Standard: Distribution Design Manual Vol 4 –
Underground Cable Distribution
Standard Number: HPC-5DC-07-0004-2014
Document Control
Author
Name:
Anthony Seneviratne
Digitally signed by Anthony
Seneviratne
DN: cn=Anthony Seneviratne, o, ou,
email=anthony.seneviratne@horizon
power.com.au, c=AU
Date: 2014.06.23 11:18:02 +08'00'
Position: Standards Engineer
Document Owner
(May also be the Process Owner)
Approved By *
Name:
Digitally signed by Justin Murphy
DN: cn=Justin Murphy, o=Horizon
Power, ou=Asset Management
Services,
email=justin.murphy@horizonpow
er.com.au, c=AU
Date: 2014.07.07 12:10:28 +08'00'
Justin Murphy
Position: Manager Power System Services
Name:
Digitally signed by Justin Murphy
DN: cn=Justin Murphy, o=Horizon
Power, ou=Asset Management
Services,
email=justin.murphy@horizonpo
wer.com.au, c=AU
Date: 2014.07.07 12:10:57 +08'00'
Justin Murphy
Position: Manager Power System Services
Date Created/Last Updated
June 2014
Review Frequency **
3 yearly
Next Review Date **
June 2017
* Shall be the Process Owner and is the person assigned authority and responsibility for managing the whole
process, end-to-end, which may extend across more than one division and/or functions, in order to deliver agreed
business results.
** Frequency period is dependent upon circumstances– maximum is 5 years from last issue, review, or revision
whichever is the latest. If left blank, the default shall be 1 year unless otherwise specified.
Revision Control
Revision
Date
Description
A
17/06/2014
Initial Document
STAKEHOLDERS
NOTIFICATION LIST
The following positions shall be consulted if an update or
review is required:
The following shall be notified if an update or review is
required
Manager Engineering Services
Engineering & Projects
Manager Assets Management Services
Operations
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TABLE OF CONTENTS
FOREWORD .............................................................................................................. 7 1 INTRODUCTION .......................................................................................... 8 1.1 General .................................................................................................................... 8 1.2 Design Objectives .................................................................................................... 8 2 DESIGN PROCESS AND INPUTS ............................................................. 10 2.1 Safety in Design ..................................................................................................... 10 2.2 Network Requirements ........................................................................................... 10 2.2.1 Planning ................................................................................................................................ 10 2.2.2 Equipment ............................................................................................................................. 10 2.3 Installation Requirements ....................................................................................... 11 2.3.1 Cable Route .......................................................................................................................... 11 2.3.2 Termites ................................................................................................................................ 11 2.3.3 Equipment Location .............................................................................................................. 11 2.4 Equipment Compatibility......................................................................................... 12 2.5 Environmental and Approval Management ............................................................ 12 3 ELECTRICAL EQUIPMENT USED FOR UDS INSTALLATIONS ............. 13 3.1 Medium Voltage Ring Main Unit (RMU) Switchgear .............................................. 13 3.1.1 Outdoor RMU Kiosks ............................................................................................................ 13 3.1.2 Indoor RMUs ......................................................................................................................... 13 3.2 MV Cables, Joints and Terminations...................................................................... 13 3.2.1 MV Feeder Cables ................................................................................................................ 13 3.2.2 MV Transformer Cables ........................................................................................................ 13 3.2.3 MV Cable Joints .................................................................................................................... 14 3.2.4 MV Terminations ................................................................................................................... 14 3.2.4.1 Non-Loadbreak Terminations .............................................................................................................. 14 3.2.4.2 Load break Terminations..................................................................................................................... 14 3.3 Types of Substations .............................................................................................. 14 3.3.1 Modular Packaged Substations (MPS) ................................................................................. 15 3.3.2 Non MPS Arrangements ....................................................................................................... 15 3.3.3 Customer Owned Substations .............................................................................................. 16 3.3.4 Single Phase Padmount Transformers ................................................................................. 16 3.3.5 25 kVA Single Phase (SPUDS) Transformer ....................................................................... 16 3.3.6 10 kVA Rural Underground Transformer .............................................................................. 17 3.4 Service Pillars ........................................................................................................ 17 3.5 LV Cables, Joints and Terminations....................................................................... 18 Page 3 of 47
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3.5.1 LV Feeder Cables: ................................................................................................................ 18 3.5.2 LV Service Cable .................................................................................................................. 18 3.5.3 LV Street Light Cables .......................................................................................................... 18 3.5.4 Other LV Cables ................................................................................................................... 18 3.5.5 LV Cable Joints/Terminations ............................................................................................... 19 4 VOLTAGE REGULATION .......................................................................... 20 4.1 Voltage Tolerance Limits ........................................................................................ 20 4.1.1 Statutory Voltage Tolerance Limits ....................................................................................... 20 4.2 Voltage Drop Criteria .............................................................................................. 20 4.2.1 Effect of Different Load Cycles ............................................................................................. 21 4.3 Voltage Drops and Line Currents in LV Feeders.................................................... 21 4.3.1 General ................................................................................................................................. 21 4.3.2 Effect of Load Unbalance ..................................................................................................... 21 4.3.3 Voltage Drops/Line Currents in Meshed Networks............................................................... 21 4.3.4 Voltage Drop Limits for LV Networks .................................................................................... 22 4.4 MV Voltage Regulation .......................................................................................... 22 4.4.1 Design Approach .................................................................................................................. 22 4.4.2 Computer Modelling .............................................................................................................. 22 4.4.2.1 Voltage Control Equipment ................................................................................................................. 23 5 UNDERGROUND DISTRIBUTION SCHEMES (UDS) ............................... 24 5.1 Design Procedure .................................................................................................. 24 5.2 Transformers .......................................................................................................... 24 5.2.1 Initial Requirements .............................................................................................................. 24 5.2.2 Transformer Selection .......................................................................................................... 25 5.2.3 Mixed Loads.......................................................................................................................... 25 5.2.3.1 Example 1 ........................................................................................................................................... 25 5.2.4 Regions other than Esperance ............................................................................................. 26 5.3 LV Network Design................................................................................................. 26 5.3.1 Primary Aim .......................................................................................................................... 26 5.3.2 Challenge for Network Designers ......................................................................................... 26 5.3.3 Use of Computer Packages .................................................................................................. 26 5.3.4 Aspects of Electrical Design ................................................................................................. 27 5.3.5 Determination of Cable Size ................................................................................................. 27 5.3.6 Selection of LV Feeder Routes ............................................................................................. 27 5.3.6.1 Proximity to Loads ............................................................................................................................... 28 5.3.6.2 Utilisation/Loading ............................................................................................................................... 28 5.3.7 Typical Route Lengths .......................................................................................................... 28 Page 4 of 47
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5.3.8 Interconnection with Other Feeders...................................................................................... 28 5.3.9 Pillar/Cabinet Positioning and Alignment .............................................................................. 29 5.3.10 Other Considerations ............................................................................................................ 29 5.3.11 Typical Design Issues ........................................................................................................... 29 5.4 MV Design .............................................................................................................. 29 5.4.1 MV Cable Requirements ....................................................................................................... 29 5.4.2 MV Network Systems ........................................................................................................... 30 5.4.2.1 Radial Feeder System ......................................................................................................................... 30 5.4.2.2 Ring Main System ............................................................................................................................... 30 5.4.3 Hybrid System....................................................................................................................... 30 5.4.3.1 Satellite Substations ............................................................................................................................ 30 5.5 Automation ............................................................................................................. 31 5.6 Design Outputs ...................................................................................................... 31 5.6.1 Outputs - MV/LV Layouts ...................................................................................................... 31 5.6.2 Outputs - Cable Ducts .......................................................................................................... 31 6 DETERMINATION OF RECOMMENDED LOAD DEMAND VALUES ....... 32 6.1 Estimation of Load Demand ................................................................................... 32 6.2 Effect of Load Diversity on Maximum Demand ...................................................... 32 6.3 Residential Load ADMDs ....................................................................................... 32 6.3.1 Determination of ADMD when standard values are not used .............................................. 33 6.3.2 Non-Residential Load Demands ........................................................................................... 34 6.3.3 Residential Lot Classification ................................................................................................ 34 7 LV FEEDER PROTECTION ....................................................................... 35 7.1 Introduction ............................................................................................................ 35 7.2 Feeder Protection Policy ........................................................................................ 35 7.3 LV Fuse Selection Policy........................................................................................ 35 7.4 Prescribed Fuse Sizes (MV and LV) ...................................................................... 36 7.5 Maximum Lengths of LV Feeders .......................................................................... 36 7.5.1 General ................................................................................................................................. 36 7.5.2 Equivalent Length of LV Feeders ......................................................................................... 36 7.5.3 Feeder Equivalent Length Calculation .................................................................................. 37 7.5.4 Maximum Equivalent Lengths............................................................................................... 37 7.6 What if the Maximum Allowable Length is Exceeded?........................................... 38 7.7 Calculation of Fault Currents at End of LV Feeders ............................................... 38 7.8 Fault Current Ready Reckoner............................................................................... 40 7.9 Typical LV Fuse Time-Current Characteristics....................................................... 42 Page 5 of 47
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8 INSTALLATION REQUIREMENTS ............................................................ 43 9 STREET LIGHTING.................................................................................... 43 APPENDIX A – REVISION INFORMATION ......................................................................................................... 44 APPENDIX B – CURRENT RATING OF UNDERGROUND CABLES .................................................................. 45 B.1 Continuous Current Rating ............................................................................................. 45 B.1.1 Rating Factors for depth of laying direct in the ground ................................................ 45 B.1.2 Rating Factors for depth of laying direct in a duct ....................................................... 46 B.1.3 Rating Factors for variation in Thermal Resistivity (3 core cables laid directly in the
ground) ............................................................................................................................... 46 B.1.4 Rating Factors for variation in Thermal Resistivity (1 core cables laid directly in the
ground) ............................................................................................................................... 46 B.1.5 Rating Factors for variation in Thermal Resistivity (3 core cables laid in duct buried in
the ground)............................................................................................................................ 46 B.1.6 Rating Factors for variation in Thermal Resistivity (1 core cables laid in duct buried
in the ground) ........................................................................................................................ 47 B.1.7 Rating Factors for Variation in Ambient Temperature ................................................. 47 B.1.8 Rating Factors for Variation in Ground Temperature .................................................. 47 Page 6 of 47
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FOREWORD
This volume is one in a series of five volumes, which together, form the Horizon Power
Distribution Design Manual. The DDM is intended to be a comprehensive reference manual
for distribution design work carried out by professional engineers and technical support staff.
The five volumes are:
Volume 1: Quality of Electricity Supply
Volume 2: Low Voltage Aerial Bundled Cable
Volume 3: Supply to Large Customer Installations
Volume 4: Underground Residential Distribution (URD)
Volume 5: Overhead Bare Conductor Distribution
The DDM will also serve to initiate "newcomers" to distribution work in Horizon Power
without them having to start from scratch. It serves to establish "standards" for design work
to ensure that we get the best value from our facilities - not only in terms of initial cost, but
also in terms of component availability, length of service life and cost-effective maintenance.
In addition to this, the DDM will also serve as a teaching aid for courses run by Horizon
Power.
This volume describes the engineering process involved in designing and providing
electricity supplies using underground cables.
It describes the design process in detail, making use of standardised design information for
use with routine work.
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1
INTRODUCTION
1.1
General
This document describes the engineering process involved in designing
distribution underground networks. These networks typically originate from Zone
substations as medium voltage feeders and are stepped down to low voltage
networks through distribution transformers. Low voltage distribution networks
then transmit power to customer installations, though some customers are
supplied directly from the medium voltage networks.
Although, underground cables do not account for a significant proportion of
Horizon Power's networks at present, Horizon Power's policy mandates
underground power supplies in all sub divisions including residential, rural
residential, commercial and industrial. There are variations to this policy in the
case of larger lot sizes greater than 10 hectares, which can be found in Horizon
Power’s Underground Distribution Schemes Manual.
Underground assets are capital intensive, both for Horizon Power and its
customers and they need to be properly designed and constructed. It is
imperative that a high level of engineering is put into their designs, particularly
because cables are buried and are not visible. Effort expended during design
could avoid unnecessary expenses and ensure that the requirements (Horizon
Power's and its customers’) are catered for.
Each cable network may require different design considerations, configurations,
layouts, etc. As such, there may be many different ways to approach a design.
The information contained in this manual will assist the designer to develop a
structured design approach, and ensure that the optimum configuration is
selected at all times.
1.2
Design Objectives
The objectives of underground cable design are to:
a)
b)
c)
d)
e)
f)
g)
h)
Reduce cost to customers;
Reduce life cycle costs;
Provide greater durability, with due consideration to location in rocky,
saline and marshy soils;
Ensure safety of workers and the general public (safety in design);
Promote environmental compatibility;
Ensure electromagnetic field compatibility;
Promote public acceptance (e.g. easements); and
Attain and exceed the required supply quality and reliability standards.
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The following factors are to be considered before the design can commence:
a)
b)
c)
d)
e)
f)
g)
h)
Potential number of customers and total load;
Estimation of potential load growth;
Availability and/or requirement for interconnections;
Selection of voltage for line operation;
Size and location of loads (Bulk supply, transformers);
Selection of route;
Length of cable route; and
Life cycle costs.
Note: The size and type of cable to be used will be dictated by the capacity
(load) to be carried by the cable during its lifetime together with voltage
drop, thermal rating and fault rating considerations.
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2
DESIGN PROCESS AND INPUTS
This section covers the various considerations and inputs needed as part of
design. The steps involved in the design of an underground cable network will
depend on the individual project and the context in which the design is
performed.
It is an iterative process, with the designer making some initial assumptions, e.g.
cable type and rating, which may later be adjusted as the design is checked and
gradually refined. Delivering an optimum arrangement that meets all constraints
as the final outcome. Horizon Power mainly uses underground cable simulation
software to aid the design process.
2.1
Safety in Design
Whenever design work is undertaken to construct new distribution network
assets, or modify existing assets, demonstration of due diligence with respect to
safety is required. This must cover the full life cycle of the asset that is created or
the remaining life of the modified asset and thought about early in the design
stage.
The essential elements covering a designer’s responsibility in ensuring the safety
of the asset during its life cycle are addressed in the document, Guideline –
Safety in Design - HPC-2DC-17-0001-2014.
2.2
Network Requirements
Design shall take into account both present and future network requirements.
This information is typically covered in the relevant planning report, design
specification and equipment specifications.
2.2.1
Planning
For new distribution networks or extension to existing distribution networks,
planning is carried out during concept development stage. Details covered in the
planning reports that need to be considered include but not limited to:
a)
b)
c)
d)
e)
f)
g)
2.2.2
Load size;
Load distribution centres;
Load cycle;
Nature of load;
Required transfer capacity;
Potential interconnection point; and
Automation requirements.
Equipment
Design specification and equipment specifications play a role in capturing
requirements that need to be addressed during design. This includes the
following:
a)
b)
Equipment and cable rating for normal load, emergency load and for fault
conditions (selection of medium voltage cables as feeders based on
continuous current rating is covered in Appendix ‘A’);
Equipment or cable operating conditions (e.g. Broome versus Esperance);
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c)
d)
e)
Network tolerance limits (e.g. statutory voltage tolerance limits);
Standard installation requirements (refer to clause 2.3); and
Protection grading requirement.
In special cases, there may also additional requirements such as:
i. Customer request for a higher security supply; and
ii. Coordination with road lighting design
2.3
Installation Requirements
Installation condition has a significant impact on the overall technical design of
an underground distribution network. Factors that must be considered by
designer include but not limited to the following:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
2.3.1
Ambient air and ground temperature;
Soil type and terrain such as: sandy, rocky, water table, etc.;
Soil or backfill thermal resistivity;
Cable installation arrangement (i.e. numbers of circuit within same
easement, use of conduit, installation depth, etc.);
Space requirement for installation of ground mounted equipment;
Termites activity;
Environmental risk such as fire, flood, acid sulphate soil and erosion;
Pollution such as dust, salt and noise;
Proximity to other utility assets and congestion level from existing services;
Proximity to metallic/conductive structures;
Proximity to occupiable structures; and
Soil salinity.
Cable Route
Evaluate the terrain to determine issues with ground. For example, suppose a
medium voltage underground cable is to be constructed to supply a customer
remote from a zone substation, and the line route traverses an area of rock, it
would seem prudent for the designer to consider the issues involved in
embedding cables in rock and the associated cost.
2.3.2
Termites
Termite protection must be installed in all areas prone to termite attack.
2.3.3
Equipment Location
Equipment must be suitable for the environment in which it operates. For
example, a ground mounted transformer with open bushings may not be suitable
for use outside a cement plant or quarry, where the build-up of fly-ash or dust
may lead to nuisance tripping or a disproportionately high level of maintenance.
Others include mines sites, with open air blasting, etc.
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The location of ground mounted substations and other equipment shall take into
consideration access, fire separation, touch and step potential, and other related
issues. (Refer to Substation Installation Technical Requirements –HPC-9DJ-230001-2012 and Distribution Line Earthing Standard –HPC-9DC-08-0001-2012).
2.4
Equipment Compatibility
Standard equipment shall be used as much as possible. In certain cases
however, use of non- standard equipment may be required to deliver the
required outcome or to deliver the most cost-effective solution. In such cases,
equipment compatibility must be considered. Where unusual conditions or other
circumstances warrant using alternative equipment the disadvantages in terms of
readily available replacements and operational issues must be considered.
Cable accessories such as joints and terminations for example, can only be used
for cables within a certain size range. Other factors that need to be considered
include but not limited to:
a)
b)
Equipment’s rated voltage
Equipment’s normal load and fault rating
In cases where non-standard equipment is required as part of the design, the
designer should seek formal approval from the Standards Group prior to
proceeding with the final design.
2.5
Environmental and Approval Management
Environmental sensitive areas, land usage, condition and ownership issues
along a cable installation route can have a significant impact on the overall
project cost and timeline. Relevant factors that need to be considered by
designers include but not limited to the following:
a)
b)
c)
d)
e)
f)
g)
Aboriginal heritage sites or areas;
Area with bio-security weeds, pests and disease spread risk (i.e. dieback
disease);
Threatened ecological communities, sites with declared rare flora and
fauna;
Land with native title;
Protected wetlands;
Waste management areas; and
Registered and/or private lands.
Prior approvals are typically required to perform work at or close to these sites.
Where vegetation clearing is required, a permit shall also be obtained prior to
proceeding with the clearing.
Current statutory processes require a range of approvals to be obtained prior to
commencement of works. Due to the time taken to obtain these approvals, these
issues must be considered at the commencement of a project.
As per the Western Australian Distribution Connections Manual (WADCM
Section 6.12) environmental and heritage impacts must be investigated and
managed by the applicant for power supply and their agent.
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3
ELECTRICAL EQUIPMENT USED FOR UDS
INSTALLATIONS
3.1
Medium Voltage Ring Main Unit (RMU) Switchgear
22 kV and 33 kV RMU switchgear is used for switching of the medium voltage
network. They are used as a "standalone" kiosk or incorporated into a RMU
integrated package substation (only 2 + 1 option is currently available as a
packaged substation).
11 kV networks shall make use of 22 kV switchgear.
3.1.1
Outdoor RMU Kiosks
The commonly used outdoor RMU combinations at 22 kV and 33 kV are:
a)
b)
c)
d)
e)
f)
g)
2 switches plus 1 fuse switch (2+ 1);
2 switches plus 2 fuse switches (2+2);
2 switches plus 3 fuse switches (2+3) – only at 22 kV;
3 switches plus 0 fuse switches (3+0);
3 switches plus 1 fuse switch (3+1);
3 switches plus 2 fuse switches (3+2); and
4 switches plus 0 fuse switches (4+0);
22 kV and 33 kV RMUs are incorporated into either 3, 4 or 5 way kiosks (e.g.
2+2).
RMUs are installed in a freestanding aluminium kiosk mounted on a steel frame.
This steel frame is buried in the ground to provide a firm foundation and allows
easy access to the cables and terminations below the switchgear.
3.1.2
Indoor RMUs
Indoor compounds comprising brick enclosures with roof are used to house
RMUs and transformers. They are generally used to cater for larger loads
(> 630 kVA). Extensible and non-extensible MV RMUs are also installed within
buildings owned by customers.
3.2
MV Cables, Joints and Terminations
3.2.1
MV Feeder Cables
(a)
(b)
3.2.2
3 x 1 core, 95 mm2, 185 mm2, 400 mm2 aluminium and 240 mm2 copper
XLPE insulated, PVC/HDPE sheathed cables are used on 22 kV networks.
3 x 1 core, 185 mm2 aluminium and 240 mm2 copper XLPE insulated,
PVC/HDPE sheathed cables are used in 33 kV networks.
MV Transformer Cables
(a)
(b)
3 X1 core, 35 mm2 aluminium XLPE insulated, PVC/HDPE sheathed,
cables are use on 22 kV systems
3 X 1 core 50 mm2 aluminium XLPE insulated, PVC/HDPE sheathed,
cables are use on 33 kV systems
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3.2.3
MV Cable Joints
Currently, all MV straight-through joints, including transition types, use heat
shrink materials, except where otherwise approved.
All cable joints shall be installed in accordance with details outlined in the
Underground Cable Installations Manual: HPC- 5DJ -03-0001-2012, and the
manufacturer's instructions supplied with joint kits. Where not published in
specific detail clarification shall be sought from the supplier. In any case, sound
engineering practice shall be used.
3.2.4
MV Terminations
All pole top cable terminations and some transformer terminations use heat
shrink materials, except where otherwise approved.
Separable insulated connectors (non-Ioad break type) are used for terminations
on transformers (satellite and ringmain), Modular Packaged Substations (MPS),
small single phase pad mount transformers (SPUDs) and ringmain switchgear
with integral bushings.
3.2.4.1
Non-Loadbreak Terminations
The MV connectors, bushings and apparatus used in Horizon Power's
underground system are shown in table below:
3.2.4.2
Type of Connector
Connector Function
Non-Load
Bushing
break
Mounted on the MV side
transformer to connect cables
Non-Load
Elbow
break
Terminates the XLPE cable to allow
connection with the MV bushing
of
Dead-End Plug
Used to protect the non load break
elbow when it is not connected to a
transformer bushing
Dead-End
Receptacle
Used to protect the transformer
bushing when there is no non load
break elbow connected to it.
Load break Terminations
Load break terminations are currently not used by Horizon Power.
3.3
Types of Substations
Horizon Power may require that the supply arrangement to an installation be via
a particular "type" of substation, i.e.:

"District" Substation (With LV street feeds to/from the substation);

"Sole Use" Substation (With no LV street feeds); or

"Customer Owned" Substation (Supplied at distribution MV voltage levels).
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Consideration may also be given to installing a pad-mount or Modular Package
Substation (MPS) in lieu of the more common brick enclosure substations. There
may be cost advantages as well as land/space advantages with this option.
The decision as to which type is selected depends on several factors, including:
a)
Size of customer's load;
b)
Location of customer's load centre on the property, and distance of the
same from the street boundary;
c)
Type and nature of loads within the installation (disturbing, passive, etc.);
d)
Nature of Horizon Power's existing distribution network and loading levels
on other substations in the vicinity;
e)
Horizon Power's need to connect LV street feeds to/from the substation;
f)
Fire separation requirements
LV street feeds may be required to/from a substation for the following reasons:
3.3.1
3.3.2
a)
The customer requires a "back-up" LV supply to the installation;
b)
There will be future developments and load growth in the immediate area;
c)
The customer's load is expected to increase in the future;
d)
Ease of maintaining equipment in the substation (e.g. with street feeds into
the substation, the customer's load can be partly met while the MV
switchgear is being maintained); etc.
Modular Packaged Substations (MPS)
a)
A MPS comes complete with a single transformer and LV switchgear. It is
housed in a self contained metal enclosure and is installed on an inverted,
direct buried concrete culvert. If MV switchgear is required, this is also
housed in a self-contained metal enclosure which is installed adjacent to
the transformer on a direct buried steel mounting frame.
b)
MPS’s are used only as District Substations. They are not used as Sole
Use substations and are not fire rated.
c)
MPS is the preferred arrangement for a District substation with a maximum
load of 630 kVA after allowing for future load growth, and where there is no
requirement for the substation to be fire rated.
Non MPS Arrangements
a)
A Non MPS arrangement comprises a combination of one or more
transformers plus LV switchgear and MV switchgear as required. Each of
these items is a separate component housed in a self contained metal
enclosure. The transformer is installed on an inverted, direct buried
concrete culvert. The LV and MV switchgear enclosures are installed on
direct buried steel mounting frames.
b)
Non MPS components are not installed as a single package. They can be
installed either as a “cluster” substation or in a fire rated enclosure (see
Section 6). In the latter case, the culvert and switchgear mounting frames
are not required.
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c)
Non MPS arrangements can comprise multiple transformers, with
1000 kVA being the largest individual transformer size. They can be used
as both District and Sole Use substations.
A Non MPS arrangement shall be used where;
3.3.3

A Sole Use substation is required; or

Multiple transformers are required; or

The maximum load is greater than 630 kVA after allowing for future load
growth; or

The substation is to be fire rated.
Customer Owned Substations
Typically, for loads greater than 4 MVA, a Customer Owned substation shall be
provided. Neither the MPS nor the Non MPS arrangements are suitable for
Customer Owned Substations. Horizon Power shall provide extensible MV
switchgear necessary for connection to the network. The equipment shall be
installed in a switch room constructed by the customer, along with the customer’s
own MV switchgear (See DSM-3-22 for details).
Where a customer’s load is less than 4 MVA, MV outdoor ground mounted
switchgear can be considered. (See DSM-3-23 for details). A Customer Owned
Outdoor Ground Mounted Substation cannot be upgraded for loads above
4 MVA. In the event that the customer’s load increases above 4 MVA, the
substation shall be converted to a MV Indoor Ground Mounted Substation, which
will require a switch room to be built.
In areas with overhead networks only, MV outdoor aerial mounted switchgear
may be used (See DSM-3-24 for details).
3.3.4
Single Phase Padmount Transformers
The padmount single phase transformer is available in both 10 kVA and 25 kVA
units for 12.7 kV Single Wire Earth Return (SWER) operation or for 22 kV "Two
Phase" operation.
The transformers are supplied configured for 240 volts, but can be re-configured
to 480 volts for "sole use" applications.
3.3.5
25 kVA Single Phase (SPUDS) Transformer
The transformer is mounted on a hot-dipped galvanised steel base. The HV side
of the transformer is equipped with either 2, 3 or 4 x 200 A tapered bushings
which allow connection with separable non loadbreak elbow connectors. An
internal, oil-immersed HV fuse is fitted inside the transformer tank.
The unit is fitted with an externally operated, off-load tap-changer with steps of
0, ±2.5% and ±5.0%.
The LV feeder cables, outgoing from the LV compartment, are protected by one
"Red Spot" fuse (100 Amp) for 240 V or two "Red Spot" fuses (63 Amp) for
480 V.
For further information refer to SPUDS design and operation manual.
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3.3.6
10 kVA Rural Underground Transformer
The transformer is mounted on a concrete base. The HV side of the transformer
is equipped with 2 x 200 A tapered bushings which allow connection with
separable non-loadbreak elbow connectors. An internal, oil-immersed HV fuse is
fitted inside the transformer tank.
The unit is fitted with an externally operated, off-load tap-changer with steps of
0, ±2.5% and ±5.0%.
The outgoing mains from the LV compartment, are protected by one "Red Spot"
fuse (50 Amp) for 240 V or two "Red Spot" fuses (32 Amp) for 480 V.
3.4
Service Pillars
Customer service pillars facilitate the connection of house services, customer
bulk supply cables or interconnections of main LV street cables. In dusk to dawn
street lighting areas, some may also provide supplies to streetlights. The service
pillars are of a dark-green, polyurethane construction, with base partly buried in
the ground. Figure 3-1 shows typical pillars.
Figure 3.1 Typical Pillars
There are two types of service pillar:
1)
"Mini" Pillar (with tunnel terminal blocks):
Tunnel block accepts up to 5 outgoing circuits (to 35 mm2 copper cable),
usually connected as follows:
Pillar on cable side of road:



Connect incoming 3 core 25 mm2 cable from LV feeder; and
Connect outgoing 2 x 3 phase or 2 x 1 phase services; and
outgoing 3 core 25 mm2 road crossing service cable
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Pillar on other side of road:


2)
Connect incoming 3 core 25 mm2 road crossing service cable; and
Connect outgoing 2 x 3 phase or 2 x 1 phase house services
Universal" Pillar (with links and tunnel terminals):
The universal pillar can be used as:



an "interconnector" pillar with provision for solid links, to form a
normally open or closed network point.
a location to reduce the size of LV cable;
to service a large single load (e.g. multiple dwelling lot) with LV HRC
fuses (J type) to 315 A
The universal pillar also contains tunnel terminals for service cables, as per
"mini" pillar.
Note: both "mini" pillars and "universal" pillars have provision for a fused
terminal for an adjacent lighting column.
3.5
LV Cables, Joints and Terminations
3.5.1
LV Feeder Cables:
(a)
(b)
(c)
(d)
3.5.2
120 mm2, 3 core, solid Aluminium conductor, Copper screened
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed
185 mm2, 3 core, solid Aluminium conductor, Copper screened
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed
240 mm2, 3 core, solid Aluminium conductor, Copper screened
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed
630 mm2, 1 core, solid Aluminium conductor, Copper screened
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed
neutral
neutral
neutral
LV Service Cable
25 mm2, 3 core, solid Copper conductor, Copper
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed
3.5.3
neutral
screened
neutral
LV Street Light Cables
Single core 10 mm2 and 16 mm2 stranded copper, XLPE insulated, helical
copper wire neutral screen, PVC sheathed.
3.5.4
Other LV Cables
For minor branch and road crossing services use:
25 mm2, 3 core, solid Copper conductor, Copper screened
(Wave Wound), 0.6/1 kV, XLPE insulated, PVC sheathed cable.
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3.5.5
LV Cable Joints/Terminations
There are many variations of joints and terminations being used which are
summarised below:
1)
feeder cable straight through and breeches joints;
2)
feeder cable to service cable tee joints;
3)
feeder cable pole terminations (to bare and LV ABC overhead conductors);
4)
feeder cable termination at universal pillar;
5)
feeder cable termination at fused switch;
6)
service/street light cable termination at mini pillar;
7)
service/street light cable terminations (to bare and LV ABC overhead
conductors); and
8)
service/street light cable straight through and tee joints.
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4
VOLTAGE REGULATION
4.1
Voltage Tolerance Limits
4.1.1
Statutory Voltage Tolerance Limits
Horizon Power declares the voltage level at a customer’s point of supply as
within ± 6% of the nominal 240 V single phase and ± 6% of the nominal 415 V
three phase.
As such, the maximum and minimum phase-to-neutral voltage levels at any
point of supply on the LV network shall be within 225 V and 254 V for single
phase supplies and within 390 V and 440 V for three phase supplies (under
normal network conditions).
In accordance with AS 61000.3.100 – 2011, Horizon Power expects to adopt the
new voltage standard 230 V +6%, -10% for single phase and 400 V +6%, -10%
for three phase supplies sometime in the future.
When planning and designing a residential distribution network, the designer has
to ensure that the voltages at any point of supply on the network will be within
the statutory voltage tolerance limits, under normal network conditions.
4.2
Voltage Drop Criteria
Impedance in each of the following components of the distribution system leads
to voltage drop:
1)
Medium Voltage Feeder;
2)
Distribution Transformer;
3)
Low Voltage Network;
4)
Customer Service Leads/Cables.
After a distribution system has been constructed, there are only two locations
where voltage levels can be adjusted:
a)
at the zone substation (bus-bar voltage set-point and the use of Line Drop
Compensators), and
b)
at the distribution transformers (off load tap changers).
It is therefore important that the non-adjustable parts of the system be designed
adequately to fully utilise the voltage control equipment at these locations to
keep the customers’ voltages within the statutory voltage tolerance limits.
Table 4.1: - Voltage Drop Limits with respect to nominal voltage
Non-Adjustable System Components
Medium Voltage Feeder
Distribution Transformer
Low Voltage Network
Customer Service Cable
Maximum Voltage Drop Limits
5.0%
4.0%
5.0%
2.0%
Thus to compensate for voltage drops caused by components in Table 4.1, the
Automatic Voltage Regulator (AVR), Line Drop Compensator (LDC) and
distribution transformer taps are set accordingly.
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With a 2% voltage drop assumed for customer service cables, “coincident”
voltage drops, when taken together with zone substation LDC Buck/Boost and
distribution transformer tap options are considered a reasonable balance to
achieve:


4.2.1
a customer’s voltage at the meter panel between ± 6% of the nominal
240 V.
Maintenance and Emergency Voltage Limits are shown in Table 4.2.
Effect of Different Load Cycles
The majority of customers in a “typical” area will have similar, “normal” load
patterns. Some, however, will have load patterns which vary and in extreme
cases could be completely opposite to the “normal” pattern.
These are usually single customer loads. Such loads of relatively small
magnitude with respect to the total feeder load (or of relatively large magnitude
with respect to the total distribution transformer load) can be catered for by
adjusting the tap settings on the transformer supplying the load.
Instances could also arise where a particular MV feeder load profile becomes
dominant and “masks” the normal load profile of the remaining feeders on the
zone substation. Such a feeder could influence the response of the LDC, to the
detriment of the remaining feeders and their individual loads. This problem falls
into network load modelling and is not dealt with in this manual.
4.3
Voltage Drops and Line Currents in LV Feeders
4.3.1
General
A three phase, four wire distribution system servicing a large proportion of single
phase residential loads together with three phase commercial/industrial loads is
subject to rapidly fluctuating currents. These currents produce corresponding
rapidly fluctuating voltages on the system.
4.3.2
Effect of Load Unbalance
It is inevitable that an imbalance between the line currents on the three phases
of a feeder will occur if the feeder services a large number of single-phase loads
(e.g. residential loads).
This imbalance in the line currents leads to a current which flows in the neutral
conductor, which adds to the voltage drop caused by the current flowing in the
phase conductor.
The voltage drop calculation (in LV DESIGN software) takes into account this
added voltage drop caused by the load unbalance , as necessary.
4.3.3
Voltage Drops/Line Currents in Meshed Networks
A “Null Point ” is a point on the meshed portion of the network, through which no
line current flows - the voltage drop from the transformer to either side of the null
point is also the same.
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In practice, the location of the null point in the meshed portion of the network can
change as the loads on the meshed portion vary during the day. However, during
times of peak load, the location of the null point would be approximately at the
same position.
The location of the null point in the meshed portion of the network signifies that
the voltage drop from the transformer to either side of the null point is within the
maximum allowable limit. Hence, once the location of the null point is known, the
network can be assumed to be “opened” at this point and the cable sizes are
appropriate to ensure that the voltage drop to the null point (and hence to all
other points on the meshed portion of the network) remains within the maximum
allowable limits.
4.3.4
Voltage Drop Limits for LV Networks
One of the voltage drop criteria is that the maximum allowable voltage drop limit
for the LV network is 5.0%. This translates to a phase-to-neutral voltage drop of
12 V between the transformer LV terminals and the Point of Supply of any load
on the network. This limit, however, applies for normal or steady state conditions.
In general, the network designer shall ensure that the design of the network
conforms to the voltage drop limits shown in Table 4.2.
Table 4.2 - Maximum Voltage limits for LV Networks
Condition
Normal or Steady State
Maintenance
Emergency
%
±5.0
±7.0
±9.0
Voltage Limits (Phase to Neutral)
Volts
Max (V)
Min (V)
12
252
228
17
257
223
22
262
218
When designing the network, maintenance or emergency conditions must also
be considered. Interconnection with adjacent networks is necessary to maintain
the supply.
4.4
MV Voltage Regulation
4.4.1
Design Approach
The design approach is generally as follows:
4.4.2
(a)
Determine loads for maximum, lightly loaded and maintenance conditions.
(b)
For least cost option, check that voltage remains within limits for the
various loads.
(c)
If voltage goes outside limits try various options.
(d)
Compare options to determine optimum solution.
Computer Modelling
In many instances the cable electrical data is entered into a suitable computer
program for analysis such as Horizon Power’s Power Factory (Digsilent)
program. This calculates the voltage variations for each option. The designer still
needs to compare the options.
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4.4.2.1
Voltage Control Equipment
Some voltage control is built into the standard system equipment as follows:
(a)
Distribution Transformers:
Out of service manual tap changes of ±2.5% and ±5%.
(b)
Zone Substation Transformers:
Typically ±10%, ±13% or +10 - 20%.
In urban areas it has been standard practice to utilise the above two measures
only and choose appropriate conductor sizes and distribution transformer
location/quantity to provide satisfactory voltage regulation. These are covered in
Clause 5.3 - LV Network Design.
Where longer lines are used it can become uneconomic to increase the
conductor size. Additional forms of MV voltage control may become the lowest
cost option.
The three options usually considered are as follows:
a)
Capacitors -typically used for lines of moderate length
(effective when permanently in service)
b)
Reactors - typically used for very long lightly loaded lines
(effective when permanently in service)
c)
Regulator - can be used to raise or lower voltage
(output voltage varies to suit load conditions)
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5
UNDERGROUND DISTRIBUTION SCHEMES (UDS)
5.1
Design Procedure
The Underground Distribution Schemes Manual (UDSM) sets out the procedure
for a subdivision that is to be supplied with electricity from Horizon Power’s
network. The UDSM covers the policies, processes, practices, requirements and
equipment that are relevant to designing underground distribution systems and it
shall be referred to when designing Underground Distribution Schemes.
The challenge for the designer is to produce the most economical selection of
equipment, location and cable size that will adequately service the loads within
the constraints of achieving Horizon Power’s "quality of supply" objectives.
(Refer to HPC-5DC-07-0001-2012: Distribution Design Manual Volume 1 –
Quality of Electricity Supply).
Section 6 – Determination of Recommended Load Demand Values, provides
information about determining loads for underground distribution schemes.
General steps involved in design are summarised below:
5.2
Transformers
5.2.1
Initial Requirements
Count the lots in the development from which the total number of transformers
can be calculated (refer to HPC-3DC-07-0001-2012: Information – Electrical
Design for Distribution Networks: After Diversity Maximum Demand).
Based on the number of lots to be serviced by a transformer, do a rough
grouping of the lots and select tentative transformer locations.
Relocate transformers after Step (b) to optimise loading on the LV distribution
cables available.
Identify non-residential loads such as pumps, shops, schools etc. Identify
discrete or sole use transformer loads.
The transformer substation should be located as near as possible to the
electrical load centre of a group of lots in order to best balance the loads
between feeders. This is achieved by locating the transformer close to road
intersections and junctions. The designer must be prepared to regroup lots and
change transformer locations as the design develops.
Standard transformer ratings used by Horizon Power for underground distribution
schemes are:
i.
160 kVA MPS and non MPS
ii.
315 kVA MPS and non MPS
iii.
630 kVA MPS and non MPS
iv.
1000 kVA non MPS
Note: The above transformers are available as indoor or outdoor units
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5.2.2
Transformer Selection
The designer must also be aware of other factors that may affect transformer
selection and location unique to the subdivision under development, such as:
1)
5.2.3
Isolated pockets of UDS that may be serviced from:

Satellite substations;

Spare capacity from adjacent non residential loads; and

Spare capacity from adjacent separate developments.
2)
Pockets never likely to be expanded and satisfactorily serviced from a
315 kVA substation (i.e. no future 630 kVA requirement).
3)
Topographical features and ground conditions prohibiting installation.
4)
Strategies adopted by developers for different schemes.
Mixed Loads
If the full utilisation of Horizon Power assets is to be achieved then mixed loads
are inevitable. Different peak load times for mixed loads must be taken into
consideration when selecting transformer ratings or grouping of lots.
5.2.3.1
Example 1
How many residential lots can be supplied from a substation feeding a high
school?
Available information:
School maximum demand is 220 kVA (clause 6.3.2)
School is supplied from a 315 kVA transformer
ADMD for Esperance is 3 kVA
Residential peak occurs between 5:30 and 6:30 PM.
School load has been measured at 20% peak during domestic peak.
Transformer kVA
= (No. of lots x ADMD) + (220 x 20%)
Hence, No. of lots
= {315 - (0.2 x 220)} / 3.0
= 90 lots now
and ultimately No. of lots = {630 - (0.2 x 220)} / 3.0
= 195 lots with 630 kVA transformer
It appears that about 195 lots may be mixed with a high school load provided
that a 630 kVA transformer replaces the 315 kVA transformer sometime in the
future.
However, the school peak occurs at 11:30 am when the domestic load has been
measured at 50% maximum demand,
Therefore, No. of lots
= (315 - 220) / 1.5
= 63 lots now
and ultimately
= (630 - 220) / 1.5
= 273 lots with 630 kVA transformer
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An analysis of the above shows that certain options exist depending on the
particular circumstance of the subdivision requirements.
1)
63 lots could be serviced now from a 315 kVA substation to be upgraded
later to a 630 kVA substation. This would underutilise the 630 kVA
transformer unless, as is often the case, there was a requirement for spare
capacity (up to 132 lots) at a later stage of development.
2)
195 lots could be serviced now from a 630 kVA transformer saving the cost
of upgrade but incurring additional capital costs and early transformation
losses.
In general, it may be prudent not to take advantage of the 120% overload
capacity of the transformer. This allows for contingencies, common in mixed load
applications.
5.2.4
Regions other than Esperance
It is standard practice to install 630 kVA transformers initially because the flat
load profile due to air-conditioning allows little scope for cyclic rating use.
5.3
LV Network Design
5.3.1
Primary Aim
The primary aim when designing a LV network is to select and locate equipment
that will adequately service both present and future customer loads and also
satisfy the reliability and quality of supply standards stipulated by the Electricity
Industry (Network Quality and Reliability of Supply) Code 2005.
5.3.2
Challenge for Network Designers
The challenge for any network designer is to avoid over/under design of the
network. Over design is costly in terms of unnecessary capital investment, whilst
under design leads to high losses, costly investigation and rectification of Quality
of Supply related complaints.
Extra effort expended in optimising the design of LV networks results not only in
the efficient utilisation of capital costs but also impacts on the MV network,
affecting the number and location of distribution transformers.
5.3.3
Use of Computer Packages
Typically, the design studies and calculations are carried out using specially
written computer programmes, for the more complex cases or where accurate
results are required. Alternatively, manual calculations can sometimes be used,
especially for simpler cases or where only estimates are required.
LV DESIGN is a PC based computer program, written specifically for studying LV
networks. It is particularly suited for underground distribution scheme designs,
with distributed loads along the LV feeder. The program automatically accounts
for load unbalance and diversity.
However, it can also be used to calculate the voltage drops and line currents
caused by large commercial loads. LV DESIGN can be used to investigate the
impact of new large loads within residential estates, e.g. shopping centres,
pumps, etc.
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GIS (Geospatial Information System) is one of Horizon Power's prime computer
systems. Various distribution plant items are recorded in the system for most
parts of the state, e.g. transformers, MV and LV cables and many other assets.
Customer property boundaries are also recorded in the GIS database.
GIS can be used by the designer to obtain information quickly about the existing
supply system around a new proposed installation, from which, various supply
alternatives can be considered.
GIS can also be used to down-load information on the supply system onto Power
Factory (Digsilent) for later analysis.
5.3.4
Aspects of Electrical Design
The electrical design
aspects:
of distribution feeders generally involves the following
1)
Estimation of load demands;
2)
Selection of distribution transformer;
3)
Planning of network layouts;
4)
Calculation of Voltage Drops and Cable Currents;
5)
Selection of cable sizes to satisfy the voltage drop and current capacity
requirements; and
6)
Selection of fuse/protection device (if applicable).
These aspects are explained in the following sections.
5.3.5
Determination of Cable Size
The size of LV cable is chosen to ensure that all of the following criteria are
satisfied:
5.3.6
1)
Voltage drops during peak network load times being within maximum
allowable limits (and during minimum load times being within minimum
allowable limits) - Refer to Section 4
2)
De-rated current carrying capacity of cable being adequate so that load
currents will be within the capacity, not only during steady state conditions,
but during maintenance/emergency conditions when the LV network is
interconnected with others (Refer to Appendix B);
3)
Other cable current ratings (e.g. summer, winter) not being exceeded,
wherever applicable ( Refer to Appendix B and cable manufacturers data);
4)
Cable impedance satisfying the LV fuse/protection requirements (so that at
times of fault at the end of the feeder, the fault current will be large enough
to be “seen” by the LV fuse and hence, cleared in time to prevent damage
to the cable ( Refer to Section 7).
Selection of LV Feeder Routes
When selecting LV feeder routes, the designer should take the following into
account:
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5.3.6.1
Proximity to Loads
The feeder route should be chosen such that it will “start to be loaded” as close
to the transformer as possible. This is facilitated by locating the transformer as
close to heavy load centres as possible or as close to the “centre of gravity” of a
group of loads.
Feeder routes where the feeder only “picks up” loads after a considerable
distance away from the transformer should be avoided (as this causes larger
voltage drops than necessary in the initial part of the feeder).
5.3.6.2
Utilisation/Loading
LV feeder routes must be chosen such that the transformer will service the
required number of loads determined on the basis of design load demand values
(refer to Section 6 – Determination of Recommended Load Demand Values and
HPC-3DC-07-0001-2012 Electrical Design for Distribution Networks: After
Diversity Maximum Demand).
5.3.7
Typical Route Lengths
The length of a LV feeder affects the:
1)
voltage drop on the feeder; and
2)
fault current at the end of the feeder.
Very long LV feeders should generally be avoided since this would only result in
medium voltage drops than necessary, cause improper operation and lead to
possible conductor burnouts.
Designs may require up to 500 m route lengths. However, route lengths in
excess of 400 m are unusual and may indicate poor substation location.
Cable routes should be selected so that feeders start to pick up load as close to
the substation as possible. This can be achieved by locating the substation close
to the electrical load centre of a group of residential loads or non residential
loads.
Cable routes that pick up loads at significant distances from the substation entail
substantial voltage drops to occur. This can impact adversely on conductor costs
and losses.
5.3.8
Interconnection with Other Feeders
If a transformer becomes unserviceable, its LV network has to be supplied by
adjacent transformers until repairs can be effected or a replacement put into
service. As such, the LV network should be provided with sufficient numbers of
“interconnecting” points (e.g. via the use of removable solid links, fuse switches)
to allow lateral interconnections between LV networks of adjacent transformers.
When selecting LV routes, the designer should select routes which can assist in
the provision and location of these “interconnecting points”, if possible.
The interconnection criteria generally used by Horizon Power is to ensure that
the backbone feeder of any transformer can be interconnected with other LV
feeders from adjacent transformers, at least twice.
If the number of interconnections cannot be provided due to certain constraints,
the designer should consider using a smaller transformer size instead.
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5.3.9
Pillar/Cabinet Positioning and Alignment
When selecting the LV feeder route, the designer must also give consideration to
the positioning of Pillars and Cabinets.
5.3.10
Other Considerations
Sometimes, in order to mitigate the excessive voltage drops caused by large
motor starting currents, it may be necessary to connect up large motors (e.g.
large reticulation and sewerage pumps) via a dedicated LV feeder.
A similar requirement may be called for to mitigate any interferences caused by
“potentially disturbing electrical loads” to other customers on the same LV
feeder, e.g. light industrial customers with arc-welders, thyristor controlled motor
speed drives, large motors.
On the other hand, from the nature of the load itself, or due to special requests
from the customer for a more “secure” supply arrangement, certain loads may
need to be serviced via dedicated LV feeders or from “sole-use” transformers
(e.g. small hospitals, retirement villages, bulk cold food storages).
5.3.11
Typical Design Issues
When designing the LV feeder and street lighting, voltage drops with various
cable sizes are calculated.
If the voltage drop at the end of a radial LV feeder exceeds the prescribed limits
the following alternative design choices are possible:
a) Adjust lot grouping or change transformer boundaries
b) Relocate the transformer to a site nearer the electrical load centre of the
grouped lots.
c) Upgrade cable size
d) Check current flows against the current rating of the cables
e) Check LV feeder protection fuse size
5.4
MV Design
5.4.1
MV Cable Requirements
When designing the MV layout, the shortest and most direct MV cable routes
should be selected.
If the design is for a large UDS:
a) a detailed and comprehensive study of the existing and proposed MV
feeders supplied from adjacent zone substations shall be carried out to
determine the effect of the new load on the overall MV network and
system security;
b) in the overall area concept plan, the location of all the transformers and
all existing MV mains adjacent to the subdivision (obtained from GIS)
shall be marked;
c) transformers that are to be supplied as "satellites" from the adjacent
overhead MV mains and the transformers that are to be "ring main"
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supplied requiring new underground MV mains, switchgear etc shall be
marked;
d) MV cables to service distribution transformers, with MV interconnections
as required shall be indicated on the layout plans, indicating the ring main
switchgear "types" (i.e. 2+2, 3+1 etc.) transformer sites, all new and
existing underground and aerial mains with interconnection points.
e) Consider aspects of electrical design in clause 5.3.4.
5.4.2
MV Network Systems
5.4.2.1
Radial Feeder System
Failure of a radial feeder interrupts the supply to a substation for the period it
takes to repair the faulted equipment. Therefore, duplicate feeders to important
customers e.g. hospitals, may be considered. Ideally the zone substation should
be at the "hub" of the feeders. In built up areas this is rarely possible.
5.4.2.2
Ring Main System
In the ring main system, failure of a feeder will interrupt supply only for the time it
takes to isolate the faulted equipment and relocate the open points. The zone
substation does not need to be central. Ring main systems are associated with
high initial costs and used in areas where high reliability of supply is required.
5.4.3
Hybrid System
Horizon Power has adopted a hybrid system approach in high density areas to
take advantage of the economic and the in-service benefits from both the radial
and ring main systems. MV distribution is by ring main normally open at one
point in the ring effectively producing two MV feeders. MV Ringmain Switchgear
units are "looped in" around the ring and typically will feed two distribution
transformer substations.
This system can be achieved by installing a 2+2 ring main switchgear unit
In-service benefits are gained from layout arrangements that allow sections of
the ring network to be isolated or reconnected in order to maintain supplies,
under fault and maintenance conditions.
5.4.3.1
Satellite Substations
The use of the Satellite Substations is an economic method of supplying small
"pockets" of UDS both at the edge of large subdivisions and inside extensive
overhead networks.
The satellite substation is radially connected from an overhead feeder protected
by drop out MV fuses (expulsion type). The fused tee-off is connected to a
satellite substation via 11 or 22 kV cable (3 x 1 core 35 mm2 XLPE).
In-line pole top switches should be added to the overhead feeder on each side of
the tee-off or in an economical arrangement to provide adequate isolation under
fault or maintenance conditions.
If the overhead feeder is likely to be undergrounded in the near future, the
designer may consider an alternative to the fused tee-off arrangement and
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Satellite Substation. This could be a cable "loop in, loop out" of the main feeder
circuit through a Ring Main Unit.
The 3 x 1 core XLPE cable would be sized to match the future feeder fault and
current ratings.
5.5
Automation
Automation facilitates the interconnection of adjacent feeders and switching of
loads during faults and under abnormal conditions. (Refer to DSM - 08 for
installation requirements)
5.6
Design Outputs
5.6.1
Outputs - MV/LV Layouts
LV Substation locations and site details
Cable and Aerial main routes, sizes and lengths;
Service pillar locations;
Switchgear (MV) types (2+2, 3+ 1 Package etc);
Fuse sizes, Tee-off points (if satellite subs);
Interconnection points;
Street lighting details;
Boundary of subdivision;
Land requirements
5.6.2
Outputs - Cable Ducts
Location, numbers, sizes
Alignments;
Construction and Installation requirements;
Depth in ground;
Cable pulling pits
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6
DETERMINATION OF RECOMMENDED LOAD DEMAND
VALUES
6.1
Estimation of Load Demand
Maximum demand is the all important parameter in system design because this
value directly determines component sizes (e.g. cables, transformers), voltage
drops, line currents and ultimately the cost of servicing the loads.
The fluctuating nature of electrical loads, particularly that of residential peaks,
makes the measurement of instantaneous demand difficult, and sometimes,
undesirable. System components are rated in terms of their thermal (overload)
capacity and thus their “average demand ” over a period of, say, 15 minutes is
far more meaningful than the moment by moment fluctuations which actually
occur. For this reason the demand on electrical equipment is often obtained by
the use of special instruments (e.g. load data-loggers) which can provide an
average reading for a certain period. The information provided by this type of
meter is often employed in system design.
Subject to predefined conditions, maximum demands can be measured, adjusted
and projected to become the basis of design for new systems. While easily
understood in principle, maximum demand can be expressed in various terms
and measured in various ways. Unless these aspects are fully understood and
appreciated, confusion and inaccurate design may result.
6.2
Effect of Load Diversity on Maximum Demand
The peak load of any installation is characterised by the demand fluctuations
from the switching in and out of appliances within the installation. It is improbable
that every appliance will impose its maximum demand at the same instant. As
such, the maximum demand of the installation is generally less than the sum of
the individual maximum demands of all the appliances within that installation.
Similarly, the maximum demand of a LV feeder is characterised by the demand
fluctuations from the varying load demands of all the loads on the feeder. The
maximum demand of the feeder will generally be less than the sum of the
individual maximum demands due to the “diversity ” between the loads.
It is conceptually possible that if the “average maximum demand” of a “typical”
load in a group is known, then the maximum demand for the whole group can be
obtained by simply multiplying the average maximum demand of this typical load
by the number of loads and also by an appropriate “multiplication factor” chosen
for that particular number of loads.
This multiplication factor is commonly referred to as the “diversity factor ”. Used
in conjunction with the number of loads, the diversity factor “scales” the “average
demand” of a “typical” load within a group, to the maximum demand for that
group of loads.
6.3
Residential Load ADMDs
ADMD values for residential loads are provided in Horizon Power document
HPC-3DC-07-0001-2012 (Information – Electrical Design for Distribution
Networks: After Diversity Maximum Demand). While the ADMD values are
applicable only to standard sized lots, there may be cases where the actual
ADMDs could be even higher than these values (e.g. for larger lots, beach front
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houses, riverside lots, canal developments, etc.). Similarly, it may be necessary
to reduce the recommended ADMD values. Changes to recommended ADMD
values must, at all times, be made in consultation with the technical staff in the
relevant Regional Area office prior to the design being carried out.
Since these ADMD values are averaged, they must be “scaled up” to obtain the
maximum demand for a group of loads before the LV feeder can be designed.
The “scaling” of the ADMD values is automatically taken into account in Horizon
Power’s Voltage Drop and Line Current formulae.
6.3.1
Determination of ADMD when standard values are not used
The maximum demand on a residential substation, when divided by the number
of loads supplied, provides a value which is in essence the “average contribution
per customer” to that maximum demand, or simply the “average demand” for a
“typical” customer. The larger the number of customers involved, the nearer to its
ultimate value will be this “average demand”.
For practical purposes, groups of 60 or more loads are considered to produce a
figure sufficiently close to the ultimate for it to be considered as the “After
Diversity Maximum Demand” or ADMD.
Because the load ADMD is the all important basis of residential distribution
design, this matter must receive full and careful consideration, concerning its
value at the initial loading of the system, the provision for future growth and the
repercussions of having to alter the system as a result of a poor choice of design
ADMDs
Among the factors influencing the choice of the ultimate design ADMD values
are:
1)
Limited capital resources;
2)
Apprehension concerning the future;
3)
Penetration of natural gas in traditionally all-electric areas;
4)
Climatic, socio-economic and/or geographic influences;
5)
Load growth, changing standard of living;
6)
Trend towards more efficient appliances/equipment; and
7)
Tariff structure.
Whatever the ultimate design ADMD figures are, the designer must endeavour to
ensure that the system is not under/over designed for the reasons given in
clause 5.3.
Optimum design requires optimum choice of ADMD. In most cases, a designer
has to make a value-judgement as to what value of ADMD is most appropriate
for the particular distribution system, after having considered all relevant issues.
For most instances, the load demand can be estimated based simply on the
designer’s previous experience with similar developments. However, careful
thought must still be given to this crucial design parameter for each residential
development, rather than simply using highly conservative “standard” values.
It is not uncommon for a designer to find himself/herself in the position of having
to be a mixture of an engineer, an economist and even a prophet at the same
time!
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6.3.2
Non-Residential Load Demands
As mentioned earlier, maximum demand values are expressed in a variety of
ways, e.g. amps, kVA, kVA/hectare, kW etc. The following load demand values
for non-residential loads are a mixture of “average demand” type figures
(kVA/hectare figures) as well as “maximum demand” type figures (kVA, kW, hp
etc. figures).
Typical design load demand values for non-residential loads are as follows:
1)
High Schools: 220 kVA;
2)
Primary Schools: 82 kVA;
3)
Neighbourhood Shopping Centres: obtain the load kVA based on an
average load density 200 kVA/hectare. (Alternatively, enquire from
consultant or measure maximum demand);
4)
Large Shops/Business Centres: enquire from consultant;
5)
Pumps and other large 3-phase fixed equipment: obtain full load kVA from
equipment name-plate or specifications;
6)
Small Shop Groups: 200 kVA/hectare;
7)
Light Industrial Lots: 100 kVA/hectare.
More information is available in Horizon Power document HPC-5DC-07-0032012 (Distribution Design Manual Volume 3 – Supply to Large Customer
Installations).
6.3.3
Residential Lot Classification
Some lots have an “Rn” classification (e.g. R25, R30). This classification relates
to the “density” of houses on the lot. The “n” index refers to the Number of
Units/hectare, so that an R25 lot classification refers to 25 units per hectare.
Since 1 hectare = 10 000 m2, each unit on a R25 lot would occupy approximately
(10 000 ÷ 25) m2 = 400 m2.
The number of units in a given “Rn” lot of area, A (m2), can then be calculated as
follows:
No. of Units = A (m2) × n ÷ 10 000
For example, if an R25 lot has an area of, say, 4898 m2, the number of units in
the lot would be 4898 × 25 ÷ 10 000 = 12 units.
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7
LV FEEDER PROTECTION
7.1
Introduction
Protection devices are installed on underground networks to prevent or to
minimise the risk of:
(a)
fires caused by uncleared faults;
(b)
thermal damage to the cable insulation and to the transformer;
(c)
annealing of the conductor;
(d)
mechanical damage to the cable due to electromagnetic forces caused by
high fault currents.
Horizon Power uses LV High Rupturing Capacity (HRC) fuses in its LV networks,
to protect against:
7.2
(a)
three phase faults;
(b)
phase-to-phase faults; and
(c)
phase-to-neutral faults.
Feeder Protection Policy
''Any segment of LV cable installed within a network, existing or new, shall be
protected using appropriately rated LV HRC fuses".
For underground networks, the LV feeder shall be protected by LV fuses
installed immediately after the transformer.
The fuse rating for residential street circuits shall not exceed 315 amps at any
substation. Downstream fusing shall not be used to extend the length of a feeder
backbone.
7.3
LV Fuse Selection Policy
The MV fuse links (full range) exhibit low temperature rise characteristics and are
capable of interrupting both overload and short circuit currents up to their rated
breaking capacity. The MV fuse is selected essentially for short circuit protection
of the transformer.
The LV fuse elements must be graded with the MV fuse links but must primarily
protect the LV feeder cable against damage due to short circuits and as much as
possible, high impedance faults, e.g. physically damaged cable leading to a
phase to earth fault with significance impedance (low fault current).
Sustained earth faults (not cleared by fuse operation because of the incorrect
use of fuses rated at 400 A) have led to overload conditions on the transformer,
and in turn fuses of the MV switchgear, resulting in catastrophic failure of MV
fused units.
The LV fuse/protection concept used in Horizon Power is based on the following
assumptions or "rule of thumb":
For satisfactory protection, the prospective phase-to-neutral current at the end of
the LV feeder should be at least three (3) times the LV fuse current rating.
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7.4
Prescribed Fuse Sizes (MV and LV)
Standard LV supply arrangements and fusing are shown in HPC-5DA-07-00012012: Distribution Substation Manual Section 1: Customer Supply Arrangements.
Fusing of street circuits at all underground substations shall not exceed 315 A.
7.5
Maximum Lengths of LV Feeders
7.5.1
General
For any given LV fuse/transformer combination, the maximum length of feeder
can be calculated such that if a zero resistance, phase-to-neutral fault were to
occur at the end of the feeder, the estimated fault current would be at least three
(3) times the LV fuse current rating.
This maximum feeder length is mainly dependent on the resistance (R) and
reactance (X) of the whole of the LV feeder, as well as the impedance of the
transformer.
Note:
The system fault level at the transformer also affects the maximum
length calculation. The equivalent MV system impedance, however, is usually
much smaller than the LV feeder and the transformer impedances. As such, its
overall effect on the calculated maximum LV feeder length is small.
Since the LV feeder can be made up of several sections, each of a different
conductor (with different resistances and reactance’s), the actual maximum
length of any particular feeder must be calculated individually1.
7.5.2
Equivalent Length of LV Feeders
In order to select maximum lengths of LV feeder and for particular fuse sizes, the
designer can use the technique termed "equivalent length".
For example, by comparing circuit impedances, an LV feeder constructed using
several different conductors, can be expressed as a particular length equivalent
to a feeder constructed solely of 95 mm2 LV ABC conductor. (The prospective
phase-to-neutral fault currents at the end of both feeders will be the same).
This length is termed the equivalent length (of 95 mm2 LV ABC) of the LV feeder.
Using the "equivalent length" concept, we can express any feeder in terms of an
"equivalent length" of 95 mm2 LV ABC. This is particularly useful since this length
can then be compared with calculated maximum permissible "equivalent lengths"
for various LV Fuse/Transformer combinations (see Table 7-1).
1
Horizon Power's LVDESIGN PC based computer programme has a Feeder Fuse/Protection Check
option which can be used to calculate the maximum feeder length for a given LV fuse/transformer
combination
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7.5.3
Feeder Equivalent Length Calculation
To calculate the "equivalent length" for a LV feeder constructed using several
different conductors, simply divide the individual conductor length in the
particular section with the appropriate "scaling factor" appropriate for the type of
conductor, and add up the resulting lengths.
The appropriate “scaling factors” are shown in Table 7-1.
Table 7-1 LV Feeder Equivalent Length Scaling Factors
CONDUCTOR
CLASS / TYPE
EQUIVALENT LENGTH
CONDUCTOR SCALING FACTOR
25 mm² COPPER
2.57
2.08
1.45
0.52
95 mm² LV ABC
1.00
240 mm² ALUM
UNDERGROUND
CABLES
LOW VOLTAGE ABC
185
mm²
ALUM
120
mm²
ALUM
Example
Suppose a LV feeder was constructed as follows:
1)
80 m of 185 mm2 AL U/G cable,
2)
300 m of 120 mm2 AL U/G cable, and
Then, the "equivalent length" (of 95 mm" LV ABC) of the feeder is:
E.Length = (80 + 2.08) + (300 + 1.45) = 245 m
7.5.4
Maximum Equivalent Lengths
The maximum "equivalent length” for a given LV fuse/transformer combination
can be calculated, as shown in Table 7-2.
These lengths of 95 mm2 LV ABC are equivalent to the actual feeder length, at
the end of which, the phase-to-neutral fault current will be at least three times the
fuse rating.
Table 7-2: Maximum Equivalent Lengths of 95 mm2 LV ABC
Transformer Size
(kVA)
1000
630
315
160
63
315 A
310
310
305
290
240
Maximum Equivalent Length (m)
(of 95 mm2 LV ABC)
LV Fuse Size
160 A
610
610
610
595
565
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100 A
980
980
975
965
940
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When designing LV feeders, the network designer has to ensure that the LV
feeder's "equivalent length" (of 95 mm2 LV ABC) does not exceed the lengths
shown in Table 7.2.
LVDESIGN automatically calculates the equivalent length of the LV feeder.
Note:
7.6
From the above tables it is seen that 440 m of 120 mm2 feeder can be
protected by a 315 A fuse for 315 kVA and above transformers. This
means that lower rated fuses need rarely be considered on
underground schemes.
What if the Maximum Allowable Length is Exceeded?
If the maximum allowable length is exceeded for any LV feeder, the fault current
at the end of the feeder will be less than three times the rating of the LV fuse
installed to protect the feeder. This may lead to unsatisfactory fault clearance
times.
The options available to the designer when the feeder's "equivalent length"
exceeds the maximum allowable length are:
1)
Consider Shortening the LV Feeder
Shorten the feeder so that, its "equivalent length" (of 95 mm2 LV ABC)
corresponds to the appropriate length in Table 7-2. This will mean that
some of the loads previously serviced by the feeder must then be serviced
from an adjacent or new transformer.
In some circumstances, this may not be an economical option, particularly
if additional expenses are incurred (e.g. MV mains extensions to connect
up an additional transformer).
2)
Consider Using Larger LV Conductors
By using LV conductors with a larger cross sectional area, the feeder's
"equivalent length" may be "shortened" to within the maximum allowable
length.
For example, a 450 m feeder constructed entirely of 120 mm2 cable has an
"equivalent length" of 310 m. However, the same feeder constructed using
185 mm2 cable in its first two segment lengths (of, say 40 m each) will have
an "equivalent length" of 293 m.
(80 / 2.08) + (450-80) /1.45 = 293 m
This option may not be economic if the length of the larger conductor
needed is excessive.
7.7
Calculation of Fault Currents at End of LV Feeders
The "equivalent lengths" in Table 7-2 are calculated by determining the total
impedance to the end of the feeder which will result in a fault current of at least
three times the fuse rating.
Conversely, for a given feeder made up of several different conductor types, the
prospective phase-to-neutral fault current at the end of the feeder can be
calculated.
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The formula for the fault current is given by:
where:
I= prospective phase-to-neutral fault current at the end of the LV feeder (amps)
Xs = source/system impedance from the zone substation to the MV terminals of
the transformer (ohms)
Xt= transformer impedance (ohms)
R= total resistance from the transformer LV terminals to the end of the LV feeder
(ohms)
X= total reactance from the transformer LV terminals to the end of the LV feeder
(ohms)
The R and X values are obtained by multiplying the conductors' resistance and
reactance values in (Ω/km) with the length of the conductor (in km).
When a feeder is made up of several types of conductors, then the resistance
and reactance values of each conductor are calculated as described above and
added together to give the feeder’s total reactance.
where:
= resistance (Ω/km) for conductor "i"
= length (km) of conductor "i''
K = total number of different conductors on the feeder
Similarly, the reactance X is given by:
where:
= reactance (Ω/km) for conductor "i"
The transformer impedance values are as shown in Table 7-3.
Table 7-3: Typical Distribution Transformer Impedances
Transformer
Size (kVA)
1000
630
315
160
63
Impedance, Xt
(%)
5.5
4.0
4.0
4.0
3.3
(Ω)
0.01
0.011
0.022
0.043
0.09
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Note:
The transformer impedances are calculated as follows:
Xt(Ω) = {Xt(%) x 4152} ÷ {100 x Tx kVA}
The % impedances are from Horizon Power’s current Technical Specification for
Transformers (HPC-8DJ-07-0001-2013).
The source or system impedance, Xs can be derived from the fault level at the
HV terminals of the transformer. In most instances, the source impedance is so
small compared to the transformer impedance that it can usually be ignored
altogether in the fault current calculations.
Example
If a feeder is fed from a 315 kVA transformer, and is constructed as follows:
100 m of 240 mm2 Al cable,
80 m of 185 mm2 Al cable and
40 m of 120 mm2 Al cable,
The (phase-to-neutral) fault current at the end of the feeder can be calculated as
follows:
1)
Transformer Impedance, Xt:
The transformer impedance, from Table 8-3 is 0.0267 Ω.
2)
Total Resistance, R:
The total resistance, R, is:
R = (0.1365 x 0.1) + (0.1787 x 0.08) + (0.2739 x 0.04) = 0.03891 Ω
3)
Total Reactance, X:
X = (0.0690 x 0.1) + (0.0690 x 0.08) + (0.0620 x 0.04) = 0.01490 Ω
Hence, the phase-to-neutral fault current is:
.
.
.
= 2497 amps
7.8
Fault Current Ready Reckoner
The prospective fault current at the end of any length of LV feeder can be
approximated using a "Ready Reckoner".
Having found the feeder's Equivalent Length, the curve shown in Table 7-4 can
be used to obtain the approximate fault current at the end of the LV feeder. From
this:
1)
the magnitude of the fault current as a "multiple" of the fuse size can then
be determined.
(The "ideal" multiple of the fuse size is three (3) times. However, multiples
as low as 2.5 times may also be satisfactory for some circumstances).
2)
the fault clearance times can be estimated (using Figure 7-1).
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Figure 7.1: Fault Current Ready Reckoner
Note:
The minimum curve is for a 63 kVA Tx, while the maximum curve is for
a 1000 kVA transformer.
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7.9
Typical LV Fuse Time-Current Characteristics
The calculated phase-to-neutral fault current (in amps) at the end of the LV
feeder can be checked against the typical time-current characteristics of the LV
fuses to determine the fusing times (in seconds).
The time-current characteristics for most commonly used LV HRC fuses are
shown in Figure 7-2.
Figure 7.2: Time-Current Characteristics of LV HRC Fuses
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8
INSTALLATION REQUIREMENTS
Refer to the following documents:
9
1)
Western Australian Distribution Connections Manual (WADCM)
2)
Underground Distribution Schemes (UDS) Manual
3)
Underground Cable Installation Manual
4)
HPC-9DJ-23-0001-2012: Substation Installation Technical Requirements
5)
Distribution Substation Manuals ( DSM -1, DSM – 3 & DSM – 6)
6)
HPC-9DC-08-0001-2012: Distribution Line Earthing Standard
STREET LIGHTING
Refer to Section 13 of HPC-5DC-07-005-2012: Distribution Design Manual
Volume 5 – Overhead Bare Conductor Distribution.
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APPENDIX A – REVISION INFORMATION
(Informative) Horizon Power has endeavoured to provide standards of the highest quality
and would appreciate notification if any errors are found or even any queries raised.
Each Standard makes use of its own comment sheet which is maintain throughout the life of
the standard, which lists all comments made by stakeholders regarding the standard.
The document HPC-5DC-07-0004-COMM can be used to record any errors or queries found
in or pertaining to this standard, which will then be addressed whenever the standard gets
reviewed.
Date
17/06/2014
Rev No.
A
Notes
Original Issue
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APPENDIX B – CURRENT RATING OF UNDERGROUND CABLES
B.1 Continuous Current Rating
Cable manufacturer’s catalogues provide continuous current ratings for cables installed in
the following standard installation conditions;



In air
Buried direct in ground at a depth of laying of 0.8 m to the top of cable or group of
cables
In a duct with depth of laying of 0.8 m to the top of the duct
The continuous current ratings of cables provided in cable manufacturer’s catalogues are
generally based on the following operating conditions:
a)
Maximum conductor temperature 90°C (XLPE insulated cables)
b)
Ambient air temperature 40°C
c)
Ambient soil temperature 25°C
d)
Soil thermal resistivity 25°C.m/W
(Refer to HPC-9EJ-01-0001-2013, Horizon Power Environmental Conditions Standard, for
temperature conditions in various parts of the network)
Cable manufacturer’s catalogues also provide modifying factors when cables are installed in
environments which are different to the above conditions. Cable manufacturer’s catalogues
should be used to perform the cable rating calculations in different installation conditions to
the above using the formula:
Continuous current rating = Rating in Table x factor for depth burial x factor for thermal
resistivity x factor for ground temperature
Continuous current ratings are used to calculate the cyclic and emergency ratings and are
rarely used otherwise in practice.
The rating modification tables in this section shall be used only when the cable
manufacturer’s catalogues are not available.
B.1.1 Rating Factors for depth of laying direct in the ground
Depth of
Burial (m) (to
top of cable)
Low Voltage Cables
High Voltage Cables
≤ 300 mm2
≤ 300 mm2
> 300 mm2
> 300 mm2
0.6
1.0
1.0
0.8
0.98
0.97
1.0
1.0
0.9
0.97
0.96
0.99
0.98
1.0
0.96
0.95
0.98
0.97
1.25
0.95
0.93
0.96
0.95
1.5
0.93
0.92
0.95
0.93
1.75
0.92
0.91
0.94
0.91
2.0
0.91
0.90
0.92
0.89
Page 45 of 47
Print Date 23/06/2014
© Horizon Power Corporation – Document Number: HPC-5DC-07-0004-2014
Uncontrolled document when printed. Printed copy expires one week from print date. Refer to Document No. for current version.
B.1.2 Rating Factors for depth of laying direct in a duct
Depth of
Burial (m)
(to top of
duct)
Low Voltage Cables
High Voltage Cables
Single Core
Single Core
Three Core
Three Core
0.6
1.0
1.0
0.8
0.97
0.98
1.0
1.0
0.9
0.96
0.97
0.99
0.99
1.0
0.95
0.97
0.98
0.99
1.25
0.92
0.96
0.95
0.97
1.5
0.91
0.95
0.94
0.96
1.75
0.92
0.95
0.92
0.96
2.0
0.89
0.94
0.91
0.95
B.1.3 Rating Factors for variation in Thermal Resistivity (3 core cables laid
directly in the ground)
Conductor
size (mm2)
Up to 400
Thermal Resistivity (°C.m/W)
1.0
1.2
1.5
2.0
2.5
1.07
1.0
0.92
0.82
0.74
B.1.4 Rating Factors for variation in Thermal Resistivity (1 core cables laid
directly in the ground)
Conductor
size (mm2)
Above 400
Thermal Resistivity (°C.m/W)
1.0
1.2
1.5
2.0
2.5
1.08
1.0
0.90
0.79
0.71
B.1.5 Rating Factors for variation in Thermal Resistivity (3 core cables laid in
duct buried in the ground)
Conductor
size (mm2)
Up to 400
Thermal Resistivity (°C.m/W)
1.0
1.2
1.5
2.0
2.5
1.04
1.0
0.95
0.87
0.82
Page 46 of 47
Print Date 23/06/2014
© Horizon Power Corporation – Document Number: HPC-5DC-07-0004-2014
Uncontrolled document when printed. Printed copy expires one week from print date. Refer to Document No. for current version.
B.1.6 Rating Factors for variation in Thermal Resistivity (1 core cables laid in
duct buried in the ground)
Conductor
size (mm2)
Thermal Resistivity (°C.m/W)
Above 400
1.0
1.2
1.5
2.0
2.5
1.06
1.0
0.93
0.84
0.77
B.1.7 Rating Factors for Variation in Ambient Temperature
Air Temperature (°C)
Rating Factor
25
35
40
45
50
55
1.14
1.05
1.0
0.95
0.89
0.84
B.1.8 Rating Factors for Variation in Ground Temperature
Air Temperature (°C)
Rating Factor
15
20
25
30
35
40
1.07
1.04
1.0
0.96
0.92
0.88
Page 47 of 47
Print Date 23/06/2014
© Horizon Power Corporation – Document Number: HPC-5DC-07-0004-2014
Uncontrolled document when printed. Printed copy expires one week from print date. Refer to Document No. for current version.