3 : PVC ducts – double circuit
Three or more ducts having the proper diameter and wall thickness are placed in a trench at the predetermined depth and configuration. A layer of special bedding material having low thermal resistivity is
placed on the bottom of the trench prior to placing of ducts. Thinner wall ducts could be encased in
concrete to form a duct bank. Ducts could also be stacked in two or more layers to accommodate the
required number of cables to be installed. Special spacers are used to ensure the exact configuration
and to allow concrete to flow between ducts.
Reinforcing steel rods should be used in special cases such as crossing under railways in order to
increase the rigidity of the duct bank .
In some cable sections, the space between cables and ducts could be filled using special materials to
enhance cable current carrying capacity or restrict its movement. This is recommended in excessively
deep installation or when difference in elevation between manholes is substantial.
Manholes are underground chambers built to house the joints and other auxiliary equipment such as
fluid feeding tanks, sheath cross bonding cables and sheath protection surge arresters. Access to
cables and joints is easy using fixed or removable ladders installed in two or more chimneys depending
on manhole design.
Manhole dimensions depend on the number of cables to be jointed as well as the circuit voltage.
Metallic structures are usually used inside the manholes to support cables and joints. All metallic steel
members inside manholes should be properly connected to a solid ground rod or bare ground cable loop.
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Joints should be protected from mechanical forces due to cable expansion under load cycles either by
expansion loops or by a rigid clamping system. Manhole dimensions could be reduced if joints and
cables are rigidly clamped.
Manholes should be designed in accordance with local standards to withstand normal road traffic loads.
Most manholes are built in place using reinforced concrete. However recent development in pre cast
concrete made it possible to use high quality prefabricated manholes. This would reduce the time
needed for assembling the factory pre cast concrete slabs forming manhole walls and the roof. Floors
are still poured in place to allow for proper ground leveling and ground water drainage.
Joint bays offer a more economic way to house and mechanically protect the joints. They could be
regarded as the lower half of manholes. They could also be built in place or assembled using pre cast
concrete slabs Temporary shelter should always be used during jointing operations to protect workers,
cables and joints from the elements and ensure a clean environment for jointing operations. Once the
joints are completed , the joint bay is filled with thermal back filling material and top slabs placed over
the entire length. A warning tape is usually placed about 30 cm below grade level.
Joints are not accessible in joint bays. Sheath testing is possible using link boxes located either above
ground or in below ground accessible pits where cross bonding cable leads are connected.
High Voltage cable circuits are normally installed in dedicated duct banks, often one cable per duct.
However, for economical reasons, three cables could be installed in the same duct in case of lower
voltages. Also two circuits (six cables ) could be installed in the same duct bank. It is not recommended
to install more than two circuits in order to reduce the risk of cable damage due to accidental
excavation. This would enhance underground system availability as well as maintain a reasonable cable
load rating.
Laying cables in ducts is considered one of the safest type of installation regarding safety in case of
short circuit. It should be noted that a good earth cover over the duct bank is necessary to ensure
public safety. It is also worth mentioning that manholes could present a safety hazard in case of cable
or joint explosion.
Empty ducts could be used for a reserve cable provided that sheath bonding is designed accordingly.
Fiber optic communication cables could also be installed in the same duct bank .
3.2.1.2 Limits of the technique
•
Civil work
Civil work includes excavation of trenches and shoring them if necessary, relocation of existing
services, placing of ducts and spacers, pouring of concrete to form duct banks and covering them with
the proper back filling materials as well as reinstating of all surfaces to their original conditions. It is
recommended that construction of necessary manholes or assembling prefabricated ones is often
carried out after ducts have been securely placed. Compacting of back filling materials as well as of
the soil layers is essential in order to obtain a low thermal resistivity.
Long cable lengths could be pulled through straight duct sections provided that cable reels could be
transported to site. However, due to factors related to cable route that have to follow existing road and
street network, land topography and existing subterranean services, almost all cable routes include
bends and offsets that would increase the required cable pulling tensions and thus limit distances
between manholes. Cables installed in ducts rarely exceed 800 meters. In major cities the maximum
length of open trenches at any given time may be limited by local authorities to a few hundred meters.
•
Drying of the soil
Over the years, soil drying may occur due to change in back filling materials properties, presence of
tree roots or higher than normal cable operating temperatures. This could be avoided by using a proper
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back fill, compacting of different layers during installation, keeping a tree-free zone along cable route
and ultimately by monitoring of cable's temperature or that of the surrounding soil.
Use of thermocouples or fiber optic cables particularly during peak load periods and hot and dry
weather spells would ensure this feed back.
•
Water drainage
Water table level varies with location. In some areas, abundant surface water could hinder civil work
progress. Water seeping through the ground during construction should be pumped out, using
appropriate equipment, to ensure personnel safety as well as quality of work.
Although the presence of water around cables and accessories could be considered somewhat
beneficial, many utilities do not allow it to accumulate in ducts or manholes to prevent possible
premature deterioration of cables and accessories. Ducts would be installed with a continuous slight
slope towards manholes. Manholes would be connected to city sewage or storm draining systems
through an anti pollution arrangement particularly in the case of fluid filled cables. Local regulations
should be followed and authorization should be obtained for these connections.
•
Temperature of the soil/environment
Ducts could be installed in soils that are naturally warm provided that some forced cooling arrangement
is foreseen.
•
Hardness of the soil
In hard rocky soils it would be advantageous to consider alternative techniques to install cables such as
micro tunneling described in this document. Technical and economic studies should be carried out in
order to compare different viable alternatives.
•
Stability of the soil
Different soil formation could exist along any cable route. Soil should be tested and its properties
investigated by carrying out on-site and laboratory tests. Soil stability should be ensured prior to
installation of ducts or duct banks.
•
Thermal resistivity of the soil
Soil resistivity should be measured along cable route using appropriate instruments to determine the
need for replacing native soil by special thermal back filling. Some laboratory measurements could also
be useful in establishing the maximum thermal resistivity and percentage of water content by weight of
soil samples.
Back filling materials having higher thermal resistivities than that assumed in cable design calculations
could lead to higher cable operating temperatures, soil drying out and eventually dielectric breakdown
due to thermal runaway. Back filling of trenches should be done in layers that are properly compacted.
Local regulations could influence the choice of back filling materials.
•
Seismicity
Ducts could be used in seismic risk areas provided that they have been designed to withstand the
expected earth tremors. Both rigid and flexible designs would be acceptable. Some experimental work
on a model are advisable.
•
Frost
Frost and ground freezing occurs for short or long duration in many countries. Ducts and duct banks
should be placed below the expected frost line in order to avoid damage due to ground movement
caused by (severe and frequent) freezing and thaw cycles.
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In extreme cases where ground is permanently frozen, special arrangement such as placing insulating
materials underneath the duct bank is recommended. Non insulated duct installation would risk being
damaged due to soil instability caused by heat dissipated from cables. Cable failure might result.
•
Archaeology
Sensitive archaeological areas should be avoided when cable route is selected. However should
archaeological finds be encountered during excavation, work should be immediately stopped and local
authorities advised .Depending on the importance of the findings, some countries would allow work to
continue after proper investigations and documentation are completed. In other cases ,an alternative
cable route might have to be chosen.
•
Presence of termites
Cables should be designed to have an anti-termite protection and ducts should be blocked using a
proper sealing material such as" ductseal".
•
Laying in National Park
Local authorities should be consulted and if necessary, alternative techniques such as directional drilling
be used to minimize digging in sensitive areas of national parks.
•
Duration of the work
Civil work duration depends on many factors. The major factors to be considered are, access to site,
the nature of soil, depth of excavation, presence of underground services, type of equipment used,
weather conditions and restrictions imposed by local authorities.
Average construction duration vary also according to the size of the project. Some values, including
cable installation, were reported in CIGRE joint working group 21/22-01 report issued in may 1996.
•
Maintenance and repairing process
Manholes should be periodically inspected to ensure their structural integrity.
Although cables installed in ducts are inaccessible, joints could be inspected at manholes. Visual
inspection of joints and cables could be done after pumping out any water from manholes. At joint bay
locations, only sheaths transposition cables could be reached through hand-holes.
Periodical jacket testing could be performed from these points. Insulation or jacket faults could be
localized using different techniques. Repair should be carried out. This work would require some
excavation at fault location.
In case of major problems, an existing cable section, between two manholes, could be replaced without
any excavation.
•
Cable removal after operation
With the introduction of new international standards for environment protection cables would have to
be removed at the end of their useful service life and their components disposed of and recycled. It is
usually possible to remove cables from ducts without excavation. However, some sections might prove
difficult or impossible to remove due to cable snaking, accumulation of dirt, deterioration of ducts and
ground up-heaving. In these cases new excavation permits would have to obtained to gain access to
cables at locations between existing manholes.
Structural integrity of empty manholes should be investigated. Local authorities could impose the
demolition of manholes for safety reasons.
3.2.1.3 Adaptation of the technique to the cable system design
Duct and manhole system is well suited for cable installation in congested city core areas. In designing
cable systems to be installed in ducts many electromechanical factors should be carefully considered
together with civil engineering aspects, such as :
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• proper cable size for the required load (larger cables are required for duct installations as compared
to directly buried or in air)
• maximum pulling tensions required for cable installation
• metallic and non metallic sheaths for cable protection
• maximum induced sheath voltage and allowable sheath currents
• clamping of cables and joints in manholes if necessary
• sheath permutation and protection schemes
• grounding in manholes
• size and location of manholes
• size and type of ducts
3.2.2 Direct burial
3.2.2.1 Description of the technique
This method consists of digging a
trench and directly placing the cables
in it.
This technique is extensively used
world-wide for extruded cable as well
as for fluid filled cable. Indeed, in the
60 to 170 kV range it comes second
only to laying in ducts, whereas for
the voltages between 200 to 500 kV it
ranks just after the laying in tunnels
and the laying in ducts.
Picture 4 : Direct burial
This solution is particularly interesting economically, since apart from digging and backfilling the trench
no other heavy works are necessary. This is why the technique is used in urban as well as in rural
areas. HV cables are usually installed along the public ways. As far as possible installation in private
ground is avoided.
An advantage of this method is that the route of the link can easily be deflected to avoid unforeseen
obstacles.
The depth of the trench is such that in most cases the cables have an earth cover at least one metre
thick (this often is a legal requirement or this can also depend on the short-circuit levels).
Cables are usually laid in trefoil formation. Every metre an adequate non-corrodable clad or rope is
wrapped around the cables to keep the trefoil formation during the backfilling of the trench. The other
type of laying configuration is the flat formation which is used mainly for cables in the 220 to 500 kV
range (depending on the carrying capacity).
Trench width obviously varies according to the type of formation and the voltage level of the cables :
• width <0.8 m (60 to 170 kV) and close to 1.0 m (220 to 500 kV) in trefoil formation;
• width close to 1.0 m (60 to 170 kV) and >1.0 m (220 to 500 kV) in flat formation.
Over the backfilling material cable-protective slabs are placed.
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Above these slabs the telecommunication cables are usually placed (running mostly in ducts).
3.2.2.2 Limits of the technique
•
Civil work
The civil works are identical to those required for duct laying, except that the cables are laid directly in
the trench on a bottom layer of materials intended to protect them from any sharp rocks likely to be
present in the bottom of the trench.
The backfilling materials used to fill the trench are composed, starting with the protective bottom layer
referred to above, of sand, special backfill or possibly lean concrete. It is not so frequent that the
excavated soil or concrete are used for backfilling.
Weak mix may be used instead of the normal backfill to increase the mechanical protection around the
cables.
In many countries, a special backfill (so-called controlled backfill) is used in order to create a low
thermal resistivity environment to dissipate the heat released by the power cables (this greater
dissipation allowing an increased power rating of the link).
In this respect, the use of fibre-optic cable or of optical fibres in HV cable, although not yet
generalised, brings certainly advantages in the future as it will make possible permanent thermal
monitoring of the link (giving precise knowledge about the thermal environment of the cables).
Civil works include also the excavation of joint pits. The size of these pits is naturally larger than the
trench (width and depth) itself, and may vary according to the voltage level, the type of joint and layout
(in parallel or longitudinally).
The length of the trenches is often defined by the size of the cable drums, drum-size itself being often
dependent on the means and possibilities of transport and handling (but the length can also depend on
the calculated pulling tensions if they exceed the limits, environmental aspects, …). Furthermore, in
urban situations, especially taking into account of the traffic, opening a trench several hundreds of
metres long may give rise to problems. Accordingly, local authorities may restrict the length of open
trenches (with increase of joint's number), restricting the periods of the year or week and sometimes
the hours of day during which work may be done.
This problem is the more critical, in terms of site planning and organisation, when controlled backfill is
being used as this material must be very carefully applied (degree of humidity, of compaction) and
closely inspected by a laboratory.
The length of the trench may be some hundreds of metres, hardly ever exceeding 800 metres.
•
Drying of the soil
Soil drying can occur in the immediate vicinity of cables in service, due to migration of the humidity
from the hot zone to the cold zone (so increasing the thermal resistivity of the dry zone).
This phenomenon can also be observed when the link runs parallel to certain types of vegetation (trees
that have deep roots, …).
In certain regions the soil may be permanently dry naturally.
Various solutions can be considered :
• use of controlled backfilling material;
• creation of a ‘root-free’ corridor;
• a different configuration for the cables (in flat formation instead of trefoil formation) to improve the
rate of heat dissipation of each cable.
These points can have influence on the type or the width of the trench but don’t really change the
manner of working.
•
Water drainage
Conversely to the soil drying phenomenon, wet soil may seem more favourable to laying of HV cables
because it constitutes a natural cooling system.
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If the water can flow out, there is nevertheless a risk of erosion of the soil and the materials embedding
the cables.
The execution of trenches in wet soil often calls for installing substantial means of drainage (pumps,
etc.) in order to avoid erosion of the trench edges leading to collapse or "flooding".
•
Temperature of the soil/environment
Construction of trenches in a soil that is naturally or artificially (old mining areas) hot does not
apparently cause any particular problems.
However, it remains obvious that too hot a soil will significantly reduce the power transit capacity of
the link.
•
Hardness of the soil
Laying of cables in particularly rocky soil, although technically feasible, is not advisable, as excavating
the trench will require costly heavy equipment (and entails risk of equipment damage).
Work planning has to make due allowance for the difficult conditions, to avoid falling undue delay.
•
Stability of the soil
Construction of trenches in unstable soil such as in marshlands implies that the trench must be
completely shored (with sheet piling if necessary) and that drainage systems must be installed.
Considerable cost savings are possible by carrying out a survey of the subsoil along the intended route,
so that the costly or difficult route can be avoided by selecting another route for the trench.
•
Thermal resistivity of the soil
Soil resistivity considerations have little influence on the choice of the construction method. However, it
does have a considerable impact on the power-transit capacity of the link.
Accordingly, soil samples have to be taken for analysis well in advance of the start of the works in
order to determine the characteristics of the cables (cross-section, material, …), their configuration
(trefoil or flat formation) and decide whether or not to use controlled backfilling. For a selected backfill
to achieve an average resistivity, the size of the trench can also be influenced by the results of the
analysis.
•
Seismicity
Cables laid in plain soil incur the stresses generated by soil movements. However, the soil movements
do not normally seriously affect the cables as the cable components have a certain elasticity.
Nevertheless, it is obvious that a large crack in the soil at right angles to the link could seriously damage
the cables.
•
Frost
Frozen soil renders trench digging difficult (or impossible) due to the hardness of the soil. The
conditions are also arduous for the workers who build the trenches, as they have to work in subzero
temperatures and for longer times at the site as work progress is slower in hard soil.
In particularly cold regions there may be prolonged periods of the year where work is not reasonably
possible.
It can also be mentioned that the mix of frost and water during the thaw can swell the soil (with
possibly the collapse of the trench).
•
Archaeology
If during route selection for the link it is not possible to avoid a potentially archaeological area, it must
be borne in mind that, due to local authorities, any finds made during excavation will make it
mandatory to stop the work, at least in the portion where the find has been made.
Work at the site is then delayed until the archaeologists have investigated the find. The length of the
portion affected and the duration of the stoppage depend on what further discoveries are made.
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•
Presence of termites
In soil infected with termites there are two possible ways of protecting the cables :
either
• by placing an anti-termite chemical in the outer covering of the cables, or wrapping an anti-termite
cloth ribbon under the outer covering. However, international environmental guidelines increasingly
forbid these options.
or
• by inserting each cable in a steel pipe (these being different techniques from direct burial) or
providing a steel covering around all the cables.
For other pests it may be effective to lay the cables at a depth where these pests are not usually active
(for instance, rodents do not normally stray below a depth of 80 cm).
•
Laying in National Park
Certain local or national authorities may make mandatory a different laying technique in order to
preserve the natural environment (for example, directional drilling instead of direct burial).
Assuming digging a trench is allowed, a number of particular recommendations or stipulations will have
to be complied with restoration of the soil and the vegetation.
•
Duration of the work
When the duration of the work would otherwise be too long, a different technique may be imposed.
This may be the result of the already being other rights of soil occupancy (e.g. utilities, telecom) or of
local circumstances (important road crossings, residential areas) where the local authorities would
impose different techniques (for instance, laying in ducts or directional drilling).
•
Maintenance and repairing process
Once the link has been built with the direct burial method, the only points where access remains
possible are the extremities where the cables emerge from the soil for connection to terminals, and
possibly the cable shields at the joints between cables. These are the only places where direct visual
inspection is possible.
Nevertheless the link operator can still perform tests on the outer covering (generally a DC. test).
Defect location and repair will always necessitate some excavation work. If the cable defect is a
substantial one, the civil works will also be substantial, as the deteriorated length of cable will have to
be replaced, and two new joints made.
•
Cable removal after operation
Cable removal at the end of his life has been definitively stopped represents a huge amount of work
and cost, because the trenches will have to be completely reopened. This is the reason why these days
the disused links are usually abandoned in place.
However, this may result in environmental concern if the cables are the fluid-insulated type. The fluid
should be regularly drained by pumping it from the central channel so as to avoid the risk of fluid
leaking into the soil.
New developments to remove the cables with a trenchless method are at the present time under
investigation.
•
Adaptation of the technique to the cable system design
For a well-defined cable system the choice made concerning the conductor materials has a significant
effect on the link construction method and cable laying technique, on account for instance of the
difference in weight between copper and aluminium.
The lighter cables (aluminium) allow to have drums with longer length of cable (but with a different
carrying capacity for the same size in copper), in turn allowing the laying of longer lengths.
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During laying of lighter cable the drawing strain is applied to the entire cable by using stockings,
whereas for the heavier cable the drawing strain is applied to the conductor itself by means of a
specially designed pulling grip.
3.2.3 Tunnels
The reason for selecting the tunnel instead of ducts.
The tunnel is usually used when a lot of cables will be laid out in that particular section, which results in
difficulty to ensure the transmission capacity required.
Shield method
Shield is a kind of tunnelling method designed to operate even in poor subsoil. Tunnels are excavated by
a tunnel driving machine known as a "shield machine" and tunnel wall is constructed by fixing a prefabricated circular pre-cast members called "segments" against each other using bolts.
By using the Shield method, circular tunnels with diameters from Ø‘1800mm up to Ø‘14000mm could
be bored.
3.2.3.1 Description of the technique
A tunnel is used for cable accommodation when a lot of circuits must be installed along one particular
route, when it is difficult to secure the required transmission capacity using direct burial or ducts.
It is also used within urban areas where the logistics of using other techniques at ground level are
insurmountable.
A tunnel is constructed by open-cut method, shield method, or pipe jacking method. Pipe jacking
method is described in Section 3.3.4. Shield method and pipe jacking method are similar in their shapes.
The difference of them is just construction method and only shield method is described here.
a) Open Cut Method
Open cut is a method of constructing a tunnel. First, excavate from the ground surface and then build
the tunnel in required location and restore the ground surface by the backfill.
The most common method is generally the full face one.
b) Shield Method
When Open-cut method cannot be used, the Shield method should be used. It may be applied where
the road traffic is too heavy or the tunnel to be constructed too deep to excavate from ground surface
because of keeping away from other underground equipment such as telephone cables, gas pipes,
water pipes, sewage pipes, subways, etc.
Shield method can be used when the subsoil is poor. A shield tunnel is excavated by a tunnel driving
machine known as a "shield machine" and tunnel walls are constructed by fixing pre-fabricated circular
pre-cast members called "segment" against each other using bolts. Circular tunnels with diameters from
Ø 800 mm up to Ø 4000 mm have been constructed in Japan.
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Picture 5
: Open cut gallery
Figure 9 : Tunnel boring methods
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Figure 10 : Shield machine
Ventilation is generally used in tunnels for human safety. Ventilation also dissipates the heat generated
by the cables, thus increasing the transmission capacity compared to direct burial or ducts. When larger
transmission capacity is required, cooling system may be applied.
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Figure 11 : Cooling system in tunnel
Cables in tunnels may be installed in a rigid configuration (normally when there is not much space
available) or, more commonly, in a flexible configuration. In the latter case, both vertical and horizontal
snaking are used, depending on practical considerations. Unfilled troughs with horizontal snaking may
also be adopted.
3.2.3.2 Limits of the technique
•
Civil Work
Since tunnel construction method is much more expensive than construction of ducts by open cut
method, it is necessary to carefully evaluate the construction cost. Construction of a tunnel is
economically unfavourable when there are only a few circuits to be installed. At the time of
constructing shield tunnel, all the route need not be excavated but land for shafts is necessary. Land for
ventilating facility is necessary for both open cut tunnel and shield tunnel.
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•
Drying of the Soil
When the tunnel temperature is low enough for human beings (e.g. 40 degree C), drying of the soil
need not be considered.
•
Water Drainage
Tunnel is designed as an almost waterproof structure. Seepage water into the tunnel is drained into a
pit and pumped to the outside.
•
Temperature of the Soil / Environment
Transmission capacity is decreased if the surrounding soil temperature is high or if the air temperature
within the tunnel is high.
•
Hardness of the Soil
Much power is required for excavation in case the soil is hard. But it is not a fatal problem.
•
Stability of the Soil
Stability of the ground is fundamental to the construction of the tunnel since any settlement must not
compromise the integrity of the tunnel and shafts.
•
Thermal Resistivity of the Soil
Compared to direct burial or ducts, thermal resistivity of the soil does not have as much influence on
the transmission capacity because heat from the cables is mainly transferred to the air within the
tunnel.
•
Seismicity
In Japan, a tunnel is designed to have stability against an earthquake with an acceleration of 0.3G and
with safety margin for earthquake is more than 2. According to the past experience, it can be said that
a tunnel has enough strength against earthquakes.
•
Frost
No need to be considered.
•
Archaeology (prehistoric sites)
If prehistoric ruins are found, it should be reported to relevant authority and site investigation should be
carried out before the construction.
•
Presence of Termites
Cables should be protected with anti-termite sheath.
•
Laying in National Park
It is necessary to get permission from relevant authorities.
•
Duration of the Work
Required construction period is as follows.
Shaft :
6-9 months (depth 30m)
Shield Driving :
10 - 15 m/day
Invert concrete, Cable Supporting Material, Lighting : 15-20 m/day
•
Maintenance and Repairing Process
By monitoring for cracks produced in the tunnel wall, erosion rate may be estimated and the most
suitable repair method determined. Repairing method varies from simple repair like filling the cracks to
large construction projects such as building a steel reinforcement to support the tunnel itself from
inside.
•
Cable Removal after Operation
When the flexible installation is applied, it is rather easy to remove the cables.
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3.2.3.3 Adaptation of the technique to the cable system design
•
Planning
At the time of the planning, various items should be considered such as, number of circuits, supporting
material, ventilation, cooling system, working space, road condition, countermeasure for fire,
environmental impact since the construction of a tunnel may involve major earth movements, etc.
These items affect one another and should be considered systematically.
•
Basic Design
The height of the tunnel needs to be such as to allow adequate space for the installation and
maintenance work. Joints are generally positioned within the tunnel with the distance between joints
being as long as is possible based on the longest length of cable that can be transported to site and
installed. Ventilation is provided by shafts along the route as needed to satisfy the ventilation
requirements for personnel access and safety requirements and to ensure the capacity of the link.
•
Snaking Design
It is very important to evaluate the thermal expansion of the cables. To cope with thermal expansion
and contraction of single-core cables installed on shelves in tunnels, pits, etc., a snaking installation
technique is generally used.
This technique enables thermal cable expansion and contraction to be absorbed by lateral
displacements of the cable initially laid in waves at a certain pitch and width. There are horizontal and
vertical snaking installations.
Selection between them is made depending on site conditions, available space, economy, etc. Horizontal
snaking installations are widely used for fluid-filled cables and both snaking installations are used for
XLPE cables.
3.2.4 Troughs
3.2.4.1 Description of the technique
A trough is a generally prefabricated U-shaped covered housing which is used to protect the installed
cable from mechanical damage.
The trough can be cast in place as a single element composed of precast sections of approximately one
meter long installed end to end or by means of continuous concrete casting process with the top of the
sides permitting a structural cover such as concrete or steel or fibre reinforced plastic, to be used to
protect the installed cable. The troughs are generally filled with special backfill in the form of selected
sand or weak mix mortar to aid heat dissipation.
Once the trough path has been assembled, the cables may be installed as in an open trench, either by a
pulling or a laying process from joint to joint or from joint to termination. Then covers are placed.
3.2.4.2 Existing installation techniques
There are three types of cable installations in troughs :
1)
Direct buried troughs
2)
Filled/unfilled surface troughs
3)
Unfilled troughs in air (in tunnel)
(1) Direct buried troughs
Cables are laid in reinforced concrete troughs which are installed in a trench.
The troughs are filled by sand and then backfilled completely.
Internal dimensions of the trough must be such that enough space exists between cable(s) and internal
walls of the trough element :
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Bottom : ≅ 1 cm, Side walls : ≅ 1 cm, Cover : ≅ 4 cm
Picture 6 : Cables in trough
(2)
Filled/unfilled surface troughs
Reinforced concrete troughs are installed at the surface of ground as shown in the drawing below and
cables are installed in the troughs.
In the case of filled troughs (sand filling in the troughs), the most likely movement of cable for
thermomechanical behaviour is in the vertical direction where there is least resistant and lifting of
trough lids can occur.
Care is therefore necessary to ensure that the trough lids are either heavy enough or sufficiently well
fixed to the trough to prevent movement.
In the case of unfilled troughs, cables are necessary to be snaked and fixed with cable cleats to cater
for thermomechanical behaviour same as the unfilled trough in air of class (3) described below.
This surface trough type has been used running along side railways and in substations.
Figure 13 : Unfilled troughs
Figure 12 : Filled troughs
(3)
Unfilled troughs in air (tunnel)
In the case of the reduced transmission efficiency of many cable circuits installation, cables are laid in
tunnel and fixed with cleats on hangers.
To limit the extension of fire or prevent from external damage, cables could be laid in closed FRP
troughs as shown below.
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Figure 14 : Unfilled troughs in air
Picture 7 : Unfilled troughs in air
3.2.4.3 Installation methods
Cable installation using troughs is classified with regard to installation method : namely direct buried
trough and filled surface trough are of the rigid type, while unfilled surface trough and unfilled trough in
air are of the flexible type.
(1)
Rigid type
The bottom of the trough is filled with a layer of thermally suitable sand backfill or weak mix mortar
before laying the cable.
If the cables are in trefoil, then after laying the bottom cable, the trough is backfilled to the top of the
installed cable to eliminate the air remaining in empty spaces and in preparation of the next cable to be
installed.
After installation of all three cables, the trough is completely filled with sand, then covers are installed,
sealed and eventually fixed.
For buried troughs, backfilling is achieved in several successive layers carefully compacted.
(2)
Flexible type
When single core extruded cable is installed in the straight line in unfilled surface trough or in unfilled
trough in air, irregular thermal cable movement occurs due to longitudinal thermal expansion. So,
snaking installation, where cables are laid in waves at a certain pitch and width, is applied to absorb
thermal expansion and contraction. The dimension of snake is determined by considering the cable
occupied space, axial force at the end of snake section, workability of snaking and so on.
•
Kinds of snake
There are two type of snaking, one is horizontal snake and the other is vertical snake. Horizontal snake
is applied in cable installation into the trough.
•
Sheath distortion
Sheath distortion of cable with metallic sheath is in most cases sufficiently less than the permissible
value, when the above mentioned parameters are adopted. This is confirmed theoretically and
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practically. Whenever necessary, precise calculations can be performed according to the design criteria
described in chapter 4.1.1.
•
Other
There are some cases that the cables are bound together at regular interval against electrodynamic
force at the occurrence of short circuit.
3.2.4.4 Limits of the technique for buried troughs
•
Civil work
For buried troughs, civil work include excavation of a trench and shoring when necessary. After
installation of the cable as described before, the trench is backfilled and different layers of the native
soil are compacted. Limits are the same as for directly buried cables with an additional limit concerning
bending radius which is generally 70 times the cable outside diameter.
•
Drying of the soil
The buried troughs method is better than direct buried method because of the improved heat flow
provided by the use of concrete trough
•
Hardness of the soil
Same limits as for directly buried technique
•
Stability of the soil
If the soil is not stable, it is necessary to anchor the troughs on a concrete sole.
•
Thermal resistivity of the soil
Troughs material and backfilling inside and outside the troughs can be selected to take account of the
thermal resistivity of he soil.
3.2.4.5 Limits of the technique for surface troughs
The use of this technique is strictly limited to these cases where the right of way is the utility property.
3.3
Description of innovative techniques
These techniques have been developed more recently, mainly to reduce cost and to accommodate the
increasing demand to transmit more power using high voltage cables. They are categorized as
innovative as very few companies have used them to date. Some special applications such as
underground hydroelectric power stations required the use of shafts to house high voltage cables.
Lately, the horizontal drilling technique was borrowed from the oil and gas industry and used as a
trenchless method to lay cables. It is mostly suited for environmentally sensitive locations as well as
river crossings. Mechanical cable laying has also been developed to install high voltage cables quickly
and economically over long distances. It is expected that more innovative techniques will be developed
in the future to meet the increasing demand for underground high voltage cable laying.
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3.3.1 Bridges
3.3.1.1 Description of the technique
It is common to use existing bridges where the cable route is
crossing rivers, railways, road junctions etc. Some bridges
have a natural space for placing cables, either inside the
bridge or in the sidewalk. On concrete bridges the cables are
placed in precast troughs in the sidewalk where the cables are
directly laid. Single core cables are laid in trefoil to reduce the
magnetic field (reduction of the circulating current in the
sheath and then of the losses). The trough is filled with
cement bound sand which has a low thermal resistivity. The
space available is often limited and does not allow making
joint pits.
Picture 8 : Dedicated tunnel for cables
On steel profile bridges, the cables can be laid in steel profiles or on cable ladders. In this case
additional protection is needed, especially at the piers.
The cables can also be directly cleated on the bridge or installed into ducts.
Before deciding to use an existing bridge as a crossing, a careful study should be made. The designer
has to take into consideration the dynamic mechanical stress caused by its vibrations, elongation and
bending at junctions and the environmental stress such as sunlight heat and wind pressure.
When constructing new bridges there should always be a design with a space for possible future
cables. Cast-in ducts make it easy to pull the cables through the bridge. But again the offset of the
cables at the piers is very important.
Vibration
If cables, that have an extruded metallic sheath, are installed in bridges the vibration generated by
automobiles and trains may introduce strain into the sheath, which could lead to fatigue. To reduce this
strain to an acceptable value it is necessary to design the cable supporting method and cable supporting
intervals with regard to their resonance frequency.
3.3.1.2 Limits of the technique
•
Civil work
The civil work will normally be to make modifications or extensions of the existing bridge structures.
This should be done in close co-operation with the owner of the bridge to avoid reducing the
mechanical properties of the bridge. The transition zone at the piers is the most critical points. Here the
cable needs to be installed with an offset to compensate the thermal movement of the bridge. It should
also be considered if the magnetic field or the increased temperature may affect the lifetime of the
bridge. A research made in Norway in 1995 show that the risk is low. The cable racks/ trays and sun
shielding should allow the maintenance of the bridge.
If the cables are laid in unfilled troughs, a drainage system should be provided.
•
Temperature of the soil/environment
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The temperature variations have to be considered when calculating the ampacity of the cable. The
temperature is significant in assessing the thermal expansion of the bridge and hence the cable offset to
cater for this expansion.
•
Seismicity
In areas with seismic activities, the cables should be laid with an larger offset at the transition zones
•
Frost
If the cables are laid in unfilled troughs, a drainage system should prevent ice in the through.
•
Presence of termites
The cable designer should take care of the protection against termites. Tin-bronze tapes are widely
used together with a PVC protective covering with an anti-termite repellent additive.
•
Maintenance and repairing process
The space for repair is often very limited. If a damage occurs, replacement of the cable on the whole
length of the bridge may be needed.
•
Cable removal after operation
If cables are laid along a bridge it is normally easy to remove the cables if they have to be replaced.
3.3.2 Shafts
3.3.2.1 Description of the technique
Shafts are generally used in hydraulic generation plants where the power generated from the
underground equipment have to be brought up to the beginning of the aerial lines.
Shafts may also be part of cable routes in cities where the cables are running in deep tunnels and must
be connected to aerial lines or substations.
Cables may be fixed with clamps at the shaft walls or to metallic structures.
Several circuits may be installed in the same shaft; in this case walls or special structures are used to
reduce the possible damages in case of problems of one of the circuits.
Joints, if present, are normally installed in horizontal configuration in special chambers to be purposely
created.
Sometimes the shafts are used as vent of the production plant, and the air temperature during normal
operating conditions has to be considered when designing the installation layout.
3.3.2.2 Limits of the technique
In case of FF cables the limits of this technique are represented by the high internal fluid pressure
values inside of the cable. Stop joints will have to be used to reduce this pressure, and the installation
technique of the joints must be carefully evaluated. Accessories design or the hydraulic pressure limit
the maximum cable length that can be installed in the shaft between two stop joints.
Also the supporting structures along the cable route and at the terminations may have an impact on the
design of the system.
In case of extruded cables attention must be paid to the significant expansion coefficient of the cables,
limiting the restraining force that each clamp may transfer to the cable and hence requiring special care
while designing the supporting structures.
Special clamps will have to be used in this case.
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For both types of cables, but especially for FF cables, the laying operations, requires the use of special
procedures and tools to be adopted. The design of the installation shall carefully consider the laying
aspects in terms of space and sufficient working areas.
Laying of the cables is normally carried out from the top of the shaft, and enough space for reel and
cable laying equipment handling has to be present. Transportation of the cables inside the terminal
station may be also a critical aspect if for example the station is completely underground.
In shaft the safety aspects have a significant impact, because short circuits or explosions may lead to
the complete failure of the circuits.
Special structures or precautions shall be taken to minimise the effect of fire inside the tunnel.
Special laying tools and ancillaries structures will have to be in place and available during the whole
service life to allow the recovery and replacement of a faulty phase.
•
Civil work
Shafts are often designed as part of the power plant or tunnel structures and the cable installation
design shall consider the existing facilities. Sometime calculation have to be made to be sure that the
existing facilities can withstand the weight of the cable and the relevant structures.
In case of FF cables, following the maximum allowable cable length, joint chambers will have to be built
on purpose along the shaft. The size of the chambers shall take into account the size of the joints, the
number of cables and the structures to be installed.
•
Water drainage
Water may permeate through the shaft walls, and heavy moisture may condense over the cables.
Corrosion problems has to be carefully considered when selecting the materials for the supporting
structures.
•
Temperature of the soil/environment
The design of the whole system requires the knowledge of the shaft temperature in operating
conditions and the annual excursions to evaluate the thrust developing in the cables.
•
Thermal resistivity of the soil
In shafts the cables are installed in air that can circulate from the bottom to the top of the shaft.
Generally no cooling problems are present
•
Seismicity
Accelerations imposed by earthquakes may have to be considered when designing supporting
structures. Being the shaft part of the station or tunnels civil works, the civil structures are already
designed considering these accelerations.
•
Duration of the work
As the laying and jointing operations are carried out in substations and in the shaft itself, the operations
can be planned quite easily.
On the other side, installation works may last more than usual, due to the particular care to be taken for
cable clamping.
•
Maintenance and repairing process
Maintenance for cable in shaft may consist of a periodical visual inspection on:
cable sheath
supporting structures and fixing devices
joints
In shaft the repair of a faulty phase will involve the recovery of the faulty cable and the lay of a new
cable. Repair of an existing cable putting joints in the middle of a cable length is generally not possible.
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•
Cable removal after operation
Cable removal is possible but with difficulty due to access restrictions caused by access stairways and
other features installed after the original cable installation was undertaken. Removal is in the same
manner as installation but precautions must be taken to ensure personnel safety and that no damage
can occur to the remaining cables in the shaft.
3.3.3 Horizontal drilling
3.3.3.1 Description of the technique
•
Introduction
Augers, vibratory pipe reamers, micro-tunnellers, directional drilling, pneumatic moles; all of these are
devices for installing underground facilities with a minimum of digging required.
Many of the techniques and equipment have been around for many decades, however the most
interesting and versatile trenchless technique, in use, has increased exponentially over recent years,
guided horizontal drilling.
•
The technique
Guided horizontal drilling, sometimes referred to as guided boring, is a construction technique that
provides for faster product placement with less disruption to the surrounding urban and suburban
neighbourhood.
Drilling can be initiated on the job either by being placed into a pre-dug launching pit, or starting from
the road or soil surface, commonly called surface launch. The following outlines the procedure
commonly followed to install a conduit or small conduit bundle:
Step One:
Lay out of entrance and exit pits for determining drilling path and segment lengths.
(highly dependent on machine size and capability)
Step Two:
Pilot hole drilling between the pits at the proper depth to avoid "frac-outs" or situations
where the drilling fluids used might bubble to the surface.
Step Three:
With the drill steel left in the ground, a back reamer is attached to the exit end of the
steel to widen the hole to the proper dimensions for the conduit.
Step Four:
Repeat step three as often as is necessary to obtain the desired hole diameter. On the
last back ream, a high tensile strength swivel and packer will be added to pull in the conduit or conduit
bundle behind the reaming system.
In all jobs, the proper drill heads, reamers, fluid handling systems and pumps must be carefully selected
in order to have an efficient and cost effective product installation. Drilling fluids, sometimes referred to
as "mud", must be mixed using proper densities for the job at hand. Fluids normally are comprised of
water, bentonite and sometimes a polymer oxide additive to provide for better performance. The
bentonite "mud" or slurry, serves various purposes; it is the medium that provides a path for rock
cuttings and surrounding soil to flow back to the launch pit, it provides cooling of the drill head and
lubrication when drilling into rock substrata and it stabilises the hole.
For long lengths, an installation to the recycling of the mud is to be considered.
•
The equipment
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Horizontal drilling rigs can be classified into four equipment sizes based on there capability : service
tools, mini-rigs, midi-rigs and river crossing units.
Depending on the installation, the size of the tool needs to be chosen carefully for maximum economic
benefit.
Service tools are used for very small installations of gas and underground residential distribution (URD)
services. These are easily transportable and require very little set-up space. Service tools can drill small
diameter holes for about 60 m.(200 ft) effectively. Many are "dry borers", meaning that they use no
slurry to act as a coolant or for hole stabilisation.
Mini-rigs, currently represent the largest market segment for drill rigs. These are small versatile tools
that employ a minimum amount of slurry for drilling in relatively easy to moderate soil conditions. Minirigs come in various sizes and capabilities just as back hoes. In many respects these small machines
are the equivalent of backhoes while midi-rigs, which are approximately the same physical size, are
equivalent to excavators. The Table 1 below shows the performance ranges to the four categories of
drilling units.
Table 1 : Horizontal drilling references
Rig Type :
Range : m (ft) :
Hole Size : cm (in):
Pull Back kN : (klbf)
Torque : kN-m (ft-klbfs)
Crew Size (#) :
•
Service
60 (200)
10 (4)
17-31 (4-7)
0.6-0.9 (0.5-0.64)
2
Mini
180 (600)
30 (12)
44-178 (10-40)
0.8-4 (0.6-3)
3-4
Midi
900 (3000)
61 (24)
222-400 (50-90)
13.5-27(10-20)
4-8
River
1800 (6000+)
132 (52)
534-3337 (120-750)
34-122 (25-90)
6-12
Benefits
Safety - Open trenches endanger pedestrians and traffic; including cave-ins, trench debris, and failed or
improperly installed cover plates
Convenience - Businesses, homes, and commuters are less inconvenienced by traffic backups, dust and
uneven pavement due to removal.
Productivity - Installed lengths can exceed open trenching by as much as 5 times on a per day basis.
This translates into less traffic disruption, noise and reduces the construction times in any one location.
However, in many countries, the horizontal drilling is mainly used to cross obstacles and not to replace
the open trench where it is possible to work with direct burial.
Conflict Reduction - Increasingly congested utility corridors and easements make it very difficult to
place cable or conduit. Directional drilling can be a solution to this dilemma through the use of
measure-while-drilling, MWD, electronic tracking systems accurately drill beneath existing utilities.
Route Selection - Drilling may allow for different or shorter routes to be taken for the installation of a
cable circuit. This is because in many cities there are moratoriums on cutting open recently paved or
refurbished road surfaces. The local governments in many cases make allowances for the use of
drilling instead of trenching.
Reduced Environmental Issues - Run-off from job-site excavation is minimised as is the risk of
excavating and disposing of soils that may be contaminated. Regulatory restrictions related to
excavations in wetlands and other sensitive areas will be reduced.
Cost Savings – Faster installation time, less backfill materials used, traffic control issues, pavement
removal, separation and disposal / refurbishment effectively eliminated, reduced spoil handling and
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trucking cost; all of these provide for significant savings when guided horizontal drilling is effectively
employed.
3.3.3.2 Limits of the technique
Although the use of guided horizontal drilling can be a faster more efficient means of installing services,
there are cases where technical limitations out weigh the potential benefits. In those situations hybrid
installation practices should be employed, i.e. trenching and drilling used in the proper locations.
•
Civil work
Civil limitations to drilling are fairly obvious. The following limit the effectiveness or are the cause of
problems during job performance :
The necessary hole diameter exceeds the capability of the available equipment
The depth of installation doesn’t allow enough cover to prevent the drilling fluids from percolating to the
street surface.
Segment lengths between pit locations are too long to be done in a cost effective manner.
The underground environment contains flowing sands with high water table, or contains a rocky
environment not conducive for the proper directional controls needed.
Attempting to drill around tight bends or intersection corners in a street.
Many of the above can be overcome using lesser known drilling methods, however in some cases the
better approach to take would be to use microtunnelling.
•
Temperature of the soil/environment
Depending on the depth of drilling, temperature of the ambient soil or rock where the cable circuit
would eventual reside, can actual be more constant and at a lower temperature than for shallower
installations done using open cut methods. Because of the greater installation depths, the circuit
ampacity may need a de-rating. This disadvantage can be overcome through the use of dynamic
monitoring on the circuit or through the use of a larger conductor cable.
•
Hardness of the soil
As previously mentioned under civil limitations, the hardness of soil can affect the drilling effectiveness.
•
Stability of the soil
Unstable soil conditions such as flowable sands and cobble mixed with softer soil materials are
probably the worse conditions for drilling operations to be effective. In the case of flowing sands, or
very sandy soil conditions where the bentonite slurry cannot stabilise and maintain the drill hole, a
washover technique can be performed (for small diameters). This is where sections of a larger
diameter pipe are connected and pushed into the bore hole during the pilot drilling operation. Once the
drill rod has gone the desired length, it can be removed from the washover pipe and also be used to pull
in the cable. The washover pipe is sacrificial and it stays in the ground to act as a conduit. For larger
diameter holes, a technique known as forward reaming is used. During the pilot hole drilling, reamers
are attached every so often onto the pilot drill rod faced in the forward direction (smaller reamers first,
eventually having the last reamer at the desired diameter). Behind the last reamer is attached a sleeve
or large pipe, which is pushed into the large hole in parallel with the pilot hole drilling and reaming
operations. This sleeve or large pipe provides a path for the cuttings and slurry back to the drill rig. The
fact that it is pushed or spin into the hole during the drilling results in a stable hole. This however does
limit the advance rate of the drilling and as a result dramatically increases to cost. This same method
may also be needed where cobbles and covered river bottom rock is encountered.
•
Thermal resistivity of the soil
Installation of circuits at greater depths tend to reside in surround soil and or rock that have better
native thermal resistances than the soils closer to the surface that contain higher organic content. Thus
the thermal resistivity of the native environment is not a limitation on the guided horizontal drilling
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technique. However, it is more careful to take soil samples for analysis before the start of the works
(to avoid hot spots). The only limitation is on the hole size and the proper sizing of the hole with the
spacing of the conduits that the cables would eventually be pulled into. In this case the interstitial
regions between the conduit walls and the hole wall might need to be filled with a good thermal grout
similar to flowable backfill used in trenching.
•
Archaeology
Trenchless techniques in general are preferred methods for installing services where archaeological
sites have been noted. This is because less surrounding soil is removed during drilling and the drill path
can be planned to go deeper under these sites.
•
Laying in National Park
National Parks, golf courses, old growth forested areas where services may need to be installed, have
much less change of having environmental damage if guided drilling is use over open cut methods.
•
Duration of the work
Guided horizontal drilling when properly planned and deployed to the field can be 5 times or more faster
in placing product into the ground over open cut. This means that in most situations the duration of the
project will be greatly reduced. Even in hard rock situations, drilling is a much faster process than jack
hammering, explosives with backhoe, or carbide toothed saws.
•
Maintenance and repairing process
The preferred practice for installing cable is in a conduit. This means that maintenance and repairs
issues would be similar to shallower installations. The only situation where there are significant
limitations on repair capability is where the cable system is direct buried. If direct buried design for the
circuit are used, this technique from a repair and maintenance standpoint is probably not a good choice
to use.
•
Cable removal after operation
Should direct buried installations be the preferred method, then the use of guided horizontal drilling is an
excellent choice for placement, but is much more expensive and difficult when it comes to repair
operations where a segment of cable has to be removed. Because there is a minimum depth required
for drilling that is based primarily on the hole diameter, most drilled-in installations are generally deeper
than their open cut counterparts. Repair and removal is a limitation for this technology. The general
practice in the United States is of course to always place the cable phases in a conduit. If conduits are
installed, then removal is no more of a limiting factor than in the open cut case.
3.3.3.3 Adaptation of the technique to the cable system design
The use of any trenchless technique will more than likely dictate a change in the cable design in order
to get closer to the equivalent open cut / shallow installation ampacity. Guided horizontal drilling
contractors are also not used to installing facilities where heat transfer and product spacing is such a
big issue. Most directionally drilled facilities are still water pipes and gas distribution lines. In the
United States, there have been many efforts done to installing sewers by drilling. This is usually the one
area where micro-tunnelling has seen dominance.
In adapting the cable circuit design to the drilling installation case, the following need to be looked at in
detail before the total design is selected;
How deep will the cable be installed ?
This affects ampacity and may dictate a larger conductor for the cable.
Will the cables and/or conduits be spaced ?
This again affects the cable ampacity and also the isolation of the phases to improve reliability and help
prevent a phase-to-phase fault condition.
Will the bore hole be sleeved ?
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HDPE is used by drillers. This material has a thermal resistivity of about 4 degrees Kelvin-m/watt, a
large thermal resistance over the native soil contact. Sleeving will most likely dictate a larger bore hole,
filled with a good thermal grout and conduits spaced equally, if good ampacity ratings are to be had. A
larger conductor design will help as well.
Adaptation of cable design and trade-off analysis is complicated with a myriad of civil parameters
versus thermal performance and cable design. More and more engineering design firms, consultants
are doing detailed analyses for clients. When done properly, application of guided horizontal drilling to
the electric industry can save significant civil / construction costs. These savings by far are larger than
any increase cable cost caused by larger conductors.
3.3.4 Pipe jacking
3.3.4.1 Description of the technique
There are three different pipe jacking techniques :
i) Jacking by beating or pneumatic rocket
ii) Jacking by rotation
iii) Jacking by thrust
The last technique will be described in great detail as it is the most common used.
Pipe jacking can be considered as an environmental balanced installation technique, as it does not
effect the surroundings. The surplus soil (equal to the volume of the pipe) is removed from the ground
implying that displacement of the in situ and settled soil can be avoided.
As a rule of thumb, the following minimum depth to the upper pipe surface could be considered :
Øpipe = 200 – 500 mm 2m
Øpipe = 500 – 1500mm 3m
Øpipe > 1500mm
4m
Picture 9 : Pipe Jacking
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Pipe jacking is not applicable in hard soil as limestone, sandy shale or granite. Pipe jacking is not
recommended in heterogeneous soil with blocks (clay with flint, sandstoned sand).
The nature and composition of the soil must be investigated before the pipe jacking method is selected.
Without this study, the pipe jacking may be halted due to a block, which will impose a manual and time
consuming action at the front of the first pipe when the block shall be cut to pieces. Such action is not
possible with pipe diameters less than 800 mm. Only excavation from the surface or use of explosive
within the pipe (if possible) can prevent that the installation must be given up. In such a case a new
pipe jacking operation must be started at a new depth or location.
Cathodic protection shall be considered if the pipe jacking is done with steel pipes.
i) Jacking by Beating or by Pneumatic Rocket
This system consists of beating horizontally steel tubes. The technique is characterised by the speed in
the progression in non consolidated and homogenous soil (clay, silt, sand).
The excavated soil is taken out of the tube by compressed air or by using a flush.
The saturated steel tubes can have a diameter ranging from 100 to 500 mm. The penetration speed can
reach 3 to 10 m/hour in unconsolidated soil. The pipe jacking lengths in these soils are between 30 and
40 m but this technique by beating is not very accurate.
ii) Jacking by Rotation
The pipe jacking installation is performed from a work shaft and consists of pushing into the soil steel or
concrete bore tube with a drill inside that turns the drill bit. The function of the rotating drill is to
transport the soil from the first pipe backward to the work shaft and not to perform excavation in the
nature soil. If the soil is removed by the drill at small depths and in loose soil a risk of undermining the
surface layers exist. Such undermining shall be avoided since it can cause damage on constructions
situated on the ground surface or reduce the mechanical stability of roads, railways, etc.
When a tube enters the soil the subsequent pipe is electrically soldered on to it.
This technique can only be contemplated in homogenous and soft soils (clay, silt, sand, etc.) with
diameters ranging from 400 to 800 mm and with 40 to 50 m lengths. It is simple and quick, useful when
the soil is suitable and there is no need for great precision (about 0.5m for works of 40 to 50 m).
iii) Jacking by Thrust
The technique consists of pushing into the soil prefabricated tubes having the exact diameter of the
final tube. The tubes are pushed from the work shaft. This shaft’s walls will be mainly shored up with
overlapping planks adapted to the subsoil and have a concrete slab which evacuate rain and
underground water to a draining well. It will also have a support system capable of receiving and
transmitting to the subsoil the jacking thrust, which can sometime be very strong.
As the pipe jacking progresses the earth works are done, either manually or mechanically, according to
the requested diameter. The first tube is equipped with a steel drum curb, which bites into the subsoil
while protecting the workers cleaning the earth.
The drum curb is equipped with correcting screw jacks that direct the unit of assembled tubes.
Topological measurements are done with a theodolite or more usually with a laser. The extracted soil at
the working face is taken to the thrust by a winching tip truck.
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When the first tube is completely pushed in the subsoil the second is taken down into the shaft and is
soldered to it. The thrust and extracting then continues.
When the thrust linked to the friction load becomes too high, it is possible to resort to intermediary
stations. During the pipe jacking, bentonite is injected between the soil and the tube in order to reduce
the friction coefficient. The substance is injected into the tube and goes out through holes on the side of
the tube. Towards the end of the pipe jacking the bentonite is replaced by cement grout to "solder" the
tube to the subsoil and spread the earth’s thrusts.
When the pipe jacking is finished, the drum curb is retrieved in the exit shaft. The screw jacks at the
intermediary station are dismantled.
This technique is applicable for pipe diameters between 1000 and 3200 mm.
The thrust station at the work shaft is equipped with 4 to 6 screw jacks which each are capable of
developing 1000 to 3000 kN. The average friction of the pipe surface / soil is approximately 1.2 kN/m2
of the external surface.
The maximum permissible thrust for a standard 2 m reinforced concrete pipe is:
Table 2 : Pipe jacking figures
Diameter [mm]
1000
1200
1400
1600
1800
2500
Maximum permissible thrust (kN)
2000
2600
3500
5200
7000
10500
3.3.4.2 Limits of the technique
•
Civil work
Pipe jacking is possible up to 100 m length without intermediary stations. With intermediary stations
lengths up to 500-600 m may be installed, but the work is limited by the time consumption for
transporting of the soil backward in the pipeline. Health and safety concern for the workmen might also
give reason to limit the length of the pipe. However the length can be doubled if it is possible to
excavate a central receiving shaft and perform two pipe jacking installations (one from each side).
Pipe jacking precision :
Speed of progress :
50 mm around the theoretical axe.
4,5 m/day if the soil is removed by a mechanised system.
2,5-3 m/day if the soil is removed manually.
Pipe jacking by thrust is not applicable for rocky soil. The technical limit of the cutting head is 300 kN.
For pebbled soil situated below the ground water (sand and gravel, sandy silt) measure must be taken
to lower the groundwater level and the pipe jacking shall be done by compressed air.
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•
Drying of the soil
The design of the pipe jacking installation shall consider the possibility of drying out the soil at the
external pipe surface which might result in a thermal run away causing an insulation cable failure if the
system is heavily and continuously loaded.
Drying out of the soil is not of any importance if the permanent groundwater level is above the pipe
installation. Therefore the groundwater level must be verified at the geological survey before the
detailed design of the pipe installation is completed. Additionally the soil characteristics (thermal
resistivity and drying out performance) must also be clarified at the geological survey.
Drying out of the soil can be expected to start at a continuous temperature of 50 oC depending on the
soil characteristics. (In sand material the drying out phenomena will most properly start at a lower
temperature).
Cable installation with pipe jacking will always result in that all three phase conductor cores are pulled
through the same concrete or steel pipe. The pipe surface temperature varies (among others) with the
current, phase distance, depth of the pipe, and the outer diameter of the pipe.
Drying out of the soil must be considered in particular for cable systems designed for a continuously
load and with a conductor size being equal along the whole cable route. In such a case a large depth
(above 3m) might result in a bottleneck for the current capacity of the whole link, since available
measures against the possibilities of the drying out of the soil is limited. Spreading out of the cables
beyond the inner pipe diameter is impossible. Improving the soil characteristics by substitution of soil
with better thermal performance is not possible. Injection of a "specific developed fluid with excellent
thermal properties" within the pipes will only have a minor effect, since the major temperature raise
appears in the soil.
Above mentioned heating problem can be prevented if a cooling arrangement is applied. Natural or
forced air circulation in the cable pipes can remove the heat generated by the cable cores.
Alternatively a water cooling system can be adopted. A cooling system will result in additional
installation costs and require supervision and maintenance on a larger scale than the cable system,
which is practically maintenance free.
•
Water drainage
Water drainage shall be considered for the implementation period. Permanent water drainage is not
recommended.
•
Temperature of the soil/environment
Pipe jacking will be performed at a depth more than 2,5 meter in almost every installation. The
temperature at the upper surface layers (depths 0,2-1,0 m) varies during the day and week depending
on the sun heating, wind, and air temperature. This fluctuation in the soil temperature is less distinct at
1-3 m depth and vanishes at a larger depth. The soil temperature at large depth is almost constant and
equal to the yearly average air temperature at the particular location. This implies that a different soil
temperature can be applied for the calculation of the cable temperatures.
•
Hardness of the soil
Pipe jacking in soil consisting of rock, granite or similar soil can not be performed.
•
Thermal resistivity of the soil
Pipe jacking with three conductors within one pipe is not recommended in soils with high thermal
resistivity unless a cooling system is applied.
•
Frost
Since pipe jacking is performed at depths larger than 1.5 m frost does not affect the performance of
the pipe jacking operation.
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•
Archaeology
Pipe jacking can be applied close to archaeology sites within the cities. If large depths is required and
the available area for a sloping horizontal drilling to the maximum depth is insufficient a pipe jacking can
be performed.
•
Presence of termites
Intrusion of termites and other vermin must be prevented if an air installation is chosen for large pipe
diameters.
•
Laying in National Park
Pipe jacking requires a large working area at the working shaft and the receiving shaft compared with
the relative short length. If a cable route shall cross a national park or a similar area with sensitive
environment, it is unlikely that pipe jacking is a feasible trench less installation compared with the
horizontal drilling with PE-pipes.
•
Duration of the work
The implementation of the work will follow the phases mentioned below :
Soil investigation and report
Tendering/Contract
Site mobilisation
Excavation of shafts
Pipe Jacking
Attachment of cable pipe
Injection of bentonite & concrete slurry
Reestablishment
Demobilisation
1 week
2..3 weeks
1 week
1 week
1-3 weeks
½ week
½ week
½ week
½ week
Above mentioned duration is of course dependent on the complexity of the installation (pipe diameter,
length and dept of installation).
•
Maintenance and repairing process
Cable pipe with bentonite
No maintenance is required for the cable installation if the plastic pipes and the concrete/steel pipes are
injected with bentonite. In case of cable failure within the pipe installation the bentonite can be flushed
out and the cable core redrawn from the pipe.
Cable suspended in air
If the pipe installation can be accessed a regular visual inspection (each second year) can be
recommended in order to detect any deterioration of the cable suspension arrangement or to inspect if
objects or animals has intruded the installation and caused any damage.
•
Cable removal after operation
Cable pipe with bentonite
The bentonite can be flushed out and the cable core redrawn from the pipe when the cable system is
taken out of operation.
Cable suspended in air
Cables suspended in air to the inner surface of the pipe by mean of applicable cable clamps are easily
removed when the system fails.
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3.3.4.3 Adaptation of the technique to the cable system design
•
Installation practice
Depending on the size of the concrete/steel pipe two different installation methods can be applied:
•
Air installation
If the concrete pipe is large (above 1,5m inner diameter) the cables can be suspended at the inner pipe
surface either as a flexible or rigid installation in air. In this case the cable installation design practice
used for cables in tunnels can be applied taking into consideration the mechanical forces during short
circuit and the thermal design (heating up of the soil and the cable insulation). A proper sealing of the
pipe must be considered in order to prevent intrusion of water, insects etc.
•
Cables laid in pipes
If a small pipe diameter is designed the cables must be laid in cable plastic pipes (made of PE or PVC)
in order to avoid damage of the cable sheath during the pulling operation. Depending on the dimensions
and weight of the cables one or three cable pipes can be used. Small cable sizes can be pulled out in
one pulling operation if the three phase conductors are attached together with a suitable tape.
Unintended overheating of cables must be prevented by substitution of the residual air volume with a
material with improved thermal characteristics. Bentonite or a similar pumpable mixture is injected
between the cable and the plastic pipe. A slurry mixture with a thermal resistivity less than 1 Km/W in
dry condition is normally selected for use between the plastic pipes and the concrete pipe.
If a trefoil formation is essential for the cable system design all three plastic pipes must be attached
firmly in trefoil and pulled through the large pipe in one operation. Increasing the phase distance to the
maximum possible can reduce the thermal mutual heating between the phase conductors. In such a
installation (with pipe diameter above 1.5 m) it is necessary to adapt clearance wedges between the
cable plastic pipes.
It is not mandatory that the cable cores are located in a true trefoil formation. The current and voltage
of the screen depend on it’s bonding, the location of each phase conductor and the length of the whole
cable system. If close trefoil is applied in the standard trench a short length with flat formation will
result in an unbalanced screen bonding system. The screen voltage at the opened end for single point
bonded systems and the circulating current for cross bonded & two point bonded systems will increase
a little compared with a true balanced system. This however is of negligible importance if the total
length of pipe/cable installation is relatively short compared with the whole cable system length.
If the space is available more than one cable system consisting of three phase conductors can be
installed in the same concrete pipe. However consideration related to the mutual heating and the
possibility of a cable failure caused by one system to the other system must be taken.
3.3.5 Microtunnels
Microtunnels are one of the installation techniques which are adopted where open ditches are not
possible, e.g. crossing of obstacles like railways, rivers, duct banks, motorways etc.
Their diameters are typically between 300 and 1200 mm and their lengths 200 m maximum. In contrary
to larger tunnels they are not accessible by man and mostly contain only one 3-phase power cable
system.
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3.3.5.1 Description of the technique
Similar to the pipe jacking method this technique also consists of pushing prefabricated tubes in the
subsoil. The earth works at the front of the tubes, however, are systematically mechanised using a so
called microtunneller, i.e. a steerable drilling device, which allows to penetrate even harder subsoils
than with simple pipe jacking.
The machine is driven electrically or hydraulically and remote controlled and can dig horizontal holes of
up to 1200 mm diameter over a length of appr. 150 m (max. 200 m).
As the pipe jacking or large tunnel techniques the microtunnelling, too, starts and ends in vertical shafts,
which have to be prepared in advance to provide the necessary space for the tunnelling and thrusting
equipment.
The clearing of the dug out earth can be done in three different ways via tube systems to the decanting
box
- removal by an endless screw (earth pressure)
- hydraulic removal (mud pressure)
- pneumatic removal (air pressure)
Picture 10 : Microtunnelling
For the main tubes of the tunnel two different techniques exist:
- pipe jacking the final tubes directly
- pipe jacking temporary tubes, which will be replaced by the final tubes once the exit shaft has
been reached.
Advantages of using temporary tubes are:
- a rapid execution due to simplicity of assembling the tubes by bolt-connections
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- a lesser risk of damaging the final tubes, as the second thrust is generally reduced to a third of
the initial thrust
- in the event of being stuck in the excavations the possibility of bringing it back to the work shaft
by pulling on the temporary tubes.
Inconveniences of using temporary tubes are:
- longer completion time
- extra cost of temporary tubes
- a more complicated installation
- the need for more room on site to store the temporary tubes
The temporary and the final tubes have the same diameter. The latter are fitted into each other and can
also be soldered (when made of steel).
The main components of the microtunnel technique are:
•
on the surface, near to the entrance work shaft:
- a control system, where all controls are automatic and are backed up by manual controls
- a decanting box for the excavated mud
- a hydraulic pump or electric generator for the thrusting station
•
the entrance work shaft, which houses
- a thrusting station and the screw jacks to push the prefabricated
tubes into the pre-drilled subsoil
- a complete control system for the microtunneller
- the various materials actually needed during the process
•
the microtunneller, which is composed of the following elements
- a steerable drum curb fixed to the tunneller by guiding jack screws
- a steel covering, which comprises the tunneller’s body and the tubes
- a pre-drilling wheel and its kinematics, which is adjusted according to
- an electric or hydraulic engine, which drives the wheel
- a laser system to control the bearings
the soil properties
•
a train of different tubes which comprises
- the main tunnel tubes, either steel or concrete
- a circuit to bring the mud
- the sludge process to remove the earth
- a lubricant circuit to reduce the external friction of the tunnel tubes against the earth (injection
of bentonite)
- the passage for the laser beam
- the passage for control cables
•
the exit work shaft, which allows to remove at the end
- the microtunneller equipment
- temporary tubes (if applied)
If the drilling length becomes too long, it can be divided into two with a central work shaft (provided
that space is available) and two lateral exit shafts. In this case, the thrusting station must be turned
round.
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3.3.5.2 Limits of the technique
•
Civil work
Microtunnels are constructed where open trenches are not possible. Such technology of digging without
using a ditch requires excellent geological knowledge of the lay of the land. A geological study and a
detailed survey of the character of the subsoil including the position (or non-existence) of the ground
water table are necessary before defining the civil work to be done.
These data will determine the technological choice of the earth removal method by screw, hydraulically
or pneumatically, the type of drill head, closed for hard soil, half closed for less hard, open for soft soil,
and also the lubrication, (bentonite, polymeric mixture, foam etc.).
The possible risk of an incomplete examination and the use of an inappropriate drill head can lead to the
microtunneller being blocked. If this happens there are several systems that allow part of the equipment
to be recuperated, but in most cases it will be lost.
It is therefore necessary to take any precautions possible, as the cost of a microtunneller is in the order
of several hundreds of thousands of US $.
Apart from the boring of the underground tunnel the civil work is more or less restricted to a few
limited areas, i.e. mainly the work shafts at the start and the exit of the tunnel and their surroundings.
The shafts have to be carefully designed and executed according to size of equipment to be installed
therein, to depth, kind of soil, ground water table, thrust pressure etc. They must be kept dry from rain
and ground water. Their typical dimensions are 4 m x 2.5 m or a diameter of 3.5 m for the entrance
shaft and 2.5 m x 2.5 m for the exit shaft. Around these shafts a certain surface area must be available
for storage of material, decanting of mud, machines, cranes, control and other equipment.
Typical dimensions are 40 m x 3 m (max. 150 m²) at work site of entrance shaft.
•
Drying of the soil
The thermal situation of a power cable system within a microtunnel is determined by a large number of
different components:
- cables will be laid in filled or unfilled PE or PVC ducts
- these ducts are installed inside the common tunnel tubes, either steel or concrete
- interstices between ducts and tunnel tubes are filled with special slurry
- tunnel tubes are ”soldered” to surrounding subsoil by special fillers (bentonite, cement grout)
- surrounding subsoil can be of great variety with regard to geological composition, equality along
the route, homogeneity and, last but not least, thermal resistivity
- the position of the microtunnel in the ground will be different with each installation, especially
with regard to depth, ground water table, distance to foreign heat sources etc. The calculation
of admissible thermal ratings of the cable system has to adequately consider these parameters
to guarantee for stable thermal conditions rather than thermal runaway and drying out of soil.
•
Water drainage
The robotisation of the mechanism allows work to be carried out in the groundwater table without
lowering the water level. This is possible up to 20 m water pressure but only for microtunnellers which
remove the earth hydraulically. The only problems can arise at the work shafts, but there are methods
to seal these watertightly.
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•
Temperature of the soil/environment
As microtunnels typically will be positioned in greater depths the average yearly ground temperature of
the soil will be dominant. This will be especially true, when the tunnel is below the ground water table.
If drying out of soil is avoided (see clause b) stable soil temperatures can be assumed.
•
Hardness of the soil
The microtunneller does not have problems in hard soils because its tools can be adjusted to the type of
soil.
Of course, there are limits to the hardness and/or the abrasiveness of the tools, e.g. with extremely
hard rocks. Screw machines are well adapted to clay conditions and therefore are used in any sort of
soil from sand to soft rocks.
Hydraulic and pneumatic earth removal machines are efficient in groundwater tables and very sticky
soils.
•
Stability of the soil
The microtunneller is badly adapted for heterogeneous soils. Once the microtunneller type and the
drilling head are chosen, a change is very difficult and costly.
Blocs or obstacles that exceed about 30 % of the drill’s diameter can not be dealt with and it is
therefore impossible to use this method in soils containing large blocks.
Certain clay soils, which are too sticky, should be avoided either.
•
Thermal resistivity of the soil
The detailed knowledge of the thermal characteristics, especially the thermal resistivity of the soil along
the microtunnel is a must before such a cable system can be designed. If changes of these parameters
cannot be excluded over the life time of the cable system a real time temperature monitoring system
with temperature sensors along the cables can be installed to identify changes and to adjust cable
ratings accordingly.
•
Seismicity
The tunnel tubes, whether steel or concrete, provide a certain mechanical protection for the cable
system inside. The degree of seismic impact, which can be withstood without damage is hard to define.
•
Frost
Since microtunnels are positioned in larger depth, frost does not influence its performance.
•
Archaeology
As microtunnelling avoids opening of trenches, it could be a favourable technique to keep
archaeological sites of limited area (< 200 m) undisturbed.
•
Presence of termites
The part of a longer cable link, which is laid inside the microtunnel, will be much more protected against
termite attacks than the remainder outside.
•
Laying in National Park
Although microtunnelling avoids open trenches, it does not seem to be a favourite for application in
National Parks, as their standard lengths are limited to 100-200 m. It might be too short for extended
areas and the impact of the construction work at the shafts on the environment could be considered too
high.
•
Duration of work
The duration of the installation of a cable system in a microtunnel is hard to estimate, as it depends on a
number of individual parameters, the most important of which are:
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- geological study, either conventional with drilling and core sampling or advanced with georadar
or seismic examination
- definition and approval of tunnel route and working areas
- erection of shafts, depending on width, depth, kind of soil, ground water table etc.
- diameter and length of tunnel, kind of tubes
- progress by thrust is about 10 m/day, it can vary from 30 m/day in very soft soils (chalk) to 4 to
5 m/day for harder soils (clay, hard marl), if temporary tubes had been used, the later laying of
the final tubes is about 50 m/day
- installing of ducts inside tunnel including slurry to fill the interstices
- laying and connecting of cables inside and outside the microtunnel
- reestablishment of the shaft and the working area
Altogether a duration of 2 to 3 months seems to be realistic.
•
Maintenance and repairing process
Once the microtunnel and the cable installation is completed no maintenance is required.
As the tunnel tubes provide a strong mechanical protection, damages from outside are very unlikely,
thus excluding the need of mechanical repairs. If the cable should fail internally it can easily be pulled
out of the duct for exchange. Repair by cable joints in the section of the tunnel is not possible due to the
limited diameter of the ducts.
•
Cable removal after operation
As the cables are laid in ducts inside the microtunnel they can easily be removed, provided that the
filling is removable (e.g. bentonite).
3.3.5.3 Adaptation of the technique to the cable system design
The microtunnel provides the mechanically well protected tube for the cable system. Typically one
PVC or PE duct for each cable phase is either thrust or pulled into the tunnel and fixed by drowning in
injected cement (slurry). A fourth duct can be laid and used as a reserve in the event of a problem with
one of the other phases. The position of the ducts is maintained by clearance wedges.
Putting in place the slurry is a delicate operation. This material must be spread in all the volume in order
to avoid the presence of air bubbles, which could create hot spots. The injection of the slurry must be
done without damaging the ducts.
To avoid ovalisation during injection of the slurry it is recommended to strengthen the plastic ducts by
air, water or helium pressure.
Lastly, the slurry should have a low and consistent thermal resistivity of preferably 1 Km/W.
Once this installation of ducts and filling is completed, the three single core cables can be pulled into the
ducts within the microtunnel. The ducts can be left unfilled or be filled with e.g. bentonite to improve
thermal heat dissipation.
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3.3.6 Mechanical laying
Picture 11 : Mechanical laying
3.3.6.1 Description of the technique
This technique, coming from the traditional trench technique is suitable for buried cables and consists of
opening the excavation and simultaneously laying the three phases, and possibly earthing cable and
telecommunication cable as well as their backfilling. When combined with the use of weak-mix mortar
it offers:
good cable protection from external damage
good control of the direct heat environment of the cable
good protection of the environment in case of short-circuit
reduction of the size of the trench compared with conventional technique
reduction of work duration
Laying principle ( HV cable systems)
Cable laying conditions
The cables are laid in a trefoil or flat position. The depth is 1.30 m to the bottom of the excavation,
which on the one hand enables any effects of a zero phase-sequence short circuit to be controlled, and
on the other hand, it protects cables from third party damage.
The thickness of the mortar around the cables and in particular under the cables (raft) must be at least
50 mm.
For trefoil laying, fastening must be used if the cable guides cannot maintain the trefoil position until the
covering is in place.
A telecommunication cable may be laid if necessary in a separate duct above the power cables (on the
weak mix mortar) and directly laid in the soil.
A warning plastic netting is laid on top. Then the trench is backfilled and the soil is compacted.
The width of the trench which takes into account the diameter of the cables is between 300 and 400
mm for trefoil cables and 450 and 500 mm for cables laid in a flat position (H V Cables).
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Cable laying :
The cables can be laid using two different methods :
- simultaneously : the cables are laid with the help of cable laying machines or a cable drum carrier
(Figure 15, •1) which is a few metres in front of the trenchdigger,
- beforehand : the three phases are laid beside each other along the future route. The cables must not
come into direct contact with the ground. They can be laid on rollers or on a polyane sheet.
Joints :
Depending on the voltage and the accessory technology, joints might not be laid by machine. Indeed,
their preparation and completion time might not be compatible with the works progress. In this case,
they are placed in special excavations (joint chambers) that are then filled once the joints have been
made.
Trenchdigger (Figure 15, •2) :
The trenchdigger has to dig the trenches at the dimensions indicated previously and must be equipped
with a cable guiding system to maintain the cables above the trenchdigger. Different kind of trenching
machines can be found :with sawing wheel or sawing chain.
This cable guiding system is necessary to obtain the correct positioning of the cables at the entrance to
the cubicle tray . If the cable guides are not able to maintain the cables in a trefoil position until the
covering is in place, a fastening system becomes necessary.
Cubicle Tray (Figure 15, • 3):
The cubicle tray which follow the digger must carry out the following tasks :
- Positioning the cables in the excavation,
- Vibrating the weak mix mortar as necessary,
- Covering the cables with weak mix mortar,
- Compacting the covering.
The equipment must allow a permissible route curve radius of approximately 12 to 15 m and must be
capable of being dismantled when necessary.
Mortar hoppers (Figure 15, • 4) :
Although they are not specific to this technique, the mortar hoppers carry out the following tasks :
- Carrying to the works site the weak mortar mix necessary for the covering of the cables. One cubic
meter of mix allows a trench length of about ten meters,
- Pouring the mortar into the cubicle tray while operating the trenchdigger.
Backfill (Figure 15, •5) :
The backfill is identical to that used for conventional methods. In order to reduce the works time, its
synchronisation is made with the progress of trenching.
Equipment :
All the above equipment is part of a laying "train" about 50 m long, an example of which can be seen in
Figure 15.
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Figure 15 : Mechanical Laying
6) Examples of utilisation of this method
Since August 1993 many sites have used this technique in Europe. Thus we can consider that this
technique has moved from the experimental to the industrial stage in the HV cables range
3.3.6.2 Limits of the technique
•
Civil work
This technique imposes the use of dedicated equipment which are expensive and heavy . The
transportation of the equipment from one site to another one must be carefully considered (cost ,
duration.) . The ground occupied by the works site is larger than for a conventional site for several
reasons:
- the width must include excavated earth,
- digging equipment,
- pathway for the mortar hoppers.
Consequently, an access strip of about 7 m is necessary. Nevertheless , a local complement of
backfilling is still possible.
On the other hand, advantages of this technique compared to conventional open trench are :
- important reduction of time (time duration divided by 2 or 3)
- less cable handling and consequently reduction of the risk of damaging the cables.
For good achievement a few points have to be carefully taken in consideration :
- management of the supply of weak mortar mix which is conditioned by three parameters :
the distance,
climatic
progress
- management of the quality of the weak mortar mix.
A few conditions are necessary to envisage the use of this technique :
- rural type lands or along roads provided only a few obstacles are present
- route more than a few hundred meters long .
- slopes with less than 25 % .
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•
Drying of the soil
As the heat is first dissipated through the weak-mortar mix, thermal resistivity of the soil as well as
drying of the soil are not so critical. The use of specially formulated weak-mix mortar can be a solution
for local problems.
•
Temperature of the soil/environment
As for any laying operation, special care should be given to cable temperature during laying.( minimum
as well as maximum temperature, sun beams protection )
•
Hardness of the soil
This technique is available for a rocky soil but the time is increased consequently, tractors with
caterpillar tracks should be used in marshy areas for better carrying performance.
•
Seismicity
The use of weak-mortar mix seems to be a good improvement in case of seismicity.
•
Archaeology
The technique is not suitable for archaeological sites.
•
Presence of termites
The technique is similar to other buried techniques.
•
Laying in National Park
This technique could be interesting in some cases (re-use of native soil, duration of the work )
•
Duration of the work:
A speed of 50 m/day (rocky soil ) to 300 m/day can be expected.
3.3.7 Embedding
3.3.7.1 Description of the technique
This technique consists of excavating a riverbed from a barge or with an amphibious machine, burying
a tube or cables and filling up the trench.
Burying of cables presents the effective, definitive protection against mechanical damage.
3.3.7.2 Limits of the technique
•
Civil work
The methods that can be used to bury cables in riverbeds vary widely; the choice depends on such
factors as river-bed conditions, operating depth, route obstructions, depth of burial desired or required,
total length to be installed, cable size, and tools available.
Trenching can be performed by:
a)
b)
c)
Dredging
Blasting
Jetting operated by divers
Usually, the use of these methods is limited to rather shallow waters, practically up to depths of about
20 m, which is usually enough for river crossings.
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As the technical brochure is limited to land cables, the equipment dedicated for submarine cables, such
as submersible equipment which can operate down to a water depth of 1000 m and even more is not
described in this document.
Picture 12 : Embedding
•
Method of operation:
Pre-trenching
Post-embedding
Simultaneous laying and embedding
Plugging or cutting of cable trenches before laying the cables requires precision laying of the cable and
is usually limited to a depth where divers can work for some time and where the river is comparatively
calm.
•
Method of excavation:
a)
b)
c)
d)
e)
f)
g)
h)
Static plough
Static plough water jets (injectors)
Water jets (fluidisers)
Suction-pumps
Cutting-chain
Cutting-wheel
Mechanical disintegrators
Various combinations of the above
Methods a), b), c), and d) may only be used where the riverbed is soft, i.e. sand, shingle or clay.
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•
Propulsion:
a) Towed from the surface
b) Self propelled:
•
Operators:
a) Operated from the surface
assisted by divers
not assisted by divers
b) Operated by divers
at bottom pressure
at atmospheric pressure
It is to be pointed out that if the riverbed changes its morphology along the route, different types of
equipment might be needed.
•
Hardness of the soil
Depending of the type of soil vehicles can accommodate three distinct cable-trenching tools:
A rock wheel cutter which require a cable route over the top of the vehicle will create a trench 1.2
meters deep in any riverbed up to quite strong rock. This is a robust device, but the work rate can be
low with wear rate and consequently is time consuming.
A chain cutter can provide trenches more than 2 meters deep in quite hard material, but is subject to
significant wear, with low work-rates in difficult conditions.
A powerful jet tool creating trench up to 2 meters. It can provide high work-rates in sandy riverbeds.
•
Maintenance and repairing process
Due to the recent technological progress in the field of embedding machines and other ancillary
equipment (remote operated vehicles (ROV), etc.), cable burial is presently possible, with various
methods, up to a considerable depth and practically in every kind of riverbed. However in many cases
the cost of the embedment is very high therefore the right way to proceed is to limit it to the sections
where the risk of a cable damage is so high that it offsets the embedment cost.
In case of limited risk, and where the power availability of the link is high, the possibility of a cable
repair has to be considered as preferable, its cost will be paid only if, and when, the cable will be
damaged.
Of course the cost of a repair, weighed with its probability, has to be compared with the cost of
protection. The result of this approach could be different case by case: for example in a short
connection where extensive human activities (shipping, with anchoring) are present. A total embedding
may be preferable, whilst in a long connection with limited local activity, an exposed cable (except
limited portions with special protection) may be much more economic, even taking into account the cost
of a possible repair.
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Picture 13 : ROV machine
•
Environment
When crossing navigable waterways, this method implies that river traffic be stopped or deviated
during the excavation and laying operations.
3.3.8 Use of existing structures
3.3.8.1 Description of the technique
Finding new layouts for high tension lines is more and more difficult, especially in urban or conservation
areas. The use of existing structures is very attractive to solve integration problems in the landscape
and may lead to drastic reductions in cost and start-up delays. The cohabitation of energy lines with
railway and road constructions is considered in previous items (tunnels and bridges). This section is
dedicated to existing pipes, including an application to pipe-type cables retrofitting.
Pipe-type cables are the most commonly used in the United States to transmit power at high voltages.
Three phase conductors are insulated with layers of fluid-impregnated paper and housed in a coated
steel pipe. The free area in the pipe is pressurised with a dielectric fluid (oil or gas filled) to increase
the dielectric strength of the system, to suppress ionisation in the insulation, and to defer moisture
ingress in the event of a leak in the pipe.
This mode of installation offers several advantages : the pipe itself is very tough and can be installed
with short and narrow roadway openings, minimising traffic disturbances. When the pipe sections are
welded together, the cables may be pulled at a later date, and the maintenance requirements are low
compared with self-contained fluid-filled cables.
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The first pipe-type cable system was installed as far back as 1932. Retrofitting of systems is planned,
and steel pipes are very suitable for the replacement of old cables or to increase the cable size. Existing
pipes can stay on site, only the cables have to be changed by pulling.
By adopting another technology, utilities reduce their environmental exposure to fluid leaks. An idea is
to replace the old fluid-impregnated paper tapes cables by cables with extruded dielectric insulation
such as polyethylene. Due to electrical stress design considerations, the outer diameter of extruded
cables may be larger than for the previous fluid-impregnated cables. Therefore, the substitution is not
always feasible because of the minimum clearance between the top of the upper cable and the pipe.
With extruded cables designed with a moisture barrier as a thin metallic sheath, no pressurised
dielectric fluid is required. The technological change affects cable ratings because insulation and the
free area in the pipe are modified.
3.3.8.2 Limits of the technique
•
Civil work
The use of existing structures offers the advantage of reduced civil work operations, without trench
opening or disturbance. Nevertheless it may be important to empty the pressurised dielectric fluid in
addition to removing the existing cables, if it is to be feared a risk of chemical incompatibility between
the remaining fluid and the replacing cables.
If a grout is injected after the pulling of the new cables, some vents have to be placed along the link.
•
Drying of the soil
The air gap issued from the lack of pressurised fluid is prejudicial to the efficient heat flow dissipation
from conductors towards the surrounding soil. Special injection grouts, with low well characterised
thermal resistivity, can decrease the risk of overheating and thermal instability due to moisture
migration.
•
Duration of the work
Since no civil work is involved, the use of existing structures is very favourable to shorten the site
duration. Retrofitting operations can be anticipated and planned to optimise installation.
•
Cable removal after operation
Any operation to have access to cables after laying is similar to ducts configuration.
3.3.8.3 Adaptation of the technique to the cable system design
The electrical stress design is the first element to design extruded cables to be pulled in place of
existing insulated conductors in steel pipes. A second design point of view concerns ampacities and is
of great interest. It rules the performance and the final acceptance of the proposed new solution.
Fluid-filled cables have been designed for the voltage stress at lightning impulse. The main insulation is
the fluid which fully impregnates the cable. Its breakdown strength in the butt gaps between the paper
tapes determines the insulation thickness.
The critical design parameter for extruded insulation cables is generally not the lightning impulse
voltage but the maximum stress at the alternating current operating voltage to achieve an expected
nominal lifetime of more than 30 years. Historically, the ageing parameters were not accurately
established. Low insulation design stress levels resulted in high insulation thickness and large cable
diameters.
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Today, the improvements of design, materials and manufacturing techniques led to cables with
synthetic extruded insulation for higher voltages up to 500 kV. The insulation thickness has been
reduced for high performance materials, and the maximum continuous operating temperature of 90°C
of XLPE permits to be competitive with fluid filled paper cables, and with polypropylene paper laminate
cables to a certain extent.
Improved ampacities are not the only consequence of the external cable diameter reduction. Longer
shipping lengths are achieved, and the overall system cost may be positively affected.
Cable clearance :
A critical parameter for pipe-type cable design is the clearance between the cables and pipe to ensure
that the cables can be pulled through the conduit. A minimum clearance of about 0,5 in. (12,7 mm) is
recommended by most utilities for straight pulls.
For three single-core touching cables in trefoil formation :
(
where :
De
=
Dd
=
C
=
(
)
)
1

D − 1 + 3 De + Dd Dd − 2 De 

2  d
C=
external diameter of cable (mm),
internal diameter of duct or pipe (mm),
clearance between the cables and the pipe (mm).
The value of the external cable diameter De can be increased by a few per cent to allow for variations
in cable and pipe dimensions or ovality at bends.
Reciprocally, the maximum external cable diameter for a given clearance value is :
De =
[(
] (
)
)


 Dd 4 1 + 3 C + 3Dd − 2 1 + 3 C + 3Dd 
2 2+ 3
(
1
)
140
12"
De (mm)
120
10"
100
80
8"
60
40
6"
5"
4"
20
100 120 140 160 180 200 220 240 260 280 300 320
internal diameter of pipe Dd (mm)
C=1/4 in.
C=1/2 in.
C=1 in.
Figure 16 : Maximum external cable diameter
in terms of internal pipe diameter and clearance
Jam ratio :
When the ratio of the internal diameter of the duct or pipe to the cable external diameter is higher than
3.0, one of the cables in a group of three or four may slip between two other cables, causing the cables
to jam in the conduit. The limit on jam ratio should be modified to take into account variations in cable
or conduit diameter and ovality in conduit diameter at bends.
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4. CABLE INSTALLATION DESIGN AND LAYING TECHNIQUES
4.1
Cable installation design
4.1.1 Installation design in air
Basically two types of cable installation must be considered.
In the first type, the cable is rigidly supported and restrained from any movement due to thermal
expansion or contraction. This is the type of support provided by closely spaced cleats in air. This type
of installation is described in Chapter 4.1.1.1.
The second type of support includes all systems in which the cable is free to move as a result of its
thermal expansion or contraction. The cable may be supported in cleats with a spacing wide enough to
allow it to deflect vertically or horizontally as it expands or contracts. The cleats are usually supported
from below, but can also be suspended from above, depending on the local situations. This type of
installation is described in Chapter 4.1.1.2 or 4.1.1.3.
There is another situation where the cables are in air, being installed in pipes not filled with solid
material. From the thermo-mechanical point of view the cable may be rigid or flexible, depending on the
specific installation design, as described in Chapter 4.1.1.4.
The different types of support give rise to very different mechanical stresses and strains within the
components of the cable and the design procedures are therefore quite different. In general it is
preferable that any given cable system should be designed throughout its length on the basis of either
rigid support or flexible support. If for any reason it becomes essential to mix the two types of support
within a single cable route, special precautions must be taken at the interface between the different
systems, as described in Chapter 4.1.3.
4.1.1.1 Rigid systems
When a length of cable is subjected to a temperature change, each component attempts to expand or
contract by an amount corresponding to its temperature change and its coefficient of expansion. When
the cable is installed in a rigidly restrained environment, no longitudinal expansion or contraction can
occur and the cable therefore develops a thrust when heated and its components are subjected to a
corresponding compressive strain. The conductor and sheath need be considered in practice when
calculating this thrust, if the sheath is not present only the conductor must be considered. Experiments
show that the value of the thrust developed on heating depends on the cable size and design, the
temperature rise and the rate of temperature rise, the slower the rate the more the cable elements will
relax and reduce the actual thrust.
It is generally assumed that at the time of installation the cable is in a stress free condition so that if the
cable is laid at a ground temperature (or ambient temperature for cables in air) below the maximum
design ambient temperature on which rating calculations are based it will develop a thrust when the
ambient temperature increases to the design value.
This temperature increase is likely to be very slow however and hence allows a greater relaxation to
occur. The rate of temperature increase from the design ambient temperature to the maximum
operating temperature depends upon the cable environment and the rate of load increase.
A buried cable cannot increase in temperature rapidly because of the thermal capacity of its
surrounding. A cable in air can rise in temperature more rapidly but it is most unusual to require a
newly installed cable to carry full load immediately, load growth is usually gradual and cyclic so that
some opportunity for relaxation occurs. To allow for these effects it is necessary to include relaxation
factors in the calculation of total cable thrust.
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•
Calculation of cable thrust
The evaluation of the cable thrust is essential when dealing with rigidly installed cables, or at the
interface points where flexible and rigid systems meet.
Generally speaking the total thrust of a cable can be calculated as:
C= C1 + C2 + C3 + C4
(kg)
where:
C1 = conductor thrust due to load
C2 = conductor thrust due to ambient change
C3 = sheath thrust due to load
C4 = sheath thrust due to ambient change
C1 is given by:
C1 = K1. α c. ∆Tc1. Ec. Ac
(kg)
where α is the coefficient of thermal expansion of the conductor metal
α c = 17.10-6 for copper
(1/K)
-6
α c = 24.10 for aluminium (1/K)
∆ Tc1 = conductor temperature rise from the maximum ambient temperature to the maximum
conductor temperature (K)
Ac
= conductor cross section (total cross section for three-core cables)(mm2)
K1
is the relaxation coefficient, which is of the order of 0.75 for load temperature variations,
depending on cable constructions
Ec
is the equivalent Young’s modulus for the conductor which depends upon its construction and
materials and on the constraint provided by the insulation surrounding the conductor. Experimental
measurements are necessary to obtain accurate results.
C2 is given by:
C2 = K2. α c. ∆Tc2. Ec. Ac
(kg)
where the symbols have the same values as above, but K2 (the relaxation factor) is of the order of 0.45
for ambient, temperature variations, depending on cable construction.
∆Tc2 is the conductor temperature rise from the laying temperature to the maximum design ambient
temperature (since the laying temperature is not usually known at the design stage the minimum
ambient temperature may be assumed). (K)
For the metallic sheath the thrust is given by:
C3 = K3. α g. ∆Tg1. Eg. Ag
(kg)
where α g is the coefficient of thermal expansion of the sheath metal
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αg
αg
= 28.10-6 for lead sheaths
= 24.10-6 for aluminium sheaths
(1/K)
(1/K)
∆Tg1 is the sheath temperature rise from the maximum ambient temperature to the maximum sheath
operating temperature (K)
= sheath cross section
(mm2)
is of the order of 0.30 for lead sheaths and of 0.65 for aluminium sheaths
is the equivalent Young’s modulus for the sheath in compression and must be found
experimentally for reasons similar to those given for the value of Ec
(kg/mm2)
Ag
K3
Eg
C4 = K 4 ⋅ α g ⋅ ∆Tg 2 ⋅ E g ⋅ Ag
(kg)
where the symbols have the same value as above, but K4 may be taken as 0.1 for lead sheaths and
0.45 for aluminium sheaths, again depending on cable constructions
∆Tg 2 is the sheath temperature rise from the laying temperature to the maximum design ambient
temperature
•
(K)
Spacing and cleating
For rigidly restrained system the spacing and cleating evaluations must be done considering several
parameters:
- At curves the restraining elements must be capable of withstanding the radial force given by
F= C/R (kg/m)
where C is the cable thrust and R the curve radius
- the radial pressure at bends due to maximum conductor thrust must be compatible with the insulation
material
- cleat spacing must be calculated considering that the cable thrust must be less than the critical load
for instability ( Ccr )
For cables with thick aluminium sheath the critical load may be calculated as follows:
π 2 ⋅E ⋅J
Ccr =
l2
whilst for cables with lead or thin aluminium sheath:
Ccr =
2⋅π 2 ⋅ E ⋅ J
l2
This difference of behaviour, which is shown in experimental tests, may be explained by the fact that
the more rigid aluminium sheathed cables behaves as though the cleat acts as a hinge, whilst for the
less rigid cables the restraint normally appears as midway between a hinge and a rigidly fixed beam.
- sheath strain must be checked if daily temperature changes are significant (> 35°)
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- At bends the cleat spacing is reduced by half compared with the straight sections.
•
Short circuit forces in rigidly restrained cables
Short circuit forces may be significant in the case of rigidly restrained cables cleated in air. In this case
the cable between cleats will already be in compression due to its temperature rise under normal load
and the electrodynamic effect of the short circuit will result in the addition of a uniformly distributed
side loading to the original compression load which, assuming a phase/phase short circuit, is given by:
µ0 ⋅I 2
F=
2 ⋅ π ⋅ S ⋅ 9.81
(kg/m)
where:
µo
= magnetic permeability of air, 1256.10-6
I
= short circuit current, rms
S
= cable spacing
(H/m)
(A)
(m)
These forces result in a bending moment in the sheath which is a maximum adjacent to the cleat and
has a value:
M=
F ⋅ l2
12
(kg.m)
where:
l
= cleat spacing
(m)
This equation is valid for the normal case where the thrust C existing in the cable prior to the short
circuit is less than 10% of Ccr the critical thrust causing deflection, where:
C * er =
4 ⋅ π 2 (EJ )
l2
(EJ)*
is the flexural rigidity of the cable based on the short term properties of the sheath.
*
(kg)
If the thrust existing in the cable before the short circuit exceeds 0.1. for a lead sheath cable or if the
sheath is of aluminium, a more elaborate calculation must be used.
Having calculated the bending moment M, the sheath strain ε is given by:
ε=
M ⋅ Ds ⋅10 −3
*
2 ⋅ (EJ )
where:
Ds
= outside diameter of cable sheath
(mm)
To avoid noticeable permanent deformation of the cable the maximum sheath strain ε should be limited
to an acceptably low level.
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4.1.1.2 Flexible systems (Western approach)
Flexible types of cable support are those systems which allow the cable to expand in length and to
deflect laterally to accommodate this expansion when the cable is heated and to return to the original
formation on cooling. In order to control the movement of the cable within pre-determined limits it is
usually installed initially in an approximately sinusoidal formation with cleats at appropriate intervals so
that expansion takes place by an increase in the amplitude of the sine wave.
Because the flexible system allows cable expansion to take place it is not characterised by the high
values of thrust which occur in the rigidly restrained system.
•
Cables cleated with movement in a vertical plane
The cable is held in widely spaced cleats with an initial sag between cleats which increases with
temperature rise. Figure 17 illustrates a system of this type.
Figure 17 : Cable cleated with movement in a vertical plan
The spacing of the cleats is not critical and within the limits given below can be chosen to suit the
fixings available.
The weight of the cable is supported by the cleat and if the cleat spacing is too large the side pressure
on the cable at the cleat will become excessive and there will be a tendency to concentrate bending at
the edge of the cleat. On the assumption that the cleat length is approximately equal to the cable
diameter and has suitably rounded edges the following practical rule is suggested
l≤
De2
65 ⋅ W
(m)
where:
l
= cleat spacing (m)
W
= cable weight (kg/m)
De
= cable outside diameter
(mm)
Similarly, to avoid concentrated bending at the edge of the cleat the cable deflection δ due to its own
weight should be at least five time less than the initial sag between cleats fo required to ensure
satisfactory expansion and contraction movement. It is therefore necessary to make an initial estimate
of cleat spacing. The following criteria for δ and fo may be followed
W ⋅l
f
δ =
≤ o
384 ⋅ ( EJ )
5
4
(m)
where:
δ
= cable deflection due to its own weight (m)
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(EJ)
fo
W
l
=
=
=
=
(kg.m2)
(m)
(kg/m)
(m)
flexural rigidity of the cable
initial sag
cable weight
cleat spacing
Having determined the cleat spacing it is necessary to fix the value of fo, the initial sag between cleats.
This sag should not normally be less than 2 De but it may be necessary to increase it beyond this value
in order to ensure that the change of strain in the sheath due to thermal movements does not exceed
the maximum imposed by the fatigue properties of the sheath.
To simplify the calculation of the sheath strain it is assumed that the longitudinal expansion of the
complete cable follows the expansion of the conductor.
The total sheath strain is then the sum of the absolute values of the strain due to the movement of the
cable together with the strain due to the differential expansion of the conductor and the sheath.
On this basis it can be shown that the maximum sheath strain change ∆ε max will not be exceeded
provided:
f0 ≥
2 ⋅α c ⋅ ∆Tc ⋅ Ds ⋅10 −3
∆ε max − α c ⋅ ∆Tc − α g ⋅ ∆Tg
where:
αc
=
∆Tc =
αg
=
∆Tg =
(m)
coefficient of thermal expansion of the conductor
daily temperature rise of the conductor
coefficient of the thermal expansion of the sheath
daily temperature rise of the sheath
(l/K)
(K)
(l/K)
(K)
Ds
= outside diameter of the metal sheath (or average outside diameter for a corrugated sheath)
(mm)
∆ε max = maximum allowable sheath strain change due to daily load cycles.
For a typical system designed for a life of 30, 40 years the standard values of 0.1% for lead and 0.25%
for aluminium sheathed cables are normally adopted, particularly for fluid filled cables.
Taking into account the excellent experience during many years, however, slightly less conservative
values such as 0.12% for lead and 0.35% for aluminium can also be considered, particularly for
extruded cables.
The system described above is suitable for straight or gently curved cable routes. If it becomes
necessary to install the cable around a small radius bend in the route it should be supported on a
horizontal plane within the bend and with suitable means of minimising friction as the cable moves due
to thermal changes.
Flexible system with cable movement in a horizontal plane
In this type of installation the cables are arranged in a sinusoidal formation in a horizontal plane with
cleats fixed at the points of flexure of these sinusoids, as shown in
Figure 18.
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Figure 18 : Plan view of cables installed with movement in a horizontal plan
Swivelling cleats may be used, capable of rotating on a vertical axis as the cable moves, but it is
preferred to use fixed cleats with a length approximately equal to the cable diameter and with a rubber
lining of 3 to 5 mm thickness.
These cleats must be installed at an appropriate angle.
The movement of the cable due to thermal cycles will be largely influenced by the friction between the
outside surface of the cable and the support between cleats. It is essential that the cable should be
supported so that it moves only in the horizontal plane using a low friction support and allowing
adequate air movement around the cable to avoid de-rating.
As a practical rule the cleat spacing should be:
De
(m)
20
l=
where:
De
= outside diameter of the cable (mm)
The initial deflection of the cable fo should be fixed following the same rules as given in paragraph for
cable moving in a vertical plane.
•
Calculation of cable thrust
As already mentioned the cable thermal expansion in a flexible configuration give rise to small axial
thrust, while the initial sag is increased.
Simple formulae can be used to calculate these parameters, assuming that the initial configuration is a
sinusoid.
The sag is given by the formula:
f =
f 02 +
4 ⋅ α c ⋅ ∆Tc ⋅ l 2
π
2
Where the symbols are the same used as before.
The axial thrust is given by the formula:
4⋅π 2 ⋅ E ⋅ J f − f0
C=
⋅
l2
f
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It may be easily verified that the axial thrust in a flexible system is much lower than in a rigid system
and may be practically neglected in most applications.
•
Short circuit forces in flexible type cable installation
Short circuit forces are of much greater significance in cable installations of the flexible type because
of the wider cleat spacing used compared with rigidly restrained cables.
It is normally necessary to provide straps around the three cables at intervals between the cleats to
hold the cables together during a short circuit. It therefore becomes necessary to consider the spacing
of these straps and the strength of the strap necessary to withstand the forces involved.
As in the case of rigidly restrained cables, the length of cable between restraints will be subjected to a
uniformly distributed side loading and assuming a phase/phase short circuit this is given by:
µ o ⋅ l2
F=
2 ⋅ π ⋅ S ⋅ 9.81
(kg/m)
where:
µo
= magnetic permeability of air = 1256.10-6
I
= short circuit current (rms)
S
= cable spacing
(H/m)
(A)
(m)
This force results in a bending moment in the sheath adjacent to the restraint of:
M=
F ⋅ l1
12
2
(kg.m)
where:
l1
= distance between restraints
(m)
Since it is usually necessary to fit at least one restraining clamp around the cables between the cleats, l1
corresponds to the distance between these clamps or between a cleat and a clamp.
Hence, as before, the maximum sheath strain is given by:
ε=
M ⋅ D s ⋅10
−3
2 ⋅ (EJ ) *
where the symbols have the same meaning as before and to avoid noticeable permanent deformation of
the cable the value of the sheath strain ε must be limited to an acceptably low level.
The strength of the strap can be calculated from the equation for F above but since this ignores the
instantaneous value of current, which may substantially exceed the rms value and also ignores
resonance effects which may occur, a factor of safety of 2 should be allowed so that the minimum
strength of the strap is given by:
2.F.l1
(kg)
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Using these equations the number of strap within a span of cable between cleats may be calculated.
The same equations are valid for both types of flexible installation with movement in the vertical or
horizontal planes.
4.1.1.3 Flexible systems (Japanese approach)
Generally, there are uniform basics for the snaking design, as described below.
Horizontal snaking installation
1) Initial snaking width : 1 De or more
Figure 19 : Horizontal snaking
Picture 14 : Snaking in a tunnel
2) Pitch (2 L): 6-9 m
3) Occupied width (W): W = D + B + n + σ
Where D = Cable occupied width (2D e De = outside diameter of the cable when trefoil installation)
B = Initial snake width
n = Lateral snake displacement
σ = Tolerance
4) Lateral displacement (n): n =
B² + 2 L.m x 0.8 − B
Where: m = Cable expansion = α.t.L
α = Coefficient of linear cable expansion
t = Temperature rise
5) Formulas for axial tensions generated (Fa):
Depending on types with or without metal sheaths, the formulas in Table 3 are used.
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Table 3 : Horizontal snaking calculations
Formulas for Calculating Axial Snaking Tensions Generated (Horizontal Snake)
Metal Sheath
With Metal Sheath
Low Temperature
High Temperature
µ. w. L ²
x 0.8
2B
−
Without Metal Sheath
−
8. E . I α. t µ. w. L ²
.
+
x0.8
B²
2
2. B
8. E . I α. t
8. E . I α. t
.
−
.
B²
2
( B + n )² 2
µ. w. L ²
−
x 0.8
2 ( B + n)
8. E . I α. t
µ. w. L ²
−
.
−
x 0.8
( B + n )² 2
2 ( B + n)
Note: + tension; - : compression
EI= Cable bending rigidity
W = Unit cable weight
µ = Coefficient of friction between cable and installation surface
6) End section of snaking installation
The necessary number (N) of fastening cleats is generally determined as follows:
N = Fa/F + 1 (or Fa/F × Sf)
Where: F = Restraining force of terminal fastening cleats
Sf = Safety factor
7) Middle section of snaking installation
The snake formation is fastened at inflection points with intermediate cleats at every or several pitches.
Vertical snaking installation
Figure 20 : Vertical snaking
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Initial snake width (B) : 1 De or more
Pitch (2 L) : snake pitch
Bending distortion :
1.5% or less
Radial surface pressure : 3.33 kg/cm² or less
Lateral displacement (n) : n = B² + 2 L.m x 0.8 - B
Formulas for axial tensions generated (Fa):
From the same viewpoint as for horizontal snaking installations, the formulas in Table 4 are used with
or without metal sheaths.
Table 4 : Vertical snaking calculations
Formulas for Calculating Axial Snake Tensions Generated (Vertical Snake)
Metal Sheath
With Metal Sheath
Low Temperature
High Temperature
w. L ²
x0.8
2. B
−
Without Metal Sheath
−
8. E . I α. t w. L ²
.
+
x0.8
B²
2
2. B
8. E . I α. t
8. E . I α. t
.
−
.
B²
2
( B + n )² 2
w. L ²
−
x 0.8
2 ( B + n)
8. E . I α. t
w. L ²
−
.
−
x 0.8
( B + n )² 2
2 ( B + n)
End section of snaking installation
The necessary number (N) of terminal fastening cleats is generally determined as follows:
N = Fa/F + 1 (or Fa/F x Sf)
Where: F = Restraining force of terminal fastening cleats
Sf = Safety factor
Middle section of snaking installation
The cable is supported by direct cable rests at crests of the vertical snaking. In some installations,
restraining cleats are used at every several pitches.
Vertical Installation Design
Fluid-Filled cables involve fluid pressure rises due to their vertical installation. XLPE cables do not
involve such difficulties and are easy to install upward on a tower. Vertical installations used on towers,
in vertical tunnels, etc. can generally be classified as follows:
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Table 5 : Vertical cable installation at shafts
Item
Triplex cable
Single-core cable
Shaft height
6-10m or less
Shaft height
6-10m or more
Shafts where cleats
can not be used
Method
- Straight installation
- Fastened with cleats at several m intervals
- Straight installation
- Fastened with cleats at several m intervals
- Snaking installation (6-8m pitch)
- Fastened with cleats at snake inflection points
(For some sizes, movable cleat supports are also
used at snake crests.)
- One-point fastening using tension member cable
- Steadying cleats are used (in special installations case).
4.1.1.4 Cable in ducts
The ducts may be filled with solid material such as Bentonite or not filled.
The first solution is often preferred for relatively short ducts (normally less then 100 m) used to cross
roads, railways or other obstacles, inserted in sections where the cable is directly buried. Any
movement of thermal origin is completely prevented and the cable behaves as if it were directly buried.
In other situations according to different practices or due to other constraints, the ducts are not filled
and three different design concepts may be adopted.
a) Large diameter duct, cable blocked at the extremities.
If the inner diameter of the duct is significantly larger than that of the cable (typically 1.5 to 2 times),
the thermal elongation results in cable snaking. The geometrical configuration is similar to a sinusoid or
a helix with a certain pitch and amplitude, depending on duct and cable diameters, weight, axial rigidity,
flexural rigidity of the cable, friction coefficient between cable and duct, temperature variation. Thanks
to the snaking effect, the axial thrust is drastically reduced with respect to the thrust developed by a
rigidly restrained cable. As a consequence of the thermal cycling, the amplitude of the deformation
varies cyclically and the resistance of the cable to fatigue phenomena must be considered.
b) Small diameter duct, cable blocked at the extremities.
If the diameter of the duct is only slightly larger than the cable (minimum clearance to allow the cable
pulling), the very limited snaking is not sufficient to reduce the axial thrust and the thermal movements
are negligible. In practice this case may be considered a rigid installation.
c) Small diameter duct, cable free to move at the extremities.
In this type of installation there is a certain movement of the cable from the duct towards the manholes
as a consequence of the thermal expansion. In the manholes the cable is installed in a snaking
configuration properly designed in order to maintain the cyclic bending of the cable within acceptable
limits. This situation is dealt with as a transition in Chapter 4.1.3.
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4.1.2 Installation design for buried cables
4.1.2.1 Backfill
To improve heat dissipation, only sand or special backfill shall be used around cables or ducts.
Sand :
There are specifications available how to select sand for cable trenches. During performance of the
work the constructor shall take random samples for clay tests and sieve tests. The thermal resistivity
characteristics of the sand shall also be verified by testing. During installation of the cable the
excavated trench shall be kept in dry condition by dewatering until cable laying and backfilling are
completed. Backfill material shall be placed in uniform layers and compacted. Moisture content of
backfill material shall be adjusted as required to obtain the specified density. To protect the cable shall
be covered by concrete tiles or plastic sheets. On top of the tiles native backfill may be used.
•
Special backfill
The thermal resistivity of a carefully selected Cement Bound Sand (CBS) for electrical cables has
several advantages as a good thermal backfill material. It will eliminate the risk of voids due to water
erosion or movement caused by thermal expansion. CBS will also keep the thermal resistance of the
backfill surrounding the cable at a very stable level.
The thermal resistivity of a good backfill for electrical cables when hardened is 0.35 K.m/W or less in
moist condition and 1 K.m/W in its totally dried out condition. Testing of the thermal resistivity and
compressive strength has to be performed to be sure it reaches the specified values.
The thermal backfill shall be composed of fine and coarse aggregates, cement and fluidising agent. The
fluidising agent consists normally of fly ash and water.
The thermal backfill should be installed by pouring it into the trench or by use of grout pumps.
Natural backfill should not be placed in the trench until one day after pouring and inspection.
4.1.2.2 Cooling systems
To increase the capacity of a cable circuit forced cooling can be used for the direct buried cables and
cables in duct banks. There are two main systems that have been used. External cooling, where cooling
water runs in four pipes separated from the three single core cables and laid parallel to them. The other
system is surface cooling where water is in direct contact with the outside surface of the cable. Each
cable and its cooling water being contained in a pipe. Cooling stations are normally placed at the ends
of the cable route. The cooling station consists of water pumps, water storage and expansion tank, and
a heat exchanger for cooling the water circulated round the cable route. The cooling stations can be
operational from either local or remote positions.
To increase the load capacity of cables installed in tunnels and shafts, forced ventilation can be used.
Temperature sensors are monitoring the surface temperature of the cables in various places along the
route. The ventilation fans starts at pre set levels to cool the cables during the high load periods.
A range of articles describing design and calculation of forced cooling of cable installations are
available in Electra and IEEE.
4.1.3 Transition between different installation types
Along the route of an underground link, different installation techniques may be used. If all the
techniques are all rigid or all flexible, the behaviour of the link seems to be homogeneous, but even in
this case, problems may appear around the jointing areas.
If they are different, i.e. unfilled duct and direct burial or filled trough and manhole, damages can
happen on the link if transitions between flexible and rigid installations and of course around the jointing
are not treated or badly treated.
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4.1.3.1 Transition between ducts and manholes (open air)
Cable thermal expansion of the span appears in the manhole by duct method.
Therefore the “Offset” part is made, and it must absorb thermal expansion so that the thermal
expansion does not have a bad influence on the cable and joint part.
Offset design of the XLPE cable in the manhole uses a geometrical analytic technique so that the cable
bend radius do not to become less than allowable radius when the cable come out from duct face.
And, the consideration is necessary about the amount of occurrence sheath distortion in the case of the
cable that has a metal sheath.
So, allowable amplitude distortion has only to find amplitude condition by the day data, from the S-N
curve (amplitude Strain - Number of times to the destruction), because the thermal expansion number
of times of the year is smaller than the thermal expansion number of times of the day.
1) Design of straight offset
The experiment was done about movement of jointbox offset form that absorbed thermal expansion of
one core XLPE cable, based on the technique of SCFF cable experiment in the beginning.
The cable spiral deformation occurred, and the inclination of the jointbox and direction movement in the
core occurred as a result of this examination.
Therefore, it confirmed that the rigid of the jointbox was necessary, because the deformation is
concentrated on one bend part and an allowable radius can not secure it.
Such an experiment is continued, and an examination is done on the jointbox fixed condition, and the
design technique of the following 3 forms is applicable at present :
a) Equal arcs range offset type
b) Long offset type
c) Three equal arcs with pendency type
Each calculation technique is shown in Table 6.
a) Calculation technique of “Equal arcs range offset type”
This form is the way of designing that suppose to absorb thermal expansion by two circular arcs
without change the turning point.
The attention is needed with this form that the cable contact to the other cables and the other jointbox
because of spiral deformation offset by thermal expansion.
b) Calculation technique of “Long offset type”
This form has straight line part in the offset section.
It is the way of designing that suppose to absorb thermal expansion by forms three circular arcs by
straight line part's being curved.
c) Calculation technique of “Three equal arcs with pendency type”
Large conductor size cable hangs down greatly between support points at the time of the thermal
expansion, the part where a cable in the neighbourhood of the bottom hung down point can not satisfy
the allowable radius.
So, this form takes that problem into consideration.
This form does not contain a straight line in the offset section in the same way as “Equal arcs range
offset type”.
It is the way of designing that supposes to absorb thermal expansion by three circular arcs threedimensionally.
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Table 6 : Offset calculations
Equal arcs range offset type
Three equal arcs with pendency
type
Long offset type
Pendent part at the time of
thermal expansion
Joint box
Joint box
Plane distance between the support points
Initial form
Bend radius R at the time of thermal
expansion
 F 2 + L2 
1 
F 
 tan − 1   
R =
m + 

2θ 
F
 L  


 F 2 + L2 
F
m + 
 tan − 1  
F
θ
 L


=
θ
F 2 + L2
sin
2
2
Where,
R: Bend radius at the thermal
expansion length of the annual
maximum (mm)
L: Offset length (mm)
F: Offset width (mm)
θ : Centre angle at the time of
thermal expansion (rad)
Bend radius R is approximated by
three arcs.
 F 2 + L2

2 LF

sin−1 2
+ l 
2
2
F
F
+
L


θ
sin
(L + l − m)2 + F 2
2
=
2
2
F +L
2
LF
θ
sin−1 2
+l
2F
F + L2
R=
1
2θ
Where,
R: Bend radius at the thermal
expansion length of the annual
maximum (mm)
L: Offset length of arc part (mm)
l : Offset length of straight part
F: Offset width (mm)
θ : Centre angle at the time of thermal
expansion (rad)
Where,
R1 :Radius decided more than the
interior radius rate of both ends
support hardware
R2 :Bend radius at the thermal
expansion length of the annual
maximum
d : The amount of pendent
a1 ,a2 :The position to the bottom
point
As for the details, it is omitted.
d) Method of supporting cable in the manhole
The cable guide is set up at duct face and joint side to avoid bad influence for cable against thermal
expansion in the all forms.
Support method for the cable that can be put between the cableguide, varies in the kind of the each
form.
In the case of “Equal arcs range offset type”, It is supported with the cable guide in the duct face and
the jointbox side, and the cable between that is not supported.
In the case of “Long offset type”, It is supported the middle point for the prevention of hang down the
cable, because the offset length sometimes becomes long by the existence of the straight line part at
the time of thermal expansion.
And, In the case of “Three equal arcs with pendency type”, It is supported with the suitable support
hardware to keep the cable allowable radius in the pendent part at the time of the thermal expansion.
On the other hand, the Triplex XLPE cable of the horizontal part, is supported with the pillows
manufactured by the porcelain material, and so on in the interval of about 1-1.5m.
2) Offset design of the bend part
This form is the way of designing that supposes to so that the degree of radius might become a
constant, as shown in the bottom figure.
The bend radius changes in "R" from "R 0" by thermal expansion quantity "m", then the straight line part
which is equal on both sides of the bend part is established.
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Straight part
R = R0 −
m
2 tan(θ / 2) − θ
Straight part
Figure 21 : Shape of bend part
4.1.3.2 Transition between flexible and rigid systems (open air)
As already stated it is desirable that any given cable installation should be designed as a wholly rigid or
wholly flexible system. It may sometimes be necessary, however, to mix the two above designs within
a single cable route and special consideration must be given to the interface between the two systems.
Rigidly restrained cable systems are characterised by the presence of a substantial mechanical thrust in
the conductor when the cable is heated, whilst flexible systems have a low value of conductor thrust.
At the interface between these systems the conductor will tend to move from the rigidly restrained
section into the flexible section. The amount of the movement depends on the cable characteristics and
particularly on internal friction between the conductor and the other cable components. The movement
may extend over a few meters on both sides of the transition section and may cause damage or
disturbance to the insulation and unacceptable sheath strains, if appropriate precautions are not taken.
In order to reduce the movement and its effect on cable integrity, it is good practice to install the cable
in a series of rigidly fixed curves at the extremity of the buried section, in order to provide a high
frictional resistance to the movement of the conductor within the cable.
If a joint is installed at the transition section, the behaviour of the joint itself must be carefully
considered in relation with the above mentioned phenomena of movement and axial thrust of the
conductor.
In the most common joint design there is no mechanical restriction to the conductor movements,
whereas in other designs a mechanical block of the conductor is provided.
It should be verified that the movements or the mechanical thrust do not exceed acceptable limits.
4.1.3.3 Transition between flexible and rigid systems (buried)
This case can be found where cables are partly laid in ducts and partly directly buried or laid in filled
troughs.
As already stated before, at the interface between rigid and flexible systems, the conductor will tend to
move from the rigidly restrained section into the flexible section ; Ducts of appropriate size are then
required in order to allow a kind of snaking inside the conduit.
One must be aware that the movement of the whole cable inside the duct may extend on both sides of
the duct section. If a joint is installed at the transition section, the behaviour of the joint must be
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carefully considered in relation with the above mentioned phenomena of possible movement and axial
thrust of the cable. Even when the joint is buried, additional fixing of the joint is required, for example
by the use of weak mix or by appropriate cleating.
4.2
Cable laying and installation techniques
4.2.1 Cable pulling calculations
The basic calculations relevant to cable pulling are reported hereunder.
4.2.1.1 Clearance in ducts
Cables pulling in ducts or pipes requires that the duct or the pipe have an internal diameter in excess of
the cable diameter to allow for a safe operation.
The free space between the cable and the duct inner size is called clearance.
The clearance to be considered is not the result of the implementation of precise formulae, but the
result of the practical experiences.
For single core cable the inner size of the ducts should be normally at least 1.5 times the cable size,
particularly with long ducts with some bends along the route.
For three cables pulling in the same duct, the duct size should not have a ratio with the cable size of
less than 2.8 to 3, because jamming may take place at bends.
According to a different industrial practice a standard clearance of 30 mm is adopted. Even smaller
clearance may be adopted for straight pulls.
4.2.1.2 Pulling tension
The main parameter to be evaluated when assessing the cable laying aspects is the cable pulling
tension.
The knowledge of the pulling tension is not only essential to plan the actual lay, but also to assess the
suitability of cable design / route design / laying methodologies.
The following equations are applicable to single cables, nose pulled into trenches or into long ducts or
pipes.
The route should be first divided into specific sections of straight, curved, uphill slope and downhill
slope. The pulling tension required for each section is then calculated, starting at the drum and taking
the exit tension for each section as the entry tension for the next. The formulae are as follows:
Straight pull
T2 = T1 + W ⋅ K ⋅ L
(kg)
where:
T2
=
T1
=
W
=
L
=
K
=
(kg)
(kg)
(kg/m)
(m)
(m)
exit tension
entry tension
cable weight
length of section
coefficient of friction for that section
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Horizontal bend
T2
R
T1
ϑ
Figure 22 : Horizontal bend
T 2 = T 1 cosh K θ + sinh K θ
T 1 2 + (W ⋅ R )2
(kg)
where:
θ
= angle subtended by the bend
R
= bend radius
(radians)
(m)
Vertical bend
Pulling up the bend
T3
ϑ
R
R
T2
ϑ
T1
Figure 23 : Vertical bend (pulling up)
T2 = T1 ⋅ e K .θ −
[
W⋅R
2 ⋅ K ⋅ sin θ − 1 − K 2 e K .θ − cos θ
2
1+ K
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(
)(
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)]
(kg)
T3 = T2 ⋅ e K .θ +
[
W ⋅R
2 ⋅ K ⋅ e Kθ + 1 − K 2 1 − e K .ϑ ⋅ cosθ
1+ K 2
(
)(
)]
(kg)
Pulling down the bend
T1
ϑ
R
ϑ
R
T2
T3
Figure 24 : Vertical bend (pulling down)
[
)]
K ⋅θ
T2 = Te
+
1
W⋅R
2 ⋅ K ⋅ sinθ − 1 − K 2 e K .θ − cosθ
2
1+ K
T3 = T2e K .θ −
W⋅ R
2 ⋅ K ⋅ e K ⋅θ ⋅ sinθ + 1 − K 2 1 − e K ⋅θ ⋅ cosθ
2
1+ K
(
[
)(
(
)(
(kg)
)]
(kg)
Upward slope
L
T2
ϑ
T1
Figure 25 : Upward slope
T2 = T1 + W ⋅ L(sinθ + K ⋅ cosθ )
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Downward slope
T1
ϑ
T2
L
Figure 26 : Downward slope
T2 = T1 − W ⋅ L(sinθ − K ⋅ cosθ )
(kg)
In the formulae given above the value of K, the coefficient of friction, for the part of the route in
question will depend on the material of the cable outer sheath and the surface with which it is in
contact.
It is essential to have good reference values for the friction coefficient to have reliable values, while
simplified formulae can be used to calculate the pulling tension.
Having established the pulling tension required it must be checked that this tension is within the
acceptable limits for the cable.
To avoid relative movement between conductor and sheath with possible disturbance of the insulation it
is essential to fit a cable pulling grip which is anchored to the conductor or conductors and to the sheath
at the leading end of the cable. A pulling grip is also fitted at the trailing end of aluminium sheath
cables. However it is assumed that the tension is withstood by the conductor.
As reference the following values could be considered:
single core cables
single core cables
3 core cables
3 core cables
- copper conductors
-
6 kg/mm2
aluminium conductors 3 kg/mm2
copper conductors
5 kg/mm2
aluminium conductors 3 kg/mm2
but alternative values could be considered.
For example, France considers
- for aluminium single core cables, 5 kg/ mm2,
- for all single core cables, a limitation on the pulling tension of 4000 kg.
In any case, the maximum permitted levels of conductor tension have to be checked with the cable
supplier.
4.2.1.3 Side wall pressure
Having established the pulling tension required it must be checked that this tension is within the
acceptable limit for the cable and that the side pressure on the cable at bends is also acceptable.
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At bends in the route the compression force between the roller and the cable is given by:
F=
T ⋅d
R
where:
F
=
T
=
R
=
d
=
(kg)
compression force on roller
tension in cable
bend radius
distance between rollers
(kg)
(kg)
(m)
(m)
If iron skid plates are used at the bend, the compression force between cable and skid plate is given by
F=
T
R
(kg/m)
The maximum permissible values of F are different and dependant on the type of sheath and insulation.
These values have to be checked with the cable manufacturers.
4.2.2 Installation Methods
4.2.2.1 Introduction
The following five techniques are now used: Nose pulling by winch, synchronised power drive rollers,
caterpillars, mechanical laying and bond pulling and the most common is nose pulling followed by power
rollers, caterpillars , bond pulling and finally mechanical laying.
Picture 15 : Cable pulling in duct
A brief description of each of the five techniques is given below:
4.2.2.2 Nose pulling
With this technique the cable is installed by using winch with a pulling hawser directly connected to the
cable end, or "nose".
In this case the tension required to install the cable is taken by the cable itself and hence it is important
that the pulling tensions are calculated beforehand to ensure the design limits are not exceeded.
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4.2.2.3 Synchronised power drive rollers
This technique relies on the use of multiple powered rollers positioned at regular intervals along the
cable route to install the cable. The frequency of the rollers is dependent upon the cable construction
and the route itself.
Since each roller has to provide an equal force they require to be synchronised to operate effectively
and to avoid any damage to the cable due to compressive forces.
Normally a winch and hawser are used to supplement the rollers by nose pulling but the tension on the
cable end is very low due to the effects of the powered rollers.
4.2.2.4 Caterpillar or hauling machine
Caterpillars apply a pushing force directly onto the cable outer sheath and can be used to install the
cable directly or in conjunction with power rollers or winches.
4.2.2.5 Bond Pulling
With this technique the pulling tension applied by the winch is taken by a wire bond to which the cable
is tied at regular intervals.
At bends the bond is passed through a snatch block and the ties attaching the cable are removed before
the bend and reapplied after the bend in a continuous operation.
The tension required to install the cable is therefore distributed along its full length and sidewall
pressure at bends is reduced to a minimum.
4.2.2.6 Mechanical laying
There are three ways of organising the mechanical laying site:
- mechanically excavated narrow trench, and separate laying of the cables: laying and backfilling is
done by traditional methods after the trench has been mechanically excavated;
- trench excavation and cable laying both mechanical : trench excavation, cable laying and sometimes
the backfilling are performed by a machine;
- trench excavation, cable laying, backfilling all continuous and mechanised : with this method, trench
excavation, cable laying and sometimes trench backfilling can all be done simultaneously in a
continuous process over the full length of a homogeneous portion of the link (the joints have to be
prepared beforehand).
This technique is only used for voltages under 170 kV.
The cables are usually buried directly in trefoil formation with a minimum cover of one metre.
4.2.2.7 Other installation methods in tunnel
•
By magnetic belts
When laying a cable in a tunnel, many electric powered
caterpillars are placed in the tunnel. Caterpillars are operated
synchronously to pull the cable in the tunnel. Recently, in order to
shorten the construction period and lower the cost by decreasing
the number of joints, the cable span becomes longer and longer.
For quick and steady cable drawing of such long cables, a cable
transfer system with magnetic belts may be used.
Picture 16 : Cable installation
in tunnel
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Figure 27 : Cable installation in tunnel
Cable
Position of the setting
machine in the tunnel
Rail
Guide roller
Driving gear
(Magnetic belt)
Figure 28 : Magnetic belt pulling machine
•
By locomotive and trolley system
An innovative technique is being developed for the installation of high voltage cables in a 3m diameter
10km long tunnel beneath Auckland (New Zealand). The cables in 1350m long sections are to be
located on racks at varying height on either side of the tunnel. The tunnel has a 710mm gauge
conventional light rail track on the floor.
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The installation is to be achieved by lowering 30 tonne drums into the cable shafts where an hydraulic
Caterpillar Cable Pusher feeds the cable onto 700 custom-made trolleys attached to a wire rope. The
trolleys are guided by one of the rails and support and tow the cable along the tunnel into position. The
empty trolleys are parked on one rail and travel around to a turntable near the Caterpillar where they
accept the cable from the drum. A diesel hydraulic locomotive (tow-shoe unit), driving on solid rubber
tyres and guided by the light rail track tows the cable and trolleys from the turntable along the floor into
position. The locomotive uncouples from the trolleys and reverses over them while simultaneously
lifting from the trolleys up to the cable brackets. The cable is snaked before being lowered onto the
wall brackets.
The Locomotive has a maximum draw bar pulling capacity of 30 kN and can lay cable at 2 km/h, with
a top travelling speed when not working of 4 km/h. The maximum tensile load imposed on the cable
during handling is only about 1 kN allowing for lightweight cable brackets to be utilised.
Picture 17 : Cable laying Locomotive undergoing trials.
The photo shows the Locomotive with trolleys in left foreground. The simulated tunnel and brackets
are to the right.
4.2.3 Installation process
4.2.3.1 Transportation of cable to site
Cables are traditionally transported to site on cylindrical drums. The size of these drums being
determined by the length of cable to be delivered, the minimum internal diameter of the drum required
to satisfy the cable minimum bending radius, the maximum size and weights allowed under existing
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transport legislation, handling limitations in the cable makers facilities and specific loading and size
limitations relevant to the particular project for
which the cable is supplied.
For HV and EHV cables this normally limits the
maximum lengths of cables that can be delivered
on drums and by road to between 1500 metres
for HV and 1000 metres for EHV cables.
Where access to the installation site is possible by
sea then the cable can be delivered on turntables
or very large drums. In these circumstances, the
cable length is only limited by the cable makers’
factory facilities or by the cable system design.
Picture 18 : Cable reel
4.2.3.2 Cable Bending Radius
The bending radius of the cable both on the despatch drum and especially during the installation and
final positioning has to be controlled to avoid damage to the cable during transport and installation to
ensure the long term reliability of the cable circuit.
The minimum bend radius of the cable is normally specified by the cable manufacturer and varies in a
site environment dependent upon whether the cable is bent in a controlled manner or not.
The minimum bend radius when the cable is bent around a former or by using a formed support is
generally significantly smaller than when the bend is formed naturally by applying lateral force to the
cable.
During the installation phase the minimum bend radius is also dependent upon the need to limit the cable
sidewall bearing pressures to within acceptable limits as discussed previously and the cable installation
design needs to take all these aspects into consideration.
4.2.3.3 Cable Temperature
The range of acceptable temperature for cable installation is generally defined by the properties of the
material used for the cable outer sheath.
This range is more restricted when PVC is used for the cable outer sheath than when PE is used.
Although not normally a significant issue the scheduling of installation activities and selection of the
installation techniques used need to take this into account in countries where temperature extremes are
experienced.
4.2.3.4 Pulling Length
The maximum pulling length that can be achieved when using nose pulling is fundamentally a function
of the allowable pulling tension and the maximum sidewall pressure (see chapter 4.2.1.3) that the cable
can withstand.
For other pulling techniques the maximum pulling length is more often determined by either transport or
handling difficulties, than the maximum size or weight of the cable drum, or the system design – section
lengths for special bonded circuits for example.
4.2.3.5 Route Profile
The route profile is significant as it effects the magnitude of the forces needed to install the cable.
It is important that a full route survey is available to the installation system designer at the early stages
of the project to enable the profile to be taken into account since this could effect the maximum
allowable pulling lengths of the cable sections being installed.
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4.2.3.6 Obstacles
In most situations the cable route selected will encounter obstacles of one kind or another along the
route.
Various installation techniques have been developed to allow all eventualities to be overcome in a
controlled manner. It is normal therefore to find a number of installation techniques used along a given
cable route.
Again it is important that all obstacles along the proposed route are identified and known at the early
stages of the design phase since they may effect both the cable design and the selection of the
installation and pulling techniques
4.2.3.7 Setting Up
To ensure the successful implementation of the installation activities the logistics and facilities required
need to be carefully considered.
Adequate provisions need to be made for the storage and handling of cable drums generally weighing
between 15 tonnes and 30 tonnes.
Access to the site by heavy transporters, availability of lifting equipment and suitable hard standing for
storage and during cable installation needs to be carefully considered and the decision on pulling
direction and method must take all these issues into consideration.
Prior to actually installing the cable the necessary installation equipment needs to be set up along the
cable route.
The actual equipment needed is dependent upon the installation technique used but must in all cases be
positioned to satisfy the installation design criteria and must be supported and fixed in a manner that
can withstand the mechanical forces generated during the installation activity.
4.2.3.8 Installation of Cable
The cable drum is moved into position and mounted on a purpose made stand that allows the drum to
rotate. This stand is often motorised to overcome the forces required to turn the drum, hence
minimising the "backtension" on the cable, and also to allow controlled braking of the drum.
A braking facility must always be available to ensure the drum only rotates at the required speed.
The cable is then installed along the route in accordance with the selected method under close
supervision at all times to guarantee that the process runs smoothly and that the design criteria are not
exceeded.
Upon completion of this stage of the installation and where the cable is accessible, such as in troughs,
trenches and tunnels the cable is carefully positioned into its final position in the trench or trough or on
its support systems, as determined during the system design phase. For flexible systems the cable is
offset either manually or by use of jigs at this stage.
On successful installation of the first cable the equipment is moved as necessary for the next cable and
the process is repeated until all cables are successfully installed.
4.2.3.9 Final Installation Stages
On completion of the installation process for all cables the final stages of the installation are carried out.
For open trench and trough type installation this requires the installation of stabilised thermal backfill
around and above the cables, the application of protective mechanical barriers and warning tapes and
finally the reinstatement of the upper layers of the excavation in accordance with local requirements.
For other types of installations this requires the installation of mechanical restraints such as cleats and
short circuit straps, filling and sealing of ducts etc.
4.2.3.10 Site Quality Assurance
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The quality of the materials used in manufacturing the cable and accessories and the quality of the
manufacturing process itself can be closely monitored and is assured by rigorous type and routine
testing of the product prior to delivery to site.
Long term reliability of the cable circuit can only be guaranteed if the same attention to quality is
transferred into the installation phase.
It is therefore essential that the installation design and the laying and installation techniques are
engineered correctly such that the laid down criteria are complied with. During the actual installation
phase it is important that these criteria are complied with. This requires the correct use and positioning
of all equipment, appropriate site controls and monitoring throughout the process to record and check
that the design standards are complied with.
4.2.3.11 After Laying Tests
Before the cable circuit is connected to the power transmission system it is normal to carry out a series
of tests to confirm the integrity of the cable system including the cable and accessories.
The tests vary across the industry but generally include HV tests on the cable outer protection
(outersheath) and on the primary insulation. Further tests are carried out such as conductor resistance,
bonding and earthing tests.
Historically the HV tests have been DC tests and whilst DC testing is still used for SCFF cables and
for the outersheath tests it has been recognised that DC testing is not suitable for extruded cables being
both unable to consistently detect defects in the system and potentially causing damage to the cable
under test.
With the availability of mobile on-site AC test sets, of the resonant frequency type, there is a general
move to testing extruded cable on site by AC only.
Further developments in partial discharge testing have shown that the combination of a PD test with a
high voltage AC test is the most reliable means of confirming the integrity of extruded cables prior to
connection to the power transmission system.
4.2.4 Adaptation of the Cable System Design to the Technique/Environment
4.2.4.1 Adaptation of the Cable System Design to the Technique
•
Ducts
Factors that need to be considered for the cable system design and cable design :
1. Flexible or Rigid System
With the cable installed in ducts the system design depends upon whether the ducts are filled or
unfilled.
If the ducts are filled, usually with bentonite, the cable is effectively restrained and the system
design is considered as a rigid system.
If the ducts are unfilled then the cable can move to an extent, dependant upon the relative
proportions of the cable and the duct. The system is therefore generally described as a flexible
system however where the movement of the cable is limited by the size of the duct then it is
important to be aware that the cable will develop thrust due to thermomechanical stress.
2. Pulling tension
Since the only practical method of installing cables through a fully ducted system is by “nose”
pulling it is essential that the necessary design studies are completed to calculate the pulling
tensions that will be required to install the cable and to check that the cable limits are not
exceeded. If necessary the route and system will have to be modified to ensure the pulling
tensions are within the cable design limits.
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3. Thrust in Manholes
It must be recognised that cables installed in ducted systems will develop thrust. In manholes
the system design needs to take this into consideration to avoid problems with the accessories.
4. Cross bonding
Transposing of cables in a ducted system is more difficult to execute than for other laying
techniques and the system design needs to provide for this.
5. Conductor cross sectional area
Due to the poorer thermal performance of the unfilled duct the cable rating will be lower and
therefore it may be necessary to increase the cross sectional area of the cable conductor to
carry the required current.
6. Metallic Sheath
With the cable unrestrained the sheath fatigue performance over the life of the cable needs to
be carefully considered and the cable system design and cable design need to be reviewed to
ensure the integrity of the metallic sheath.
7. Cable Oversheath
It is essential that the installation method avoids damage to the cable oversheath and as an
added precaution it is normal for a more robust material to be used such as MDPE rather than
PVC. Dependant upon the cable route and type of ducts it may be necessary to increase the
thickness of the cable oversheath to provide greater protection during the installation phase.
•
Direct Burial
Direct burial is the most commonly used cable laying technique and since the cable is restrained
throughout the route the system is always a rigid system.
Factors that need to be considered for the cable system design and cable design: 1. Route details
Careful planning of the route is required to ensure that the rating and long-term performance of
the cable circuit can be assured. Details of any obstacles along the route need to be provided to
allow the system to be designed to avoid these.
The route details will allow an assessment to be made of the positioning of joint bays and
location of installation equipment, drums etc to allow the optimum solution to be engineered.
2. Environment
Knowledge of the environment through which the route is passing is essential. The thermal
resistivity and make up of the indigenous soil should be understood to allow the cable cross
section, cable spacing, depth of laying, backfill requirements and bonding arrangements to be
defined to achieve the required rating.
3. Cable Oversheath
The cable oversheath acts as a corrosion barrier for the cable metallic sheath. Depending upon
the location and environment additional precautions may be necessary to provide an oversheath
that is resistant to local ground contaminants or lifeforms such as termites and rodents that
could damage the normal oversheath materials.
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4. Duration of the works
Trench have to stay open between two joints up to the cable pulling. This can lead local
authorities to ban this technique in urban areas to avoid having trenches open too long.
•
Tunnels
By their nature, tunnels allow the cable system and cable design to be optimised and enables the
designer to adopt the most cost effective form of design for the support systems and the use of long
cable lengths to minimise the number of joints within the system.
In all cases it is essential that the long term performance of the system is not compromised and the
risks associated with each stage of the process must be fully assessed and understood.
Factors that need to be taken into consideration for the system and cable designs :
1. Flexible or rigid
Tunnel installations are normally installed as flexible systems although in some circumstances
the systems may be installed in troughing or cement bound sand surround making a rigid
system.
2. Support System
3. Cable Lengths
4. Sheath Voltages
5. Bonding
6. Metallic Sheath
7. Oversheath
It is common for cables installed within tunnels to be required to have enhanced fire
performance capability. This can be provided by low flame type materials or by the addition of
addition flame retardant coatings applied after installation.
8. Fire Performance of the system
The possibility of fire within a tunnel is extremely serious and the system design and
components of the system need to be assessed to minimise risk to personnel and assets in the
event of a breakdown within the cable system.
This is a significant issue for SCFF cables installed within a tunnel and often such cables are
surrounded by cement bound sand to reduce the risk of damage which could lead to cable
failure.
•
Troughs
This installation technique is fundamentally identical to that of direct burial and as such the system and
cable designs are the same as for the direct burial technique.
•
Bridges
Factors that need to be taken into consideration for the system and cable designs :
1. Flexible or Rigid
Cables installed in or on bridges may be installed as flexible or rigid systems in general the
system design is dependent upon the particular requirements of the route.
2. Transition Design
Careful consideration has to be given to the design and installation of facilities within the route
to cater for the movement of the bridge structure due to thermal expansion or other possible
movement. There is generally a need to design a transition area between the fixed portion of
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the route and the route on the bridge to allow for the inevitable movement between the two
systems.
3. Oversheath
4. Fire Performance of the system
•
Shafts
Installation of cables in shafts introduces the problem of potential differential movement between the
cable core and metallic sheath. The severity of the problem being directly related to the depth of the
shaft.
Cable can be installed in either flexible or rigid systems although generally the cables are rigidly fixed at
the top and bottom of the shaft.
In either case particular attention needs to be paid to ensuring the clamping system is designed to
prevent any slippage of the core within the cable sheath.
The system and cable design is more complicated for the fluid filled cable since the hydraulic system
design needs to be taken into account. Again dependent upon the depth of the shaft it may be
necessary to include special reinforcing of the cable metallic sheath to withstand the hydraulic
pressures or to introduce stop joints within the shaft to ensure the maximum acceptable hydraulic
pressures are not exceeded.
Factors that need to be taken into consideration for the system and cable designs :
1.
2.
3.
4.
•
Flexible or Rigid
Cleating
Metallic Sheath
Oversheath
Horizontal Drilling
For this technique the cables are usually installed within ducts that are installed during the drilling
process.
In this instance the system design and cable design adaptations are as described for the ducted
installation technique (see above).
•
Pipe Jacking
The installation technique adopted is dependent upon the diameter of the pipe jack and can be any of a
number of alternative techniques.
For example the cable may be installed in ducts pulled trough the pipe jack after completion of the pipe
jacking operation, alternatively the cables may be installed in air on steelwork installed after the pipe
jacking process is complete.
The system design chosen may therefore be flexible or rigid with the decision being based upon the
installation technique selected for the particular application.
•
Microtunnels
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Microtunnels can be treated in the same manner as pipejacks except that generally the diameter does
not allow personnel access into the microtunnel and therefore the installation of support steelwork is not
possible.
The installation technique adopted is therefore generally that of ducts pulled into the microtunnel after
completion of the tunnelling process
•
Mechanical Laying
For this technique the cable is installed in a direct buried environment and therefore the system is a
rigid system.
This technique is best suited to light cables which allow longer lengths to be installed.
Whilst it is possible to surround the cable with cement bound sand the process is not as controllable as
other techniques, such as direct burial and therefore the cable design needs to take this into account.
This may effect the sizing of the conductor due to a degree of uncertainty regarding the thermal
resistivity of the backfill material.
In addition the metallic sheath and cable oversheath design may be adapted to provide a light cable
design which will allow longer cable lengths to be transported and installed but which will be robust
enough to withstand the rigours of this installation technique without effecting the performance of the
system in the long term.
Factors that need to be taken into consideration for system and cable design :
1.
2.
3.
4.
•
lightweight construction
oversheath
rating
bonding/sheath voltages.
Embedding
For this technique the cables are usually installed in ducts which are embedded into the ground during
the embedding process.
Generally therefore the system design and cable design are as for the ducted technique.
•
Use of Existing Structures
Factors that need to be taken into consideration for system and cable design :
1.
2.
3.
4.
5.
6.
7.
8.
Flexible or Rigid
Thrust at transitions and joint positions
Duct Sealing
Cleaning and Assessment of asset
Pulling Tensions
Overall dimensions
Lengths
Oversheath
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4.2.4.2 Adaptation of the Cable System Design to the Environment
•
Drying of Soil
For highly loaded cables drying of the surrounding soil is a strong possibility and the cable rating
calculations need to take this into account with the conductor cross sectional area being selected on this
basis. The difference between a fully dry backfill and the same material with even a very small
moisture content is dramatic. For example research has shown that a 2% moisture content reduces the
thermal resistivity of normal backfill such as sand or cement bound sand by 50%.
Failure to recognise this possibility will result in the cable exceeding its design temperature limits due to
the dramatic increase in the thermal resistivity of the surrounding material. This will lead to eventual
failure of the cable.
Factors that need to be taken into consideration for system and cable design :
1. Depth of laying and separation of cables
2. Special backfill requirements
•
Water Drainage
Water drainage may have a number of effects upon the cable system. It is possible that over time
water draining into the cable route could wash away the cable system backfill material compromising
the cable rating.
Inadequate drainage could lead to the cable and accessories being immersed continuously in water and
special precautions need to be taken to ensure that adequate sealing is provided to prevent moisture
ingress into the cable and accessories such as joints and link boxes.
Factors that need to be taken into consideration for system and cable design :
1. Special backfill requirements
2. Sealing for joints and other accessories
•
Temperature of the Soil/Environment
The temperature of the medium surrounding the cable circuit is a fundamental factor in determining the
rating of the cable system.
It is essential therefore that the temperature profile is known throughout the route.
Under extreme conditions of either high or more normally low temperature installation of the cable may
not be possible and work may have to be planned at a time when the ambient environment
temperatures are within acceptable limits.
Factors that need to be taken into consideration for the cable system design:
1.
2.
3.
4.
Cable cross section and spacings
Depth of laying
Special backfill
Oversheath material
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•
Hardness of the Soil
The hardness of the soil in the main effects the construction of the route and may influence the route
plan and selection of installation technique.
•
Stability of the Soil
Where unstable soil conditions are expected then the installation design needs to allow for this. This can
take the form of the use of civil construction techniques to stabilise the soil in the vicinity of the cable
circuit.
Where settlement of the route is expected to take place then the installation system design can make
provision for this along the route. This is especially important at points of known discontinuity where
there is the potential for shear stress to be imposed on the cable system.
•
Thermal Resistivity of the Soil
The thermal resistivity of the soil surrounding the cable circuit has a direct effect upon the rating of the
cable circuit.
It is therefore important that this information is available to allow the system design to proceed.
Where the surrounding soil is found to have a high thermal resistivity then it may be necessary to
excavate beyond the normal area required to install the cable and replace with material with a more
suitable thermal resistivity.
Factors that need to be taken into consideration for the cable system design:
1.
2.
3.
4.
•
Cable cross section
Depth of laying
Cable separation
Special backfill
Seismicity
Where seismic activity is expected, it is possible to accommodate possible ground movement by
adopting the same techniques as would be used for unstable ground conditions.
•
Frost
In general high voltage cables are installed at depths which are not normally effected by frost. During
operation frost will have little effect on the cable circuit although frosts occurring during period of deenergisation could lead to cracking and disturbance of the backfill surrounding the cable. This could
lead to voids being generated which would effect the thermal performance of the cable surround in a
direct buried situation.
Frost during installation may mean that the ambient temperature is below the minimum installation
temperature. In which case installation of the cable will have to be delayed until a temperature increase
occurs.
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Factors that need to be taken into consideration for the cable system design:
1. Depth of laying
2. Backfill
3. Oversheath material
•
Archaeology
Evidence of archaeological remains along a planned cable route would not influence the cable design
but would influence the technique used for constructing the route.
This could in turn effect the cable design in line with comments previously made depending upon the
installation technique selected.
•
Presence of Termites
Termites will attack the outer sheath covering and compromise the corrosion protection system for the
metallic sheath.
The outer sheath material needs to be impervious to termite attack or provided with suitable chemical
deterrents if possible. Otherwise an alternative installation techniques such as ducted techniques will be
required.
•
Laying in National Park
The cable system design is influenced by the installation technique used to overcome any restrictions
placed as a prerequisite to approval of a cable route through the National Park.
The factors that need to be taken into consideration are dependent upon the technique selected as
indicated before.
•
Duration of the Work
The duration of work has no influence upon the cable system design but may influence the choice of
installation technique.
SCFF cables require longer to install since the hydraulic procedures are an additional complexity when
compared with extruded cables. However selection of the type of cable to be used is not normally
influenced by this factor.
The factors that need to be taken into consideration are dependent upon the technique selected as
indicated before.
•
Maintenance and Repairing Process
Cable systems are designed for 40 years operation and are generally very reliable. Maintenance
procedures for extruded cable systems are generally limited to inspection of the cable and associated
equipment and periodic checking of the integrity of the cable oversheath and bonding systems. The
introduction of partial discharge monitoring techniques should allow the condition of the cable system to
be assessed over time.
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Maintenance for SCFF cable systems is more complex due to the periodic checks required on the
hydraulic system and components. Routine sampling of the dielectric fluid and analysis of the dissolved
gases within the fluid allow a degree of condition assessment to be undertaken and comparisons made
over time.
Considerations need to be made at the design stage as to how the system will be maintained and to
ensure access and provisions are made such that the maintenance regime can be carried out.
Although cable systems are very reliable the need for repair cannot be discounted and again this needs
to be considered at the design stage. Whilst not directly effecting the cable design or installation system
design the need to cater for a future repair will influence the choice of installation technique with the
resulting effect on the cable system design as mentioned earlier.
•
Cable Removal after Operation.
The factors associated with removal are very similar to the issue of cable repair noted above.
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5. EXTERNAL ASPECTS
5.1
Location (Urban vs. Rural)
The high-voltage cable installation methods are obviously adapted depending on the location of the
laying site so as to take into account the local and environmental limitations and constraints.
Therefore, not surprisingly, in urban areas the installation in ducts is the most frequent method,
followed, in this order by :
• conventional installation in open trench;
• visually unobtrusive methods (tunnels, microtunnels and, to a lesser extent, pipe jacking and
horizontal drilling);
• utilisation of existing structures, e.g. bridges.
In rural areas, however, the constraints regarding time-limits on disruption due to execution of the
works, and with respect to available space, are much less important than in urban areas.
Therefore, in rural areas the more traditional laying methods are most frequently applied, such as ducts
and direct burial (of which costs are lower than those used in urban areas).
At special crossings, placing on bridges and directional drilling methods may be applied, though special
laying methods are not the general philosophy for cable laying in rural areas.
As regards laying depth there is not much difference between cable laying in urban or in rural areas,
since a minimum depth is usually set by regulations, as we will see later (chapter 5.5.).
5.2
Right of way
The rights of way are usually settled by joint agreement between the utility and a private owner or the
utility and one or several public authorities (Roads, Railways, Bridges, …).
When using a public authority property, the construction techniques used are generally agreed with the
partners, before beginning the works. Then, it has to be decided under whose responsibility the works
have to be done, the utility or the public authority.
5.3
Magnetic fields
Reference: "Magnetic field in HV cable systems: systems without ferromagnetic component" – Electra
CIGRE – Technical Brochure n°104 - Joint Task Force 36.01/21 - June 1996.
Although the delicate question of magnetic fields is usually discussed regarding overhead power lines,
increasingly attention is being paid to magnetic fields when selecting the configuration of the cables and
the routes of buried links.
Indeed, in more and more countries now exist recommendations, limits and possibly even standards as
to the level of magnetic fields. These concerns may eventually dictate changes in planned routes, but,
above all, they may increase the burial depth or implicate some precaution disposition.
We shall bear in mind that buried cables (contrary to overhead lines) do not generate electric fields
outside their metallic screen. As such sheaths are earthed, an electric field only exists between the
conductor and the sheath.
Several three phase single core cable configurations can be considered.
A lot of factors have a influence on the magnetic field, e.g. phase spacing, burial depth, load current
amplitude, phase arrangement in systems of several three-phase circuits, distance between them and
induced currents in the sheaths (which are strongly affected by a lot of factors).
5.3.1 Flat arrangement
A system of three single core cables in flat formation is first considered. It is characterised by
geometrical parameters which are phase spacing (s) and burial depth (d). Height considered for
calculations above ground (h) is also defined.
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Figure 29 : Flat arrangement, 1 circuit
The currents in cables are assumed to be balanced, i.e. IA = I < 0°, IB = I < -120 °,
IC = I < -240 °, and the frequency is 50 Hz. The current will be fixed to a reference value of I =
1000A.
The two diagrams below represent the magnetic flux density along a horizontal line at 1 metre above
the ground surface, considering various burial-depths and spacings of the phases.
Figure 30 : B rms profiles with various s
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Figure 31 : B rms profiles with various d
The highest magnetic field value immediately occurs above the cables. The distance from the cables (h
+ d) as well as the phase spacing appear to have an important influence on the flux density whose
values are higher for low burial depths and high phase spacings. Moreover, it particularly appears that,
for a fixed phase spacing, burial depth has no effect on the flux density at horizontal distances from the
system centre line that are greater than several times this depth. Further, the reduction of magnetic
field away from the centre line is higher for a low burial depth.
Two systems of three single core cables in flat formation can also be considered, assuming the same
current of 1000 A in both circuits. Their geometrical parameters are phase spacing (s), burial depth (h)
and distance between systems (g). Height above ground (h) is also defined.
Figure 32 : Flat arrangement, 2 circuits
The hypotheses are identical to those referred to above. Furthermore, two configurations have been
retained : ABC-ABC (same order of phases) and ABC-CBA (inverted order of phases).
The figure below illustrates the evolution of the magnetic flux field considering various heights above
the ground, at the set parameters g, d and s.
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Figure 33 : B rms profiles for two cable system configurations
with various h
The ABC-ABC configuration appears to give a lower magnetic flux density near the cables than the
ABC-CBA configuration. However this last configuration gives the lowest magnetic flux density from
a certain distance from the cables (breakpoint for h = 1 m). Such a breakpoint distance depends on g
and s.
The next figure shows profiles of magnetic flux density along a horizontal line one meter above ground
for both configurations and for several system spacings.
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Figure 34 : B rms profiles for two cable system configurations
with various g
It can be seen that increasing system spacing respectively decreases or increases the magnetic field
for ABC-ABC and ABC-CBA configurations.
We must also mention that the magnetic field is often higher at locations where junctions are made, i.e.
where connections are made between power cable screens and the ground wires (if any), especially if
these connections are made in an aboveground junction box (for paralleling of the screens).
Judicious connection of the screens and ground wires (connection between portions of the buried link
made underground instead of in an aboveground junction box) or connection made in a buried junction
box can significantly reduce the value of the magnetic field.
5.3.2 Trefoil arrangement
A system of three single core cables in trefoil formation is now considered. Its geometrical parameters
are phase spacing (s) and burial depth (d). Height above ground (h) is also defined.
Figure 35 : Trefoil arrangement, 1 circuit
In fact, usually, the three cables touch each other and variations of phase spacing allow to consider
cables of several outer diameters.
The first of the two next figures compares the magnetic flux density profiles along a horizontal line one
meter above ground with a burial depth of one meter for both flat and trefoil formations with several
phase spacings. The second one compares the magnetic flux density profiles along a horizontal line one
meter above ground for several burial depths and a fixed phase spacing for both formations.
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Figure 36 : B rms profiles for both flat and trefoil formations
with various s flat and s trefoil
Figure 37 : B rms profiles with various d
for both flat and trefoil formations
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The trefoil formation gives clearly the lowest magnetic field which is more than 30 % lower than the
one of the flat formation whatever phase spacing is.
Two systems of three single core cables in trefoil formation are also considered.
Figure 38 : Trefoil arrangement, 2 circuits
The hypothesis are the same as those made in paragraph 5.3.1.
The next figure shows again the profiles of the magnetic fields for the configurations ABC-ABC and
ABC-CBA considering various heights above the cables, at the set parameters g, d and s.
Figure 39 : B rms profiles for two cable system configurations
with various h
The ABC-CBA configuration appears to give a lower magnetic flux density then the ABC-ABC
configuration.
The next figure shows again the profiles of the magnetic fields along a horizontal line 1 metre above
ground level, for the two configurations and with various spacings.
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Figure 40 : B rms profiles for two cable system configurations
with various g
Unlike in the horizontal arrangement, here we can see that the increase of spacing between the two
systems results in a lower magnetic field in the two configurations ABC-ABC and ABC-CBA.
5.3.3 Vertical arrangement
Line X
Sv
Sv
d
h
A system of three single core cables in vertical formation can also be considered. Its geometrical
parameters are phase spacing (s) and burial depth (d). Height above ground is also defined (h) :
Figure 41 : Vertical arrangement, 1 circuit
This configuration is in fact an artifice considered for the purpose of enabling to compare the magnetic
field values of this configuration with those of the two configurations discussed above (trefoil, flat). In
reality a vertical configuration is only adopted in tunnels, bridges or other structures, practically never
for buried links.
The next figure shows the magnetic flux density profiles along a horizontal line 1 metre above ground,
with a burial depth of one metre for flat, trefoil and vertical configurations and several phase spacings.
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h=1m; d=1m
s=Phase spacing (flat)
st=Phase spacing (trefoil)
sv=Phase spacing (vertical)
Magnetic flux density
(Brms) [10e-6T/kA]
30
25
st=0.08m
20
st=0.12m
s=0.12m
15
s=0.3m
10
sv=0.12m
sv=0.3m
5
0
-10
-5
0
5
10
Distance from center line (x) [m]
Figure 42 : B rms profiles for flat, trefoil and vertical formations with various s flat, s trefoil and
s vertical
The values of magnetic field for the vertical configuration are lower but close to the flat configuration.
They rapidly match when departing from the vertical axis.
It results that the trefoil formation clearly remains the most advantageous option with respect to
magnetic field.
Two systems of three single-core cables in vertical formation are also considered.
Sv
Sv
d
h
Line X
g
Figure 43 : Vertical arrangement, 2 circuits
The hypotheses are identical to those made in paragraph 5.3.1.
The last figure below shows the magnetic field profiles for the ABC-ABC and ABC-CBA
configurations for vertical, trefoil and flat formations.
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Magnetic flux density
(brms) [10e-6T/kA]
h=1m; d=0.5m;g=0.7m
s=Phase spacing (flat)
st=Phase spacing (trefoil)
sv=Phase spacing (vertical)
70
60
50
40
30
20
10
0
ABC ABC ; st=0.3m
ABC CBA ; st=0.3m
ABC ABC ; s=0.3m
ABC CBA ; s=0.3m
ABC ABC ; sv=0.3m
ABC CBA ; sv=0.3m
-5
-4
-3
-2
-1
0
1
2
3
4
5
Distance from center line (x) [m]
Figure 44 : B rms profiles for two cable system configurations with fixed h, d,
g and s = s t = s v = 0.3m
It appears that in the ABC-ABC assumption the magnetic field of the vertical formation is higher in the
middle of the 2 systems that it is with the flat formation.
Conversely, for the ABC-CBA configuration, the magnetic field is clearly less in vertical formation
than in flat and trefoil formations. At higher distances of the centre line, it appears that the decreasing
of the values of magnetic field for the trefoil configuration is slower.
5.3.4 Comparison between overhead lines and buried links
Reference: "Magnetic field in HV cable systems: systems with ferromagnetic component" – Electra
CIGRE – Technical Brochure n°104 - Joint Task Force 36.01/21 - June 1996.
In addition, it is important to remind that an electric field is present around overhead lines, whereas in
cables, the electric field is completely confined inside the electric screen.
The assumptions regarding the types of cable are the same as in the previous paragraphs, and we have
considered three configurations : trefoil, flat and vertical.
As regards the overhead line we considered a line composed of one circuit and an earth wire. The
magnetic field calculations were performed for a distance of 10 m between ground level and the
conductor and with a base current of I = 1000A.
In order to establish a parallel between the buried links and the overhead lines we imagined the three
overhead line conductors to be in vertical and flat formations.
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Magnetic flux density
(Brms) [10e-6T/kA]
30
h=1m; d=1m
h overhead line=10m
s=Phase spacing (flat)
st=Phase spacing (trefoil)
sv=Phase spacing (vertical)
25
st=0.12m
20
sv=0.12m
s=0.12m
15
s=0.3m
line v-config
10
line h-config
5
0
-10
-5
0
5
10
Distance from center line (x) [m]
Figure 39 : B rms profiles for flat, trefoil and vertical formations (buried links)
B rms profiles for flat and vertical formations (overhead lines)
The graph clearly indicates that the magnetic field of overhead line and buried link are of the same
order on the axis, the difference being that the magnetic field of the overhead line diminishes much
more slowly than that of the buried link. With the buried link the magnetic field becomes very low at
only a few metres apart from the link axis.
It also appears that for a given formation the magnetic field values of a buried link may be higher in a
buried link compared to an overhead line.
However, the information supplied by the graph cannot be considered totally reliable, because magnetic
fields are known to be sensitive to many parameters (order of the phases, number of circuits, current in
the screens or groundwire(s), configuration of the cables, …) which may significantly affect the values.
For example, if the value of h decreases, the value of the magnetic field will be increase for
underground cables and decrease for overhead lines.
5.3.5 Conclusion
As a conclusion it must be said that, whatever the cables formation, the magnetic fields induced by
buried links are lower than those specified in the national or international recommendations generally
accepted.
Also, there are now several ways in which the magnetic fields of buried links can be further reduced,
for instance by placing steel or aluminium sheets around the cables in the trench, or by placing the
cables in steel pipes.
In this respect, we can mention the work done by the Joint Task Force 36-01/21 "Magnetic field
calculation in underground cable systems with ferromagnetic components" (Electra n° 174,
October 1994).
5.4
Existing services
The proximity of buried power lines to other services in ducts, sewers, cables and other utilities'
networks is these days practically unavoidable, particularly in urban areas.
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Generally, power cables are laid as much as possible at sufficient distance from other services in order
to prevent the damage of existing installations during the laying of HV cables. In urban areas this
becomes increasingly difficult.
The effects of a short-circuit of a phase on its environment (gas, telecommunications, …) are
discussed in paragraph 5.6.
•
Gas
The clearances to be observed between gas pipes and HV cables, and to a lesser extent the laying
techniques, are generally imposed by the gas utility. The gas utility’s stipulations naturally differ
depending on the type of gas transmitted, the pipe diameter and the gas pressure in the pipe.
Using ducts in which later the cables should be laid, is strongly advised against in the vicinity of gas
pipes, because the former ducts may be a source of accumulation of gas in the event of a gas pipe leak
(hence a risk of explosion).
Problem of parallelism of a gas pipe to HV cables : see l Telecommunications
•
Electrical cables
The proximity of various electrical links can have both an electric impact (in case of defects) and a
thermal impact. They contribute to soil heating and in this way they reduce the carrying capacity of the
electrical links.
A deeper investigation of these situations is highly recommended prior to laying the cables.
•
District heating
Like in the above case, the thermal impact of the steam pipes on the carrying capacity of the buried
power line must be investigated.
•
Telecommunications
Telecommunication cables (like gas pipes) are a typical and frequent example of a system affected by
the HV cables that run parallel to them.
Indeed, any electric current transiting in a conductor generates a magnetic field around that conductor.
If the current is the alternating type this magnetic field will in turn induce a potential rise between the
extremities of an open circuit surrounding the conductor, or the circulation of an induced current in a
closed circuit surrounding the conductor.
However, we shall bear in mind that the most critical situation arises in the event of a fault. If the
conditions are acceptable during a fault situation, they are obviously acceptable during a normal
situation.
When installing a buried HV link, this impact can be first reduced by the presence of the cables’ metal
screens which are earthed at either ends (circulation of a screen current) and also by placing ground
wires connected to the earthing network of the line, so facilitating the returning of a fault current to the
source, this reducing the magnetic field perceived by the world outside the cables.
World-wide, protection of telecommunication infrastructure is one of the main concerns of electrical
utilities.
•
Water
Water pipes do not present a particular risk, except when leaks are sprung.
However, even if the protective screen of the HV cables is damaged, most modern cables have
sufficient radial and longitudinal leak-tightness to protect them (but the cable has even so to be repaired
if damaged).
In turn, the erosion of the backfill following an accidental leak of water presents a certain risk
(particularly if it is a special backfill that is being washed away by the leak).
•
Sewers
Sewers do not present a specific risk except possibly in relation to mechanical damage during work at
or around sewers on cables that have been laid too close to the sewers.
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It is worthwhile to mention also that especially main (large diameter) sewers may release quite some
heat, which may negatively affect the carrying capacity of the cables.
•
Trees
Trees and cables may have an impact on each other.
If cables have to be laid very close to threes, excavation must be extremely careful (at extra cost and
time) so as not to damage the root systems of the trees, and, later
when the buried power link is in service, the drying of the soil may affect the growth of the trees (and
possibly eventually kill the trees);
the drying of the soil may also negatively affect the carrying capacity of the cables.
Also, the root systems of the tree may get entwined around the cables, making difficult later
intervention on the line.
Accordingly it is recommended (or imposed by local authorities) that sufficient distance be adopted (at
least 2.5 m) between the link and the trees, or that the cables be placed in ducts.
•
Railways
The phenomenon most feared in the vicinity of railway or tramway substations is that of corrosion of
the metal screens around the buried power line (which corrosion may be incurred even at considerable
distance from these substations).
Corrosion develops when a direct current strays from the screen and flows through the soil to a directcurrent source.
In order to protect a metal screen like a gas pipe against this type of corrosion, the utilities install
cathodic protection stations to bring the ducts to a sufficiently negative potential compared to the soil,
so that no current can escape from it. The direct connection of a steel structure to a cathodic
protection station protects this structure against any electrochemical corrosion.
Although in the past the metal screens of HV lines have been connected to such cathodic protection
stations but the method has the disadvantage that the electrical utilities can depend on other companies’
installations.
Moreover, a very simple passive protection method of the metal screens exists, which consists of make
use of the normally existing plastic outer sheath (medium or high-density polyethylene). This sheath
thanks to its high electric resistivity impedes the leakage of any currents to the soil.
However, it will be able to provide this protection only as long as the sheath is not damaged. Indeed if a
defect appears, the density of DC current through the outer sheath defect could reach very high values
and causes very rapidly important damage to the metallic sheath. This implies regular inspection (at
least annually) of the dielectric strength of this sheath.
5.5
Legal aspects
Among the great choice of laying techniques, there are none that are systematically forbidden by the
local authorities.
The two techniques that cause the most controversy are the laying in trenches, which naturally causes
quite some local disturbance, and the placing of cables on bridges (which may be historic or
architectural monuments, bridges that require constant maintenance work, …).
It can be observed that national authorities or the utilities hardly ever forbid one or the other method,
unless there are very particular reasons.
It must be borne in mind that in certain countries the law is such that the owner of the land also owns
the subsoil under that land. During link construction requiring for instance tunnel jacking or horizontal
drilling, it is in those cases absolutely necessary to secure the authorisation from the person that owns
the land under which the link construction will take place.
An alternative option may be to build the link under the public roadway, in which case only
authorisation by the public authorities is necessary (but this may entail a longer route for the link).
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Also, as said before, magnetic field calculations more and more dictate, through guidelines, or even
through standards, the choice of the route and, above all, the depth at which the cables will have to be
buried.
The usually regulatory-imposed minimum depth is 1 metre. In turn, there are hardly ever any legal
stipulations regarding the width of the trench.
Finally, it should also be borne in mind that, world-wide, there are more and more legal restrictions
regarding the duration of opening of the trench, this resulting in the necessity to adopt shorter lengths,
and restrictions concerning the periods of the year or day during which civil works and cable-laying
work can be done.
5.6
Safety aspects
Safety aspects must be taken into account as soon as at the early stages of link design. A risk analysis
becomes increasingly necessary that clearly identifies the potential risks.
A first step in this analysis consists of collecting all information about the various utility networks in the
vicinity of the link, and evaluating them with respect to possible risks.
Safety implies that precautions be taken in order to protect :
• the HV link;
• the other links or networks in the vicinity;
• the workers working near the HV links or other networks.
• the public
The safety aspects relating to terminations are considered by Cigre Working Group 21-19.
5.6.1 Protection of the link from external damage
Apart from being at a depth that gives them some protection, the HV cables are usually protected all
along their route by a cover of durable and mechanically resistant materials that protect them against
damage from excavation tools.
This cover extends on either sides above the cables and may consist of concrete or polyethylene or
other slabs.
Furthermore, the cables are often signalled by non-corrodable markers placed above the line. This may
consist of coloured plastic strip showing the voltage level and the name of the utility, sufficiently
resistant not to be ripped by a mechanical excavator.
It is clear that in particular cases additional safety devices may be installed like, for instance:
• placing of steel plates above the protection slabs when the minimum regulatory depth is impossible
to comply with;
• placing of warning panels above ground level on a bridge or on the river banks (embedding, pipe
jacking, …);
• simply placing the cables in strong ducts or troughs.
Lateral protections are rarely used because the depth of the cables is considered to be (nearly) enough
large to avoid aggressions due to works carried by the most other Utilities.
Joint pits when in plain soil are protected similarly to the cables, but their protection extends at least
over the entire surface of the joints.
When joints are made for instance in prefabricated pits or in tunnels, these joints are naturally protected
by these structures that house them. Only the access to these structures needs to be designed against
intrusion by unauthorised persons.
Also, in certain cases, particularly safety measures may be required (such as lining) in order to protect
a link situated in a tunnel from other cables or pipes running in that tunnel.
It is clear that placing slabs or markers is not possible when using trenchless laying techniques.
Moreover, it is not easy to give the exact route in the x, y, z axis. Nevertheless, it has to be noticed that
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with these techniques, cables are usually laid at great depth and so are protected from usual external
damage.
5.6.2 Protection of the environment from a system fault
In the event of a short-circuit in a cable the best protections against an explosion remain the adequate
depth at which the cables are buried and the presence of protection slabs.
Moreover, in the case of a phase-to-earth fault (e.g. in the event the cable is punctured by a
mechanical excavator), the surrounding soil and the installations of third parties situated in the close
vicinity of the HV line will incur a rise in potential.
Most often the installations of third parties are grounded or connected to a cathodic protection system.
Accordingly it is the external protection cover of the installation, i.e. the layer that insulates the metal
part (screen or pipe) from the soil that will have to sustain the fault voltage.
Generally these protections are not particularly designed to have high dielectric strength, as they are
primarily designed for protection against electrochemical corrosion by the soil. Accordingly the possible
local rise in potential has to be reduced to an acceptable value.
The only component on which it is possible to act in this respect is the linear resistance of the screen,
as it can be demonstrated that the maximum value of the fault voltage is as follows :
Vf =
LR
If
4
where
R = linear resistance of the screen
L = length of the link
If = fault current
This being, the value of R can be reduced by two methods, which may both be applied simultaneously :
• paralleling of the HV cable screens between each other,
• paralleling of all the HV cable screens with the ground wire.
Such paralleling may be done in aboveground or in buried joint boxes.
5.6.3 Protection of the workers
Workers are particularly exposed to mechanical hazards (deep trenches, …) and electrical hazards
(voltage step, …).
During work in plain soil, the depth of the trench (for instance > 1.5 m) can be such that it needs
stabilising (lining). Furthermore, the workers must wear their individual protective equipment (hard hats,
gloves, …).
For other laying techniques such as in tunnel, bridge or shaft, various philosophies may be envisaged:
• bar access and install fixed or mobile CCTV cameras inside, in order to verify the proper condition
of the cables. Once the link installed, human interventions will be few and far between (only in case
of repair or other).
• authorise access, either after putting the HV link(s) out of service or if the links are left live,
restricting the access to authorised persons only.
In this case, particular systems have to be installed in the tunnel (ventilation, emergency hatches or
exits, ..).
In the event where there are other users of the tunnel, bridge, shaft who installed cables or pipelines in
it, other collective protection equipment may be necessary depending on the type of cables and
products involved. For example, steel sheets may be placed around the cables in order to protect them
from external aggression (piercing by screwdrivers, …) or the cables may be placed inside ducts or
concrete troughs in order to limit the impact of a defect on personnel or the installations of other
utilities.
Also, the case of prefabricated joint pits deep in the ground is similar to that in tunnels, except that the
space is less. Therefore, particular measures for venting and extraction of possible toxic gases may be
necessary, as well as provision of an extra exit hatch as a backup.
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The electrical risks must be considered also.
We can mention the case of laying of pipe or an other underground power link parallel to buried power
links without any particular protective measures. When the cables of a portion of link are placed on
insulating supports prior to backfilling the trench, there may appear a considerable voltage on the pipe
which is dangerous to individuals working on the pipe or the new link. Local grounding during the work
on the pipe is effective for protecting the workers from that type of impact.
It should be also noted the accessible parts of pipes of third parties (valve control stations, cathodic
protection stations, measurement stations, …) situated near HV links must be earthed in order to limit
the induced voltages and so protect their personnel.
Interventions in joints boxes must also be carefully considered on account of the considerable currents
that may flow in these.
5.6.4 Protection of the public
As already mentioned in paragraphs 5.6.1. and 5.6.2., the cables are usually buried at such depth that a
defect arising in them is not noticeable at the ground surface, except perhaps for a slight noise.
At the joint boxes, an adequate and possibly strengthened grounding circuit enables to eliminate any
electrical risk (step voltage, …) to the public.
Although the joint boxes limit the mechanical impact of a defect on the outside, there remains the fact
that a short-circuit may provoke an explosion generating such a pressure that no aboveground or buried
boxes can resist.
It may therefore be highly advisable that joint boxes be placed in a concrete or steel containment.
5.6.5 Safety of the different laying techniques
Among the twelve laying techniques described in this brochure, some are more safe than others
depending on which aspect the designer looks at.
Globally, techniques where cables and joints are laid at a sufficient depth in the soil and completely
protected (i.e. cable in duct, joints in jointbays) are the best ones. As soon as cables or joints are in
open air, (i.e. cables laid in tunnel without protection, joints in manholes), the security of the public or
the workers is at a lower level.
More details are given in the previous chapters.
5.7
Environment
Installation of power cables entails environmental impacts which depend on which installation method is
applied.
For instance, the results of techniques such as horizontal drilling or tunnel thrust jacking are practically
‘invisible’ to the outside world.
Conversely, when trenches are dug (direct burial, ducts, tunnels built with the open cut method, etc.)
the natural environment may be substantially altered.
It is important not to underestimate the environmental impact during the construction itself, and
therefore it is wise to consider it already at the preliminary study stage, before it becomes the main
visible point for the population.
The ultimate objective is to restore an environment identical to what it was before the installation of the
HV link.
In order to analyse the environmental effects arising from the construction and operation of a power
link, a distinction is made, following the usual practice in this kind of studies, between the physical
medium (soil, water, air), the biological medium (flora, fauna), the social medium (population, economic
sectors, …) and the landscape.
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The potential alterations which may be attributed to the construction and operation of a power link,
classified according to the element of the impacted, are as follows :
• Soil
direct damage due to the excavations
deposition in the water (embedding) and/or land medium of the materials excavated from the
trench
movements of plant and machinery on the banks of a river, for example for embedding and
horizontal drilling
possible contamination of the soil (Fluid Filled cables)
•
-
Water
alteration of the water quality by materials or products during the construction work (embedding)
alteration of the water quality by pollutants (embedding, bentonite injected during horizontal
drilling)
•
-
Air
release of pollutants to atmosphere during the construction work
noise generated by plant and machinery during the work
vibrations generated by plant and machinery during the work (tunnel, …)
generation of magnetic fields
•
-
Flora
direct destruction of the vegetation cover
indirect destruction of the nearby plant communities
damage to unique or interesting species
•
-
Fauna (embedding)
direct disturbances to benthic communities
indirect damage arising from changes to the water ecosystem
•
-
Socio-economic aspects
difficulties caused to parking and access to shops etc. during the work (direct burial, …)
temporary effects on tourism trade during the construction work
rights of way affected during the construction work
effects on fisheries (embedding)
•
-
Landscape
Alteration of the landscape during the construction phase.
It clearly appears that most of the disturbances can be prevented or mitigated at the outset, i.e. during
the studies. The use of particular installation techniques can also harm to the environment by
propagation of polluted materials (for example, horizontal drilling going through a gas or oil pipe) but the
major impact arises most of the time during the construction phase and can be mitigated by using, for
instance, less disturbing plant and equipment (low noise, low vibration equipment or measures during
digging of shafts) and by informing and consulting with the local authorities and population.
However, it should be borne in mind that using SCFF cables may under certain accidental
circumstances (leaks) cause some impact to the environment by the fluid leaking into the subsoil.
Finally, it is worth noting that more and more international or local guidelines stipulate that the cables be
removed at the end of their service life and that the materials be recycled.
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6. DESIGN OF A LINK
When an engineer is at the beginning of a new project, the problem is always : how could it be
managed in order to be the most effective on the technical and economical points of view.
The following chart will help the inexperienced engineer in the management of his project. If you are in
an organisation accustomed with the underground cable system project management, it is clear that
some stages have to be jumped over.
It can be seen that the exercise is very much an integrated process with the impact of the various
stages being considered and steps taken to modify previous and subsequent stages of the process to
achieve an optimised end result.
6.1
Methodology
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STAGE 1 : Preliminary design of the cable system
Operation voltage
Ampacity (normal operation, emergency)
Load curve
Cable aim temperature during operation
Short circuit level and duration
Impulse levels
Touch voltage
Cost of kWh, cost of losses
Estimated length of the link
Type of soil
Soil temperature
Maximum allowed temperature
at soil-cable contact
Soil resistivity
Frost depth
Environmental hazards (earthquake, flood,...)
Cable selection among existing cables
Accessories selection among existing accessories
Preliminary design of cable cross-section
Design of earthing (grounding) method
Determination of number of cables per phase
Need of a cooling system ?
Yes
New cable design
No
Stage 2
Figure 45 : Stage 1
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STAGE 2 : Preliminary cable route design
In the global studied area
Identification of obstacles to cross :
Roads, railways,rivers,
National parks, archeological sites,
Thermal proximities (steam,...)
Electrical proximities
Services
Trees
Division of the global studied area
in sections
Local and national regulations
In each section
Allowed civil work techniques
Allowed time of trench opening
Location
Right of way
Available civil work techniques in
the country
Choice of options
Route section / Possible techniques
Soil stability
Soil hardness
Soil resistivity
Soil seismicity
Stage 3
Figure 46 : Stage 2
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Identification of the possible civil
work techniques in each section
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Stage 3 : Checking of the cable design
Stage 1
Stage 2
Identification of the sizing point
(thermally speaking)
Modification of cable cross section
Modification of earthing method
Modification of cable architecture
Checking of the cable design
Is the ampacity still
good ?
No
Yes
Stage 4
Figure 47 : Stage 3
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Stage 4 : Choice of options Route section / Civil work technique
Site "prejudice"
Site duration
Site working time (24 h/24 h possible)
Magnetic field
Laying depth
Maintenance and repair process
Cable removal
Construction cost
Site prejudice cost
Maintenance and repair cost
Operation cost
Link non availibility cost
Repair cost
Choice of a technique for each section
with cost and technique aspects
Stage 5
Figure 48 : Stage 4
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Stage 5 : Cable installation
Stage 1
Laying technique
Installation technique : rigid or flexible
Pulling method
Pulling tension
Sidewall pressure
Drum transportation
modification of the design
Cable system performance
Thermomechanical performance
Good
Choice of the cable
Choice of the accessories
Choice of the length on each drum
Final design of the cable
Final design of the civil works
Final design of the installation
Writing of link specifications
Administrative authorizations
Works
Final ampacity, according to works
Commissioning
Figure 49 : Stage 5
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6.2
Study cases
A R1
C1
L1
L2
C2
R2
flooded
area
L3
C3
R3
river
national
park
C4
C5
hill
C6
L4
L5
C7
B
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R5
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bridges
R4
Figure 50 : Possible routes
Let us assume that we have a new project to design : Join substation A to substation B.
We will go through the flow chart to optimise the project.
Stage 1 : Preliminary design of the cable system
Electrical data collection :
Operation voltage : 225 kV,
Ampacity : Normal : 800 A in winter and in summer,
Emergency : 1000 A during 6 hours in winter and in summer,
Cable aim temperature during
normal operation : 90 °C,
emergency operation : 100 °C,
Short circuit level : 50 kA, 0.3 second,
Impulse level : 1050 kVc,
Estimated length of the link : 6 km,
Allowed sheath voltage : 400 V. This information is only useful to determine how the link will be
earthed : solid, single point or cross bonding.
Civil work data collection :
Soil : mainly sandy clay,
Soil resistivity : native soil : 1.2 K.m/W in winter, 1.6 K.m/W in summer,
Available special backfill : 0.7 K.m/W in winter, 1.0 K.m/W in summer
Soil temperature : 15 °C in winter, 25 °C in summer,
Maximum allowed temperature at soil-cable contact : 55 °C,
Maximum allowed air temperature in bridge or in tunnel : 20 °C in winter, 30 °C in summer,
Frost depth : 0.8 m,
Environmental hazards : flood along the river,
Cable selection :
From the cable temperatures, we have to choose an extruded cable and premoulded joints.
Preliminary design of cross-section cable : 1600 mm² Al, 1 cable per phase with special backfill or 1600
mm² Cu, 1 cable per phase, with native soil.
No need of a cooling system
Earthing method : cross bonding, coming from the allowed sheath voltage.
Stage 2 : Preliminary cable route design
Three main routes were found by the design team.
They will be called Left (L), Central (C), Right (R) in all the study.
Identification of obstacles to cross :
Left :
Private land,
a river, 50 m wide and an area liable to flooding, 150 m wide,
a national park,
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Central :
Private streets,
Narrow streets with low traffic,
electrical cables in a street, crossing of a 225 kV existing link laid at 1.8 m deep, cross bonded,
1000 mm² Cu, which ampacity is 900 A in winter, 770 A in summer,
a bridge,
a sloping hill,
Public rural road,
Right :
Public streets, but with heavy traffic,
numerous services in the urban streets but not in the rural road,
a bridge,
Public rural road,
Available civil works techniques in the country where the link has to be built : all,
Trench opening : 300 m at the same time in the city (urban streets),
Working hours : not allowed at night in the city (urban streets).
Civil work techniques selection :
From the collected data, some techniques are of non interest :
Troughs and direct burial in urban areas, where the maximum allowed length to be opened is 300 m.
The length between two joints is then too short to be cost-efficient.
Division of the route in sections :
The different routes can be divided in seven sections : urban streets, river, national park, hill, bridge,
land, rural road.
The following table identifies the possible civil work techniques in each section :
Duct
Direct burial
Tunnel
Trough
Bridge
Shaft
Horizontal drilling
Pipe jacking
Microtunnel
Mechanical laying
Embedding
Use of existing structures
Street
Y
N
Y
N
N
Y
Y
Y
Y
N
N
Y
River
N
N
Y
N
Y
Y
Y
Y
N
N
Y
N
National park
Y
Y
Y
Y
N
Y
Y
Y
Y
Y
N
Y
Hill
Y
Y
Y
N
N
Y
Y
Y
Y
Y
N
Y
Bridge
Y
N
N
Y
Y
N
N
N
N
N
N
Y
Land
Y
Y
Y
N
N
Y
Y
Y
Y
Y
N
Y
Road
Y
Y
Y
Y
N
Y
Y
Y
Y
Y
N
Y
Stage 3 : Checking of the cable design
At this stage of the study, it is necessary to identify the sizing point. According to the collected data, the
sizing point is the native soil with the bad resistivity. The cross section necessary to meet the ampacity
requirement is 1600 mm² Cu.
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This cable being of current manufacturing, nothing has to be changed in the cable architecture or in the
type of soil to be used for the backfill.
If it was decided to change the native soil by a special backfill, the sizing point will also change and
become the two 225 kV links crossing.
Stage 4 : Choice of options route section / civil work technique
The laying technique to adopt in each section depends on the cost structure in the country where the
link has to be laid. Nevertheless, we propose an a priori choice without knowing in which country we
are.
• Private streets on route C : Duct filled with air, laid in special backfill at 2 m deep (bottom of the
trench). It will be necessary to have a reinforced block to cross the 225 kV link, as the depth is less
than the normally allowed one.
• Public streets on route L : Tunnel. It was not allowed by the local authority to open trench in these
streets, due to heavy traffic,
• River : Horizontal drilling with native soil and water inside the duct. Max depth : 7 m under the river
bed,
• National park : Mechanical laying with native soil. Trench bottom at 1.7 m deep,
• Hill : Direct burial with native soil. Trench bottom at 1.50 m deep,
• Bridge : Ducts filled with air,
• Land : duct laid in special backfill at 2 m deep as in the streets.
• Rural road : Direct burial with special backfill. Trench bottom at 1.50 m deep.
It must be noted that in case of native soil surrounding, the very vicinity of the cable is made of sand or
special backfill to avoid direct contact with stones or other materials that could hurt the cable.
The cables are systematically laid in trefoil formation, one cable per duct when any, except at the 225
kV link crossing where cables are in flat formation, to improve the heat diffusion, taking into account
the mutual heating of the two links..
Comments :
In the sloping hill, direct burial could be preferable to duct as the installation will be considered as rigid
and so, the creeping problem can be avoided.
The width of the area liable to flooding is 150 m large. Microtunnel is not the best technique in this case
as the maximum length is considered by experts to be around 100 m on the present site.
The land section, in accordance with the private land owners, will be done with ducts.
In private streets, the same technique will be applied (ducts), the only difference will be in the final
layer, good soil for land and asphalt for streets.
In the public streets, the tunnel has been chosen after discussions between utility and local authorities in
order to limit traffic problems.
In the national park, an important environmental impact study was realised by independent specialists
and it concluded that it was possible to cross it. According to the type of soil, land, animals, flowers
found in this park, the best solution was to use mechanical laying at a precise period (Fall for example).
The main criteria that convinced the specialists were : speed of the works, narrow width, no big soil
movements.
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With the different laying techniques encountered along the routes, transitions between rigid and flexible
installation have to be designed.
In particular, it could be reasonable in the bridge to fill the ducts with bentonite to avoid to change the
installation technique from rigid, as the contiguous sections are envisaged with direct burial, to flexible
with the unfilled ducts and so to keep the rigid one on three sections.
Cable checking :
Cable
Street or land
River
National park
Hill
Bridge
Tunnel
Road
1600 Al
1600 Cu
1600 Cu
1600 Cu
800 Al
800 Al
1600 Al
It is important to state that, along the route, we can have changes in the cable cross section, and so
optimise the cable section, but it is not easy to joint two cable sections if the difference between cross
sections is too important.
As the length of our project is rather short, we decide to homogenise the cross section all along the
route, with 1600 mm² Cu.
This approach would make the impact of the cables an invariant, but in real projects it could be a way
to reduce the cost of links. A full optimisation can give a reduction that could be up to 10 %.
Time and period to complete the site
An other important aspect of the project is the time required to complete it. Of course, the
administrative and design items need time but do not affect the inhabitants living along the routes. On
the contrary, the required time to complete the civil works, the jointing and the period during when the
works will be realised is of first importance for them.
Some local authorities have strong demand on this specific point, so strong that it could determine the
route or the laying technique that will be finally chosen by the project manager.
On the left route, the specialists propose to cross the national park during the Fall season. We can also
imagine that works on the central route can only be done during the Summer season when most of the
inhabitants are on holidays, that is to say away from their houses, to be sure that the noise coming from
the works will not hurt their ears.
Cost of the project, comparison of the different routes :
After the cable checking, you have to choose the final route among the three identified ones which
respective lengths are 6 km for left route, 5 km for central route and 7 km for right route and then
review the installation of the cable.
For a better understanding, lengths of the different routes could be identical, but it is rarely the case in
an actual project.
As soon as you know the costs of the different items, you can calculate the overall cost and choose the
best route. All the discussions with the local authorities are not recorded here, but are necessary to
officially finalise the route.
It is necessary at this stage of the project to check the completion time. A line is dedicated to this
specific point in the table so that the right decision could be taken before writing the tender.
Each of you can fill up Table 7 with the real costs of his country.
WG 21-17 Technical Brochure
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With this example, we can see that it is not easy to say which route is the best one, according to the
different criteria that can be used to establish the ranking.
An analysis was done with French costs. Regarding this criteria, the left and the central routes are at
the same price, slightly in favour of the central one, and the right one not far from twice. If we had the
site completion time, the most favourable route becomes the left one.
This analysis is only valid for France.
It is of evidence that limitations due to administrative regulations, unknown in this study, can upset the
technical and economical ranking shown here.
To optimise the project, it is obvious that we won’t change the cable section at each new route section.
One or two changes are possible but not more. Depending on the route that will be chosen, an
optimisation is then necessary, the place of the manholes being for example one of the problems.
As a conclusion, we have to underline that :
• It is necessary to follow up the different stages of the process as described in the flow chart, (see
chapter 6.1),
• It is important to be open minded at the beginning of a new project to study all possible solutions. It
is clear that some will fall as soon as the designer gets new constraints.
• The shortest route is not always the cheapest route and that savings can be done by using other
routes or innovative laying techniques.
Section
Length (m)
Type
Land
Land
River
National Park
Cable
Linear
cost
Section
cost
Laying technique
Linear
Section
Type
cost
cost
Duct
Duct
Horiz.Drilling
Mecha.Laying
L1-L2
L4-L5
L2-L3
L3-L4
450
400
150
5 000
1 600 Cu
1 600 Cu
1 600 Cu
1 600 Cu
C1-C2
C5-C6
C3-C4
C2-C3
C4-C5
C6-C7
800
2 500
100
400
200
1 000
1 600 Cu
1 600 Cu
1 600 Cu
1 600 Cu
1 600 Cu
1 600 Cu
Duct
Direct burial
Duct
Direct burial
Direct burial
Direct burial
1 200
200
600
5 000
1 600 Cu
1 600 Cu
1 600 Cu
1 600 Cu
Tunnel
Duct
Direct burial
Direct burial
Left route cost
Private street
Hill
Bridge
Road
Road
Road
Central route cost
Public Street
Bridge
Road
Road
R1-R2
R3-R4
R2-R3
R4-R5
Right route cost
Route
Left
Central
Right
6 000
5 000
7 000
Site duration (Months)
Total length (m)
Total route cost
Table 7 : Route cost
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Total cost
7.
GLOSSARY
Throughout this brochure, the terms have to be considered as follows: The term “construction
techniques ” is considered as relating to the techniques used to create the cable route, mainly covering
the civil works such as trenching. Likewise the term “installation techniques” is considered to relate to
the cable system design and cable installation methods.
Cable design issues associated with the laying and installation techniques have also been considered
under the general subject of "Installation Techniques".
The cable installation was then the rest : the pulling and backfilling, the fixing when laid in open air.
PIPE JACKING :
This technique consists in pushing into the soil prefabricated tubes having the exact diameter of the
final tube. The tubes are pushed from a work shaft. As the pipe jacking progresses the earth works are
done, either manually or mechanically, according to the requested diameter. The first tube is equipped
with a steel drum curb which bites into the subsoil while protecting the site workers clearing the earth.
This technique concerns diameters between 1000 and 3200 mm.
MICROTUNNEL :
This technique consists in pushing in the subsoil prefabricated tubes having the exact diameter of the
final tube. They are pushed from a work shaft. The earth works are systematically mechanised : a
microtunneller is put in front of the tubes. This remote controlled machine can dig small diameter pipe
jacking horizontally. By using the microtunneller tubes of a diameter between 300 mm and 1200 mm
can be put in place.
SHAFT :
vertical circular or rectangular excavation from which the tubes are pushed.
HORIZONTAL DRILLING :
Directly issued from oil drilling techniques, horizontal drilling is carried under rivers beds, railway tracks,
motorways,..., and is composed of four phases : drilling of the pilot hole from the bank for rivers or
from one side for motorways or railways tracks, casing the pilot hole, boring and pulling and laying of
the final tubes. Direct drilling is an other word that is used in some countries to design the same
technique.
EMBEDDING :
This technique consists of excavating a river bed from a barge or with an amphibious machine, burying
a tube or cables and filling up the trench.
USE OF EXISTING STRUCTURES :
sometimes, it is possible to use some existing structures, like water, gas or old fluid-fluid tubes, to put
High Voltage extruded cables inside of them.
BRIDGES :
The cables can be put in rail or road infrastructures. They can be placed in or outside bridges
structures. This avoids other techniques which remain costly and can be difficult to accomplish.
MECHANICAL LAYING :
This technique, entirely mechanical, consists in excavating a trench, and burying the cables
simultaneously in a continuous progression.
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TRENCH : Excavation in which the cables are directly buried.
Trench
Backfill
Cable or duct
Bedding material
BEDDING MATERIAL :
Material which can be put at the bottom of the trench under the cables, the troughs or the ducts.
BACKFILL :
Material used to fill up the trench.
DUCT :
Tube in PVC, PE, concrete, steel,... in which high voltage cables are pulled.
PIPE :
Tube used for some laying techniques, i.e. pipe jacking, microtunnelling,…to open a hole in the soil for
the underground link. Cables are not pulled directly in the pipes ; Usually, ducts are pulled in the pipes,
and cables pulled in the ducts.
TROUGH :
Prefabricated concrete element, placed at the bottom of the trench, in which high voltage cables are
laid.
CABLE REMOVAL :
Action of removing the cables at the end of their operation.
RIGHT OF WAY :
High voltage cables are installed in Public or private areas. In these two cases, the electrical company
has to obtain a "right of way" which allows to excavate a trench, to bury high voltage cables and
operate the underground line.
RIGID INSTALLATION :
in a rigid system, the cable is held in such a manner that virtually no lateral movement occurs and the
cable absorbs the thermal expansion by developing a high internal compressive force. To ensure a
satisfactory performance, the cable must not buckle under this force giving rise to severe local sheath
strains.
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FLEXIBLE INSTALLATION :
in a flexible system, the cable is held in such a manner that the expansion movement does not cause
excessive strain in any of the cable components and hence a short fatigue life. The basic principle of
flexible systems is that the thermal expansion and contraction are absorbed by movements of the cable
at right angles to the longitudinal axis of the cable.
CATERPILLAR OR HAULING MACHINE :
pulling or pushing machine used for the installation of cables. The cable passes between two
caterpillars and is belt-driven or braked by rubbing. They are frequently used in conjunction with cable
rollers for the installation for cables in trenches, troughs and on bridges. Motorised rollers can be used
to assist in the installation of cables around bends. When installing a cable down a steep tunnel, a
braking caterpillar is used to prevent the cable from running away down the slope. In addition, braking
rollers can be used to hold back the cable from running away on steep inclines.
caterpillar
cable
MANHOLE
Visitable bay where joints are laid. Usually, a manhole has two covers that allow the workers to enter
in.
JOINT BAY
Non visitable bay where joints are laid. Usually, these bays are backfilled after joint completion.
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8. BIBLIOGRAPHY
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laying, ducts
66,B. Gregory, J. Monteys (BICC), S. Barris (Enher-Hec), J-M. Mendez, Installation of 220 kV XLPE
cables in a tunnel, whilst minimising electromagnetic induction in communication cables, 1999,
Jicable'99, 148-153, tunnels
67,P. Lavantureux, P. Deguines, F. Gahungu (Alcatel Cables), T. Todorov (Nationalna Elektrichescka
Kompania Bulgaria), 400 kV insulated cable link installation carried out at the Chaira pumped
storage power plant in Bulgaria, 1999, Jicable'99, 154-159,tunnels
70,C. Figaret (EEE), The mechanical laying of underground high voltage cables (63 and 90 kV) in
France, 1999, Jicable'99, 160-161, mechanical laying
71,Y. Maugain, P. Hudson in name of WG 21-17, Cable installation: State of the art for the installation
design of HV and EHV cable systems, Cigre 2000, paper 21-202
WG 21-17 Technical Brochure
Page 145 / 145
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