Summary Report: Underground Transmission Cable
System Construction and Installation Practices
3002000886
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Summary Report: Underground Transmission Cable
System Construction and Installation Practices
3002000886
Technical Update, December 2013
EPRI Project Manager
T. Zhao
ELECTRIC POWER RESEARCH INSTITUTE
3420 Hillview Avenue, Palo Alto, California 94304-1338 ▪ PO Box 10412, Palo Alto, California 94303-0813 ▪ USA
800.313.3774 ▪ 650.855.2121 ▪ askepri@epri.com ▪ www.epri.com
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DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES
THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF
WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI).
NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY
PERSON ACTING ON BEHALF OF ANY OF THEM:
(A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH
RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM
DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED
RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS
SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR
(B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING
ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED
OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS
DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN
THIS DOCUMENT.
REFERENCE HEREIN TO ANY SPECIFIC COMMERCIAL PRODUCT, PROCESS, OR SERVICE BY ITS
TRADE NAME, TRADEMARK, MANUFACTURER, OR OTHERWISE, DOES NOT NECESSARILY
CONSTITUTE OR IMPLY ITS ENDORSEMENT, RECOMMENDATION, OR FAVORING BY EPRI.
THE FOLLOWING ORGANIZATIONS, UNDER CONTRACT TO EPRI, PREPARED THIS REPORT:
Electric Power Research Institute (EPRI)
Power Delivery Consultants, Inc.
NOTE
For further information about EPRI, call the EPRI Customer Assistance Center at 800.313.3774 or
e-mail askepri@epri.com.
Electric Power Research Institute, EPRI, and TOGETHER…SHAPING THE FUTURE OF
ELECTRICITY are registered service marks of the Electric Power Research Institute, Inc.
Copyright © 2013 Electric Power Research Institute, Inc. All rights reserved.
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ACKNOWLEDGMENTS
The following organizations prepared this report:
Electric Power Research Institute (EPRI)
1300 West W.T. Harris Blvd.
Charlotte, NC 28262
Principal Investigator
T. Zhao
Power Delivery Consultants, Inc.
28 Lundy Lane
Ballston Lake, NY 12019
Principal Investigator
J. Williams
This report describes research sponsored by EPRI.
EPRI acknowledges the following individuals for their contributions:
•
•
•
Pierre Argaut of Silec Cables provided a great deal of information on extruded dielectric
cable installation, including international practices.
Joseph Zimnoch, consultant, reviewed draft material and provided guidance on civil works
and pipe-type cable installation.
John Cooper and Richard Allen, Jr., consultants, provided comments and figures.
This publication is a corporate document that should be cited in the literature in the following
manner:
Summary Report: Underground Transmission Cable System Construction and Installation
Practices. EPRI, Palo Alto, CA: 2013. 3002000886.
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PRODUCT DESCRIPTION
Over a period of three years, the Electric Power Research Institute (EPRI) developed an
important report in the Underground Transmission Program entitled Underground Transmission
Cable System Construction and Installation Practices Manual—2012 Update (1024180). The
report was most recently updated at the end of 2012. Despite the report’s value but because of its
length, the report is challenging for some stakeholders to absorb. To capture key insights,
important best practices, and key stakeholder value, EPRI developed this summary based on the
original report.
Background
A reliable underground transmission line depends on acceptable cable system manufacturing,
design, construction, installation, operation, and maintenance. Within implementation,
construction and installation remain the most expensive processes. Recent advances in
underground transmission have led to more demand for best practices and innovative ways to
reduce construction and installation costs in a cable project by better planning for and designing
of these systems.
EPRI has funded many projects over the years to improve the efficiency and reduce the cost of
underground transmission cable installation. To this end, EPRI has published many reports,
including the 2006 EPRI Underground Transmission Systems Reference Book (1014840, also
known as the Green Book), which contains extensive descriptions of cable systems and their
design and installation. Other organizations, including the Association of Edison Illuminating
Companies, IEEE, and the International Council on Large Electric Systems, have also published
documents addressing transmission cable construction and installation. EPRI funded a
comprehensive project to review these existing documents, coordinate with long-standing
experts in underground transmission installation, and prepare the original version of this report
(1024180). Discussions with installers, design engineers, suppliers, and transmission cable
system users provided additional useful information. The primary author of the report obtained
substantial input from other experts in underground transmission cable system construction and
installation and worked closely with staff of EPRI-member utilities, including an active advisory
group.
Objective
The goal of this summary report is to present the material from the original, larger report
(1024180) in a more digestible length and in an organized fashion that transfers this important
information to utility decision makers.
Approach
To develop the present report, the team selected from the original report the information most
relevant to decision makers involved in cable system construction and installation projects and
distilled this information into the shorter version.
Results
For the original report, a large amount of pertinent information was found in the published
documents and installation specifications. Discussions with installers, design engineers,
suppliers, and transmission cable system users provided useful information not contained in
previously published documents. Example project descriptions provided by EPRI-member
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utilities and others demonstrated how both traditional and innovative approaches made cable
system installation proceed more smoothly and how lessons learned helped to identify potential
problems that decision makers might encounter. The project team incorporated all of this
information into the original report.
The present summary report captures the essence of this information in a shorter report format
that is better suited for utility decision makers.
Applications, Value, and Use
The original report is intended as a resource that will enable cable system owners and decision
makers to make initial plans for the best construction and installation methods and ensure that
important steps are incorporated into owner specifications and implementation. The report is
applicable to almost any transmission cable project. It was a well-organized volume that
immediately benefits experienced cable decision makers and system planners, as well as those
considering their first transmission cable installation project. It has industry-wide value because
it provides background and guidance for planning every step in the construction and installation
of a transmission cable system. Therefore, the report should find widespread use among all
owners and others who are considering installation of such systems.
This summary report is most valuable to utility decision makers during the planning, conceptual
design, and permitting stages of both extruded dielectric and self-contained fluid-filled cable
systems and pipe-type cable systems.
Keywords
Cable construction and installation
Cable construction planning
Cross-linked polyethylene insulated cable
Extruded dielectric cable
High-pressure fluid-filled cable
High-pressure gas-filled cable
Self-contained fluid-filled cable
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CONTENTS
1 INTRODUCTION AND GENERAL TOPICS: PLANNING, CONCEPTUAL DESIGN,
PERMITTING, AND CIVIL WORKS.........................................................................................1-1
1.1
Objectives ...............................................................................................................1-1
1.2
Introduction .............................................................................................................1-1
1.3
System Planning .....................................................................................................1-4
1.3.1 System Planning ...............................................................................................1-4
1.3.2 Design Criteria Document .................................................................................1-5
1.3.3 Engineering Studies ..........................................................................................1-5
1.3.4 Project Implementation Approach, Cost, and Responsibility .............................1-6
1.4
Conceptual Design and Permitting........................................................................1-11
1.4.1 General Approach for Cable Systems: Different Areas ...................................1-11
1.4.2 Duct Bank Versus Direct Burial .......................................................................1-13
1.4.3 Special Installation Conditions ........................................................................1-13
1.4.4 Routing ...........................................................................................................1-21
1.4.5 Surveys; Obstacle Detection ...........................................................................1-23
1.4.6 Manhole Placement ........................................................................................1-24
1.4.7 Traffic Control Plans .......................................................................................1-24
1.4.8 Cable Length, Reel Size, and Transportation Constraints ...............................1-24
1.4.9 Thermal Property Testing................................................................................1-25
1.5
Schedule ..............................................................................................................1-25
1.6
Specifications .......................................................................................................1-26
1.6.1 Specifications and Standards ..........................................................................1-26
1.7
Safety ...................................................................................................................1-29
1.8
Civil Works Including Manholes ............................................................................1-31
1.8.1 Initial Checklist ................................................................................................1-31
1.8.2 Survey ............................................................................................................1-32
1.8.3 Underground Locating ....................................................................................1-33
1.8.4 Geotechnical and Soil Thermal Tests..............................................................1-33
1.8.5 Excavation, Manholes .....................................................................................1-34
1.8.6 Water Removal ...............................................................................................1-39
1.8.7 Duct or Pipe Installation ..................................................................................1-39
1.8.8 Direct Burial ....................................................................................................1-40
1.8.9 Encasement ....................................................................................................1-40
1.8.10 Backfilling the Trench ....................................................................................1-41
1.8.11 Restoration....................................................................................................1-42
1.9
Equipment Used for Civil Work .............................................................................1-43
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2 EXTRUDED DIELECTRIC AND SELF-CONTAINED FLUID-FILLED CABLE SYSTEM
INSTALLATION.......................................................................................................................2-1
2.1
Introduction .............................................................................................................2-1
2.2
Overview ................................................................................................................2-1
2.3
Duct Bank Construction ..........................................................................................2-1
2.3.1 Duct Material and Size ......................................................................................2-1
2.3.2 Installing and Joining Ducts ..............................................................................2-2
2.4
Directly Buried Installations.....................................................................................2-4
2.4.1 Trenches for Directly Buried Cables ..................................................................2-4
2.4.2 Transition of Duct to Directly Buried Cables ......................................................2-4
2.5
Cable Installation ....................................................................................................2-5
2.5.1 Cable in Duct Bank Installations........................................................................2-5
2.5.2 Cable in Directly Buried Installations .................................................................2-8
2.6
Special Installation Conditions ..............................................................................2-10
2.6.1 Elevation Changes ..........................................................................................2-10
2.7
Termination Structures .........................................................................................2-10
2.8
General Requirements for Sheath Bonding and Grounding ..................................2-10
2.8.1 Multipoint Bonding and Grounding ..................................................................2-11
2.8.2 Single-Point Bonding and Grounding ..............................................................2-11
2.8.3 Cross-Bonding ................................................................................................2-11
2.9
Fiber-Optic Installation: Other Communications ....................................................2-12
2.10 Distributed Temperature Monitoring ......................................................................2-13
3 PIPE-TYPE CABLE SYSTEM INSTALLATION ....................................................................3-1
3.1
Introduction .............................................................................................................3-1
3.2
Overview ................................................................................................................3-1
3.3
Cable Pipe ..............................................................................................................3-1
3.3.1 Pipe Specifications and Size .............................................................................3-1
3.3.2 Pipe Coating .....................................................................................................3-3
3.3.3 Transporting, Handling, and Storing Pipe ..........................................................3-3
3.4
Pipe Installation ......................................................................................................3-4
3.4.1 Joining Pipe Sections ........................................................................................3-4
3.4.2 Welding Stainless Steel Riser Pipes .................................................................3-5
3.5
Installing Cable Pipe in the Trench .........................................................................3-6
3.5.1 Pipe in Manholes ..............................................................................................3-7
3.5.2 Pipe Mandreling, Evacuation, and Pressurization .............................................3-7
3.5.3 Special Considerations .....................................................................................3-8
3.6
Cable Installation ....................................................................................................3-9
3.6.1 Handling the Cable .........................................................................................3-10
3.6.2 Equipment Specifications ................................................................................3-10
3.6.3 Preparation for Cable Pulling ..........................................................................3-11
3.6.4 Pulling .............................................................................................................3-12
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3.6.5 Splicing ...........................................................................................................3-14
3.6.6 Terminating .....................................................................................................3-14
3.6.7 Final Pressure and Vacuum: Fluid-Filling ........................................................3-14
3.6.8 Connecting Cathodic Protection System .........................................................3-16
3.6.9 Pressurizing Plants .........................................................................................3-17
3.6.10 Commissioning the Circuit .............................................................................3-18
4 CONCLUSIONS ...................................................................................................................4-1
5 REFERENCES .....................................................................................................................5-1
A ACRONYMS ....................................................................................................................... A-1
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LIST OF FIGURES
Figure 1–1 General approach to a transmission cable project ................................................. 1-2
Figure 1–2 Steps for overseeing installing, testing, and commissioning of extruded dielectric
cable ....................................................................................................................................... 1-3
Figure 1–3 Steps for overseeing installing, testing, and commissioning of pipe-type cable ..... 1-3
Figure 3–1 Pipe installation progression in a city street ........................................................... 3-4
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LIST OF TABLES
Table 3–1 Mandrel dimensions for typical pipe sizes for 0.250 in. (6.35 mm) wall pipe ........... 3-8
Table 3–2 DC test voltages for pipe-type cables ................................................................... 3-19
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INTRODUCTION AND GENERAL TOPICS: PLANNING,
CONCEPTUAL DESIGN, PERMITTING, AND CIVIL
WORKS
1.1
Objectives
The objectives of this report are:
•
To serve as an overview of the detailed Underground Transmission Cable System
Construction and Installation Practices Manual—2012 Update [1]
•
To provide the decision makers with an overview of the comprehensive practices in
underground transmission cable system planning, civil works, and cable installation
•
To describe typical projects as related to these practices to aid in planning projects
•
To investigate innovative planning approaches
Users can apply this information to develop an effective implementation plan in conjunction with
Underground Transmission Cable System Construction and Installation Practices Manual—
2012 Update (1024180) and other industry guideline publications.
1.2
Introduction
The last few decades have brought significant advances in cable practices. This concise, wellorganized overview devoted to underground transmission cable construction and installation
practices helps planners make more efficient and cost-effective decisions throughout the
planning and implementation process.
This Summary Report addresses common practices for underground transmission systems,
focusing on the important issues for a successful project. Project cost and schedule are very
important for any project, including underground transmission projects. This report is not
intended to provide guidance in preparing cost estimates for projects, but following the
recommendations of this report should result in a lower installed cost as well as reduced lifetime
costs.
This report assumes that readers have a general understanding of transmission cable types and
their applications. The 2006 edition of the EPRI Underground Transmission Systems Reference
Book (also known as the Green Book, and referred to by that name throughout this document)
provides information on transmission cables; their materials and components; applications;
system considerations; and operation and maintenance procedures [2].
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Transmission cable projects use commonly accepted project management principles and
processes in five categories: initiation, planning, execution, monitoring and control, and closing.
Figure 1-1 shows the general approach for application to transmission cable project planning.
The report is organized according to Figures 1-1 through 1-3, with the understanding that
monitoring and control is applicable across all processes.
Section 1 describes objectives of the report and covers planning, conceptual designs, permitting,
and civil works of a typical cable project that are common to all cable types (see Figure 1-1).
System Planning
Design criteria document
Cable type determination
Cost estimates
Prepare specifications
Select contractors
Procure materials
Perform civil works and
underground surveys
Oversea excavation,
installation of manholes,
trenches, and cable
Commission
Figure 1–1
General approach to a transmission cable project
Section 2 covers specifications and planning for extruded dielectric and self-contained fluidfilled (SCFF) cable system construction, including installation best practices, splicing and
termination, and commissioning (see Figure 1-2).
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Figure 1–2
Steps for overseeing installing, testing, and commissioning of extruded dielectric cable
Section 3 covers specifications and planning for pipe-type cable system construction, including
installation best practices, pipe welding and testing, pressurization, and commissioning (see
Figure 1-3).
Figure 1–3
Steps for overseeing installing, testing, and commissioning of pipe-type cable
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1.3
System Planning
1.3.1 System Planning
A transmission circuit design starts with system planning. System planning considerations can
have a major effect on the selection of a cable system type and construction techniques. For
example, pipe-type cables have higher capacitance and, in most cases, lower series reactance
than extruded dielectric cables. These system considerations may also override decisions on
cable type selection based on installation approaches and cost.
The utility system planning department or other owner of the project usually provides the
following information to the engineers designing the project:
•
The steady-state ampacity requirement for ultimate loading conditions
•
Emergency ampacity durations and load levels, as well as load levels before the emergency
•
Daily load factor for ultimate loading conditions
•
Reliability/redundancy requirements; for example, two lines might be required, with the
ability to carry 100% of design power on one line for around 4 hours if the other line trips
•
Expected load growth with time (if the analysis is to include cost of losses)
•
Maximum available fault current contribution from each end and durations that will be seen
by the cable system
The design engineer must then provide the planning department with the following information
to allow the planner to evaluate the integration of the cable into the transmission system:
•
Positive sequence impedances. Negative sequence impedance is equal to the positive
sequence impedance for cables.
•
Zero sequence impedances. For pipe-type cables, zero sequence impedance is typically
provided for a range of fault currents—for example, 10, 20, 30, and 40 kA—because the zero
sequence impedance varies with the magnitude of the fault current.
•
If there are multiple circuits in close proximity, the zero sequence mutual impedance between
adjacent circuits is needed to set protective relaying.
•
Cable capacitance or susceptance and charging current.
The owner’s design engineers are often able to perform the calculations. If the cable supplier or
other organization is to perform the calculations, it is necessary for the owner engineers to
carefully define the installation conditions and to ensure that the owner is not just receiving
“handbook values” that might not be sufficient.
For extruded dielectric and self-contained fluid-filled (SCFF) cable lines, the following
information is needed to determine the impedances:
•
Type of configuration (duct bank or direct burial)
•
Sheath construction
•
Sheath connection
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•
Electrical conductivity of local soil
•
Size and location of ground continuity conductors
The planning department will often run load flow, transient stability, short-circuit, and transient
overvoltage studies. The planning processes may take several iterations early in the conceptual
stage, especially for extruded dielectric cables in which different duct bank or direct burial
configurations might be considered. Further information on the calculations is included in
section 16 of the Green Book.
Engineering and system planning personnel should agree on the cable type, number of lines,
number of cables per phase, steady-state and transient ampacity ratings, effects on power flow
and reliability, and major items that could be “deal breakers” (such as environmental
requirements, earthquake zones, and so on) at the planning stage, before the design begins, to
avoid unnecessary iterations in the time-consuming design process.
1.3.2 Design Criteria Document
Many owners prepare design criteria or a design basis document for a project, or have such a
document prepared by their consultant. This document explains the owner’s requirements for the
line (normal and emergency loading, terminal stations, approximate route, schedule, and any
other criteria that the owner has established) as well as the criteria for cable type selection,
approach to be used for conceptual and detailed design of the line, and other factors. The design
criteria document ensures that all parties involved in the project have a common understanding
of the project requirements and reduces potential project scope deviations and false starts. The
design criteria document is a living document and should be updated or modified when necessary
during the engineering design process.
1.3.3 Engineering Studies
Engineering studies are undertaken at many stages during the cable design process. Even at the
planning stage, engineering design studies are required to determine the cable type, the
approximate conductor size and number of conductors, and feasible circuit routes. The following
calculations are generally required. Typical data are entered if cable parameters and installation
details are not well known at the planning stage. Descriptions of each of these calculations can
be found in the referenced sections of the Green Book.
•
•
•
•
•
•
•
•
Impedance (Section 16)
Steady-state and transient/emergency ampacity (Section 11)
Short-circuit capability of the shield/sheath for extruded dielectric and SCFF cables
(Section 11)
Magnetic field calculations for pipe-type cables (three phases installed in a steel pipe) and
for extruded dielectric and SCFF cables (Section 16)
Pulling forces and sidewall pressures (Section 12.4.2)
Sheath voltages for extruded dielectric and SCFF cables (Section 10.2.3)
Hydraulic calculations for high-pressure fluid-filled (HPFF) and SCFF cables (Section 9.3)
Induced voltage studies for nearby metallic structures (Section 16)
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In addition, the EPRI Underground Transmission Workstation (UTW) gives the engineer the
ability to perform the following calculations:
•
•
•
•
•
Cable ampacity for a variety of installation and operating conditions
Magnetic field calculations for pipe-type cables (three phases installed in a steel pipe) and for
extruded dielectric and SCFF cables
Pulling tensions, including modeling the effect of pulling both the power cable and the
pulling line
Basic pipe-type cable hydraulic parameters
Induced voltages and currents (planned)
The UTW also has an option to perform detailed economic evaluations for trenched cable
systems. The data provided in the UTW sample cases help users to set up these types of analyses.
The UTW features a detailed bibliographic summary of more than 1000 cable-related references.
The search features permit access to a selected group of EPRI reports as well as the Green Book
topics.
1.3.4 Project Implementation Approach, Cost, and Responsibility
The overall project approach depends on the owner’s experience in underground transmission
cables, available staff, and willingness to take responsibility and risk, and project time schedule.
Potential approaches, described further in the EPRI report Lower Cost Underground
Transmission Cable [3], are summarized in this subsection. In each approach, the owner (utility,
independent power producer, or merchant supplier) should have the warranty provisions defined
very carefully. The owner’s representative for the various approaches is typically an architectengineering firm or consultant.
Cost is a major consideration for any project. There is no agreement in the industry on which of
the project approaches discussed below will result in the lowest-cost installation. Comments on
cost implications of each approach are provided; individual projects should be evaluated based
primarily upon complexity, the owner’s risk tolerance, and schedule.
Approaches for carrying out the projects vary, and individual owners often have their own
preferences for capital projects. Approaches also vary with voltage level, project worth, and
importance. The subsections that follow are devoted to the main alternatives for underground
transmission projects.
1.3.4.1 Conventional Design Bid Build (Engineered Project)
This is the type of project historically undertaken by major cable-using utilities. The owner is
responsible for all design engineering, and the owner often prepares bid packages and issues
separate contracts for materials, civil work, and cable installation—although a single contract
could be awarded for those items. The owner has full responsibility for the project, and has the
greatest control in terms of design details, material, quality control during construction, and
project cost. The owner also assumes most of the risk on the project.
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A conventional design-bid-build process might consist of the following steps:
•
The owner and/or owner’s representative prepares 100% design documents.
•
The owner or owner’s representative obtains competitive bids based on the 100% design
documents. Separate bids might be obtained for civil works, material supply, and cable
system installation.
•
The owner and/or owner’s representative evaluates bids and awards construction contracts.
•
The contractor mobilizes, orders materials if they are not procured by the owner, and
constructs the project. The owner or owner’s representative oversees the work and approves
necessary scope changes.
•
The owner is responsible for overseeing overall cable design, overseeing cable
manufacturing, meeting the design criteria, and assuming most of the project risk for
unforeseen conditions.
The engineered project can be the lowest-cost approach, provided it is well engineered and
managed. Because the owner is accepting most of the risk, the contractor(s) should have little
additional risk. If the project goes well, costs should be reasonable. If the project encounters
difficulties, the owner will be obligated to absorb the additional costs.
However, this approach reduces the incentive for the contractor to use innovative methods for
construction or installation, and strict adherence to the specifications may make responding to
field conditions difficult.
If the contracts are to be let in a manner that separates the responsibility for cable supply and
cable installation from the construction of the duct bank, direct burial trench, or pipe system that
will contain the cables, then the owner may want to consider having a bid item in the cable
installation bid sheet for inspection and acceptance of the installed ducts, pipes, or trench. If the
timing of the activities does not lend itself to this approach, the owner may want to consider the
hiring of a cable installation contractor as a consultant/inspector during the civil (duct
installation) work. This comment applies to other contract types when the activities might be
performed by different contractors. Interfaces between various contractor responsibilities must
be very well defined in the project specifications. This approach works better with duct
bank/manhole and pipe installations, where civil work can be separated from electrical work, but
can be more complicated for direct buried applications.
1.3.4.2 Design Build (Semi-Turnkey)
In this type of project, the owner performs conceptual engineering, and perhaps initial levels of
design engineering. The owner selects the design-build contractor, perhaps works with that
contractor during initial stages of detailed design, and then turns the project over to the
contractor to be responsible for details of design, procuring materials, and constructing the
project.
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The design-build process might consist of the following steps:
•
In the design-build process, the owner’s representative and a selected contractor work
together as a design-build team as described next. The owner or owner’s representative
prepares partial design documents. Design documents might range from as little as 10–50%
complete or more.
•
The owner or owner’s representative obtains qualifications and/or price proposals from
contractors based on the partial design documents. The prime contractor could be a
construction firm specializing in underground cable systems, an architect-engineering firm,
or a cable supplier.
•
The owner and/or owner’s representative evaluates qualifications and price proposals and
selects a contractor.
•
The design-build team completes design and begins cable manufacturing and construction.
Construction might begin before completion of 100% of the design.
•
The design-build team, including the owner’s representative to the extent that he or she is
involved in the design, assumes most of the risk and is responsible for the satisfactory
operation of the system for a warranty period negotiated with the owner.
The semi-turnkey project turns responsibility over to the contractor early in the design process,
and requires less owner involvement and oversight than required for the modified design-build
project (described next) or the engineered project. Because the contractor has greater flexibility
in the design and construction, the contractor has an incentive to keep the project costs lower,
although that may not always translate to a lower cost for the owner unless contractual
arrangements have been made for sharing cost savings.
1.3.4.3 Modified Design Build
A modified design-build process might consist of the following steps:
•
The owner and/or owner’s representative prepares less than 100% complete design
documents—typically, the level of completion ranges from 50–75%.
•
The owner or owner’s representative obtains qualifications and/or price proposals from
contractors based on the design documents.
•
The owner and/or owner’s representative evaluates qualifications and price proposals and
selects a contractor.
•
The owner’s representative and the selected contractor work together as a team to complete
the design documents to 100%. The team seeks methods to generate cost savings, schedule
reductions, and improvements in constructability.
•
The contractor might order long-lead materials before completion of 100% design
documents.
•
The contractor might modify the final price based on the completed design if bid items have
changed.
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•
The contractor buys or manufactures cable if the contractor is a cable manufacturer, obtains
accessories and other materials, and constructs the project.
•
The contractor has “bought in” to most of the risk and is responsible for the satisfactory
operation of the system for a warranty period negotiated with the owner.
The modified design-build approach seeks to reduce costs and promote innovation as compared
to an engineered project by bringing the contractor on board early in the process and seeking the
contractor’s innovative ideas during the final design, as well as the construction and installation.
1.3.4.4 Engineering/Procurement/Construction and Engineering/Procurement/
Construction/Management (Turnkey)
In this type of project, the contractor (often a large architect-engineering firm or large
contracting firm) takes full responsibility for the project, and turns it over to the owner in a
“ready to use” condition, with warranties and guarantees for a duration that has been negotiated
during the selection of the turnkey contractor. This differs from the lump-sum approach, because
the contractor is responsible for the engineering design on the project.
Using the EPC or EPCM contract approach, the contractor/engineer is responsible for the design,
manufacturing, material procurement, and construction process as follows:
•
The owner or owner’s representative prepares a request for qualifications for the proposed
project.
•
The owner and/or owner’s representative evaluates qualifications, prepares a request for
proposals, reviews the proposals, and selects the contractor.
•
The contractor prepares design documents to the extent required to construct the project.
Design might first be completed for long-lead items.
•
The contractor prepares procurement packages for all or some of the bid items.
•
The contractor obtains competitive bids and selects subcontractors to perform material
supply and construction activities. The contractor might elect to submit prices on
procurement packages and self-perform some or all of the construction activities.
•
Prices for construction are set.
•
The contractor serves as construction manager and oversees construction. The contractor
assumes almost all of the risk for unforeseen conditions and is responsible for the satisfactory
operation of the system for a warranty period negotiated with the owner.
The full turnkey project can be the most costly contract approach because the owner is paying for
the contractor to manage the project as well as perform the engineering, construction, and
installation work. Although this approach may be more costly, it may be the most suitable
approach for the owner with limited experience or limited time to devote to the project.
Alternatively, the full turnkey could be a cost-effective approach if full freedom is given to the
turnkey contractor to apply innovative approaches.
For each of the previous approaches the owner may also choose to define the financial terms of
the contract using one of two generally accepted methods.
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1.3.4.5 Lump Sum (Firm Fixed Price)
Lump-sum contracts for material and construction/installation provide the greatest protection to
the owner to minimize increases in project cost. The contractor will typically assign a “risk
adder” when bidding the contract to cover perceived risk for the project, and this could increase
the bid price substantially. This approach requires the owner to provide detailed engineering and
detailed specifications, because bidders must have a very well-defined scope of work. The lumpsum approach requires that the owner have a knowledgeable construction officer or field
observer on-site representing the owner to ensure compliance with the owner’s requirements.
1.3.4.6 Time and Material
Time and material contracts may be used in two situations:
•
The owner has longstanding experience and complete confidence in the contractor, and has
negotiated hourly rates, administrative and handling fees, and so on for the type of work to be
performed. Even with confidence in the contractor, the owner should still have a field
observer present to log hours spent, keep track of materials received, make sure
specifications are followed, and so on.
•
The project cannot be defined well in the design stage—there are too many unknowns and
the contractor will assign a very large risk adder if the project is to be done with a lump-sum
contract. The owner and contractor will agree upon the time and material rates and general
approach, and the contractor will use the best efforts to complete the assigned work. The
owner will be taking the risk, but generally the total cost will be lower than when the
contractor assumes all the risk of design changes.
1.3.4.7 Remarks on Project Responsibilities
Regardless of the contract type, if the contract is to be let in a manner that separates the
responsibility of cable installation from the construction of the ducts or pipes that will contain
the cables, then the owner may want to consider having a bid item in the cable installation bid
sheet for inspection and acceptance of the installed ducts or pipes. If the timing of the two
activities does not lend itself to this approach, the owner may want to consider hiring a cable
installation contractor or other qualified organization/individual as a consultant/inspector during
the civil (duct or pipe installation) work to minimize the chances of conflict between the
duct/pipe installer and the cable installer.
Note: Regardless of the project approach, it must be recognized that the owner has the
responsibility for reliable, long-term operation of the installed cable system. The contractor’s
responsibilities essentially end after the warranty period expires, and the owner should expect
many more decades of trouble-free service from the line. It is therefore critical that the owner
have good specifications, select a qualified contractor, and provide manufacturing inspections
and field supervision/field observers to ensure that the work is performed properly, regardless of
the form of the contracts during the design/installation work.
All of these approaches assume that both the owner and the contractor approach projects as
reasonable and fair organizations. In the past, some owners have been known to be unreasonable
in trying to force lower pricing on the contractor. That approach is soon understood by
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contractors, and the base bid for work with that owner will almost invariably be higher than a bid
for the same work with a reasonable owner. Similarly, if a contractor develops a reputation for
“low-balling” jobs and then demanding substantial extra charges, the contractor will probably
receive less business from owners even if the contractor is low bidder.
Some owners impose very high fees for liquidated damages, to encourage the contractors to
complete work prior to a scheduled completion date. Contractors will typically respond with
higher bid prices, to compensate for the risk that liquidated damages may be imposed. If the
owner does have a liquidated damages clause in the contract, it is often wise to also provide the
contractor with a financial incentive for completing the project before the guaranteed completion
date. It is also an important point in contract law that liquidated damages must be representative
of actual damages suffered by the owner, and not deemed to be “punishment”.
1.4
Conceptual Design and Permitting
Permitting agencies often require at the minimum a 50% cable system design as part of the
application, which means that the owner must perform significant levels of engineering at the
conceptual stage. The owner is normally responsible for securing government permits such as
county, state, and federal—including the Corps of Engineers. The construction contractor is
responsible for the construction permits at the city and local jurisdictional levels; for example,
the permits that allow the contractor to physically build the project.
This subsection of the report addresses items common to extruded dielectric, SCFF, and pipetype cable systems, unless indicated otherwise. Because SCFF cable systems are seldom used for
new installations on land, and because most of the construction and installation practices for
extruded dielectric cables also apply to SCFF cables, the report will describe only extruded
dielectric cables unless there is an area in which SCFF cables have different considerations.
1.4.1 General Approach for Cable Systems: Different Areas
Cable construction and installation methods can be substantially different depending upon the
cable system type and the mix of urban, suburban, and rural construction required. This
subsection of the document describes general approaches for each of the typical major areas.
Note that cable construction requires a significant right of way, typically a minimum of 30–50 ft
(9–15 m) for a single line. However, multiple lines might be required; especially if an overhead
line rating must be matched. Then, the right of way requirements might be significantly larger.
For installations in or next to roadways, a traffic lane might be used for movement of equipment
and materials. The required construction right of way is typically to provide a space of 12–16 ft
(4–5 m) trench edge to trench edge for multiple trenches, and owners might like to have space to
the edge of the right of way to reduce the possibility of encroachment. A narrower right of way is
suitable for short distances where the cable pipe or the ducts can be pulled into the trench or
carried down the right of way rather than set in place with a backhoe.
1.4.1.1 Construction in Urban, High-Traffic Areas
Subsurface obstructions, traffic control, speed of construction, manhole placement, allowable
work hours, and interfacing with local authorities strongly affect project schedule and cost. Over
the years, utilities serving major cities have developed construction techniques applicable to their
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unique conditions; for example, nighttime construction might be required, or construction might
be restricted to non-rush-hour periods. Routes for a new underground cable line can be extremely
difficult to identify in some major cities. In the United States, rerouting the lines to a less
congested area is typically required. Overseas, some countries use dedicated utility tunnels to
avoid problems with multiple buried utilities, and this is done occasionally in the United States.
Using utility tunnels is effective but more costly and time-consuming initially. However, cable
repairs can be made more quickly and easily than for trenched installations.
On several recent major projects, underground cable construction has been required in congested
urban areas with no previous transmission cable installations. In such areas, different
construction techniques might be required because of public involvement, nighttime construction
activities, and so on.
For underground transmission lines, pipe-type cables have historically been used in urban areas
because pipes can be installed more quickly than duct banks, and relatively long spacing can be
used between joints—reducing the number of manholes and splicing activities. The rugged steel
pipe also gives good protection against dig-ins. However, extruded dielectric cables in a
concrete-encased duct-and-manhole system have recently been installed with relatively long
pulling lengths in urban areas and continue to gain popularity for new installations.
1.4.1.2 Construction in Suburban, Medium-Traffic Areas
For construction in suburban, medium-traffic areas, permitting is often a major obstacle because
of residents’ concerns about construction activities. However, after construction work starts, it
can often proceed efficiently because roads are present for construction equipment and moving
materials, traffic requirements might not be as onerous as in urban areas, and fewer obstacles
might be encountered. Daytime construction is typical, although work hours might be restricted
because of traffic patterns.
Extruded dielectric and pipe-type cables are both appropriate for suburban areas. The selection
often depends on power transfer requirement (extruded dielectric cables have a higher rating than
self-cooled pipe-type cables, for the same conductor size), cost, speed of construction, and
environmental considerations. As of the early 2000s, the number of new extruded dielectric cable
installations has surpassed the number of pipe-type cable installations.
1.4.1.3 Construction in Rural, Low-Traffic Areas
Rural areas might seem ideally suited for underground cable installation, and this could be true in
areas such as farmland and plains. However, many rural areas are heavily wooded and have large
elevation changes and rock outcroppings. There will be challenges for movement of equipment,
materials, and personnel and for overcoming a hostile terrain or sensitive areas such as wetlands.
Extruded dielectric cables are common for this type of application. A duct-and-manhole system
is commonly used because of the protection provided by the concrete envelope, the ability to
quickly replace a cable section in the event of a failure, and the ability to reconductor the cable
system without having to excavate the entire route. However, direct burial might be feasible and
generally gives a 10–15% higher power transfer at lower cost. Note that in either case, an access
road is required along the length of the line for inspection, maintenance, and potential repairs, as
well as for cable system installation.
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1.4.2 Duct Bank Versus Direct Burial
Extruded dielectric cables are typically installed in a duct-and-manhole system in the United
States, primarily to limit the length of trench opening each day during construction and
installation in urban areas. In this way, a cable section can also be removed and replaced without
having to pinpoint the failure and excavate at the failure location, which minimizes disturbance
to the areas.
A duct bank usually contains a spare duct that can be used in the event of a cable failure that
causes damage to the duct. (There have been occasions in which testing of a duct bank before
cable installation showed that one or more ducts were unusable. Therefore, a spare duct was
required for the initial installation.) The owner should perform an economic analysis to
determine the cost of the spare duct versus the cost of repairing a duct in the unlikely event that it
is damaged, and the owner should take into account that the requirement for a spare duct might
prevent the optimum arrangement for active ducts in terms of ampacity and magnetic fields. If an
additional circuit is to be installed later, a spare set of ducts might be installed.
Direct burial for cable installation can be considered for rural and unimpeded suburban
installations in which several thousand feet or meters of trench could be kept open at one time.
However, removing the cables, or reconductoring, would be impractical and much more
expensive for a directly buried system. The owner should consider installing splices in manholes
even if the cables are directly buried, to facilitate maintenance and condition monitoring. Owners
prefer ownership or a dedicated easement of the direct buried alignment to provide protection of
the cables from encroachment. A robust road is required along almost the entire route to allow
for excavation equipment, concrete trucks, manhole placement if manholes are used, bringing
heavy cable reels to splice locations for installation into the trench, and so on. In areas where the
reels cannot be brought alongside the trench, cables can be pulled in the trench for several
thousand feet (meters) on rollers.
The planning and conceptual design should address the duct bank versus direct burial approaches
for routes in which direct burial is feasible. The latter includes cases where the route is very
irregular with many bends, as sometimes occurs in substations, or where available conductor size
is at an upper limit and ampacity must be maximized.
1.4.3 Special Installation Conditions
Although trenched installations account for a major part of the length of a transmission cable
system and there are occasional directly buried installations, special installation conditions can
be required, and they demand a significant amount of attention during design and construction.
These special conditions typically require additional permitting compared to a standard trenched
installation, they are more costly and time-consuming, and they can sometimes limit circuit
ampacity. These special installations must generally be designed on a project-by-project basis
because of their unique characteristics.
1.4.3.1 Substation Installations; Troughs
Installations within substations are commonly done with extruded dielectric cables, because of
the short length and the absence of extensive accessories. Although direct burial is sometimes
feasible, installation in a duct bank or trough is more common, the latter in view of the multiple
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buried grounds, communications lines, and so on within substations—both existing services to
contend with, and new services where excavation might cause damage to the cables. Cable
installations within substations are similar to those in street and suburban/rural areas, with a few
exceptions:
•
Although substations have an extensive ground grid, differences in potential may exist, and it
may be necessary to install a ground continuity conductor and sheath voltage limiters for
extruded dielectric cables. The owner should plan to install them unless an engineering study
shows they are not necessary.
•
Even though installing cables in troughs appears straightforward, it is prudent to fill the
trough with well-graded thermal sand to maintain the cable spacing, prevent excessive cable
motion during fault conditions, and increase short-term rating capacity. Cables should be
cleated to the bottom of the trough, or provisions should be made to have enough weight
above the cables (sand, trough cover) to prevent the cables from pushing the cover upward.
•
If the trough is unfilled, the cable design and installation should consider the proper degree of
snaking to allow thermal expansion and contraction. The cables should be cleated.
•
Cables may be installed in trefoil, lashed together, or cleated, which will reduce induced
sheath voltages and external magnetic field but will generally reduce cable rating. Most
installations in the United States have the cables installed flat, with spacing of one cable
diameter or greater between the cables.
Trough installations are not limited to substations; troughs may be used in other areas where
directly buried or duct bank installations are not desired. There have been discussions of placing
the trough on the ground surface, installing the cables, backfilling the trough with thermal sand,
and then berming around the trough to create a bermed area rather than having the trough
exposed.
1.4.3.2 Bridge Installations
Extruded dielectric, SCFF, and pipe-type cables have been installed on bridges. Bridges can be a
good alternative to submarine cable crossings, but there are many detailed considerations for
bridge installations, including the following:
•
Some transportation departments prohibit installing cables on bridges, partly because of
concern about disrupting traffic for maintenance/repair of the cables.
•
Both steady-state and transient cable ratings must be evaluated. Solar radiation can reduce
the ampacity, and the absence of the earth’s long thermal time constant can result in
unacceptably low transient ampacity ratings.
•
The cables should be located out of direct sunlight and away from areas where road or sea
salts can damage jackets.
•
Existing bridges may not be capable of accepting additional attachments such as cable
systems. Detailed structural analysis is mandatory for both new and retrofit installations,
requiring bridge design drawings.
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•
Access to the cable system for construction, inspection, maintenance, and repair must be
considered, and should not unduly affect vehicle or pedestrian traffic. Special consideration
should be given to designing access for cable feeding, cable pulling, and splice locations,
especially on long bridge structures.
•
Duct, cable, or pipe supports should be designed to accept cable weight plus expansion forces
and movement, as well as vibration and bridge movement.
•
Extruded dielectric and SCFF cables may be installed in trefoil configuration—especially if
installed in troughs—to reduce external magnetic fields and induced sheath voltages.
•
Cable system expansion and contraction is a major consideration. Expansion bends are the
most common approach, and the cable system must be designed to accommodate expansion
joints in the bridge itself. Bridge vibration should also be taken into account and proper
supports designed, because vibration can cause cable movement and sheath fatigue.
•
Extra-heavy-wall fiberglass ducts are preferred for extruded dielectric cables, because of
their greater strength and low coefficient of expansion. If metallic ducts, metallic messenger
with rings, or other metallic elements are used, electrical isolation and proper bonding must
be ensured to prevent any possibility of induced circulating currents.
•
The energy released during a cable or joint fault could cause a fire and possibly injure bridge
maintenance workers or cause structural damage to the bridge. The bridge might not have the
seismic withstand required of a strategic cable circuit, particularly if the bridge is older.
•
Circulating currents could be induced into parallel metallic bridge components, possibly
causing safety hazards or ac corrosion. Ground potential rise could be transferred from the
external utility system to bridge components during internal or external faults. For new
bridges, it can be advantageous to have a utility tunnel or cast-in-place ducts installed to
accommodate future installations of power cables as well as other cable types.
•
Coatings for pipe-type cable pipes are very important because exposed pipes, especially
those subjected to salt from road-deicing, can corrode readily. Epoxy paint is commonly used
for these sections. Cathodic protection systems need to be specially designed for the bridge
crossings, and isolation from the bridge structure is important.
1.4.3.3 Tunnel Installations
Utility tunnels are commonly used for transmission cable installations under congested streets
with multiple utilities. Many of these tunnels are constructed specifically for the transmission
cable system. A few utilities in the United States have installed transmission cables in utility
tunnels that may contain other services—natural gas lines, communication lines, petroleum
pipelines, water lines, and so on. There are many tunnel installations at hydroelectric generating
stations, to transfer power from the generators at the base of the dam to switchyards at the top.
Although conventional tunneling procedures are still followed, microtunneling equipment and
tunnel boring machines are becoming more common. In some cases, it may be feasible to opencut a large trench, install the tunnel walls in prefabricated sections, and backfill around the tunnel
walls.
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Tunnel installation requires a great deal of detailed design, taking into account considerations
such as the following:
•
Ampacity calculations are complex because of convective and radiation heat transfer from
the cables to the tunnel walls, and to the air in the tunnel that is sometimes circulated.
Multiuse tunnels often have other heat sources, and their effect on cable rating depends upon
the level of air flow, if any. Tunnels typically are designed with one end lower than the other,
giving rise to natural convection. The convection can be measured and used to refine feeder
ratings.
•
Multi-use tunnels give rise to concerns about induced voltages and induced currents on other
utilities in the tunnel.
•
Temperature and fire monitoring and control are necessary for tunnel installations, to
minimize the unlikely burning of the impregnated paper insulation and dielectric liquid in
HPFF and SCFF cables and the jacket and insulation of SCFF and cross-linked polyethylene
(XLPE)-insulated cables.
•
Cable, duct, or pipe support is a critical part of the cable system design, and thermomechanical movement must be considered, as well as the weight of the cable systems.
Extruded dielectric and SCFF cables are typically “snaked” to allow for thermal movement,
and the design of the cable offset and separation of clamps is important.
•
Careful attention must be paid to supporting the ducts/pipes and cables in tunnel shafts.
Multiple clamps are required for extruded dielectric and SCFF cables. The cables themselves
must be clamped at appropriate intervals to support the cable weight and control thermalmechanical effects.
•
Pipe-type cables typically have long-lay flat strapped stainless steel tapes that are attached to
anchor joints at the top of the shaft to help support the weight of the cables. Skid joints at the
bottom of the shaft allow the cable to expand and contract with load changes.
•
Tunnel system design must also consider the methods for installing the cable itself—
controlling its entry into the duct/pipe in the tunnel shaft, while providing the pulling
winches to install the cables in the horizontal tunnel sections. Dedicated cable feed-in tubes
are often installed in shafts in consideration of future construction and repairs.
•
Because splicing may take up to a week or longer for each splice, provisions need to be made
for long-term presence of personnel in the tunnel, and rapid egress in the event of an incident.
•
For HPFF cables, the pipe fluid-filling operation must be staged carefully to avoid too-rapid
pressurization, which may cause creasing of the cable papers for the tunnel sections of cable
as well as any other section with large elevation changes. In extreme cases, the creasing may
be severe enough to damage the cable insulation system. Pressure during fluid filling should
be measured at the lowest elevation.
•
Provisions are needed for corrosion protection for pipe-type cable pipes, similar to those for
pipes on bridges.
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1.4.3.4 Crossing Waterways, Wetlands, Highways, and Railroads
If environmental regulations permit trenching, crossings of short waterways (streams, creeks)
can be accomplished most inexpensively by installing temporary diversion pipes and damming
the waterway, and then using conventional trenching techniques. Alternatively, half of the
waterway may be dammed, with full water flow on the other half, and trenching done on the
downstream side of the dam. That dam is then removed, the other half dammed, and trenching is
completed. Riprap or other stone is used to prevent erosion of the trenched areas.
In the last 10 years, horizontal directional drilling (HDD) has become the preferred method for
crossing waterways and wetlands as long as a mile and a half (2.4 km). Installing cable on the
water bottom (typically with embedment) may be acceptable for some waterways, if
environmental requirements can be met. This is common for long sea-bottom installations.
Jack-and-bore methods have been frequently used, but horizontal directional drilling can be
faster and less costly, if there is enough room for pits and equipment at each side of the crossing,
and ground conditions are suitable (for example, there are no large boulders). Both highway and
railroad authorities almost always require a crossing perpendicular to their facilities, rather than
an angled crossing that might be more efficient for the cable system. Railroads typically require a
steel casing, although other high-strength casings may be permitted.
Cable installations along railroad rights of way might proceed more slowly and at greater
expense than city street installations, because of railroad requirements such as the possible need
to remove construction equipment from the right of way when trains pass. Owners should have a
thorough understanding of railroad requirements before considering the use of railroad rights of
way.
1.4.3.5 Extreme Elevation Changes
Extreme elevation changes, especially those associated with rock outcroppings, can be a major
challenge for buried cable systems. Extruded dielectric cables may require additional vaults
along the route, with cable clamps securing the cables in the vaults. The number and location of
the vaults is determined by the incline and distance, cable weight, and coefficient of friction
between cable and pipe. Because these calculations affect the cable performance and lifetime,
they are best performed, or at least verified, by the cable supplier.
Pipe-type cables can have long-lay stainless steel ribbon tapes applied under the skid wires, to
help the conductor support the weight of the cable. This cable is commonly known as a riser
cable. The stainless steel tapes are attached to stainless steel plates at an anchor joint at the top of
the slope, and they are attached to another stainless steel plate in a skid joint at the bottom of the
slope. This approach is common for tunnel shafts and long sloping tunnels such as the ones
sometimes found in hydroelectric generating stations.
1.4.3.6 Trenchless Techniques, Including Horizontal Directional Drilling
Trenchless installation has been used for specialized transmission cable installations for many
decades—as typified by jack-and-bore or liner-plate tunnel techniques for railroad and highway
crossings. In recent years, horizontal directional drilling (HDD) has emerged as the preferred
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installation approach for many projects. HDD may be the only feasible approach for installations
such as long river crossings, but it is seeing increasing usage for land installations to speed the
approval of projects and reduce traffic disruption on streets and roads.
Trenchless installations require careful attention to detail in site-specific design and construction,
as does any nonstandard approach. The type of trenchless equipment depends, among other
things, upon the required bore diameter, the length of the bore, the water table depth, and of
particular importance, the type of ground material that will be encountered along the bore path.
1.4.3.6.1 Horizontal Directional Drilling
HDD allows installing pipe-type cables long distances (up to 7400 ft [2255 m] as of 2010)
without disturbing the surface. This is ideal for long water crossings and wetland crossings, and
has been used for installing cables under roadways without disturbing traffic. The project’s
permitting time may be greatly reduced if HDD is chosen for sensitive areas.
As for any special installation method, a great deal of engineering design must be performed, and
specialized construction approaches are required. In a typical operation, a pilot hole is drilled
along the designed bore path, the hole is back-reamed to give a diameter that may be 1.5 times
that of the product pipe (casing, bundle of ducts, or cable pipe), and the product pipe is pulled
into the bore using the drill rig. Major construction considerations are as follows:
•
Geotechnical borings are required, approximately every 500 ft (150 m) along the proposed
alignment, depending on known ground conditions. The HDD designer uses the data to
determine the drill path, type of drilling equipment, drilling fluids, and so on. Soil thermal
analysis is performed on the samples to be used in the cable conductor sizing. Adequate work
area is required on each side of the HDD crossing.
•
Adequate pipe/duct laydown area is required on one side of the HDD crossing. Ideally, the
duct or pipe can be preassembled into one length so it can be pulled into the bore without
stopping, but it is often necessary to assemble two or more strings, and stop the pullback for
several hours to attach two sections together and test the attachment. This can increase risk of
the product being stuck in the borehole after the pause.
•
A significant quantity of fresh water is required, to be used in the “mud” mix for the drilling
operations. Provisions need to be made to handle inadvertent returns, or “frac-outs,” if the
drilling mud rises to the surface during the boring operation.
•
Bentonite, an important component of the drilling mud, is naturally occurring clay, but some
jurisdictions have limitations on its disposal.
•
Cable type selection and conductor sizing require careful attention. Extruded dielectric cables
may not be feasible for the longest HDDs, for several reasons:
− The borehole will be larger in diameter for a pipe-type cable; the larger-diameter bore
takes more time, is more costly, and has greater risk.
− For bores longer than about 3000 ft (915 m), depending upon voltage class, the reels
required for extruded dielectric cables may be too large for shipping by road, whereas
more than 7000 ft (2135 m) of 230 kV pipe-type cable can be shipped.
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•
•
− Because of the heavier weight and higher coefficient of friction, extruded dielectric
cables cannot be pulled in ducts for the long distances that are possible with pipe-type
cables in steel pipe.
− Extruded dielectric cable is typically single-point bonded to preserve ampacity; the
standing sheath voltages may be above the owner’s allowable limits. As an alternative, a
very-low-resistance shield/sheath could be provided, allowing sheath currents to
approach the value of conductor currents. The losses would be low because of the low
resistance, and sheath voltages would be low. The resulting low magnetic fields may
permit using a steel carrier pipe.
Ampacities of cables in HDD installations may be lower or higher than for trenched sections
of the same cable. Sometimes the lower thermal resistivity, lower ambient earth temperature,
long thermal time constant, and absence of external heat sources will outweigh the effect of
greater depth. It is usually not feasible to add low-thermal-resistivity material to the bore, in
the manner that is possible for a trenched installation.
The ducts for an extruded dielectric installation may not be able to withstand the high head
pressure of soil overburden, water, and drilling fluid. Filling long lengths with grout is
difficult and makes cable removal extremely difficult. Some owners have filled the ducts
with water, which balances at least some of the external pressure and also increases ampacity
slightly.
1.4.3.6.2 Auger Boring
Auger boring, sometimes called jack-and-bore, is used primarily to cross under roadways,
railroads, and other fairly limited-distance obstacles, at distances up to 100–200 ft (30–60 m).
Entrance and exit pits are dug on either side of the crossing to provide access for the boring
equipment. The equipment consists of a cutting head that is attached to a helically wound auger.
The cutting head is advanced into the hole along with a steel, concrete, or composite casing pipe
to protect the excavation from cave-in. Auger boring is not considered suitable for rock.
Construction considerations are as follows:
•
Very little deviation in straight alignment is possible.
•
A fairly large pit will be needed, especially at the entrance end.
•
The pit needs to be slightly deeper than the bore, and dewatering may be required.
•
The area inside the casing is almost always grouted.
•
Ampacity may be reduced if a steel casing is used for extruded dielectric cables, sometimes
requiring a larger cable for the manhole-to-manhole section containing the bore.
1.4.3.6.3 Pipe Jacking and Utility Tunneling
These methods of trenchless installation require worker entry into the bore, so they are restricted
to bore diameters of 48–72 in. (1.2–1.8 m) or larger. Distances can range from less than 100 ft
(300 m) to thousands of feet (meters). Utility tunneling is similar to a mining operation; a liner is
constructed by the workers as the excavation progresses.
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Considerations include the following:
•
Careful measures are required to ensure worker safety; effective dewatering is mandatory.
•
For typical casing diameters, the depth below the surface should be 10 ft (3 m) or greater to
avoid soil displacement or settling that could affect the roadbed or other surface structure.
•
Pits and intermediate jacking stations may be needed if the required thrust for pipe jacking
becomes too large.
•
The cable ducts or pipes are typically placed on wheeled spacers.
•
The inside of the pipe for pipe jacking is almost always grouted. The utility tunnel may be
treated as a tunnel installation if it is large enough, with the resulting access requirements,
need for permanent ventilation and possibly constant air flow.
1.4.3.6.4 Micro-Tunneling
Micro-tunneling includes jacking a casing pipe into the bore behind a micro-tunnel boring
machine (MTBM). The MTBM cuts a bore tunnel through the soil while the casing pipe is
jacked in behind it. Large entry and exit pits are required as well as support equipment above
grade at the entry pit. After the entry and exit pits have been excavated and prepared, jacking
plates, a rail system to support the MTBM, and a laser guidance system are installed in the entry
pit. The MTBM is lowered into the pit and pushed forward by the hydraulic jacking system via
the casing pipe. Micro-tunneling works well in most soil conditions except soils with boulders,
and dewatering may not be necessary even if the bore is below the water table. Duct, pipe, and
cable considerations are similar to those for pipe jacking and utility tunnels.
1.4.3.7 Mechanical Laying
Mechanical laying consists of opening the trench and simultaneously installing the cables—and
possibly placing controlled backfill (concrete mix) over them in the same operation. It applies
only to extruded dielectric and possibly SCFF cables, and communications cables. The ground
continuity conductor can be installed along with the power cable. Concrete encasement is
recommended to provide mechanical protection and dissipate heat. Considerations include the
following:
•
This approach is specifically applicable to direct burial, but could conceivably be applied to
installation using reels of high-density polyethylene (HDPE) duct.
•
Trefoil arrangement of the cables is preferred, to minimize trench width, external magnetic
fields, and induced sheath voltages. However, trefoil arrangement may have slightly lower
ampacity because of the higher mutual heating in comparison to cables laid in flat formation.
•
This approach is suitable for open land, where there are no shallow crossings of third-party
utilities and the progression can proceed unimpeded.
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1.4.3.7.1 Embedding Underwater Cables
If environmental restrictions permit installing cables directly in the river bottom, this approach
might be quicker and less costly than HDD. Embedding below the water bottom by dredging was
done for many water crossings in the past, but it is difficult to obtain permits for such an
operation today.
1.4.3.7.2 Using Existing Structures
This topic refers to using existing ducts or pipes for reconductoring to increase voltage or
current, replace an aged cable with a new one, and replace a pipe-type cable having a corroded
pipe with extruded dielectric cables that require no pressurization, and so on. Reconductoring an
existing duct system with new extruded dielectric or SCFF cable requires attention to physical
size to ensure that the new cable will fit with adequate clearance, enable the removal of the old
cables and the installation of the new cables, ensure the acceptability of existing splice manholes
or splice bays, and ease the calculation of the rating of the new cable system.
Replacing an HPFF pipe-type cable with an extruded dielectric cable has some additional
challenges:
•
The existing dielectric liquid must be tested, drained, and disposed of properly. Depending
upon the amount of benzene present, the fluid might be considered a hazardous waste and
require special handling. The existing paper-insulated cable must be removed and disposed
of properly. Any remaining dielectric liquid must be removed from the pipe.
•
Allowable pulling lengths for extruded dielectric cable in a steel pipe may be shorter than
those for the original pipe-type cable (because of heavier cable and higher coefficient of
friction), so additional manholes may be required. Because of the greater insulation thickness
on an extruded dielectric cable, the allowable conductor size may be smaller.
•
The ampacity will be lower than for a directly buried or duct system because of the smaller
conductors, the proximity of the three cables in the steel pipe, sheath losses that are likely to
be much higher than for a pipe-type cable, and heating in the steel pipe itself due to
hysteresis and eddy currents.
1.4.4 Routing
Cable routing can be one of the most critical activities in the conceptual design of a cable system.
The following considerations might dictate selecting a different route or adjusting construction
methods (for example, using a horizontal directional drill to bore beneath wetlands):
•
Wetlands and waterways
•
Areas with archaeological or historical significance
•
High-rock areas and steep inclines
•
Railroad and major highway crossings
•
Bridges
•
Recently paved streets
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•
Streets with high traffic density (posing a need for traffic control or short work hours)
•
Extreme subsurface congestion
•
School areas, playgrounds, and public safety facilities, such as police and fire stations and
hospitals
In addition to extending the costs and construction time, the route can affect the time needed for
permitting a line. These requirements can have a large effect on overall project cost and
schedule.
The owner must generally evaluate several potential routes in the planning/permitting stage and
might even require a contractor to bid on two or more alternate routes in the bidding stage. The
shortest route is usually the least expensive, and the preferred overhead route is sometimes the
preferred route for undergrounding as well. However, other routes might allow quicker
permitting or faster, less expensive construction.
Aerial surveys offer a valuable way for those involved in route analysis and conceptual cable
system design to evaluate the type of area that could be considered for construction, identify
potential obstacles (both natural and manmade), and quickly evaluate potential alternate routes.
The aerial surveys alone are not generally sufficient—once the aerial surveys are reviewed to
determine potential routes, the route should be walked or driven to provide a close-up analysis of
construction and installation considerations. Some owners retain an experienced construction
contractor to walk/drive the route with the owner’s engineer, because the contractors often
observe obstacles or see potential improvements that the owner might not notice.
Considerations for route selection include the following:
•
The need for the route to comply with the permits required by the controlling agencies,
franchise area, existing easements, and other existing rights of way
•
Right of way requirements for construction and maintenance
•
Access requirements
•
Minimum duct or pipe bending radius and alignment as it affects pulling tensions
•
Density of other subsurface utilities
•
Manhole placement, including absence of overhead structures and trees
•
Difficulties in handling elevation changes and obstacles such as wetlands, railroads, and
highways
•
Magnetic field considerations
•
Route length
•
Thermal coordination—to avoid underground heat sources such as steam mains and other
electrical circuits
•
Special work restrictions
•
Construction constraints; constructability
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•
Seismic and liquefaction impact if applicable
•
Route diversity for multiple circuits
•
Common-mode failure analysis
1.4.5 Surveys; Obstacle Detection
The American Society of Civil Engineers (ASCE) has developed a guide for collecting existing
subsurface utility data [4]. The ASCE guide establishes four levels of data to determine the
accuracy of the underground facility locations. These levels are as follows:
•
Utility Quality Level A. Precise horizontal and vertical location of utilities by the actual
exposure and subsequent measurement of the existing subsurface facility.
•
Utility Quality Level B. Information obtained through the application of appropriate surface
geophysical methods to determine the existence of subsurface utilities—methods such as
electromagnetic, magnetic, elastic wave, and other high-cost specialized methods.
•
Utility Quality Level C. Information obtained by surveying and plotting visible aboveground
utility features.
•
Utility Quality Level D. Information derived from existing records or oral recollections.
Because owners might need to evaluate many potential alignments, they might try to do a
minimal amount of surveying and obstacle detection during the planning and conceptual design
stage. If this is the case, Level D—in addition to walking the potential alignments—might be
sufficient at the planning stage. In addition to existing records, aerial photographs are valuable in
evaluating potential alignments, especially in suburban and rural areas. It might be necessary to
undertake a Level C analysis for the final two or so routes that are to be presented to the
permitting agency.
After the alignment is selected, a detailed analysis is performed to locate potential obstacles
along the route to provide Level A data:
•
Subsurface utility engineering (SUE) is performed by the contractor along the entire route.
SUE is the engineering process that identifies, characterizes, and maps underground utility
facilities. It includes the three major activities of designating, locating, and data management.
These activities—when combined with traditional record research, coordination with utility
owners, and site surveys—provide utility information for use during project development and
design.
•
Designating services are performed by the contractor. Designating is the engineering process
of determining the presence and horizontal and vertical (if possible) location of underground
utilities using geophysical prospecting techniques, including electromagnetic, magnetic,
sonic, or other energy fields.
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•
Locating services are then performed by the contractor. Locating services include excavating
test holes using vacuum excavation or comparable nondestructive equipment and digging test
pits at critical points along a subsurface utility’s path, exposing the utility and thereby
allowing precise measurements of vertical and horizontal position.
•
Some owners have had success using ground-penetrating radar (GPR) to locate utilities, but
difficulty in interpreting results has limited the use of this approach. However, GPR is useful
in identifying the depth to rock and the corresponding amount of rock removal required. This
information is useful during bidding.
1.4.6 Manhole Placement
Specific cable section lengths and manhole placements are not often determined during the
planning and conceptual design stages, but it is important to have an idea of “typical” section
lengths between manholes and to investigate potential constraints on manhole placement. Local
rulings might state, for example, that the manholes must be off the street, even though the cable
trench itself might be in the street right of way, or that manholes should not be placed in
intersections or in front of fire stations. It is also important to evaluate unusually long section
lengths such as river crossings, because they can affect delivery time and even availability of
cable.
Extruded dielectric cable manholes present more of a challenge than do pipe-type cable
manholes for multiple line installations—in addition to the larger manhole size, many owners are
reluctant to put two extruded dielectric lines in a common manhole because of concerns about
personnel working on a de-energized line while another line in the same manhole is energized.
Commutating failures between circuits is also a reliability consideration.
1.4.7 Traffic Control Plans
Traffic control plans are required for almost all cable installations in urban areas as well as in
some suburban areas. For larger projects, the owner or contractor may need to retain an outside
consultant organization that specializes in preparing traffic control plans. That organization will
meet with city (or local) authorities and the department of streets and highways to forecast all
lane closures that might be needed throughout different stages of the project and lay out
markings and barriers for traffic control along the cable alignment.
1.4.8 Cable Length, Reel Size, and Transportation Constraints
Owners typically want to have the maximum feasible length between splices to reduce the
number of pulls and splices, lower project costs, cut installation time, and increase reliability.
Care must be taken during the conceptual design stage to use reasonable section lengths. The
lengths might be constrained by factors such as the following:
•
The length of cable that can fit on a reel
•
Transportation restrictions for large reels
•
The weight of the cable and reel
•
Pulling tensions and sidewall pressures
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•
Sheath voltages for extruded dielectric cable
•
Access to bring the reels to the pulling location, which is a constraint that must be considered
for installations that are not on or next to streets or highways
Cable construction details will affect cable weight and outside diameter, and some manufacturers
will require less clearance between the top cable turn and the reel lagging. In some cases, larger
reels can be shipped with, or perhaps without, special permits; this is an important area that must
be evaluated on a project-by-project basis.
These considerations might dictate the type of cable and the conductor size that are feasible for a
project. For example, pipe-type cable can usually be pulled twice as far as extruded dielectric
cable in a directional drill, so a project with a long directional drill might be limited to pipe-type
cable. For extruded dielectric cable, a larger conductor size might be required due to the higher
pulling tension it permits, for example for very long pulls.
Note that larger reels with longer cable lengths may be shipped by truck if special permits are
obtained, and larger reels may be transported by rail or ship.
1.4.9 Thermal Property Testing
Thermal properties of the soil surrounding the cable have a strong effect on cable rating and can
even make the difference between one cable per phase and two cables per phase, which creates a
major cost difference when preparing the conceptual design for a transmission line. However, it
is uncommon to have a full thermal survey performed until the final one or two potential
alignments are selected. The subsection of this report “Geotechnical and Soil Thermal Tests”
provides more detail on thermal surveys.
1.5
Schedule
Schedules are developed at several stages of the transmission cable project, with increasing
levels of detail and accuracy. Typical stages are as follows:
•
Planning stage. The general schedule, with completion year (typically May or June of the
target year, so the line is in service before the summer peak loads). Duration of the
approval/permitting process is the major unknown and can be several years.
•
Conceptual design. The schedule by major task—design engineering, specification,
procurement, construction, and energization.
•
Detailed schedule during bidding and implementation. Developed by the contractor and
reviewed/approved by the owner, giving each step in the construction operation, with
interrelationships and critical paths.
•
Two- or three-week “look ahead” schedule during the implementation. Used by the
contractor for resource allocation during the project and by the owner for short-term
oversight.
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1.6
Specifications
The owner’s specification requirements for selecting material suppliers and installation
contractors depend on the approach to the project. The following discussion refers to a designbid-build project, which requires the greatest detail in specifications. Other approaches require
less specification detail on the owner’s part.
1.6.1 Specifications and Standards
The following specifications and standards are generally applied.
1.6.1.1 Cables
Extruded dielectric cables are specified in accordance with Association of Edison Illuminating
Companies (AEIC) specification AEIC CS-9, Specification for Extruded Insulation Power
Cables and Their Accessories Rated Above 46 kV through 345 kVac. Note: CS-9 describes the
complete cable system. The owner, designer, and installer must keep in mind that the goal is the
successful long-term operation of the complete system, which is especially critical at voltages
above 138 kV ac.
The following two standards published by the International Electrotechnical Commission (IEC)
may also be referenced:
•
IEC 62067, Power Cables with Extruded Insulation and Their Accessories for Rated
Voltages Above 150 kV (Um = 170 kV) up to 500 kV (Um = 550 kV) – Test Methods and
Requirements
•
IEC 60840, Power Cables with Extruded Insulation and Their Accessories for Rated
Voltages Above 30 kV (Um = 36 kV) up to 150 kV (Um = 170 kV) – Test Methods and
Requirements
Standards published by the Insulated Cable Engineers Association (ICEA) include the following:
•
ICEA S-108-720, Standard for Extruded Insulation Power Cables Rated Above 46 Through
345 kV
•
ICEA P-45-482, Short Circuit Performance of Metallic Shields and Sheaths on Insulated
Cable
•
ICEA T-34-664, Guide for Conducting Longitudinal Water Penetration Resistance Tests on
Longitudinal Water Blocked Cable
Pipe-type cables are specified in accordance with AEIC CS-2, Specification for Impregnated
Paper and Laminated Paper Polypropylene Insulated Cable, High Pressure Pipe Type.
The following guides published by the Institute of Electrical and Electronics Engineers (IEEE)
may be used for the cable system tests after installation:
•
IEEE Standard 400.1, Field Testing of Laminated Dielectric, Shielded Power Cable Systems
with Direct Current Voltage (for pipe-type systems)
•
IEEE Standard 400.3, Guide for Partial Discharge Testing of Shielded Power Cable Systems
in a Field Environment (for extruded dielectric systems)
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AEIC CS-9, IEC 60840, and IEC 62067 also include a description of after-installation tests.
It is common for the owner’s cable specification to reference these standards but also include a
section providing specific owner requirements.
1.6.1.2 Splices
Splices for transmission-voltage extruded dielectric cables are almost always proprietary designs
provided by the cable or accessory manufacturers. It is not common for the owner to specify
details of the splices. AEIC CS-9 details the specification of extruded cable accessories.
However, standards do require that the manufacturer demonstrate successful testing of the
splices with the cable system. In addition to IEC 60840 and IEC 62067 listed above, the
following IEEE standard is typically referenced:
•
IEEE Standard 404, IEEE Standard for Cable Joints for Use with Extruded Dielectric Cable
Rated 5000 V Through 138,000 V and Cable Joints for Use with Laminated Dielectric Cable
Rated 2500 V Through 500,000 V
Splice specifications are included in IEEE 404. In addition, a splice specification might have the
following requirements:
•
Manufacturer’s catalog cuts and data information for the splices (and casings, including
valves to be installed on the casing, for pipe-type cables)
•
Splice drawings, both detail and general arrangement describing each component, including a
listing of radial and longitudinal stresses
•
Test data showing that a splice of the proposed design withstands a high-voltage time test
and impulse test as specified in the latest version of AEIC CS-9 (extruded dielectric cables) or
CS-2 (pipe-type cables) for the appropriate voltage cables
•
A sample instruction book for a splice of this type
•
A detailed bill of material covering all components in the splice kit
•
Shelf storage life of components
•
A list of special tools required for installation
Splice kits for pipe-type cables from a qualified splice supplier may be applied to cable from a
manufacturer, assuming that the splice design and quality control are acceptable. Note, however,
that splices may have proprietary features that should be considered when evaluating designs.
1.6.1.3 Terminations
Extruded dielectric terminations are typically provided by the cable and accessory
manufacturers. It is not common for the specification to require details of the termination. AEIC
CS-9 details the specification of extruded cable accessories. However, an owner will generally
specify that the terminations pass a series of tests with the cable on which they will be installed.
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In addition to referencing IEC 60840 and IEC 62067 listed above, the owner might also
reference one of the following:
•
IEEE Standard 48, Standard Test Procedures and Requirements for High-Voltage Alternating
Current Cable Terminations
•
IEEE Standard 1300, IEEE Guide for Cable Connections for Gas-Insulated Substations
•
IEC 60859, Cable Connections for Gas-Insulated Metal-Enclosed Switchgear for Rated
Voltages of 72.5-kV and Above
For pipe-type cables, a manufacturer’s termination of the appropriate size and voltage class may
be applied on a cable from a cable manufacturer. In addition to the requirements of IEEE 48, a
specification for pipe-type cable might have the following requirements:
•
Manufacturer’s catalog cuts and data for the terminations
•
Outline drawings showing overall dimensions, location, and identification of all components
as well as assembled weight
•
Mounting dimensions in sufficient detail to allow support structure design
•
Test data showing that a termination of the proposed design withstands a high-voltage time
test and impulse test as specified in the latest version of AEIC CS-2 for the voltage cables on
which the termination will be installed
•
A sample instruction book for a termination of the type being purchased
•
A detailed bill of material covering all components in the termination kit
•
Recommended spare parts and shelf life
1.6.1.4 Other Accessories
Extruded dielectric cables (and SCFF cables as well) require link boxes and sheath voltage
limiters to accomplish sheath bonding and grounding.
High-pressure fluid-filled pipe-type cables require pressurizing plants to maintain the nominal
200 psig (1380 kPa) design pressure on the cable system while accommodating fluid expansion
and contraction caused by temperature changes. There are no standard industry specifications for
these pressurizing plants, although some cable-using utilities are willing to share their
specifications—this will form a starting point for preparing a new specification.
The dielectric liquid required for a high-pressure fluid-filled pipe-type cable is specified in AEIC
CS-31, Specification for Electrically Insulating Pipe Filling Liquids for High-Pressure PipeType Cable. High-pressure gas-filled pipe-type cables require a nitrogen gas control unit to
monitor and maintain the nominal 200 psig (1380 kPa) design nitrogen pressure. Nitrogen gas
for a high-pressure gas-filled pipe-type cable is specified in accordance with ASTM D1933,
Standard Specification for Nitrogen Gas as an Electrical Insulating Material.
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Pipe-type cables also require cathodic protection systems and safety grounding systems. The
National Association of Corrosion Engineers standard, NACE SP0169:2007 – Control of
External Corrosion on Underground or Submerged Metallic Piping Systems, provides good
general information.
1.6.1.5 Installation Specifications
The industry has developed installation specifications that provide good general guidance, but
most projects require owners and planners to be familiar with detailed specifications that are
specific to the work being performed. The latest revisions of industry texts as of mid-2010 are
the following.
•
AEIC CG5, Underground Extruded Power Cable Pulling Guide. This guide gives the
equations needed to calculate the cable pulling tensions before installation.
•
AEIC CG12-05, Guide for Minimizing the Cost of Extruded Dielectric Shielded Power
Cables Rated 5 through 46 kV. This guide is intended for distribution cable systems, but
some of the concepts could certainly apply to transmission cable systems.
•
AEIC CG3-2005, Guide for Installation of Pipe-Type Cable Systems. This guide provides
construction techniques for the installation of high-pressure fluid-filled and high-pressure
gas-filled pipe-type cable systems.
•
AEIC CG4-97, Guide for Installation of Extruded Dielectric Power Cables Rated 69 kV
through 138 kV. Similar to AEIC CG3, AEIC CG4 outlines construction techniques for the
installation of extruded dielectric cable systems.
1.7
Safety
Safety is an important topic that is the responsibility of each owner to address in its operations,
specifications, and contractual dealings with construction contractors. Owners often review a
potential contractor’s safety procedures, the group within the contractor’s organization
responsible for monitoring safety performance, and the experience modification rate (EMR)
statistics as part of the bid review process to verify that the contractor has good procedures and a
good safety record.
Safety involves a broad spectrum of activities in general, including construction site and cable
system safety, employee and contractor safety, and public safety. The owner should pay careful
attention to all safety issues and requirements, from the design stage through cable operation and
maintenance. Owner and site-specific safety practices should be followed.
The owner should have a company-wide safety program that provides all employees, contractors,
and vendors with education and training in all general and specific aspects of safety requirements
and regulations that may apply to the underground transmission project.
Proper adherence to all owner, company, and OSHA safety requirements including state
additions, contractor safety requirements, and requirements issued by other relevant authorities is
of paramount importance on an underground transmission project. On-site safety audits should
be conducted periodically on construction sites to find any potential safety problems and to
correct those problems before they have a chance to cause an incident or accident.
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An underground transmission installation or maintenance project involves a complex mix of
electrical, civil, and mechanical construction methods and techniques, which leaves exposure to
an accident or incident a possibility. The following is a sampling of general safety requirements:
•
Workers on an underground transmission construction project should always wear the
required personal protective equipment (hard hat, safety glasses, and safety boots; safety
vests when involved in street work; and so on). Some owners also require workers to wear
arc-flash resistant clothing.
•
Electrical hazards exist, especially on projects involving maintenance of existing lines.
Existing cables must be de-energized, discharged, and grounded before any work is started.
Work in a substation requires a conscious awareness of all the energized equipment in the
work area and the clearances (distance) required to work safely.
•
Trenches and other excavations need to be sloped, benched (stepped), sheeted, or shored
when required by regulations based on depth and type of material. Methods for access and
rapid egress from these areas must be provided.
•
Manholes may be treated as confined spaces and require air quality testing and forced
ventilation before entering, as well as air quality monitoring whenever personnel are present.
Workers entering a manhole may be required to wear a harness attachable to a rescue
retrieval device.
Work in roadways has safety issues that may require special signage, a flagman, cones, and an
approved maintenance of traffic plan. Many of the construction and installation activities require
the use of heavy construction equipment. Care should be taken to protect the workers and the
public from any incident that might involve this equipment.
Having a good safety program that is accepted and adhered to by employees, contractors, and
others will significantly decrease the chance of exposure to an accident. Steps should be taken to
keep the cable system itself safe from intrusion by others. A summary of steps that should be
taken to protect the cable system follows:
•
If the intruding utility produces heat (for example, from a distribution duct bank or a steam
main), the derating effect on the new transmission cable should be evaluated. In extreme
cases, heat pipes or a venting manhole may be required. Thermal barriers (thermal insulation)
might be effective, but in some cases they could cause further derating of the transmission
cable. A thermal analysis is required before a thermal barrier is specified.
•
The owner should ensure proper separation from other services wherever possible. Proximity
to water mains presents special problems for pipe-type cables—there have been many
instances of a corrosion pinhole in a water main creating a leak that erodes the metal in a
cable pipe, causing a leak and forcing water into the pipe. If the separation is less than 24 in.
(61 cm), the owner should consider sleeving the foreign pipe with PVC or polyethylene duct.
•
A high-strength concrete cap, sometimes with red dye in the concrete or sprinkled on top of
the concrete before it cures, is sometimes specified to provide additional warning and
protection for both extruded dielectric and pipe-type cables. Warning tapes are commonly
specified.
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•
Greater burial depth will reduce the chance of dig-in, but will also derate the cable. In recent
years there have been several instances in which HDD installations (typically by contractors
installing communications systems or water supplies) have hit the transmission cable—so
greater burial depth does not guarantee protection.
•
Use of stronger duct material such as fiberglass as opposed to PVC for extruded dielectric
cables can provide some additional protection against external damage.
•
Accurate as-built drawings are important, as is coordination with other entities that have
subsurface utilities.
It should be noted that O&M practices also have a strong effect on cable circuit safety. Some
owners have found weekly route patrols effective in identifying construction operations that
might damage the transmission cable. Proper O&M procedures such as jacket integrity tests and
bonding lead evaluation are important for extruded dielectric cables. Maintaining a good
corrosion protection system is mandatory for pipe-type cables.
1.8
Civil Works Including Manholes
1.8.1 Initial Checklist
Before work begins, the owner should do the following:
•
Ensure that the contractor has the latest copies of all specifications and drawings and that the
contractor has proper quality control and safety procedures. These are generally reviewed at a
preconstruction meeting with the successful bidder.
•
Ensure that the contractor has the proper licenses and permits and that any necessary traffic
control plans have been filed and approved. Also, ensure that all permissions and approvals
have been obtained.
•
Ensure that the contractor has the proper insurance coverage and that the contractor provides
insurance certificates before commencing work.
•
Ensure that the contractor is aware of all restrictions, for example, work hours, noise
restrictions, dust protection, public and environmental protection, allowable length of open
trench, and plating requirements.
•
Hold local kickoff meetings, if desired. Some owners have project kickoff meetings in which
local authorities (water authority, sewer authority, highway department, and public relations
department) are invited, as well as the owner and contractor personnel. All parties have a
chance to provide their comments and obtain responses. This buy-in usually makes
implementation of the project go more smoothly.
•
Notify police and fire personnel of the work locations and type of work being done and the
name and emergency telephone numbers of the responsible owner or contractor contact
persons. This should be done on a daily basis.
•
Have the owner or the contractor notify all businesses and property owners of the
construction work that will be taking place and the approximate schedule of the work.
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•
Have security personnel retained if necessary (for traffic control, material storage yards, and
personal security in some areas).
•
Even if most of these items are the contractor’s responsibility, violations will reflect on the
owner—so it is in the owner’s interest to make sure that all requirements are met. As noted
earlier, the owner should have a field supervisor or field observer present throughout the
project and more than one person available on larger projects.
1.8.2 Survey
Orthophoto mapping, which is aerial photography geometrically corrected to provide a uniform
scale, is an effective approach to evaluating alternative alignments at early stages of a project.
Some owners use orthophoto maps as the plan segment of plan-and-profile drawings.
Light detection and ranging (LIDaR) surveys may be conducted to give an accurate
representation of objects and elevations along the potential alignments.
These two approaches help select the routes to be evaluated using conventional surveying as
construction work gets closer. The owner might have performed an initial survey as part of route
selection and to determine ownership of parcels along the alignment. However, the contractor
should be responsible for the detailed survey that is part of the construction project, including
location of obstacles. Even if the owner has already performed the detailed survey, it is still
desirable to have the contractor verify the survey in a post-design survey to ensure that the
contractor accepts responsibility for the installation.
Considerations for planimetric and topographical surveying and obstacle location include the
following:
•
Survey approximately 50 ft (15 m) each side of the centerline: field changes are often needed
to avoid obstacles.
•
Show all property lines and street and utility rights of way.
•
Show curb lines, street lanes, manholes, fire hydrants, driveways, water grates, and so on.
•
Show storm sewer inlets, including width, depth, and elevation.
•
Show culverts and invert elevations on open pipes.
•
Show street addresses for properties.
•
In addition, as part of the planimetric and topographical survey, the following steps should be
taken:
–
Shoot ground elevations across the width of the surveyed area to allow a 3-D triangular
irregular network (TIN) model (file) to be generated. This will allow the route to be
relocated and a new profile to be easily developed.
–
Instruct the local underground locating service to have all of the underground facilities
located before the survey commences, so this information can also be documented by the
surveyor.
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–
Locate other unmapped, abandoned, and private underground facilities and structures,
such as fuel tanks (which might need to be purged), private lines (gas, electric,
communication, and water), and old foundations.
–
Contact all local underground utilities and obtain their facility drawings, if possible. This
is a crucial step in verifying the accuracy of the survey and identifying data missing from
the survey.
1.8.3 Underground Locating
At a minimum, the designer should develop drawings using information up to Level C of ASCE
2003 for all underground projects. If the project is in an urban area, or there is evidence of a
significant amount of existing underground facilities, it is recommended that the documentation
be to Level B or A. If the exact location is needed, Level A is required.
Ideally, the owner will have the flexibility to work around obstacles such as gas lines, water
mains, sewer lines and storm drains, distribution duct lines, and so on. Simply relocating to the
other side of the street or perhaps relocating to a nearby parallel street can sometimes eliminate
many conflicts. However, this is not possible in many developed urban areas. The owner must
adjust to local conditions by accepting separations that are less than desired, by installing the
cable more deeply than desired, or by other means to provide adequate protection. In suburban
areas, protection of tree roots may require relocating the line or placing barriers or other methods
to avoid damaging the roots during construction or operation (due to heat generated from the
cables driving moisture away).
Owners should request a video recording of the entire route within two weeks before the
beginning of construction as well as after construction is complete. This can be valuable if there
are damage claims by property owners, and it helps verify the restoration of the area to
preconstruction conditions.
1.8.4 Geotechnical and Soil Thermal Tests
Geotechnical and soil thermal tests should be conducted along the alignment, ideally before
installation specifications are prepared, so that the bidders have good knowledge of conditions
that will be encountered. The geotechnical tests will give the owner and the excavation
contractor a good understanding of the material that will be encountered and are particularly
important at manhole locations and areas of suspected incompetent soils. The soil thermal tests
will allow a more accurate design of the cable sizing as well as trench design. Generally, the
owner will retain a local soil testing firm to perform the geotechnical analysis, and a soil thermal
specialist will work with the geotechnical firm, taking in situ soil thermal resistivity
measurements while the geotechnical firm is performing its measurements.
Both firms will characterize the soils in the bores using standard soil classifications. The thermal
analysis will also include in situ thermal resistivity, moisture content, and ambient earth
temperature. Three measurements are typically made at each borehole: at the cable depth, 2 ft
(0.6 m) higher, and 2 ft (0.6 m) lower. For horizontal directional drills, measurements are taken
at the expected bore depth plus 10 ft (3 m) higher and 10 ft (3 m) lower.
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It is often possible to characterize soils along an alignment into three to five categories. The
thermal specialist will perform staged dry-out tests to develop a curve of thermal resistivity
versus soil moisture content. Because testing might not be carried out at the driest time of the
year, the thermal specialist should provide an estimate of the thermal resistivity range to use for
cable design based on expected moisture content and type of cable system to be installed.
A borehole separation of 2000 ft (600 m) or so is generally considered adequate for geotechnical
and thermal characterization, unless there are indications that the soil conditions vary within that
distance. Bores are recommended at proposed manhole locations to provide the contractor
information for subgrade preparation to support the manhole structures.
1.8.5 Excavation, Manholes
1.8.5.1 Trench Cross-Sections
Excavation requirements must reflect the owner’s specifications for the duct bank or pipe
installation because variations in installation can affect ampacity and even impedances for
extruded dielectric cables. Many different configurations can be specified, especially for
extruded dielectric cables. The ground continuity conductor for the extruded dielectric cable
systems is installed in a small (typically 2 in. [5 cm]) duct or directly embedded in the concrete
envelope. In some areas, large amounts of open space or concurrent installation of multiple
utilities permit wide, stepped-back trenches.
1.8.5.2 Manholes
Because of the size of manholes (especially the large manholes for highest-voltage extruded
dielectric cables), excavation work—or at least digging of test pits—should take place at planned
manhole locations before actual trenching and preferably before cable cut length commitments
need to be made. The excavation for a 345 kV extruded dielectric cable manhole can be 40 ft
(12 m) long, 10 ft (3 m) wide, and more than 10 ft (3 m) deep. This is also a critical item
affecting project scheduling. If a manhole must be placed at a different location, it might be
necessary to perform new pulling tension calculations and provide new cut lengths to the cable
manufacturer. Therefore, many owners install manholes before duct bank or cable pipe
installation begins. Most manholes are precast—in two or more sections for larger manholes.
However, in some areas with subsurface congestion, or if lines are to be rerouted or extended, a
cast-in-place manhole is necessary. This type of manhole is adaptable to different subsurface
conditions, but the street must be kept open much longer to form the manhole, pour, remove
forms, and cure.
Precast manholes are large and might weigh more than 100,000 lbs (45,000 kg), requiring careful
planning for larger excavations and closing of more traffic lanes, scheduling of large cranes, and
increased traffic control in comparison to what is required for trenching activities. It might also
be possible to assemble “panel-manholes,” in which each wall, floor, and roof slab is provided
separately and then assembled together in the excavation.
Note that manholes might be buoyant in spite of their size and weight. Special attention must be
paid to avoid having manholes float in areas with a high water table. Openings are left in the
bottom so that the manholes can fill with water, wide “fins” are cast on the bottom edges to
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prevent floating, or extra-thick walls, floor, and ceiling might be specified to provide additional
weight. Manholes might also “sink” in unstable soils. The owner might be required to install
special bedding before the manhole is placed or might need to place the manholes on pilings in
some areas.
Electrical grounding is required at manholes for extruded dielectric cables. Grounding design is
an important part of the cable system design. The designer should perform detailed calculations
considering system fault level, characteristics of the connected transmission network, earth
electrical resistivity, possible additional counterpoises underneath or adjacent, and connections to
reinforcing steel and cable sheaths, all to identify and eliminate any dangerous step and touch
potential hazards to workers and the public. The installer should not deviate from the design
without specific approval from the designer. Refer to the EPRI report Electrical Safety
Management for Underground Transmission Systems [5] for details on these calculations and
procedures. Resulting requirements often include the following items:
•
Copperweld-type ground rods may be driven on two or more opposite corners of the
manhole. Ground rods are sometimes installed outside the vaults rather than through the floor
of the vaults.
•
A copper or other less-corrosive ground loop may be installed around the inside walls of the
manhole and secured to the walls with less-corrosive hardware. The ground loop material is
typically copper bar or #4/0 bare copper cable. Some owners install the ground loop near the
roof and install magnesium anodes to mitigate corrosion of the bare copper.
•
The ground conductor loop in the manhole may be connected to the ground conductor for the
circuit, for extruded dielectric systems having a ground continuity conductor. A few owners
connect the manhole rebar to the ground loop either by having a small “window” in the
concrete for access to the rebar or by having copper pigtails connect to the rebar exposed in
the manhole.
•
Connections are typically exothermally welded, although some owner standards allow certain
non-weld connections because of the smoke and fumes that must be cleared out of the
manholes after exothermal welding.
The owner’s specifications for manholes should include the following:
•
The owner or owner’s representative should have copies of the design calculation, rebar
schedules, concrete mix, and drawings for the proposed manholes and should provide
approval before the manholes are fabricated. The design should account for buoyancy control
if needed.
•
Manhole structural design should be approved by a professional engineer licensed in the state
where the installation is to take place.
•
Multi-section manholes should have watertight joints, and the joints should not interfere with
cable duct or pipe entry or attachments for cable pulling.
•
The manholes should be capable of withstanding load levels determined by the owner or
transportation department. A typical highway loading limit is American Association of State
Highway and Transportation Officials (AAHSTO) HS20. Railroad rights of way have
significantly greater structural requirements.
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•
The specification should include the owner’s concrete material specifications. Many owners
require a 4000 psi (27,600 kPa), 28-day strength.
•
Steel reinforcing bars must not form a closed loop around any single-phase cable penetration
of a manhole wall, for example for extruded dielectric cables.
•
Pulling irons, eyelets, and other attachment points for pulling the power cable should be
adequately placed and sized and supported by engineering calculations. With today’s large
cables and high pulling tensions, it is not unreasonable to have the manhole end walls and the
pulling eyes be sized for three times the maximum cable pulling force.
•
A removable pulling beam, or other temporary reinforcement, can be effective for manholes
where there is concern about the strength of the end walls and pulling irons/eyes. Proper
coordination between the manhole designer and cable installer could result in cost savings.
•
Grounding connections should be made in accordance with the owner’s requirements and the
contractor’s design.
•
Adequate grade rings should be provided to ensure that the vault ring and cover can be set to
the final grade. The grade rings should contain lifting eyes.
•
Grade rings, vault rings, and covers should be specified to withstand the transportation
department traffic loading requirements. The owner may provide the cover; the installer
should ensure that supports are sized properly.
•
All manholes should have permanent identification tags, clearly visible in the chimney
without manhole entry.
•
One or more sumps and/or gutters should be provided to facilitate water removal.
1.8.5.3 Excavation
The following points should be considered in conjunction with the excavation and related civil
works:
•
Sheeting and shoring should be provided in accordance with the latest version of OSHA
Regulations 29 CFR Part 1926.
•
Structural shoring plans should be submitted and should be prepared and stamped by a
licensed professional engineer.
•
The contractor should obtain a good understanding of any restrictions on trench opening time
(the length of time a trench may be open at one location) and trench opening length (the
maximum length of trench that may be open at one time). On larger projects, it is not
uncommon to have several crews working at different locations, excavating, installing duct
or pipe, and backfilling/paving. The contractor might need to attempt to convince the
municipality that these are independent excavations rather than having to “lump” all
excavations together in terms of allowable time and length of open trench.
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•
A dewatering plan should be submitted several weeks before activities begin, if dewatering is
required. The plan should include provisions to avoid subsidence of nearby structures and
possibly ways to monitor any subsidence. Provisions should be made for water disposal in
accordance with local requirements. Plans should include methods for dealing with
contaminated water. Some owners might require the dewatering plan to be prepared by a
licensed professional engineer.
•
The contractor should submit plans for removing rock. The plans should comply with owner,
municipality, or other requirements for blasting, jackhammering, rock cutting, or other
methods for rock removal. Because rock removal is more expensive than simple trenching, it
is often necessary for the owner and contractor to agree on a schedule of unit rock excavation
costs if the amount of rock is greater than stated in the specification.
•
Plans should be submitted for erosion control and for preserving trees and landscaping that
might be affected by the excavation work.
•
A vegetation removal plan should be prepared for work on rights of way and other areas in
which vegetation must be removed to allow civil work to proceed. The plan should reflect the
owner’s requirements for vegetation removal as well as relevant requirements from other
groups that might have jurisdiction. Some jurisdictions have stringent requirements regarding
certain trees, sea grasses, and so on. Note that the owner might need to prepare a vegetation
maintenance program that is separate from construction activities and that there might also be
special environmental requirements to deal with invasive species.
•
The contractor’s excavation plan should include methods of disposing of spoil. Criteria
should be determined beforehand for quality and testing requirements (including thermal
testing) for spoil that may be returned to the trench. If spoil cannot be placed alongside the
trench, the contractor should arrange for a suitable storage or disposal area. Special attention
should be paid to identification and disposal requirements for contaminated soil. Testing of
soil samples for contaminants, and temporary storage containers, might be required in some
cases. Note that spoil alongside the trench provides a safety barrier for the public. If spoil
must be removed, the trench should be plated or other safety barriers installed.
•
The excavation plan should identify the backfill that will be used, both thermally controlled
backfill in the vicinity of the cable system and soil backfill beyond the controlled backfill
envelope. Maximum allowable thermal resistivities might be specified for material that is to
be returned to the trench. The contractor might be able to identify specific soil types along
the alignment, have those types categorized thermally, and then instruct field operators as to
which material may be returned to the trench and which material must be discarded. Spoil
with organic material should not be used as backfill because it typically has high thermal
resistivity.
•
The owner’s trench design should be followed wherever possible. If the trench design or
project specifications do not have descriptions of items such as pavement removal and
restoration, the owner should require the contractor to submit plans for owner approval.
•
If a trench cross section must be changed (for example, to avoid obstacles), the owner should
be notified as soon as possible in case changes must be made to items such as the amount of
controlled backfill necessary to achieve the desired ampacity.
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•
Having an exploratory crew excavate test holes or test pits along the route in advance—
especially at street crossings and areas in which conflicts are expected—can allow the
contractor to plan ahead if modifications to the trench design are required.
•
If at all possible, trenching should extend approximately 200 ft (60 m) forward of the current
duct or pipe installation. If obstacles are encountered that require repositioning of the duct or
pipe, it is much easier to have shallow bends if adjustments can be made well before the
obstacle.
•
Any local or transportation department regulations should be followed for the amount of
open trench, amount of plating, requirements for nonskid plating, work hours, and so on.
•
Adequate clearances should be maintained from existing utilities. Table Section 13 of the
Green Book lists the desired minimum clearances.
•
Shoring (horizontal braces using hydraulic, mechanical, or timber holding upright or
horizontal sections against the wall of the trench to prevent collapse) or sheeting (a
continuous wall of wood or metal held in place by the shoring system) is almost always
required for trenches greater than 5 ft (1.5 m) deep and might be needed for shallower
trenches, depending on soil conditions. Shoring and sheeting may not be needed in rock or if
the trench is stepped back. Design of the shoring and sheeting system must often be prepared
by a licensed professional engineer. The contractor is responsible for removing sheeting and
shoring after the work is complete.
•
If the trench bottom is over-excavated, clean fill should be installed and compacted to at least
90% of maximum dry density, or a material such as fluidized thermal backfill should be
installed to bring the trench bottom to design grade.
•
The owner’s drawings should be followed in terms of trench width and depth. The
excavation contractor and the duct or pipe installation contractor should coordinate to ensure
that there is adequate room for working in the trench.
•
All sheeting and shoring, or step-back, requirements should be carefully followed. As stated
earlier, all trenching must comply with the latest revision of OSHA Regulations 29 CFR
1926.
•
Hand excavation might be required in areas in which mechanical excavation might cause
damage to other duct banks or other buried utilities, buildings, and so on.
•
The owner should maintain, and enforce, policies addressing future parallel underground
utility installations that might cause subsidence of the duct bank or create thermal problems
that would derate the cable system.
•
Many field decisions are typically required, addressing items such as encountering
contaminated soil, whether to cut or relocate other utilities, whether to have the cable duct or
pipe be diverted, whether additional thermal backfill is required if thermally poor native soil
is encountered, and so on. These decisions typically involve the excavating contractor, the
prime contractor (the excavating contractor is often a local contractor, retained by the prime
contractor), the contractor’s project superintendent, and the owner’s field observer. The
project engineer is often involved in the discussions.
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•
A comprehensive daily log, including photographs, should be kept throughout the trenching
operation. Actual trench and duct/pipe depths should be recorded, preferably every 25 ft
(8 m) but at least every 100 ft (30 m); horizontal and vertical locations of other utilities
crossing the cable trench should be recorded and a permanent record presented to the owner.
•
The contractor should pave and patch areas disturbed by the excavation in accordance with
city or local standards as applicable.
•
The contractor should provide final as-built drawings. Generally, the contractor prepares a
markup on a plan and profile drawing and submits the marked-up drawing to the owner or
the owner’s engineer to permit modification of the formal drawings for record.
•
Final cleanup and restoration of the site are the contractor’s responsibility.
The contractor is free to select the best excavation methods while maintaining compliance with
the requirements set forth in the owner specification.
1.8.6 Water Removal
Dewatering might be necessary to provide a water-free and stable subgrade for duct or pipe
installation. Dewatering should be designed and conducted to prevent damage to adjacent
properties and structures, and dewatering plans should ensure the stability of the sides and
bottom of the excavation. The contractor must comply with all applicable requirements for
disposal of water removed during the dewatering process, with special attention paid to
contaminated water. Holding tanks might be required for contaminated water to transport the
water off-site for proper disposal.
The contractor should conduct subsurface investigations to identify groundwater conditions and
should prepare a groundwater removal plan. As noted earlier, some owners require that the plan
be prepared and signed by a licensed professional engineer.
The contract documents should ensure that the contractor has sole responsibility for the
groundwater control systems, including proper disposal of water and repair of any damage
caused by dewatering. The contractor should also have plans in place for surface water control,
for diverting the water from excavations, and for protecting nearby areas from water damage.
Note that in some locations, such as coastal areas, it is not practical to dewater the trench. Duct
bank or pipe assembly can be done above the trench and the assembly lowered into the trench.
Localized areas for joining sections in the trench can be dewatered by inserting trench boxes to
limit water inflow. Well-pointing may be used in areas of high water levels.
1.8.7 Duct or Pipe Installation
The duct or pipe installation must be coordinated with trenching operations. Requirements for
duct installation are addressed in subsection “Installing and Joining Ducts,” and requirements for
pipe installation are addressed in subsection “Pipe Installation.”
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1.8.8 Direct Burial
Because direct burial of extruded dielectric cable requires having long lengths of trench open—
as long as several thousand feet or meters—direct burial might be feasible along rights of way, in
rural areas, or in other areas in which such long trench openings are feasible and in which public
safety is not compromised. Steel plating may be placed over the trench to allow crossing or for
safety. If direct burial is used, it might be necessary to install duct sections under roads, railroads,
and other areas in which open trenching is not desired either for initial construction or for
potential failure repair. Manholes are preferred for splicing for maintenance and repair reasons,
but buried concrete splice bays might also be acceptable.
1.8.9 Encasement
Ducts for extruded dielectric cables are almost always encased in high-strength (3000 psi
[20,700 kPa]) concrete, which serves the following functions:
•
Providing mechanical protection of the ducts/cables
•
Maintaining separation and reducing the chances of a failure in one duct affecting cable in an
adjacent duct
•
Providing an area of low thermal resistivity around the cables
•
Providing an underground raceway to easily replace cable following a fault
This concrete generally has good thermal resistivity in the range of 50 K-cm/watt at natural
moisture content, but it is necessary that the specification have a statement that no air-entraining
agents are permitted or that air entrainment of less than 2% may be permitted, if air entrainment
is considered necessary. If air entrainment is permitted, there should be a field testing
requirement. These chemicals can increase the air content of the concrete, which increases the
thermal resistivity. The concrete mixture might be specified to have a thermal resistivity no
greater than 55 K-cm/watt at 3% moisture content and no greater than 95 K-cm/watt at zero
moisture content.
Owners typically have thermal testing done on the concrete mixture to be used for the
encasement, and they might specify thermal testing of samples of the material supplied, with a
frequency typically every 50 cubic yards (38 m3) of material placed. It is important to check the
batch ticket of every truck to ensure that there is no change from the material that was specified
and previously tested. The concrete should not be dropped onto the ducts from heights greater
than 4 ft (1.2 m) to avoid damaging the ducts, and it should be vibrated or a shovel used to
ensure that it completely surrounds the ducts.
If the extruded dielectric cable system is directly buried, the encasement might consist of wellcompacted thermal sand or other low-thermal-resistivity granular backfill materials.
Specifications typically require a thermal resistivity no greater than 65 K-cm/watt at 3%
moisture content and no greater than 100 K-cm/watt at 0% moisture content. The thermal sand is
compacted in lifts no greater than 8 in. (0.2 m) to at least 95% of maximum dry density. A
fluidized thermal backfill (FTB) is preferred because, in addition to its good thermal
characteristics, it prevents the contents from falling out if a foreign utility trenches beneath and
has the same thermal resistivity limits. FTB is a concrete-like material consisting of a mixture of
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a natural mineral aggregate, sand, cement, water, and a fluidizer such as fly ash. The amount of
cement should be kept small so the strength of the material is low, 100–200 psi (690–1380 kPa);
this permits easy removal with hand tools. Pay careful attention to tickets from the batch-mix
plant because adding too much cement will result in an undesirably high strength. Owners
having a soil thermal specialist can help contractors source the materials for the FTB and
perform laboratory testing to verify the thermal resistivity of the material.
Concrete slabs should be placed above the encasement for directly buried cables, to reduce the
chances of dig-in damaging the cables. Concrete sides are preferred to reduce the chance of cable
damage from adjacent excavations. Concrete bottoms could also be provided to give further
protection, especially at street intersection crossings.
Most pipe-type cables have an FTB encasement to a distance 12 in. (0.3 m) or greater above the
pipe. Thermal sand is common for owners in the Northeast. The pipes are typically supported
6 in. (0.15 m) or so above the trench bottom on bags of dry FTB or thermal sand, and the entire
trench width—or at least the area between two sides of sheeting—is filled with the material. It
flows freely, although the owner will typically require that it be vibrated. Batch tickets should be
provided for every load of the material brought to the site, and the owner’s field representative
should check to ensure that there were no changes in mixture. Thermal resistivity should be
measured on samples of the FTB taken for every 50 cubic yards (38 m3) of material placed, the
sample cylinders sent to the testing laboratory on a weekly basis, and the results provided to the
owner as well as the contractor within 48 hours after test completion.
The thermal resistivity of FTB should be no greater than 60 K-cm/watt at 3% moisture content
and no greater than 100 K-cm/watt at zero moisture content. The thermal resistivity for the
granular fill should be no greater than 65 K-cm/watt at 3% moisture content and no greater than
100 K-cm/watt at zero moisture content.
The encasement might also be a granular material, having a specific sieve analysis that ensures
good compaction and a minimum of voids. Limestone screenings form a good encasement, and
some bank-run sands naturally meet the optimum sieve analysis. Some eastern owners mix sands
and small aggregates to obtain the proper sieve analysis. Careful compaction at optimum
moisture content is important to achieve the best results, both mechanical and thermal. It is
especially important to have good compaction around the duct bank or pipe, or the cables
themselves if they are directly buried. Marker tapes or ribbons should be placed above the
encasement before backfilling to warn excavators that a high-voltage underground cable is
present.
1.8.10 Backfilling the Trench
Considerations for backfilling include the following:
•
Backfill should not be placed during freezing weather unless necessary and unless approved
by the owner. Frozen materials, snow, or ice should not be placed into the trench.
•
Backfill should not contain organic material or stones larger than 2 in. (5 cm).
•
Backfill should be placed in uniform layers no greater than 8 in. (20 cm) in un-compacted
thickness.
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•
Compaction should be done with equipment that does not transmit damaging shocks to the
cable system. Compaction should be to a minimum of 90% of maximum dry density as
determined by ASTM D698. Compaction should be carried out to a minimum of 95% of
maximum dry density under streets, parking areas, or any other facilities subject to damage
by settling.
•
Moisture content of the backfill material should be maintained to obtain the specified density
of the compacted material. The contract documents should require the contractor to repair
any areas damaged by settling.
•
If the trench thermal design requires that the backfill have a specific maximum thermal
resistivity, the contractor should test the backfill at specified moisture contents to ensure that
it meets the owner’s requirements. Thermal resistivity tests should be taken every 300 ft
(90 m) of trench length and the results provided to the owner as well as to the contractor.
•
Some municipalities require using a free-flowing material, sometimes called flow-fill, above
the duct bank or pipe encasement. This material can speed the backfilling procedure, but it
should be tested to ensure that the thermal resistivity does not result in excessive cable
temperatures.
•
For areas under pavement, municipal or transportation department requirements must be
followed for road sub-base and base.
1.8.11 Restoration
Restoration is a critical part of the cable project in the eyes of the public. Installer considerations
are summarized as follows:
•
All street restoration must be performed to applicable municipal, state, or federal
requirements. This might include repaving one full travel lane or perhaps the entire
street/road. Temporary patching is usually required to allow traffic flow as soon as possible.
The temporary patch is removed at a later time, and permanent pavement is installed.
•
In colder parts of the country, asphalt plants close in wintertime; the contractor should be
aware of the closing dates and plan the restoration accordingly.
•
Restoration of driveways, curbs, medians, and so on generally must be to the original
condition or better.
•
Right of way restoration can be problematic because road access is required along the full
trench length for maintenance and potential repair. In addition, only shrubs and other
shallow-rooted plants are permitted within 10–12 ft (3.0–3.7 m) of the cable trench to avoid
having woody vegetation remove moisture from the area of the trench, possibly overheating
the cables.
•
Some jurisdictions have specific requirements for restoration, and may have a person
functioning as an enforcement officer to ensure that sensitive fauna are not damaged by
either the construction or the restoration efforts. This enforcement officer must typically sign
off on the completed project before construction operations can be considered complete.
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1.9
Equipment Used for Civil Work
For a detailed description and uses of equipment used for underground transmission installation,
please see EPRI Underground Transmission Cable System Construction and Installation
Practices Manual—2012 Update (1024180).
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0
2
EXTRUDED DIELECTRIC AND SELF-CONTAINED
FLUID-FILLED CABLE SYSTEM INSTALLATION
2.1
Introduction
This section of the report addresses planning construction operations that are specific to extruded
dielectric cable systems and self-contained fluid-filled (SCFF) cable systems.
2.2
Overview
Extruded dielectric cables, insulated with cross-linked polyethylene (XLPE) or with ethylenepropylene rubber (EPR), are currently the most commonly installed cable types for new
installations in North America and have become the standard worldwide for transmission cable
installation on land.
This section focuses on extruded dielectric cables, since they are more frequently installed than
SFCC cables. However, most sections also apply to SCFF cables because both are selfcontained. Both types are typically installed individually in ducts or directly buried, and they
have similar accessory requirements for sheath bonding. SCFF cables have additional
requirements for their hydraulic system.
2.3
Duct Bank Construction
The majority of underground cable installations in the United States use duct-and-manhole
construction. Most installations overseas are directly buried or in utility tunnels. Although duct
bank construction is becoming more common in some European countries, most U.S.
installations have been in city streets in which the shorter street-opening times and the ability to
replace a failed cable section or splice without excavation as well as to re-conductor, if needed,
have been important. Owner and planner considerations for duct bank construction are described
next.
2.3.1 Duct Material and Size
Most ducts for trenched installations are made of polyvinyl chloride (PVC). PVC has good
strength, reasonable coefficient of friction against a polyethylene cable jacket, and easy joint
assembly. Manufacturers of extruded dielectric cables recommend a duct inside diameter that is
at least 1.5 times the cable outside diameter to allow room for cable snaking during thermal
expansion. This would require an 8 in. (20 cm) duct for a cable with a 5 in. (13 cm) outside
diameter. The EPRI report Mechanical Effects on Extruded Dielectric Cables and Joints
Installed in Underground Transmission Systems in North America [6], recommends an optimum
duct clearance of approximately 2.4 in. (60 mm) for typical transmission cables. A Schedule 40
duct (0.277 in. [7 mm] wall thickness for a nominal 6 in. [15 cm] duct) is often specified to
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ensure good mechanical strength and freedom from deformation when the concrete encasement
is poured. The 6 in. (15 cm) Type EPC40, Schedule 40, PVC duct is the most common size for
cables up to 230 kV, and 8-in. (20 cm) Type EPC40, Schedule 40, is used for some 230 kV
cables and most 345 kV cables.
NEMA TC8 for PVC duct (which has a maximum duct size of 6 in. [15 cm]) recommends using
the nominal size for the effective inside diameter rather than the calculated ID. This allows for
tolerances and deflection. PVC with fusible joints is sometimes used in horizontal directional
drills, but there are few installations as of 2010.
Two other materials are sometimes used for duct bank construction: FRE (fiberglass reinforced
epoxy or fiberglass) and polyethylene. Fiberglass duct is commonly used for aboveground
installations, such as under bridges, for its strength and its low coefficient of expansion; a few
owners also use fiberglass for duct bank construction in trenches. Although more expensive than
PVC, fiberglass duct has the advantage of being less brittle, stronger, and generally stiffer than
PVC. Fiberglass has a low coefficient of friction, allowing longer cable pulls with less reliance
on lubricants. Its joints are typically epoxied, which can be time-consuming.
High-density polyethylene (HDPE) in thick-wall “sticks” is commonly used for directional drills.
Some owners also use coiled HDPE for duct bank construction, although this is not
recommended for transmission cables because the undulations in the HDPE increase the cable
pulling tensions. Wall thickness is determined by dimension ratio (DR), which is the outside
diameter divided by the wall thickness. DR11 is a common size used for power cables, although
thicker walls may be used for some trenchless installations.
2.3.2 Installing and Joining Ducts
PVC ducts, which are the most common kind for trenched installation, are typically supplied in
straight 10 ft (3 m) or 20 ft (6 m) long sections and joined using a solvent-welded bell-and-spigot
joint (most common) or a coupling. Proper spacing is ensured by using commercial spacers.
Considerations for duct storage and installation that owners and planners must consider are as
follows:
•
Cover ducts with weatherproof covering and cap the ends if stored outdoors for more than
two weeks. Ducts with bell-and-spigot fittings must be oriented as shown on the design
drawings, and cable installation must be done in the direction shown on the drawings.
•
Stagger joints in the assembly of the duct bank.
•
Use spacers every 5 ft (1.5 m) to keep the ducts in position. Spacers should not be closer than
15 in. (0.4 m) to a coupling or joint. Standard spacer configurations may be used and, for
large projects, it might be efficient to specify custom spacers.
•
Keep duct as straight as possible to keep pulling tensions as low as possible. Alignment is
critical to keep tensions low and prevent the cable from snagging against edges.
•
Use long bells, or long couplings to help maintain good alignment, especially around bends.
Terminate ducts in manholes with flared end bells.
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•
Use a fine-tooth wood saw to cut the duct. Take care to maintain square ends. Remove burrs.
Chamfer duct ends on the inside. File the outside edge smooth. Sections should be solventglued together using normal PVC gluing techniques.
•
Keep the ducts from kinking or becoming oval, especially when making bends. Never use
elbows to make bends, no matter how large the radius. Bends down to 160 times the duct
outside diameter may be made by simply forcing the duct bundle to fit in the trench. Check
with the cable system designer to determine the minimum radius that is allowed.
•
Ensure that any glued joint coupling is fully seated and glued and that any glued joints that
fall within the radius of a field-made bend are fully inserted in long couplings and that the
glue is fully cured. Inspect closely to ensure that there are no angled joints.
Note: The allowable bend diameters depend on route geometry and thermo-mechanical design
considerations. Owners and planners should evaluate these factors during cable system design
for each potential installation. In all cases, pulling tension and sidewall pressure calculations
made for the duct profile and the cable being pulled ensure that values are within allowable
limits. If field conditions require bends of smaller diameter or more bends than the drawings
show, the responsible engineer should review and provide approval before duct installation
proceeds.
•
The inside edge of the male end of a bell-and-spigot joint (both ends if a coupling is used)
must be chamfered to approximately 45° to avoid sharp edges, regardless of the direction of
pull.
•
Duct assembly is typically done above or alongside the trench, and the duct bundle is
lowered into the trench after allowing at least 2 hours for the joints to cure under most
conditions and 4 hours if the ambient temperature is less than 5°C. Consideration should be
given to the PVC duct expansion with temperature, approximately 1.5 in. (3.8 cm) in 100 ft
(30 m), for a 20°F (12°C) change in temperature. Most duct banks are encased with concrete,
generally 3000 psi (20,700 kPa) strength. Avoid dropping the concrete drop directly onto the
ducts from a height greater than approximately 4 ft (1.2 m) to prevent damage.
•
Ducts will tend to float when the concrete encasement is poured. Two methods to prevent
floating are to tie the spacers to rebar driven into the ground approximately every 10 ft (3 m)
or to pour small sections of concrete every 20 ft (6 m) and then pour the remainder of the
trench after the initial sections have set long enough to keep the ducts from floating. Use
reinforcing where needed for extra duct bank strength. No rebar or other metallic conductor
should encircle individual ducts.
•
Open duct ends in the trench must be sealed with watertight seals or caps to prevent entry of
water and dirt. Following completion of a section, clean ducts using a duct brush with a
diameter equal to the inside diameter. Limit the use of wire brushes and steel cutter mandrels
to avoid damage to the duct.
•
Ducts should be “proofed” by swabbing and mandreling with a hardwood or metal mandrel
having a length more than twice the duct outside diameter, with a clearance no greater than
1/2 in. (13 mm) less than the duct inside diameter. For a common 6 in. (0.15 m) Schedule 40
PVC duct, a mandrel may be 18 in. (0.46 m) long and 5.75 in. (0.15 m) in diameter.
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•
The straight portion of a common mandrel is shown as 18 in. (46 cm). Adjust this dimension
based on the tightest bend allowed in the duct section or the smallest allowable bending
radius for the cable being installed.
•
If the duct bank is installed by a firm other than the cable installer, the cable installer should
be present for the proofing. The owner should receive 48-hour notice before the proofing
operation is to begin.
•
A pulling line with strength greater than 2500 lbf (1,134 kgf) should be left in the duct
section, and the ends should be capped.
•
Plan and profile drawings should include details for rotation of conduits (for example,
transition from square to in-line configuration) where required.
2.4
Directly Buried Installations
2.4.1 Trenches for Directly Buried Cables
XLPE and SCFF cables are sometimes directly buried if they are in areas in which long lengths
of trench can be excavated and left open long enough to have cable installed. The entire trench
length, splice-to-splice, is left open for cable installation, although steel plates may be placed
over the trench if required for traffic flow or safety considerations, unless the trench is too wide
to support the plates. Place a 6– 8 in. (0.15–0.20 m) layer of sand, limestone screening, or similar
material on the bottom of the trench and compacted to give a solid surface for installing rollers or
cable pushers.
Directly buried cables are typically encased in limestone screenings, thermal sand, or FTB to
allow access to them if needed for repair of a jacket or cable failure. The material is the same as
described in subsection “Encasement”. These materials are also less susceptible to dry-out
compared to uniform particle size bedding sand or most native soils.
A direct-buried system does not have the mechanical protection of concrete encasement or in a
plastic or fiberglass duct system. Owners therefore install 4–6 in. (0.10– 0.15 m) thick concrete
slabs—precast or field-poured—above and sometimes beside the thermally controlled backfill
that surrounds the cables. The concrete sidewalls provide additional protection. In this case,
place warning tapes above the top slab.
2.4.2 Transition of Duct to Directly Buried Cables
Some owners provide a transition from cable in duct to directly buried cable, within 50–100 ft
(10–30 m) from the base of the transition structure. This permits adding a bend or loop in the
cable that might give extra cable for a termination repair and therefore avoid the need for a
splice. The earth also clamps the cable, reducing any tendency for the cable to slide into the duct,
which would create extra stress on the cable clamps and termination. At the transition point from
duct bank to directly buried cable, ensure that the earth and backfill are properly compacted and
the cables exiting the ducts are supported on sand bags that raise the cables to the top of the
conduit opening as they exit the conduit. Otherwise, the cables may experience excessive stress
at the conduit exit point if settling occurs over time. It should also be ensured that cables crossing
in the loop do not inadvertently create a hot spot due to mutual heating effects.
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2.5
Cable Installation
In all cases, cable installation must be performed in accordance with the cable manufacturer’s
requirements. The bid specifications for cable and accessories should include a request for the
manufacturer’s detailed installation requirements, and the contractor’s installation procedure
should reflect those requirements. Owners can require a cable manufacturer’s representative to
be present for all cable pulls.
2.5.1 Cable in Duct Bank Installations
The installer should be sure that the ducts are properly proofed and are ready for cable
installation, and that all necessary material, equipment, and supplies are available for the cable
installation.
2.5.1.1 Cable Installation
Owners and planners must make sure the following points are addressed during cable
installation:
•
Cable reels are typically provided on flatbed trucks. Ideally the trucks would bring the cable
directly to the job site, where the reels would be placed on the reel stands or trailers. More
typically, however, the reels must be stored in a storage yard provided by the owner or the
installation contractor and are brought to the job site as needed.
•
Some installers have special trailers on which the reels can be placed in the storage yard and
brought to the job site, where the trailers are used as reel stands; then no additional handling
is required to place the reels in reel stands. Cable reels should be supported on adequate reel
stands or on trailers specifically designed for transmission cable reels.
•
The contractor should carefully inspect the cable for damage as the reel blanket is removed
and as the cable is being pulled into the duct. If damage is found, the owner representative
and cable representative should be notified, and a decision made on repair or rejection.
•
A jacket integrity test should be performed before the cable is pulled into the duct. Pulling
should be done in the direction shown in the drawings. Pulling tensions could be exceeded or
the cable jacket could be damaged if pulling is done in the opposite direction. The cablepulling winch should have a capacity at least 150% of the maximum allowable pulling
tension for the cable being installed. Caterpillar-type drives may also be used to push along
the cable to reduce pulling forces during cable installation.
•
An accurately calibrated dynamometer should be provided, with pulling force versus length
recorded for the full length of the pull, and an accurate length indicator provided.
•
The pulling rope should be of a material, size, and stranding to minimize abrasive action on
the duct wall. The rope should be clean and dry. A pulling swivel should be provided,
adequate for the cable size being installed.
•
The cable should be installed only during good weather. If the ambient temperature is going
to be lower than about 20°F (-7°C), the cable must be kept in a heated building for at least
24 hours prior to the pull and then pulled into the duct without delay.
•
The manholes at both the pulling end and the winch end should be clean and dry.
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•
The ducts should be proofed just before cable installation. The ducts may be pre-lubricated at
this step.
•
The cable bending radius should be no smaller than 20 times the cable diameter or the
manufacturer’s assigned minimum bend radius at any time during cable pulling.
•
The rigging at the pull-out manhole should include a pull-out shoe or tube to ensure that the
cable is pulled straight out the duct mouth. Adequate cable is pulled to extend well beyond
the manhole center to allow removal of any damaged cable before beginning a splice. A
brake operator is stationed at the reel to provide any braking necessary to prevent excessive
slack as the cable leaves the reel.
•
Manufacturer’s pulling bolts or pulling eyes are used for all but the shortest cable pulls.
Basket grips should be used only for very short pulls, with the manufacturer’s approval. Any
cable within the basket grips must be scrapped.
•
A feeding tube, equal to the duct inside diameter or larger, is used for feeding the cable into
the duct. Cable lubricants acceptable to the cable manufacturer are used on most pulls to
reduce friction and make the pull go more smoothly.
•
Reliable communication is vital between personnel at the pulling (winch) end and the reel
(feeding) end. The cable pulling force should not exceed the manufacturer’s stated maximum
pulling force, and the installer should have a chart of calculated pulling forces. Cable pulling
speed should not exceed 40 ft/min (12 m/min) unless prior approval by the cable
manufacturer and the owner.
•
Cables with integral distributed temperature-sensing optical fibers might require special
installation procedures provided by the manufacturer.
2.5.1.2 Post-Installation
The following steps should be taken after the cable is pulled into the duct:
•
Perform a jacket integrity test after the cable pull is complete. Both the leading and the
trailing ends of the cable should be securely sealed with a watertight cap to prevent any
moisture entry. The cable ends should be attached to temporary rigid supports before
installation of the terminations.
•
Allow the cable 24 hours of rest in the duct before splicing begins to allow relaxation of
internal tensions developed during the pulling operation.
•
As the owner, request a copy of the design calculations for cable forces resulting from cable
thermal expansion early in the project and should review all drawings before cable
installation. The cable manufacturer should be responsible for the design and furnish all cable
and splice restraint hardware, clamps, straps, and other material for restraining the cables and
splices in the manhole.
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•
Install all restraints in the manhole to properly provide adequate support of the cable without
deformation of the cable jacket or insulation. The cable should not rest with its full weight on
the duct end bell.
•
The cable should have permanent identification in the vaults showing phase and direction to
terminals, preferably using imprinted brass or embossed plastic tags.
2.5.1.3 Splicing
Proper splicing is critical to successful cable system operation. The following points should help
ensure a successful splice:
•
Splicing is almost always performed in manholes—either full manholes as used in a duct
bank system and many directly buried systems, or splicing bays with temporary enclosures as
used in some directly buried systems.
•
Splicing is typically performed by factory personnel or factory-trained contractor
personnel—and many specifications require at least five years’ experience on cables of the
voltage cable being installed. Splicing should proceed in strict conformance with the
manufacturer’s written procedures and drawings.
•
The manufacturer should provide a quality assurance checklist, and the splicers should
diligently check each item as it is completed. Clamping designs differ significantly, and the
manufacturer’s instructions must be followed explicitly.
•
Splicing might not need to proceed around the clock. In its bid, the cable manufacturer
should indicate the point in the splice installation at which operations may be suspended and
should provide detailed instructions on steps to be taken to protect the splice until operations
resume.
•
Take care to ensure proper cable phasing. The owner should normally conduct its own phase
check in addition to the installer’s phase check.
2.5.1.4 Link Box Installation
Link boxes are required for bonding and grounding connections and to house sheath voltage
limiters (SVLs) that protect the cable jacket from transient over-voltages.
The following points should help ensure owners of successful link box installation:
•
Link boxes should be installed as close to the splices or terminations as feasible to minimize
voltage drop during transient currents. Use submersible link boxes should be for manhole
installations.
•
Some owners prefer to have the link boxes in separate small vaults outside the manhole,
allowing easier access for maintenance, testing, and replacement. Coaxial cables connecting
the splice enclosures to the link boxes should not be longer than approximately 30 ft (9 m).
Refer to AEIC Specification CS-9 for background information on requirements for bonding
cables—but note that the installer must follow instructions provided by the manufacturer and
cable system designer.
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•
Because some link boxes have sheath voltage limiters and others do not, the installer must
take care to install the proper box at each location as well as make the proper connections.
The installer should carefully follow the manufacturer’s or designer’s drawings regarding
link box placement and connections. Permanent nonconductive labels should be affixed to
each bonding lead, identifying the phase and the direction relative to one transition site to
assist with later maintenance operations.
2.5.1.5 Terminations
Considerations for terminations are as follows:
•
Termination (pothead) installation is typically performed by factory personnel or factorytrained contractor personnel—and many specifications require at least five years’ experience
on cables of the voltage being installed. The owner or installation contractor often supplies
assistants.
•
Proceed in strict conformance with the manufacturer’s written procedures and drawings in
termination installation. The manufacturer should provide a quality assurance checklist, and
the splicers should diligently check each item as it is completed. Perform termination in a
clean, controlled environment, including humidity control if required by the cable/splice
manufacturer. Terminating may be done using one of four assembly approaches, depending
somewhat on voltage level:
•
–
Working from substation support structures
–
Working from scaffolding set up to a height to surround the termination (for pole-mount
terminations)
–
Working from a bucket truck or manlift
–
Assembling on the ground and then lifting the assembled termination into place
Provide sufficient support clamps every 5 ft (1.5 m) or closer as specified by the designer.
2.5.2 Cable in Directly Buried Installations
2.5.2.1 Cable Installation
Considerations for cable installation in directly buried applications can be summarized as
follows:
•
A jacket integrity test should be performed on the cable before installation begins.
•
The entire length of trench, manhole-to-manhole, must be left open to install the cable, but
sections may have steel plates installed over the open trench if needed and can safely span
the trench and accept necessary mechanical loading.
•
Care must be taken to maintain proper and constant spacing between the cables, in order to
give consistent mutual heating, minimize the chances of failure on one phase affecting a
companion phase, and reduce the chances of damage to a companion circuit if work is
occurring on a de-energized circuit in the same trench.
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•
For directly buried installations, cable is generally pulled into the trench using rollers,
although in cases in which there are no foreign utilities to cross under, smaller reels may be
suspended above the trench and the cable paid off. On bends, rollers are spaced frequently
enough to keep sidewall pressure at each roller within the manufacturer’s allowable limits.
The rollers must be wide enough to avoid having the cables “pinched” or ride over the top of
the rollers during installation.
Other methods for installing directly buried cables in the prepared trenches are the following:
•
Synchronized power-driven rollers
•
Caterpillars or cable pushers
•
Bond-pulling
Considerations regarding completion of the cable installation process include the following:
•
A jacket integrity test should be performed before encasement with selected backfills begins.
•
Backfilling around the cables must be done carefully to avoid damaging the cable.
•
Perform a jacket integrity after encasement is complete.
•
Mechanical protection to avoid damage to the cables is extremely important, even in rural
areas.
2.5.2.2 Splices in Manholes
Splice and bonding lead inspection is much easier, and fault location is very much more rapid if
the splices are in manholes.
2.5.2.3 Splices in Splice Bays
The splicing operation itself is similar to that for a duct-and-manhole system. Considerations for
the directly buried splice in a splice bay are as follows:
•
•
Place splices in a concrete bay if a full manhole is not used. A temporary enclosure is placed
above the splice bay during splicing operations. Horizontal splice placement is common.
Provide access to link boxes to permit jacket testing and inspecting sheath voltage limiters.
Small hand-hole boxes may be placed next to the splice area.
2.5.2.4 Terminations
Terminations and terminal structures can be identical to those used for a traditional duct-andmanhole system.
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2.6
Special Installation Conditions
2.6.1 Elevation Changes
Extruded dielectric and SCFF cables may be installed in areas with steep slopes if proper
precautions are taken to avoid excessive mechanical stress on the cable. These cables pose the
concern of ratcheting into the downhill splice manhole or experiencing jacket damage or other
cable damage because of weight. SCFF cables must also contend with the pressure due to
hydraulic heads; this is an important consideration in both cable design and cable installation.
The installer should carefully follow the requirements provided by the cable system designer.
Determining the requirements for restraints to accommodate slopes is a complex topic that is
addressed long before the installation contractor begins work.
2.7
Termination Structures
Extruded dielectric cables, because of their relatively light weight, flexibility, and absence of
fluid-feeding systems at the terminations, combined with the common use of polymer terminals,
can be installed on a variety of termination structures, ranging from typical substation structures
to rather elegant-looking transition poles. Considerations for transition structures include the
following:
•
If a spare termination is to be installed, connected to a fourth, spare phase, the transition
design should accommodate methods to quickly connect the spare termination at each end to
replace the damaged cable section. The termination structure should have a low-resistance
ground, 10 ohms or lower, to avoid transient over-voltages on the cable sheath and also to
help limit step and touch potentials around the structure. The structure should have
mechanical protection for the cables up to at least 8 ft (2.4 m) above the surface.
•
Properly sized surge arresters should be provided, located in an appropriate position relative
to the terminations and the structure itself. Surge arrester sizing and specification should be
provided by the owner or cable system designer.
•
Heat dissipation should be checked carefully if the cables are to be placed inside transition
poles.
2.8
General Requirements for Sheath Bonding and Grounding
Design of the sheath bonding and grounding system is a critical part of the system design for
extruded dielectric cable and SCFF cables because it affects safety (primarily related to induced
sheath voltages during steady-state and transient conditions), voltages induced on proximate
utilities (such as pipelines and communications circuits), cable rating, and the integrity of the
cable jacket. Make certain the installation contractor carefully follows the instructions of the
designer and cable/splice/termination manufacturers for making grounding connections and
performing sheath bonding.
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2.8.1 Multipoint Bonding and Grounding
For a multipoint bonded system, the cable sheaths are solidly bonded together and connected to
ground at both terminals at least, and at other locations such as splice housings. Currents are
induced in the sheath and can cause electrical losses as great as those of the conductor itself—
thereby reducing the rating of the cable circuit. However, induced sheath voltages are small.
2.8.2 Single-Point Bonding and Grounding
With single-point bonding, the sheaths of the three cables are connected and grounded only at
one end of each cable section. This interrupts the flow of sheath currents, allowing a higher
ampacity than with a multipoint bonded system. However, a voltage now appears between the
sheath and ground, with a value equal to approximately 50 volts per thousand feet per thousand
amperes (150 volts per thousand meters per thousand amperes) with the actual value depending
on spacing among the phases and configuration of the three phases. Owners typically allow
open-end voltages of 100–200 volts, which limits the section length to a few thousand feet
(1000 m or less) for most lines. If there is a midpoint splice with shield/sheath interrupts,
distances twice that long can be accommodated by grounding at the terminations and connecting
the two sides of the splice shield interrupt through sheath voltage limiters to ground.
Single-point grounding/bonding requires both a ground continuity conductor (also called an earth
continuity conductor) installed in parallel with the power cables and sheath voltage limiters at
each ungrounded shield/sheath end, to limit transient voltages on the jacket and shield/sheath
interrupts to acceptable levels. Select the location of the ground continuity conductor to limit
circulating currents in that conductor. For buried installations, the ground continuity conductor
should be transposed for the same reason. The cable system may be multiple single-point bonded
to allow long lengths (many miles or km) to be installed. One end of each shield/sheath section
would be grounded/bonded, and the other end would be connected to ground through sheath
voltage limiters.
2.8.3 Cross-Bonding
The three cable sheaths are transposed electrically (and sometimes physically as well) so that
sheath voltages are approximately zero at the end of each three sections, or triad. The induced
sheath currents are very small, so the cross-bonded system has a higher rating than the multipoint
bonded system, essentially equal to the single-point bonded system. Physically transposing the
cables is uncommon because it is difficult to do with ducts and manholes, which are the
predominant installation system. Transposing is easier with directly buried systems.
The cross-bonded system does not require a ground continuity conductor, although many owners
install one to have a single end-to-end ground conductor and a good location for making
grounding connections in manholes. The cable installer does not have as much freedom in
relocating manholes with cross-bonding; if the imbalance in minor section lengths is greater than
15%, there is enough circulating current that the degree of cable derating might be unacceptable.
Use of the ground continuity conductor requires both technical and economic considerations and
should be evaluated carefully during the design stage.
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If the number of cable sections (termination-to-manhole and manhole-to-manhole) is not
divisible by three, it is possible to have one or two sections single-point grounded/bonded and
have the remainder of the line cross-bonded, in which case a ground continuity conductor is
needed for the full length of the single-point bonded sections.
2.9
Fiber-Optic Installation: Other Communications
Most duct banks include ducts for communications, temperature monitoring, or other owner
requirements in addition to ducts for the power cables. The owner might also make ducts or
fiber-optic cables within the ducts available for outside communication company use.
To allow unimpeded access by communications personnel, the fiber-optic communications
cables are generally not spliced in the power cable manholes. Furthermore, some owners prohibit
communications cables in manholes because it would be necessary for the owner to de-energize
the power cables to allow communications personnel into the manhole for inspection,
maintenance, or repair. Typically, owners route ducts for fiber-optic cables around transmission
cable splice manholes, and the ducts are installed to pull-through hand-holes. The contractor
should submit information on the fiber optic ducts and fiber optic installation similar to the
information on the cable ducts (proofing procedures, direction of pull, and splice locations).
Fiber-optic cables installed in duct banks should have a nonconductive construction. Choose the
location of the cables location to limit the operating temperature to 65°C if possible. The
communications cables might be installed by the cable installation contractor or by another
installation contractor, often at a later time. Follow the specifications provided by
communications. General procedures are described as follows:
•
The conduit is mandreled and cleaned.
•
Attenuation tests are performed on the fibers to ensure that they are acceptable.
•
The cable is pulled or blown into the duct, depending upon its size and weight. The
manufacturer’s requirements for minimum bending radius and maximum pulling tension
must be followed.
•
Fiber-optic splices are made with the fusion-splicing method, attenuation tests are performed,
and the splices are placed in splice trays contained within waterproof splice housings. The
housings are pressure tested to ensure that they are leak tight.
•
A power meter test is performed when the fiber optic cable installation is complete, and the
total system loss in dB at the stated frequencies (1310 nm and 1550 nm) must be less than the
values determined based upon the cable length and number of fusion splices.
•
Optical time domain reflectometer (OTDR) tests are made on the completed cables, and the
contractor must remake any splice or termination that does not meet the required loss.
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2.10
Distributed Temperature Monitoring
Many new extruded dielectric cable circuits have distributed fiber-optic temperature monitoring
to monitor the cable temperature along the complete length of the circuit.
Most extruded dielectric cable circuits have a multimode optical fiber installed in the cable
construction, typically in a stainless steel or plastic tube replacing a drain wire or under the cable
sheath. An optical fiber cable might also be installed in a separate duct. Directly buried cable
circuits can have the optical fiber cable installed alongside one of the power cables—typically
the center-phase cable, which is the hottest.
The instrument measures temperatures along the optical fiber with pulses of light into the optical
fibers. The light that is backscattered from impurities in the glass fiber is used to measure the
temperature at discrete locations along the cable. Multimode fibers are most common and can
monitor lines up to about 6 miles (10 km) long.
Single-mode fibers may be used for temperature monitoring on longer lines, exceeding 5 miles
(8 km) or so up to 18 miles (30 km). The spatial resolution is not quite as good as of the
multimode.
Most owners have the temperature measurement performed off-line by having a firm with the
instrument take the measurements, prior to initial energization, to determine the ambient earth
temperature profile along the line, and at a later time when the circuit loading is high enough to
give a temperature rise of 10°C or more above ambient earth temperature. The distributed
temperature monitoring system is designed during the initial cable system design. Principal
considerations include fiber placement, fiber characteristics, and locations of monitoring points.
Lines longer than several miles using multimode fibers might require access points at
intermediate manholes because of attenuation of the signal in the fibers.
Distributed temperature monitoring is most commonly applied to extruded dielectric cable
circuits because of the relative ease of applying the fiber and because extruded dielectric circuits
do not have the temperature averaging that is possible with circulation of fluid in HPFF cable
circuits. This temperature monitoring system has the following implications for the cable
installer:
•
If the optical fiber is integrated into the cable construction, avoid damaging the fiber during
power cable installation.
•
Power cable splices must also include optical fiber splices, requiring different specialists and
increasing the cost, time, and complexity of the splicing operation. Sometimes the fibers are
damaged during power cable splicing and terminating.
•
Remove the optical from the cable or duct at monitoring locations and run to a separate fiberoptic splice box.
•
Special “pigtails” must be connected to the optical fibers at each measurement point, again
requiring a fiber-optic splicing specialist.
•
Distributed temperature monitoring systems, versus the power cable systems require testing.
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0
3
PIPE-TYPE CABLE SYSTEM INSTALLATION
3.1
Introduction
This section addresses construction operations that are specific to pipe-type cable systems. In
keeping with the focus of this report, the work described focuses on the installation contractor’s
activities—all design activities are assumed to have been completed properly. This information
is included to aid owners and planners in overseeing the construction activities.
3.2
Overview
Pipe-type cables are installed primarily because of their successful operating history, ruggedness,
greater amenability for installation in city streets, ability to uprate, and the possibility for much
longer installation lengths in horizontal directional drills than extruded dielectric or SCFF cables.
There is now only one domestic manufacturer. However, it is expected that there will be
continued need for manufacturing, installation, and engineering expertise in order to maintain
and modify existing systems in addition to new installations.
Three cables are pulled simultaneously into a previously installed coated and cathodically
protected steel pipe. The area between the cables and pipe wall is filled with a dielectric liquid
(for HPFF cables) or dry nitrogen gas (for high-pressure gas-filled [HPGF] cables). The liquid or
gas is pressurized to provide the required electrical strength for the impregnated paper insulation.
3.3
Cable Pipe
3.3.1 Pipe Specifications and Size
Two types of pipe are used for pipe-type cable systems. Carbon steel line pipe is used to house
the three cables for the major part of the route. Stainless steel riser pipe is used for the singlephase lengths that take cable to the terminations.
3.3.1.1 Carbon Steel Line Pipe
ASTM Specification ASTM A523 (2005), Standard Specification for Plain End Seamless and
Electric-Resistance-Welded Steel Pipe for Pipe-Type Cable Circuits, was developed specifically
for the high quality control requirements for pipe to be used for pipe-type cables. ASTM
Specification ASTM A53 (2007), Standard Specification for Pipe, Steel, Black and Hot Dipped,
Zinc-Coated, Welded and Seamless, is commonly specified for petroleum pipes. Owners should
specify that the pipe be supplied in accordance with A523. However, steel mills produce pipe to
A523 specifications in minimum-quantity pours, and the owner might need to accept the A53
pipe. If that is the case, the owner should inspect the pipe interior carefully to ensure that it is
satisfactory.
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Grade A line pipe is preferred, regardless of whether the pipe is provided to A523 or A53
specifications. Grade A pipe is more ductile than Grade B pipe and is therefore better suited for
the flaring of pipe ends and for bending without rippling or kinking. Grade B pipe is stronger but
more brittle and prone to cracks during the flaring operation and rippling/kinking when bending
to radii less than 50 times pipe diameter. ASTM A53, Grade B pipe is typically specified for
joint casings, which benefit from the extra strength and do not require flaring or bending. If
Grade B pipe is provided, sleeves are typically used for welding sections together. The sleeve
requires twice as many welds.
3.3.1.2 Stainless Steel Riser Pipe
Nonmagnetic Type 304 stainless steel riser pipe in accordance with ASTM A312, Standard
Specification for Seamless, Welded, and Heavily Cold Worked Austenitic Stainless Steel Pipes, is
typically specified. Stainless steel pipes are joined with couplings and fillet welds.
3.3.1.3 Pipe Size
3.3.1.3.1 Carbon Steel Line Pipe
Pipe sizes for cable use are 5, 6, 8, or 10 in. (13, 15, 20, or 25 cm) nominal diameter, IPS (iron
pipe size) scheduled pipe. Nominal 8 in. pipe is the most common for new installations at
69–345 kV, but nominal 10 in. pipe is used for 345 kV kraft-paper insulated HPFF cables and is
sometimes used for 345 kV laminated paper-polypropylene (LPP) insulated cables. Wall
thickness for most pipes on land is 0.25 in. (6.35 mm), and 0.375 in. (9.52 mm) is specified for
directional drill and submarine cable installations. Wall thicknesses is selected based on the
system pressure or the need for additional strength or negative buoyancy, as well as for greater
resistance to corrosion leaks or burn-through from faults, desired for long water crossings.
Thicker-wall pipe may be required for forced cooling or other high-pressure application. Pipe is
supplied in double random lengths from 35–50 ft (10.7–15.2 m). To reduce the number of field
welds, lengths should average 48–50 ft (14.6–15.2 m). Jointers (two shorter sections factorywelded together) should not be permitted. Order pipe with no mill coat.
Draft AEIC specification CS-2, 7th Edition (2010) provides a great deal of information on test
and quality control requirements for cable pipe. The owner or contractor might order flared,
coated pipe from the coating facility or order the pipe from the fabricator and have a separate
contract with the coating facility to flare, clean, and coat the pipe.
3.3.1.3.2 Stainless Steel Riser Pipe
Riser pipes should be sized to provide at least a 1/2 in. (13 mm) clearance for the single pipetype cable to be installed in the pipe, and it is very important to ensure that the riser pipe size is
coordinated with the termination pipe stub. Schedule 10S stainless steel pipe is typically
specified. The gap between stainless steel pieces should be held to a minimum to ensure a
smooth bore, eliminate possible edges, and maximize the strength of the joint.
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3.3.2 Pipe Coating
The coating facility should inspect, clean, and grit-blast or sand-blast the pipe prior to coating
application. The interior of the pipe should be coated to prevent rust during shipping and storage.
A dry film thickness of Dearborn Endcor 745 Amide-Cured Epoxy coating or approved dielectric
fluid inert equivalent coating should be applied.
The most common exterior pipe coating for new installations is a plastic coating, consisting of
butyl rubber applied to the pipe to a thickness of approximately 0.010 in. (2.5 mm), followed by
an extrusion of polyethylene or polypropylene to a thickness of 0.06–0.08 in. (1.5–2 mm). The
last 4 in. (10 cm) of each end of the pipe, both interior and exterior, should be left uncoated so
that welding can be performed on the pipe ends.
Owners will typically have their independent inspector visit the coating facility to inspect the
pipe and coating, including test-fitting a chill ring into the flares to ensure good fit. The pipe
should be inspected in the coating facility for damage to the pipe itself in accordance with ASTM
A523 for damage to the flared ends, and for damage to the corrosion coating. This includes
visual inspection, fitting of a sample chill ring into the flare for trial test fit, and a test of
electrical integrity performed with a 12–15 kV holiday tester.
The polyethylene or polypropylene coating does not have sufficient abrasion resistance for pipe
to be installed in a horizontal directional drill. The commonly applied coating is a fusion-bonded
epoxy followed by a polymer-concrete coating, applied after the pipe is thoroughly cleaned and
grit-blasted. The last 4 in. (10 cm) at each end (6 in. [15 cm] or more if pipe sleeves are to be
used instead of chill rings) should be left uncoated to avoid interference with the welding
operation.
The fusion-bonded epoxy and polymer concrete coated pipe should be inspected at the coating
facility as is done for the plastic coatings, although the test voltage is lower, recognizing that the
electrical strength is not as high as that of plastic coatings.
Place tight-fitting plastic pipe caps on each end of the pipe and taped closed after the pipe is
inspected and cleaned if necessary. Keep the pipe in place until pipe installation begins.
Approximately 1 in. (2.5 cm) diameter rubber rings or loops of rope should be placed every 8 ft
(2.4 m) or so along the pipe length to prevent pipe—and especially pipe ends—from banging
together.
3.3.3 Transporting, Handling, and Storing Pipe
Cable pipe is commonly shipped by trailer. All pipe should be protected with bedding and
spacers to prevent shipping damage to the coating or pipe flares. The pipe must be lifted with
wide (8 in. [20 cm] or wider) slings any time it is handled, to avoid damaging the pipe coating.
Pipe ends should not be allowed to come into contact with the ground during pipe handling.
Pipe may be stored for many months in multiple trailers at the job site until needed if necessary.
If pipe must be unloaded and installation is not to be done within a week or two, the pipe should
be supported in tiers a maximum of six high on sandbags or timbers and secured to protect
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against rolling. If pipe is to be stored for more than about two months, the owner should contact
the coating supplier to obtain any special storage procedures. Protect coated cable pipe,
especially polyethylene and polypropylene coated pipe, from contact with petroleum products
such as gasoline, oil, and kerosene.
3.4
Pipe Installation
Figure 3–1
Pipe installation progression in a city street
Figure 3-1 gives a representation of the progression of pipe installation in city streets, from
pavement cutting through pavement restoration.
Owners and planners should see that the following the pipe installation procedures are followed:
•
The contractor should describe the pipe installation procedure to the owner before beginning
the work.
•
Welders should be qualified in accordance with American Petroleum Standard 1104, with the
following additional requirements. Burn-through is not permitted. Elongated slag inclusion
should not exceed 1 in. (25 mm) in length or 1/32 in. (0.8 mm) in width. In any 12 in.
(30 cm) weld length, the total length of elongated slag inclusions should not exceed 2 in.
(51 mm). “Wagon tracks” of parallel slag lines should be considered as individual defects if
their width is greater than 1/64 in. (0.4 mm). No cracks are allowed.
3.4.1 Joining Pipe Sections
Carbon steel line pipe should be inspected before installation. Pipe welding can be most
efficiently done above or alongside the trench. For installations in city streets, space restrictions
typically force the welding to take place in the trench itself.
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3.4.1.1 Welding for Flares and Chill Rings
A chill ring (backing ring) is inserted into the flared ends of pipe to serve three to help align the
two pipe sections; to prevent weld slag from entering the pipe during welding; and to provide a
smooth profile, with the inside diameter of the chill ring equal to the inside diameter of the cable
pipe
3.4.1.2 Sleeve Welds
The following considerations apply to working with sleeve welds:
•
The pipe sleeve should be approximately 1/8 in. (3 mm) larger in its inside diameter than the
cable pipe outside diameter, with a wall thickness of approximately 3/8 in. (9 mm) and
should have a length at least equal to the cable pipe diameter. Pipe ends should be rounded
slightly on the inside edges and carefully marked to ensure that the sleeve is centered over
the two ends.
•
After sliding the sleeve over one end, the contractor may bring the pipe ends together to
ensure that there are no gaps and then carefully tack-weld the ends (making sure that no slag
enters the pipe) after verifying that the pipes are aligned. The tack welds should be ground so
that the sleeve will fit over the pipe ends. The sleeve should then be returned to the proper
centered position over the pipe ends and the fillet welds performed.
•
X-ray inspection is not effective for the sleeves and fillet welds. Inspection typically consists
of dye penetrant and magnetic particle detection testing. Dye penetrant testing is normally
used on stainless or other nonmagnetic steel applications. Corrosion coating should be placed
over the weld area after the weld has passed the weld integrity tests.
•
The entire pipe section, including the weld, should then be “jeeped” in its final location at
12–15 kV in accordance with NACE Standard RP0274-2004, High Voltage Electrical
Inspection of Pipeline Coating, and repairs made to the coating if needed.
•
For fusion-bonded epoxy and polymer concrete coatings, a two-part epoxy mix should be
used to restore the coating in the weld area after successful weld integrity tests. The
manufacturer’s instructions should be followed carefully.
•
The entire pipe section, including the weld areas, should be “jeeped” at 2000–8000 volts
before the pipe is installed, and any damaged areas should be recoated according to the
coating manufacturer’s instructions.
3.4.2 Welding Stainless Steel Riser Pipes
Stainless steel riser pipes are welded using couplings that match the diameter of the riser pipe.
Above-ground riser pipes from spreader-head to pothead stub are typically not coated, or they
are coated to a height of about 6 ft (2 m) above the ground. For underground trifurcators,
stainless steel pipes are wrapped with two layers of a high-quality corrosion tape, from the
trifurcating assembly up to a height 6 ft (2 m) above the ground to isolate them from the touch
potential on the pipes.
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3.5
Installing Cable Pipe in the Trench
Installation above or in the trench should follow these procedures:
•
For many installations where the pipe is welded, tested, and coated above the trench, the pipe
may simply be lowered into the trench using the backhoe or crane that was used to support
the pipe for welding. Coffin hoists may also be used to lower the pipe.
•
If the pipe is welded, tested, and coated in the trench, coffin hoists may be used to raise and
lower the pipe as required for the various operations.
•
If the trench has obstacles that the pipe must pass under, the pipe may be pulled along the
trench. Rollers, sandbags, or other methods should be used to prevent damage to the pipe
coating as the pipe is pulled.
•
The pulling line must be attached to the pipe itself using a pulling head; it is not acceptable to
use ropes or grips to grab onto the pipe coating.
•
Bends of a radius of greater than 50 ft (15.2 m) for nominal 8 in. (20 cm) pipe and 100 ft
(30 m) for nominal 10 in. (25 cm) pipe can be accommodated by simply pulling the pipe
around the bend, taking special care to avoid coating damage. Once the pipe is close to final
position, air bags or trench jacks may be used to nudge the pipe into proper position to
maintain the correct spacing from a companion pipe and the trench wall.
•
Smaller-radius bends in the field should be made with a pipe-bending machine before the
pipe section is installed. Great care must be taken to ensure that there are no kinks or ripples
in the pipe. EPRI testing in the 1980s demonstrated that ripples in the pipe greatly exacerbate
sidewall pressure damage when cables are pulled around bends [7]. A mandrel should be
pulled through the pipe before its installation to check for ovality and kinks.
•
Radii approximately twice as large are required to avoid damage to fusion-bonded epoxy and
polymer concrete coatings, because these coatings tend to crack at smaller radii. It is of
course imperative that proper pulling tension and sidewall bearing pressure calculations be
performed for the cable that is to be installed, especially when small-radius bends are
required.
•
Welding a section of pipe bent outside the trench to a section of pipe already installed in the
trench is difficult. Typically, the ends of the two sections are installed side-by-side with a
short overlap. Measurements are made, and the end of one pipe section is square-cut using a
pipe cutter. A pipe pup (field flare) is carefully welded into place and the weld area cleaned,
and then the two sections are joined using chill rings as described in subsection “Welding for
Flares and Chill Rings”. In all cases of field bends, the pipe section should be mandreled
before it is placed in final position and welded to other pipe sections.
•
For installations in which two cable pipes are installed in the trench, care should be taken to
maintain proper spacing at bends to avoid high mutual heating effects that might derate the
cable.
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•
At the end of the workday, a volume of nitrogen equal to the pipe volume should be forced
through the pipe at low pressure, approximately 5 psig (34.5 kPa).
•
Pipe ends should not be left open in the trench. All open ends should be sealed with a
plumber’s plug or other airtight cap and pressurized to 5 psig (34.5 kPa) or higher with
compressed dry air or ASTM Type III compressed dry nitrogen before the end of each
workday. Use steel cylinders or cylinder trucks compressed dry air or dry nitrogen with a
dew point of -40°C or lower. Using air compressors is not permitted because of the
possibility of introducing contaminated oil from worn compressor seals. The entire length of
exposed pipe in a trench should be “jeeped” as described earlier, before encasement with
FTB or other controlled backfill. Any coating damage should be repaired and the pipe
“jeeped” again.
After a manhole-to-manhole section of cable pipe is completed, a coating resistance test should
be performed. This test typically applies 12 volts between the pipe and a driven ground. The
resulting current is measured. The battery polarity is reversed and the current measurement
repeated. The coating resistance is calculated using the average of the two current magnitudes.
The number of square feet (square meters) of pipe in the section is calculated and compared to
the minimum coating resistance determined by dividing the minimum resistance in the table by
the pipe length in thousands of feet.
3.5.1 Pipe in Manholes
The points listed next should be followed for installing cable pipe in the manhole:
•
•
•
•
It is important to have the cable pipes on either side of the manhole accurately aligned so that
joint casings can later be placed with a minimum of difficulty.
Link-Seals should be placed between the pipe and the circular opening in the manhole wall to
reduce water infiltration and to keep the pipe corrosion coating from chafing against the
concrete.
Boiler caps are often welded to the cut pipe ends and the pipe subjected to burst and pressure
drop testing before reducers are installed.
After careful measurement to ensure that the reducers are at the proper separation for the
splice that is to be installed, cut the pipe ends square and remove any burrs or projections.
The reducers should be welded in place. It is not possible to X-ray these welds, but they
should be dye-penetrant tested and subsequent vacuum and pressure testing performed to
reveal any leaks.
3.5.2 Pipe Mandreling, Evacuation, and Pressurization
After a section of pipe is installed manhole-to-manhole and reducers are welded in place, a
blank-off plate with fittings and O-ring seal is bolted in place on the face of the reducer.
A pig should be blown or pulled through the pipe. The pig should include a steel or wooden
mandrel with dimensions given in Table 3-1.
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Table 3–1
Mandrel dimensions for typical pipe sizes for 0.250 in. (6.35 mm) wall pipe
•
•
•
Pipe OD,
in. (cm)
Pipe lD,
in. (cm)
Mandrel OD,
in. (cm)
Mandrel Flat
Length,
in. (cm)
Mandrel Length
Without Eyes,
in. (cm)
6.625 (16.8)
6.125 (15.6)
5.78 (14.7)
8.5 (21.6)
16.5 (41.9)
8.625 (21.9)
8.125 (20.6)
7.60 (19.3)
10.5 (26.7)
20.5 (52.1)
10.750 (27.3)
10.250 (26.0)
9.80 (24.9)
12.5 (31.8)
25.0 (63.5)
The pig should pass easily through the pipe, and the swab should show no indication of
tearing.
The pipe should then be pressure-tested with dry nitrogen or dry air. Pressure should be
supplied from cylinders or cylinder tankers. Air compressors are not permitted. Pressure
should be brought to 500 psig (3450 kPa) for a minimum of 1 hour. The pressure should then
be reduced to 275 psig (1900 kPa) and left for 24 hours. No pressure drop other than that
resulting from temperature changes is allowed.
Following the pressure test, all of the installed pipe should be vacuum tested. Pressure
gauges, 0–30 psig (0–210 kPa), should be installed, remain in place, and the gauges read
weekly until cable installation begins, which can be many months later. If the pressure falls
to 5 psig (34.5 kPa) or lower, the cause should be determined and repairs made.
3.5.3 Special Considerations
3.5.3.1 Piping from the Pressurizing Plant
HPFF cable systems typically have 2 in. (5 cm) steel pipe between the pumping plant and the
line pipe, trifurcator, or spreader-head. This pipe is typically field-taped with corrosion-resistant
tape, cathodically protected because it is connected to the cable pipe, and electrically isolated
from the pressurizing plant by an insulating flange typically located just outside the pressurizing
plant.
3.5.3.2 Fluid Return Lines
If an HPFF cable is to have fluid circulation, the owner will typically install nominal 4, 5, or 6 in.
(10, 13, or 15 cm) steel pipes to provide a return path for the fluid flow. These steel pipes are
installed when the cable pipe is installed, and the same care is taken in maintaining a goodquality corrosion protection system. Because cable will not be pulled into the pipes, requirements
are not as strict for bending radii or preventing weld slag penetration.
3.5.3.3 Bonding Leads
Cathodic protection bonding leads for measuring are typically attached to the cable pipe from the
end of a manhole wall. Leads should be brought to a waterproof junction box in or near the
manhole. They should be Cadwelded to the pipe, and strip adhesive should be placed under and
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over the leads for a distance of 15 in. (38 cm) along the pipe axis. A heat-shrinkable wraparound
sleeve s is positioned to overlap the line pipe coating a minimum of 6 in. (15 cm) on the end
away from the lead. The sleeve is heated and shrunk according to the manufacturer’s
instructions.
One lead can be used as a pipe connection to measure the pipe-to-soil potential as part of
maintenance to check effectiveness of the cathodic protection system. The second connection is
provided to form a pipe shunt that is used to measure direct current flow on the pipe.
3.5.3.4 Thermocouples
Thermocouples may be attached to the cable pipe, near the 103 ft (31.7 m) bonding lead and at
other locations that might be “hot spots” on the cable line. The thermocouples are typically
Type T copper-constantan and are attached to the outside of the corrosion coating with
conductive epoxy. They are taped to the pipe to prevent their being pulled free during
backfilling. Leads near manholes are typically brought to the same junction box as used for the
cathodic protection bonding leads. In areas away from manholes, the leads are typically brought
to a valve box that is installed just off the roadway.
3.5.3.5 Temperature Sensing Fiber Optics
Some owners attach small plastic pipe to the cable pipe. Fiber-optic cables for distributed
temperature sensing are then pulled or blown into the plastic pipe and connected to a distributed
temperature sensing instrument to allow a determination of the pipe temperature along the length
of the line.
3.5.3.6 Communications Ducts
It is common for owners to have nominal PVC ducts installed in the trench as the cable pipe is
being installed, to be used for communications cables. Inner ducts may also be installed to allow
multiple communication cable installations. The ducts may be routed inside the power cable
manholes or routed to separate service boxes, depending on working rules of the particular
owner.
3.6
Cable Installation
The owner should hold the installation contractor responsible for determining cut lengths—the
lengths of cable provided manhole-to-manhole or manhole-to-termination. The contractor should
have full responsibility for the cable, even if the owner has placed the order.
A listing of the cut lengths and the sequence and approximate delivery requirements should be
provided to the cable manufacturer early in the installation project because many months are
required to fabricate, impregnate, and supply the cable. The cable manufacturer typically will not
place the order into production until final cut lengths are received. Follow the cable
manufacturer’s instructions when installing the cables. The following sections describe general
procedures.
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3.6.1 Handling the Cable
The following points should be noted by owners and planners with regard to handling the cable:
•
Ideally, the cable would be taken directly from the manufacturer to the job site and installed
without having to be stored near the job site. However, this is seldom possible. The
contractor should obtain a secure storage area for the cable reels. Note that the owner
sometimes provides the storage area.
•
When installing cable during freezing weather, consult the cable manufacturer for any special
requirements. Follow the manufacturer’s recommendations for protecting and monitoring the
cable during storage.
•
The contractor should be responsible for unloading the reels and placing them in the secure
area. The contractor should make a thorough visual inspection of each reel to determine
whether any damage might have occurred during shipping.
•
The dew point of each cable reel should be checked and recorded weekly until the reel is to
be opened and the cable installed.
•
Reels containing cable should not be dropped or allowed to roll uncontrolled at any time.
Reels should be rolled only in the direction specified by the manufacturer. The reels should
be rotated 180° on a biweekly basis if they are kept in storage longer than two weeks.
•
Reels must be unloaded using either a lifting beam with flange hooks or a steel bar through
the arbor hole and using a spreader bar. Forklifts should not be used because of the size and
weight of the reels and the high risk of damage to the cable.
3.6.2 Equipment Specifications
A great deal of specialized equipment is required for installing pipe-type cable.
The contractor should furnish cable-pulling equipment capable of producing a pulling force at
least 50% greater than the maximum pulling expected for any pipe section. The pulling winch
should be sized to smoothly and continuously pull the cable from one end to the other without
stopping and without overheating, regardless of length of pull, pulling forces, or ambient
temperatures.
•
For longer pulls and higher tensions, the pulling rope should be a minimum 1.125 in.
(28.5 mm) steel rope with an exterior geometry designed to prevent pipe cutting. Hemp core
centers should not be accepted. For short pulls with a calculated pulling force lower than
15,000 lbf (6800 kgf), a 7/8 in. (22 mm) diameter pulling rope may be used.
•
The winch should have a minimum usable length of pulling rope at least 500 ft (152 m)
longer than the longest section-to-section length.
•
The pulling rope should be new or should be cleaned immediately and lubricated with cable
dielectric liquid before the project begins. The rope should be clean, smooth, and free of steel
burrs or other material that might score or otherwise damage the inside of the line pipe. A
yoke should be provided to connect the pulling rope to the three pulling bolts.
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•
An electronic dynamometer with a load cell should be provided, with a capacity to match the
pulling winch. The dynamometer should have been calibrated within the last 30 days before
the pulls.
•
A permanent record of pulling forces and length of cable pulled during the course of cable
installation should be recorded continuously on a data recorder or other electronic device.
•
An indicator should be provided to show footage (length in meters).
•
Feed-in tubes or V-guide reels should be used to guide cables into position. The minimum
bending radius of those devices should be 12 times the cable outside diameter.
•
A suitably controllable braking device should be provided at each reel to permit proper cable
payout during the cable pulling process without over-braking the reel.
•
An inspection platform should be provided as the cables are fed off the reels and gathered
together. This platform permits inspection of the cable before it enters the feed-in tubes.
•
If there is any possibility of rain, a structure should be erected above the cable reels so that
tarps can be quickly placed if rain begins.
3.6.3 Preparation for Cable Pulling
The following details should be noted with regard to cable pulling:
•
All manholes should be thoroughly cleaned and dried.
•
The direction of pulling must adhere to the drawings unless pulling force and sidewall
pressure calculations show that pulling from the opposite direction is acceptable. The owner
must approve any change of pulling direction.
•
The cable pull should not begin if the cable temperature is below the dew point temperature
of the air before the cable pull. All materials, tools, and equipment should be protected from
the weather so that all items are in a clean and dry condition for cable pulling.
•
Two independent methods of reliable communication should be established between
personnel located at the cable reels and those located at the pulling winch.
•
Nitrogen pressure should be maintained inside the pipe at all times except when swabbing or
when the cables are being installed. Exhaust fans should be operated to thoroughly ventilate
all manholes when personnel are in them, in accordance with OSHA safety regulations.
Oxygen content and combustible gas detectors should be monitored continuously.
•
No one should be allowed in the splice manhole when a mousing pig is being blown through
the pipe with a lead line because of the hazard of flying particles.
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•
Rigging located at the pulling end should be established so that the pull is as straight as
possible inside the manhole. Fairweathers or other devices to protect the reducer edges
should be employed. Rigging at the feeding end should be positioned so that each cable
leaves the reel from the upper portion of the drum. Cable reels at the feeding end are
supported on cradles or in other ways that allow the reels to turn freely, and braking devices
provided for each reel.
•
To withstand a force at least 50% greater than the maximum anticipated pulling force,
provide adequate structural supports at both the feed-in manhole and the pull-out manhole.
Use a feed tube for cables to progress smoothly from the cable guide tubes or conveyors to
the cable pipe without any projections, sharp bends, or other areas that might damage the
cable. If at all possible, position the reels so the cables do not have to undergo a reverse bend
as they enter the manhole and enter the cable pipe.
3.6.4 Pulling
The owner might request that a representative from the cable manufacturer be present for
challenging pulls. The following considerations apply to pulling:
•
Cable pulling should not begin if weather conditions are unfavorable or if inclement weather
is imminent.
•
Preloading of cable reels onto trailers is recommended to save time and to avoid using cranes
and reel jacks on site, and to avoid additional traffic control requirements.
•
On the morning of the pull, the nitrogen is vented (observing proper safety precautions), the
section of line pipe is opened, and a mousing line is blown through the pipe using
compressed nitrogen.
•
A pig consisting of a steel mandrel and snug-fitting cloth swabs is connected between the
mousing line and the winch line. The pig is drawn through the pipe until the swab emerges
clean with no indication of tearing. A new swab is used for each pass through the pipe. Take
great care to avoid creating a groove in the pipe when passing the lines back and forth.
•
Extreme care should be taken for pulls longer than a pull from a termination base plate to a
spreader-head or trifurcating joint if stainless steel skid wires are pulled through stainless
steel riser pipes. The cable should be lubricated liberally with dielectric liquid, and the riser
pipes inspected for signs of overheating during pulling.
•
From 5–10 gallons (19.5–39 liters) of dielectric liquid is admitted ahead of the pig, which is
pulled through the section from end-to-end in a direction opposite that of the cable pulling
direction. Larger amounts might be needed for especially long pulls. Provide sealed
containers of dielectric liquid for this operation.
•
If the pull is delayed more than five days, re-lubricate the line pipe interior.
•
Installation of cable, once started, should proceed without interruption until the installation is
complete and the pipe section is sealed, evacuated, or flushed with dry nitrogen and filled
with nitrogen to a pressure of 10 psig (69 kPa).
•
When the cable is fed into the feed tube, it should not be allowed to cross over. Pull at a
maximum speed of 40 ft/min (12 m/min).
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•
A brake operator is stationed at each reel to control the slack and prevent overrunning during
cable pulling. Avoid excessive braking, which would unduly increase pulling tensions.
Control cable back-tension and pulling speed to minimize surging.
•
Place oil-absorbent padding under all cable reels and between the reels and the pulling-in
manhole or terminal directly under the cable travel area.
•
Keep the cable clean and dry and do not be allow it to touch the ground. The minimum
allowable bending radius for loops is 20 times the cable outside diameter.
•
Lubricate the cable with dielectric liquid pulled into each pipe section initially and
periodically throughout the pull.
•
Inspect the cable thoroughly inspected for flaws, breaks, or abrasions of any kind as it is
pulled. The owner must be informed of all such defects immediately, and cable pulling halted
until the cable is inspected by the owner.
•
Record dynamometer readings for the first 50 ft (15 m) and every 100 ft (30.5 m) of pull. If
pulling tensions begin to approach the allowable tension for these cables, the contractor must
advise the owner immediately.
•
On completing the cable pull, the leading and trailing ends should be cut approximately 4 ft
(1.2 m) beyond the top of the terminator or center of the splice after the owner has inspected
the leading end of the cable and given approval to cut.
•
As soon as the pull is complete, night caps are installed and, if there has been no possibility
of moisture pickup during the pulling operation, the section of pipe is flushed with dry
nitrogen until the dew point at the exit end of the pipe reaches -30°C. The pipe is sealed and
charged with 10 psig (69 kPa) of dry nitrogen gas and monitored for a minimum of 1 hour for
loss of pressure. Within the next three days, the pipe section is evacuated as described in the
following step.
•
If humidity is high or it has rained during the pull, the contractor must apply vacuum to the
pipe immediately after completion of the pull to an absolute pressure of 250 micrometers of
mercury or lower, with a vacuum gauge connected at the remote end from the vacuum
pump, as soon as the night caps are installed. The vacuum pump should continue to run for
a minimum of 4 hours after reaching 250 micrometers. At the end of the 4-hour period, the
vacuum pump is stopped and isolated from the pipe. If the rise in pressure exceeds 250
micrometers 1 hour after the vacuum pump disconnect, repeat the entire vacuum treatment.
•
Dry nitrogen should be introduced into the pipe when the vacuum test is acceptable. The
nitrogen gas pressure should be checked daily.
•
For termination pulls, individual cable reels are set up near the terminations, and the cables
are fed over rollers or conveyor belts into the termination stub and the stainless steel riser
pipes.
A spreader-head requires a fairly high termination structure and enough room in the transition
station to position three large cable reels. Some applications use a trifurcating joint within
50–100 ft (15–30 m) of the termination structure. The trifurcating joint is usually placed in a
manhole and has a reducer for the steel cable pipe at one end and a trifurcating plate to give a
transition to the stainless steel riser pipes at the other end. The stainless steel pipes then bend up
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to the termination stub. This gives a lower-profile termination structure and does not require
cable reels to be positioned in the transition station—only a small winch is required to pull the
short lengths of cable. However, it is often feasible to place reels in the substation and pull
through the trifurcator as would be done through a spreader-head.
3.6.5 Splicing
Splice kit suppliers provide detailed instructions as well as templates for ensuring the proper
slopes for electrical stress control. The supplier’s instructions should be followed carefully. The
general procedures are similar to those for a straight joint, but additional requirements are
described in the splice kit supplier’s instruction books.
3.6.6 Terminating
Termination suppliers provide detailed instructions for making the terminations. Follow the
supplier’s instructions carefully.
3.6.7 Final Pressure and Vacuum: Fluid-Filling
Prior to fluid-filling, make all splices, assemble terminations, and complete pipe section welds.
3.6.7.1 Pressure Test
A pressure test runs as follows:
•
After the splicing and terminating are complete, the entire length of the circuit is flushed with
ASTM Type III dry nitrogen until the gas flowing from the remote end of the circuit has a
dew point of -20°C or lower when measured at atmospheric pressure.
•
Nitrogen pressure in the line is then raised to 20 psig (138 kPa), and all welded and gasketed
connections in manholes and terminals tested for tightness by water bath or soap bubbles.
After verification that there are no leaks, the line is left at 10 psig (69 kPa) nitrogen pressure
until the final evacuation and fluid-filling are begun.
3.6.7.2 Vacuum Test
The contractor must submit plans for final vacuum and fluid-filling two weeks before the filling.
Connect vacuum pumps having a capacity of at least 110 cubic ft/min (3115 l/min). Vacuum
pumps are run until the absolute pressure in the pipe is 250 micrometers or lower. After this
value is reached, the pumps are run an additional 8 hours or longer. The vacuum pumps are then
shut off and vacuum monitored for 1 hour. At the end of the hour, the vacuum rise in the pipe
should not exceed 250 micrometers of mercury. If the vacuum rise exceeds 250 micrometers, run
the pumps again until the 1-hour drop test is successfully completed.
3.6.7.3 Fluid-Filling: HPFF Lines
Fluid-filling is fairly straightforward for short lines with minor elevation changes. Filling longer
lines, especially those with significant elevation changes requiring semi-stop joints to allow
isolation of hydraulic sections, or with deep tunnel sections, can be much more complex. The
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owner or contractor should have detailed pressurization plans prepared, showing locations for
evacuation pumps and fluid-filling, the sequence and rate of filling each section, pressure steps
as supplied by the cable manufacturer, locations and pressures for venting terminations and joint
casings, and so on.
Follow the cable manufacturer’s fluid-filling procedure carefully. Procedures include steps for
pressure increases, duration at each step, and duration at final pressure before the line can be
tested or energized. If the owner intends to tag the dielectric fluid with PFT or other tracer gas
for future leak location purposes, do it with the initial fluid filling to ensure uniform distribution
throughout the cable.
The pressurizing plant should be in service before fluid-filling begins. Hoses, pumps, valves, and
piping used in fluid-filling should be thoroughly cleaned and dried, then flushed—first with
nitrogen, then with dielectric fluid—from the tanker being used, immediately before their use in
the fluid-filling operation.
•
Take fluid samples from each tank car or tank truck. Test dielectric strength. It should be
better than 35 kV before the fluid is allowed to enter the cable pipe. Acceptable values are
0.001 at 100°C for power factor and 30 parts per million for moisture content.
•
Provide trap tanks between the line and the vacuum pumps to indicate that the fluid column
has reached the pump and reduce the possibility that vacuum pump oil is sucked into the
cable pipe if the vacuum pump stops before it is valved-off from the pipe.
•
Fill terminals from a fluid source separate from and external to the line pipe. Filling the
terminals might require a bypass filter.
•
Introduce the fluid to the line at a rate no higher than 50 gal/min (190 l/min) at each filling
location. Increase the pressure at the maximum rate recommended by the cable supplier. A
slower rate is recommended to allow equalization of pressure between the cable insulation
wall and the fluid in the pipe so as not to crease the insulation and possibly disrupt the
insulation shields. A 2008 EPRI report based on detailed laboratory testing and
utility/manufacturer field experience provides guidelines for re-pressurization for different
conditions [8].
•
Vent the terminals and joint casings at the pressure levels indicated in the pressurization plan
into a bucket of dielectric liquid until the liquid from the line emerges bubble-free.
•
Carefully inspect for fluid leakage after full pressure is achieved, as well as all manholes and
exposed pipe. Immediately repair any leaks.
•
Take fluid samples and submit them for dielectric strength, power factor, moisture content,
and dissolved gas analysis, as directed in the pressurization plan.
•
Run fluid samples through appropriate gas chromatography units to capture the chemical
footprint of any fluid tagged with tracer gas.
•
On completion of venting and flushing, thoroughly clean the installed terminals, corona
shields and aerial lugs.
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3.6.7.4 Fluid-Filling Lines That Have Vacuum over the Fluid Storage Tank
Owners may maintain a vacuum over the fluid in the pressurizing plant’s fluid storage tank so
the dielectric liquid does not eventually become saturated with nitrogen the way it does with the
traditional nitrogen blanket. Filling procedures in this case are slightly different. If the line is
filled in the usual fashion, with the fluid added to the pipe (generally at the low points) while
vacuum pumps are operating (generally at the high points) to maintain a vacuum on the line.
There is no attempt to degas the fluid as it enters the pipe.
Once the line is full, or nearly full, separate pumps are used to add fluid to the storage tank using
pipe entry at the base of the storage tank to avoid filling the tank too quickly. It normally takes
several days for the vacuum pumps in the storage tank to bring the vacuum down to the desired
level.
3.6.7.5 Gas-Filling: HPGF Lines
HPGF lines have no requirements for semi-stop joints or other restrictions resulting from
elevation changes, so the filling procedure is simpler than that for HPFF lines. The gas cabinet is
connected before nitrogen-filling. Following successful vacuum treatment, the line is pressurized
by ASTM type III dry nitrogen supplied in accordance with ASTM D1933-03, Standard
Specification for Nitrogen Gas as an Electrical Insulating Material at each filling location.
Strictly adhere to the pressurization schedule provided by the manufacturer.
3.6.8 Connecting Cathodic Protection System
Most pipe-type cables have an impressed-current cathodic protection system to provide the
needed protection of the cable pipe. Pipe grounding is provided by a polarization cell or its solidstate replacement. The owner should have a cathodic protection study performed to properly size
the rectifier, anode bed, and polarization cell or isolator surge protector. Considerations for cable
system installation are summarized as follows:
•
Install anodes (either deep-well anodes or horizontally placed anodes) at the location
specified in the cathodic protection study.
•
Install a #8 copper 600-V insulated cable from the anode bed to the rectifier and connect it to
the rectifier’s positive terminal and to the anode lead.
•
Connect the negative terminal of the rectifier to the solid-state isolator with a #8 cable. Use
copper 600-V insulated cable sized for the available fault current to connect the solid-state
isolator and the selected location on the cable pipe.
•
Also use the same size copper 600-V insulated cable for the connection between the solidstate isolator and the substation grounding system. Connection length should be kept as short
as possible.
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3.6.9 Pressurizing Plants
Pressurization plants (also called pumping plants) for HPFF cable systems come from the
manufacturer preassembled, skid mounted, and ready for placement on a concrete foundation.
Although the size of each plant varies according to the size of the fluid tank and the associated
equipment, the dimensions for a commonly sized plant are roughly 13 ft (3.9 m) wide by 12 ft
(3.6 m) high by 40 ft (12.19 m) long. These units are shipped to the site on a flatbed trailer and
are off-loaded by crane onto the concrete foundation. Some owners have the storage tanks
supplied separately, located outside of the pressurizing plant, whether integral to the building or
separate, with secondary containment provided in the form of a moat or double-walled tank.
Typical pressurization plants contain a fluid tank, control panel, motors and pumps, valves,
piping, and nitrogen bottles to provide the required operations of the plant. The hydraulic,
electrical, and physical design of the pressurizing plant is completed as an early step in the
project because plants typically have a very long lead time.
Installation of the plant typically requires little field work. The owner or the installation
contractor installs the pipe and provides corrosion tape on the fluid supply lines from the
trifurcating casing or spreader-head to the pumping plant foundation. The pumping plant
installation contractor connects the plant piping to the fluid supply line through an insulating
coupling that isolates the cathodic protection voltage on the fluid supply line from the grounded
equipment in the pressurizing plant.
The owner or the installation contractor installs a single or dual distribution power supply to the
plant, and the plant installation contractor connects to that supply through an automatic throwover switch or disconnect. The pressurizing plant manufacturer typically provides an interface
box in the plant, and the owner connects its alarm, control, and supervisory control and data
acquisition (SCADA) wiring to that interface box. Since an HPFF cable must be de-energized if
it loses fluid pressure, owners provide either a separate power source (generator) or a nitrogendriven pump in the pressurizing plant sized to provide 8 hours of fluid during feeder cool-down.
Before the cable pipe is filled with dielectric liquid, the pressurizing plant is fully operational,
and initial pressure switch and alarm settings, and relief valve settings, should have been
provided based upon the hydraulic design of the line. The storage tank is generally filled
separately from the cable pipe, and has the approximate amount of fluid desired for initial
operation—approximately half full.
The cable pipe is filled with dielectric liquid while vacuum is maintained and then the
pressurizing plant is allowed to take over to maintain pressure once the line is filled. Fluid
samples are taken for dielectric strength, power factor, and moisture tests.
As part of the commissioning tests, the supplier will ensure that the proper line pressures are
maintained for all conditions (line calling for fluid, line expelling fluid, high-pressure and lowpressure conditions, and so on), operation of primary and backup pump, operation of isolation
valve if present, and nitrogen-driven pump if present. The supplier will check all relief valve
settings, alarm settings, communications systems, flow meter operation if present, leak detection
systems, and so forth. Most owners require the supplier to provide on-site training to owner
personnel at this time.
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It is common for the pressurizing plant supplier to return to the site to make final adjustments to
pressure switch settings, alarm settings, and relief settings, after the plant has been in service for
a week or longer.
These basic procedures are used for all pressurizing plants. Other considerations for specialpurpose plants are as follows:
•
Fluid circulation. Owners with a pair of cable pipes, or a cable pipe and a fluid return pipe,
sometimes use the pressurizing pump at its capacity to operate constantly and slowly
circulate dielectric fluid down one pipe and back the other pipe. This slow circulation will
help smooth out hot spots along the line, such as at a distribution cable crossing.
•
Fluid oscillation. Slow fluid movement can still be provided even if there is no return path if
there are pressurizing plants at each end of the line. The pressurizing pump is set to send
fluid from Plant A to Plant B at the 5–10 gal/min flow rate until a certain amount of fluid has
been transferred from Storage Tank A to Storage Tank B. Plant B then sends the same
amount of fluid back to the storage tank in Plant A.
•
Forced-cooling system. Forced cooling uses separate circulating pumps of much higher
capacity to circulate the dielectric liquid, and cooling is provided by a heat exchanger. The
forced-cooling system is provided as a separate unit from the pressurizing plant, and has its
own pumps, flow control and flow monitoring systems, temperature monitoring, controls,
alarms, and monitoring.
Commissioning tests for these systems require verification of fluid flow and pressure drop for the
design operating conditions. Gauges in the plants may be sufficient, or the supplier may need to
provide separate calibrated instruments. Initial tests will probably be on “cold” fluid that is at
ambient earth temperature. Testing to verify operation at design conditions may not be possible
until the line is in service for many months. Owners may attempt to perform a “heat-run” test by
forcing electrical load through the feeder with the circulation/ cooling, but that is seldom
feasible. The commissioning tests should also verify that circulation stops automatically on
electrical trip of the line, and that any abnormal hydraulic conditions generate a trip signal and
stop the circulation.
3.6.10 Commissioning the Circuit
Unlike the extruded dielectric cables, pipe-type cables are typically subjected to a high-potential
direct current test, and many owners can also require a 24- or 48-hour “soak” test at line voltage
before placing the line in service.
Relevant dc test voltages are listed in AEIC CS-2. IEEE Std. 400.1, Field Testing of Laminated
Dielectric, Shielded Power Cable Systems Rated 5 kV and Above with High Direct Current
Voltage, provides additional details for dc hi-pot testing. Most owners have the 15-minute test
performed. In accordance with AEIC CS-2, if the temperature at any point on the line exceeds
25°C, the test voltage is reduced by an amount equal to 0.0054 times the voltage given in Table
3-2 for each degree in excess of 25°C.
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Table 3–2
DC test voltages for pipe-type cables
Rated L-L Voltage,
kV ac
Acceptance Test Level
4-Hour, kV dc
Acceptance Test Level
15-Minute, kV dc
69
125
165
115
190
255
120
200
265
138
225
300
161
260
350
230
340
455
345
465
625
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CONCLUSIONS
Cable system construction, which includes all work up to the start of cable installation, and cable
installation itself, is time-consuming and costly part of a transmission cable project. Utilities and
contractors have developed and improved upon approaches and procedures and are constantly
trying new approaches to improve efficiency. This report documents the typical procedures,
including overviews of recommendations for improvement, for activities from the conceptual
design of the line through commissioning and energization.
The report references industry standards and specifications for planning purposes. Owners and
planners can find descriptions of typical practices in detail, with illustrative photographs for
much of the equipment and many of the construction and installation procedures, in the EPRI
Underground Transmission Cable System Construction and Installation Practices Manual—
2012 Update (1024180).
Report 1024180 also provides many utility examples, showing the types of projects that are
undertaken, the organizations involved, the construction/installation approaches, the challenges,
the innovations, and the lessons learned. These examples provide a good understanding of the
activities involved in planning and implementing a transmission cable installation.
Although each project must be evaluated on its own—depending upon load requirements,
distance, and local conditions including terrain, traffic conditions, soil types, and so on—the
material presented in this report should provide the decision makers with a good understanding
of the multitude of considerations required for planning a successful underground transmission
cable project.
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REFERENCES
1. Underground Transmission Cable System Construction and Installation Practices
Manual—2012 Update. EPRI, Palo Alto, CA: 2012. 1024180.
2. EPRI Underground Transmission Systems Reference Book (Green Book). EPRI, Palo
Alto, CA: 2007. 1014840.
3. Lower Cost Underground Transmission Cable. EPRI, Palo Alto, CA: 2004. 1008719.
4. Standard Guidelines for the Collection and Depiction of Existing Subsurface Utility
Data, American Society of Civil Engineers (ASCE), ASCE C-I 38-02, 2003.
5. Electrical Safety Management for Underground Transmission Systems. EPRI, Palo Alto,
CA: 2010. 1019981.
6. Mechanical Effects on Extruded Dielectric Cables and Joints Installed in Underground
Transmission Systems in North America. EPRI, Palo Alto, CA: 2004. 1001849.
7. Increasing Pipe Cable Section Lengths. EPRI, Palo Alto, CA: 1983. EL-2847.
8. Pressurization Procedures for High-Pressure Fluid-Filled and High-Pressure Gas-Filled
Cable Systems. EPRI, Palo Alto, CA: 2008. 1015930.
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ACRONYMS
AAHSTO
American Association of State Highway and Transportation Officials
AEIC
Association of Edison Illuminating Companies
ASCE
American Society of Civil Engineers
ASTM
American Society for Testing and Materials
DR
Dimension Ratio
EMR
Experience Modification Rate
EPC
Engineering/Procurement/Construction
EPCM
Engineering/Procurement/ Construction/Management
EPR
Ethylene-Propylene Rubber
EPRI
Electric Power Research Institute
FRE
Fiberglass Reinforced Epoxy
FTB
Fluidized Thermal Backfill
GPR
Ground-Penetrating Radar
HDD
Horizontal Directional Drilling
HDPE
High-Density Polyethylene
HPFF
High-Pressure Fluid-Filled
HPGF
High-Pressure Gas-Filled
ICEA
Insulated Cable Engineers Association
IEC
International Electrotechnical Commission
IEEE
Institute of Electrical and Electronics Engineers
IPS
Iron Pipe Size
LiDAR
Light Detection and Ranging
MTBM
Micro-Tunnel Boring Machine
NACE
National Association of Corrosion Engineers
O&M
Operations and Maintenance
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OSHA
Occupational Safety and Health Administration
OTDR
Optical Time Domain Reflectometer
PVC
Polyvinyl Chloride
SCADA
Supervisory Control and Data Acquisition
SCFF
Self-Contained Fluid-Chilled
SUE
Subsurface Utility Engineering
SVL
Sheath Voltage Limiter
TIN
Triangular Irregular Network
UTW
Underground Transmission Workstation
XLPE
Cross-Linked Polyethylene
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