ratings for cables.pdf

Technical Current Ratings forGuidance Note CablesTGN(T)67Issue 1 August 1996

Contents Page

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . 1Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1References . . . . . . . . . . . . . . . . . . . . . . . . 1Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . 2Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3Construction of Cables . . . . . . . . . . . . . . . . 3Temperature Limits for Cable Insulation . . . 5Cable Sheath Bonding Systems . . . . . . . . . 6Methods of Cable Installation . . . . . . . . . . . 6Thermal Resistance of Cable Environment . 9Cables in Air . . . . . . . . . . . . . . . . . . . . . . . 10Artificial Cooling of Cables . . . . . . . . . . . . 10Cable Rating Methods . . . . . . . . . . . . . . . . 11Rating Seasons . . . . . . . . . . . . . . . . . . . . . 13Rating Parameters . . . . . . . . . . . . . . . . . . 13Ratings of Cables as Part of a Circuit . . . . 16Derating of Cables . . . . . . . . . . . . . . . . . . . 18Guidance in Specifying Cable Ratings . . . 19Ratings of Typical Cable Systems . . . . . . . 21Characteristics of Cable Circuits . . . . . . . . 22Circuit Thermal Monitor Ratings . . . . . . . . 23Appendices A - F . . . . . . . . . . . . . . . . . . . . 27Tables 1 - 62 . . . . . . . . . . . . . . . . . . . . . . . 39Figures 1 - 6 . . . . . . . . . . . . . . . . . . . . . . 101

Authorised by:

Dr A WilsonTransformers & Cables ManagerTechnology and Science Division

The National Grid Company plc 1996

No part of this publication may be reproduced,stored in a retrieval system, or transmitted inany form or by any means electronic, mechanical,photocopying, recording or otherwise, withoutthe written permission of the NGC obtained fromthe issuing location.

Registered OfficeNational Grid HouseKirby Corner RoadCoventry CV4 8JY

Registered inEngland and WalesNo. 2366977

Published by:

The National Grid Company plcBurymead HousePortsmouth RoadGuildford, Surrey GU2 5BN

TGN(T)67Page 1 Issue 1

August 1996

CURRENT RATINGS FOR CABLES

FOREWORD

This Technical Guidance Note (TGN) provides information on the parameters which are taken intoaccount when a cable installation requires to be designed to meet a specific current rating. It alsoprovides information on the factors which govern the ratings of cables and which need to be consideredwhen existing cable ratings are revised or extended to meet The National Grid Company plc (NGC)needs. Unlike many other items of transmission equipment cable installations are usually individuallytailored to meet the declared rating and installation requirements of the purchaser. Whilst it is difficultto provide specific continuous current ratings for cables to match overhead line ratings which areuniversally applicable, an indication of suitable cable sizes and installation details has been provided.Methods of calculating continuous and short-term cable ratings based on IEC requirements aredescribed. Alternative simpler methods of providing conservative ratings are also discussed. Theapplication of the main cable rating programs used in off-line rating calculations and also in the CableSystem Monitor (CSM) is described. An overview of the programs is given in a separate TechnicalReport and the individual programs have their own user guides and reports.

The program GIMLI has been used to calculate a wide variety of cable ratings for directly buried cables,cables in troughs and in air and water-cooled cables of various constructions installed in specific ways.In addition a wide range of tabulated ratings are provided for directly buried cables and cables in air withmany different conductor sizes - these were supplied by one of the UK cable manufacturers and shouldonly be used for general guidance.

1 SCOPE

This TGN outlines the factors which affect the current ratings of cables and provides tables of continuousand short-term ratings applicable to directly buried cables, cables in troughs and in air and water-cooledcables for particular conditions. An indication of cable sizes required to match the overhead line ratingsgiven in TGN(T)26 is also given. The coverage of cables is generally comprehensive for voltages downto 132 kV while there is no attempt to provide current ratings for cables below 66 kV. This TGN doesnot include details of the short circuit ratings applicable to cables.

2 REFERENCES

Ball et al Self-Contained Oil-Filled Cables Systems: British Service Experience, RatingPractice and Future Potentialities. CIGRE 1972 21-02

Bungay et al Electric Cables Handbook. BSP Professional Books 1982

CEGB Standard 993208 Special Backfill Materials for Cable Installations

Electra No 104 Current Ratings of Cables Buried in Partially Dried Out Soil

Electra No 145 Determinations of a Value of Critical Temperature Rise for a Cable BackfillMaterial.

Endacott et al Thermal Design Parameters Used for High Capacity EHV Cable Circuits inGreat Britain. CIGRE 1970 21-03

ERA F/T 186 Methods for the Calculation of Cyclic Rating Factors and Emergency Loadingfor Cables Laid Direct in the Ground or in Ducts.

IEC 287 Calculation of the Continuous Current Rating of Cables (100% Load Factor).1982

IEC 853-2 Calculation of the Cyclic and Emergency Current Rating of Cables. 1989

TGN(T)67Page 2 Issue 1August 1996

Kaye and Laby Tables of Physical and Chemical Constants 1986

King et al Underground Power Cables. Longman 1982

Larsen at al Cable Rating Methods Applied to a Real-Time Cable System Monitor. IEEConference Publication No 382

NGTS 2.5 132 kV, 275 kV and 400 kV Single Core Cable.

NGTS 3.5.1 Oil-Filled 132 kV, 275 kV and 400 kV Cable.

NGTS 3.5.2 132 kV, 275 kV and 400 kV XLPE Cable & Accessories.

NGTS 3.5.3 Sheath Voltage Limiters for Insulated Sheath Cable Systems.

NGTS 3.5.4 Sheath Bonding and Earthing for Insulated Sheath Cable Systems.

NGTS 3.5.5 Cable Temperature Monitoring.

NGTS 3.5.7 Installation Requirements for Power and Auxiliary Cables.

NGTS 3.5.8 Cable Cooling Systems.

OShea et al Cable System Monitor. IEE Conference Publication No 382

TGN(T)26 Current Ratings for Overhead Lines.

TGN(T)29 Transformer Loading Guide.

TGN(T)68 Thermally Limited Continuous Current Ratings for Switchgear.

TGN(T)98 Application of Circuit Thermal Ratings.

TGN(T)109 Thermally Limited Continuous Current Ratings for 132 kV Switchgear.

TGN(T)113 User Guide to the Critical Unit Program (CUP).

TR(T)203 Factors Which May Limit the Working Life of Fluid-Filled Paper Cables.

TR(T)233 Increasing the Ratings of NGCs Lines in the UK.

TR(T)238 A Review of Cable Rating Software.

TR(T)240 An Investigation into Methods of Calculating the Cyclic and Emergency CurrentRatings for Cables.

Weedy Underground Transmission of Electric Power. Wiley 1980

Williams Natural and Forced-cooling of HV Underground Cables: UK Practice. IEE PROC Vol 129 Pt A No 3 May 1982

3 DEFINITIONS

Backfill: Material in the immediate vicinity of the cable.

Trench Filling: Material above the cable covers.

Soil: Material outside the cable trench.

Cable Environment: The total environment of the cable, ie backfill, trench filling and soil.


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Unspecified Backfill: Material, usually sand, found in early cable installations. Its thermal characteristicsare unknown but the thermal resistivity may be assumed to be 3.0 Km/W when dry.

Selected Sand Backfill: Sand obtained from selected sources chosen on the basis of density andcohesion characteristics so that the dried-out thermal resistivity does not exceed 2.7 Km/W.

Stabilized Backfill: Composite material specially selected and blended so that the dried-out thermalresistivity does not exceed 1.2 Km/W.

Laying Depths: Depth to cable centres for cables in flat formation. (Manufacturers sometimes refer todepth to top of cable and NGTS 3.5.7 refers to the depth to the top of the protective covers over thepower cables).

4 INTRODUCTION

In the design of a high voltage cable system both the electrical and thermal requirements of the cablemust be considered. The designer is required to provide adequate electrical insulation around theconductor and, whilst endeavouring to minimise the generation of heat within the cable when loaded,he must also ensure its ready dissipation. Therefore the voltage and current rating requirements of thecable system are crucial to its design. The voltage determines the insulation requirements whilst thecurrent rating is determined by the allowable temperature rise and hence the heat transfer capability ofthe cable, which depends on the installation conditions.

Heat within a high voltage cable is generated within the conductor, the dielectric and the metallic sheath.The flow of alternating current in the conductors of single core cables causes voltages to be induced intheir metallic sheaths which under certain conditions cause large currents to flow and generate heat inthe sheath. These sheath losses together with those in the dielectric and the conductor have to be takeninto account when designing a cable system to carry a specific current.

The cable system designer will attempt to minimise both conductor and dielectric losses by choice ofsuitable materials and construction. In addition to exercising similar options for the metallic sheath thedesigner may choose a method of bonding and earthing the cable sheaths which will assist in preventingor minimising sheath circulating currents, and thereby reduce the heat generated in the sheath.

There have been many articles written on UK practice for undergrounding sections of the power system.The majority of the cables were installed in the 1960s and CIGRE articles by Endacott et al and Ball etal were useful summaries. An IEE review article by Williams provided a comprehensive coverage ofmany aspects of cable installation. This TGN provides a more detailed account of thermal ratingmethodology with many examples quoted. A recent report on Increasing the Rating of NGCs OverheadLines, TR(T)233, illustrates the need for cable ratings and cable ratings methods to provide similarimprovements.

5 CONSTRUCTION OF CABLES

5.1 Conductors

These are formed from copper or aluminium. Within NGC copper is predominant and only one circuithas aluminium conductor in part of its length. A cross-sectional view of a typical 275 kV cable is shownin Figure 1. The conductor is made from stranded copper with enamel insulation and in the larger sizesthe strands are grouped in sectors around a central fluid duct in the construction named after Milliken.The objectives are to produce a mechanically stable structure with flexibility and with minimised electricallosses.


NGTS 3.5.1 describes some of the characteristics of this type of cable. A full account of power cableconstruction is given in the book by Bungay and McAllister, formerly with BICC, and shorter accountsare given by King and Halfter and by Weedy.

5.2 Conductor Screening

Carbon-loaded paper or metallized carbon-loaded paper layers are used around the formed conductorsto provide an electrical screen. This screening is intended to eliminate the enhancement in the electricalstress in the insulation that would be caused by the strands of the conductors.

It is particularly important that this screening removes the electrical stress in the fluid that is in theinterstices of the individual strands of the outer layers of the conductor.

5.3 Insulation

Under a.c. conditions the most highly stressed area of a cable is that immediately adjacent to theconductor. This stress at the conductor surface is associated both with the diameter of the conductorand its shape, in general smaller conductors have thicker insulation than larger conductors for a givenvoltage rating and maximum stress. The required thickness of the fluid-paper insulation is determinedby the maximum voltage gradient it can tolerate under impulse voltage conditions. This stress isdependent on the applied voltage, conductor diameter and insulation thickness, and under a.c.conditions is a maximum at the conductor screen surface. An alternative to fluid-paper dielectric is touse a fluid/polypropylene/paper laminate (PPL) dielectric which has the advantage of lower dielectriclosses. Several cables with this construction are now operating on the NGC system.

5.4 Metallic Sheathing Materials

Lead or lead alloy sheaths have been used for fluid-filled cables for many years. Such lead sheaths areextruded directly onto the completed and processed insulated conductor. In order to provide mechanicalstrengthening of the lead sheath tin-bronze reinforcing tapes are wound helically around it.This reinforcement is necessary in order that the sheath is able to withstand the hoop stress imposedupon it by the hydraulic pressures within the cables. These pressures are determined by both the staticpressures resulting from the cable route and by fluid volume variations resulting from temperaturechanges due to continuous and transient loading and to changes in ambient conditions.

Corrugated seamless aluminium (CSA) sheaths were developed and are now used in preference tolead. This construction of sheath is inherently strong and flexible. This strength means that the sheathcan be relatively thin and also that it does not require reinforcement. It therefore generally offerssignificant economies over reinforced lead sheaths, although in certain circumstances the higher sheathlosses result in lead alloy sheaths being preferred. Again such sheaths are applied in a continuousextrusion process.

5.5 Anti-Corrosion Protective Sheaths or Oversheaths

It is necessary to protect both lead and aluminium sheaths from corrosive attack. Initially self-vulcanisingrubber tapes wound around the cable were used, later PVC was used for the oversheath and nowpolyethylene is the material used. It is applied by a continuous extrusion process over the completedcable. In the early days of cable installation the oversheath was also known as the cable serving.

5.6 Armour

Armour is not usually applied to fluid-filled cables unless the installation conditions are particularlyhazardous or the cable requires mechanical support in deep shafts. Such armour may be comprisedof galvanised steel wires although aluminium or other non- ferrous material are used for single-corecables to avoid the losses in the magnetic materials.


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5.7 Extruded Cables

A type of cable presently in wide use at lower transmission voltages has a solid extruded polyethyleneinsulation which is normally cross linked to produce a thermosetting material with a higher maximumoperating temperature. NGC has a trial length of such a cross-linked polyethylene (XLPE) cableoperating at 275 kV and has an increasing number of 132 kV cables installed. NGTS 3.5.2 describessome of the characteristics of this type of cable.

6 TEMPERATURE LIMITS FOR CABLE INSULATION

Generally for most items of primary transmission equipment the quality of the insulation performancedecreases with increase in temperature. For this reason maximum temperatures are quoted asreferences for their design and construction. This decrease in insulation quality with enhancedtemperatures has two aspects, the reduction in the short-term electrical performance of the cable andthe enhanced rate of mechanical and chemical deterioration or ageing of the cable over many years.

The factors which are influenced by temperature and thereby impinge on the insulation performance inthe longer term are detailed below:-

(i) The dielectric loss angle, which is an indication of the quality of the insulation, increases withtemperature and also with ageing due to deterioration of the physical and chemical properties of theinsulation.

(ii) Differential expansion of the component parts of the cable system occurs and these must beconsidered in the thermo-mechanical design of the installation.

(iii) Increased electrical discharge activity may occur. Cellulose paper suffers a loss of mechanicalstrength due to thermal ageing and certain gases such as carbon monoxide and carbon dioxide mayevolve. Eventually, this gassing could lead to electrical breakdown but all the evidence suggests thatNGC cables are far from this condition. The cable fluid is generally capable of absorbing all the gasesevolved in normal services. In IEC documents it is generally supposed that the ageing rate of fluid-cellulose paper insulation doubles for every 6K rise above its design temperature.

When a cable is loaded it will be appreciated that the layer of insulation immediately in contact with theconductor attains the highest temperature. On the application of a step load change it may often takea very long time for the cable to attain a steady state temperature. In order to be able to assess theeffects of the transient heating of cables it is necessary to know how the cable and its thermalsurroundings will respond to such step function loads. This is essentially a problem of heat conductionin which both the thermal capacities of the cable and its environment have to be taken into account. Thesolutions of such transient loadings are complex and various assumptions and simplifications have tobe made for routine applications. In general for all cable current rating calculations these assumptionsinclude the following:-

(i) That the soil thermal resistivity is constant, although dual resistivity regions can also be dealt with,

(ii) That the soil thermal capacity is constant,

(iii) That the electrical resistivity of the conductor is constant and equal to that at the rated steady stateconductor temperature although in the case of calculations by the GIMLI program the effect oftemperature-dependent conductor resistivity is included.

Such assumptions enable the calculation of the transient rating of cables to be mathematicallymanageable. Presently the maximum acceptable operating temperature of the conductor of a fluid-filledcable is limited to 90oC. This value has increased from the previously accepted maximum of 85oC whichapplies to the majority of present NGC cables. These maximum design temperatures are such thatcables have a planned life expectancy of 40 years if operated continuously at such temperatures. NGCand the cable companies are collaborating in research aimed at introducing a maximum emergencyconductor temperature of up to 105oC over the next few years. This work is described in a recentreport, TR(T)203.


7 CABLE SHEATH BONDING SYSTEMS

A number of methods of bonding and earthing single core cable sheaths are commonly used. Thesecan be broadly categorised as follows:-

Solid Bonding - Sheaths are bonded and earthed at each end of the route, so that every sheath is ator near earth potential throughout its length. Cables in close trefoil configuration can often be solidlybonded since the configuration minimises induced sheath voltages and consequential circulatingcurrents. Single Point Bonding - Sheaths are bonded together and earthed at one position only. Although avoltage will be induced in the sheath, circulating currents will not occur.

Cross Bonding - Sheaths are sectioned along the cable route and cross connected to minimise thevector sum of the induced sheath voltages. Hence the circulating currents in three consecutive similarsections will sum to zero. Single point and cross bonded systems are commonly referred to as Specially Bonded Systems. Thesemethods have the following advantages:-

(i) Reduction of sheath losses and heat generated. Hence for the same current capability smallerconductors can be used than would be possible with a solidly bonded system.

(ii) Wider spacing of cables can be adopted. Improving heat dissipation enables smaller conductors tobe used or increased current ratings to be achieved.

However Specially Bonded Cables do possess several disadvantages over Solidly Bonded Systems,which are:-

(i) The cable metallic sheath and joints must be insulated from earth along the cable section length andonly connected to earth in an approved manner.

(ii) Insulation has to be inserted into the cable sheaths to create specific section lengths. This is usuallyachieved by the use of sheath sectionalising insulation incorporated in joint cases.

(iii) Dependent upon the cable section length, the phase spacing and the current being carried, astanding voltage is present on the cable sheath and joints. For 400 kV and 275 kV systems this ispresently limited to 150 volts but earlier installations may have been restricted to the 50 volt or 65 voltlimits prescribed at the time.

(iv) Transients resulting from switching surges, lightning or fault conditions may cause high voltages toappear across the joint sectionalising insulation and cable oversheath.

(v) Special measures have to be taken to limit both the steady state and transient voltages experiencedby Specially Bonded Cable Systems to within acceptable values.

A schematic diagram of a cross bonding system is shown in Figure 2. NGTS 3.5.3 describes SheathVoltage Limiters (SVLs) for insulated sheath cable systems and NGTS 3.5.4 describes sheath bondingand earthing methods for such systems.

8 METHODS OF CABLE INSTALLATION

There are many different types of cable installation and any one cable system may employ more thanone such type. The five usual methods of laying underground power cables are as follows:-

(i) Laid directly in the ground in a specially excavated trench which after the cable has been laid instabilised backfill of known thermal resistivity is backfilled with soil.

(ii) Laid in concrete surface troughs filled with a stabilised backfill of known thermal resistivity.


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(iii) Laid in open ducts or troughs which have been specially constructed for the purpose.

(iv) Laid in ducts or pipes through which the cables are drawn and which are then filled with a lowthermal resistivity material.

(v) In tunnels either specially constructed ones or ones which may have been built for other purposes,such as for road or rail traffic.

The different types are described below together with an appreciation of the factors which govern theratings of cables laid in such a manner. Most of the 275 kV and 400 kV cable circuits on the NGCsystem were laid in the 1960s. Accounts of many of the cable rating concerns and methodologies weredescribed in CIGRE papers by Endacott et al in 1970 and Ball et al in 1972.

8.1 Directly Buried Naturally Cooled Cables

The rate at which heat is transferred from the cable conductors to a heat sink depends on both thetemperature difference between them and the total thermal resistance of the cable and its surroundings.

The heat transfer by conduction in a cable and its surround is analogous to the flow of current in anelectrical system. Thus thermal resistivity, a term frequently used in the determination of cable ratings,is directly analogous to electrical resistivity. It can therefore be appreciated that different materials willhave differing thermal resistivities and the network of these that comprise the cable components and itssurroundings will determine its rating.

The ability of a material to store heat, as its thermal capacity, is the product of its density and specificheat. This storage of heat is analogous to the storage of energy in a capacitor. The stored heat maybe dissipated by conduction, radiation and convection from the cooling surfaces. We may consider thethermal network of the cable system to be composed of thermal resistances and thermal capacitancesas shown in Figure 3.

Complicated networks may be required or it may be that a single resistance, R, and capacitance, C, maybe an adequate model for the cable. For such a single network after a step change in heating it can beshown that the relationship between temperature rise and time is:

Where s = HR is the steady state temperature rise, = RC is the thermal time constant of the cable,H is the steady heat input to the cable conductor, R is the thermal resistance between the conductor andthe eventual heat sink and C is the effective thermal capacity of the system.

Most items of power equipment have temperature rise versus time characteristics which require morethan one term to model the response. When heat runs are carried out on equipment the temperaturemay appear to level out in the short term but if the test is continued the temperatures may be seen tocontinue to rise even though such a rise was imperceptible in the shorter time scale. This is often thecase with cables which take many tens of hours before reaching an equilibrium condition.

For cable installations the direct measurement of the conductor temperature whilst it is at workingvoltage is not yet a practical possibility. However, monitoring the temperature of the oversheath ispossible using thermocouples or more recently fibre optic measuring equipment. NGTS 3.5.5 describeshow such fibre optic temperature measuring equipment can be installed on power cables. The accurateprediction of the thermal conditions within a cable, usually without the benefit of any temperaturemeasurements, is of considerable importance.

Since the capital cost of cables is high in comparison to overhead lines significant savings may be madeby being able to increase the rating of a cable by what may seem quite a small amount. It is thereforedesirable that the thermal model used to predict the cable temperatures and hence its rating is asaccurate as possible. The uncertainties of the cable environment make accurate modelling difficult.The considerable efforts which have been put into this subject over recent years has been inspired bythe significant savings which can be made.The dielectric of a single core cable represents the major internal thermal resistance of the cable.The other constituent parts of a cable such as the screens, bedding layers, metallic sheaths and theoversheaths also have thermal resistances which need to be taken into account. The thermal resistance


of the cable environment is similarly influential in determining the rate of heat flow from the cable outersurface. Thus the thermal resistivity of the material which surrounds the cable and that which is usedto backfill the cable trench is important and should be considered carefully when designing or carryingout work on the cable system. These matters are discussed further in the Section 9.

8.2 Filled Surface Troughs This type of installation has been frequently used alongside railways, canals, within substations and forsome routes around cities and through the countryside. In such installations the cables are generallyplaced with a relatively small separation between the phases in concrete troughs which have reinforcedand sometimes interlocked concrete lids for mechanical protection.

The thermal resistances external to the cable are those of the trough filling and its surroundings and thethermal resistance of the trough surface to the air. Special backfill materials are again used to reducethe thermal resistivity of the material immediately surrounding the cable. It has been shown that up tohalf of the total heat dissipated by the cable may flow through the trough lid and therefore it is importantthat the lid is kept in close contact with the backfill. Solar radiation has a marked effect on the ability ofthe trough lid to dissipate heat and for shallow troughs will cause significant daily variations in the cabletemperatures.

8.3 Unfilled Surface Troughs

If the cable troughs are not ventilated then the trough is effectively a large duct in which the still air hasa relatively high thermal resistivity of 40 Km/W. They are also affected by solar radiation like filledtroughs. Such installations are of low thermal capacity and the effective time constant of the cables willbe short.

The use of grilles instead of concrete covers allows natural convection of the air in the trough to occur,thereby permitting air circulation which improves the cooling of the cable. However, unless specialmeasures are taken solar radiation will have an adverse effect on the cable rating. Solar shields havebeen used for cable in troughs with open grilles for ventilation.

8.4 Cables in Ducts or Pipes

Where such routes are available they have the advantage of allowing the cable installation to take placewithout costly and disruptive excavation. In order to preserve the required thermal environment aroundthe cable it is preferable that such ducts are filled with a grout composed of sand, cement and abentonite mixture. Such mixtures are usually of a consistency that allows them to be pumped in oncethe cable is in situ. This enables the duct sections of cables to have improved thermal ratings and alsoprovides similar mechanical support to the directly buried sections. There is however a limit on thelength of duct which may be filled in this way.

8.5 Cables in Tunnels

Cables in tunnels have been cooled by a variety of means. The major consideration is how to deal withthe cable heating when the tunnel may be several kilometres long and it is not clear how much heat willbe absorbed by the rock strata and percolated water. Cables in tunnels have usually been forced cooledby air or by water and the considerations of fire hazard and potential high tunnel ambient temperaturesneed to be included. Generally water cooled systems when initially installed have proved to beunnecessary in the longer term. Water cooling has been de-commissioned in the Woodhead tunnel, theThames tunnel and the Fawley tunnel. Forced air circulation plus other specific measures, such asremoving trough lids to enhance air circulation, have proved adequate once the actual load and theeffect of heat lost to the strata and ground water is assessed in reality. Direct water cooling of cablesin the Severn tunnel is still operational.


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9 THERMAL RESISTANCE OF CABLE ENVIRONMENT

When carrying out rating calculations the soil is assumed to have a constant thermal resistivity althoughin practice this may not be true since the soil will contain moisture which may migrate when the cabletemperature rises as it carries current. If such drying out is allowed to continue then the thermalresistivity of the soil will rise, resulting in less efficient heat transfer from the cable. Ultimately the rateof heat generation within the cable would exceed the rate of dissipation into the soil and thermalinstability or runaway might occur with consequent damage to the cable. It is therefore essential to haveknowledge of the characteristics of soils and the mechanisms of moisture migration so that cables canbe assigned ratings with confidence. With artificially cooled systems this knowledge of the behaviourof soils is less important.

In general soils may comprise of solids, air and water and their thermal resistivities are determined bythe several parallel paths that these allow through the soil. Comparison of the thermal resistivities ofvarious constituents of soil is interesting, for example quartz has a thermal resistivity of 0.11 Km/W, thatof water is 1.65 Km/W and air is highest at 40 Km/W. Comparison of these thermal resistivitiesillustrates the need to optimise the quantities of solids and water in the soil. It is essential to compactthe soil in the trench to exclude high thermal resistance air pockets. The high soil density will decreasethe permeability of the soil and decrease its ability to allow moisture migration. The density may alsobe affected by natural effects such as consolidation, shrinkage or swelling.

The particle sizes of silts, sands and gravels effect the thermal behaviour of the soils they make up andsome of their characteristics are described below.

Clays Silts Sands Gravels

Size (mm)


Special backfills provide the advantages that the effective 500C limit on the cable surface temperaturemay be raised and the use of forced cooling systems may be avoided. These backfills can be dividedinto two groups, selected sands which are obtained from approved quarries and do not have a dried outthermal resistivity greater than 2.7 Km/W and stabilised backfills of composite materials which havedried out thermal resistivities not exceeding 1.2 Km/W. The special backfills are now type tested underNGTS 3.5.7 but many circuits have backfills which were procured under the more prescriptive CEGBStandard 993208. The CEGB Standard descriptions are summarised below.

Selected Sands - Material of consistent composition not containing organic matter, pieces of clay,sharp stones or flints, and where not less than 95% by weight of the material shall pass a BritishStandard 5 mm sieve.

Stabilised Backfills - These are man-made mixes where selected sand is used and three differenttypes have been used.

Cement-Bound Sand - The cohesion of the particles is improved by mixing the sand with cement in a14:1 proportion by volume. To this mix water is added to ensure adequate compactibility with totalwater/cement ratio of 2:1 by weight.

Gravel and Sand - A selected sand is mixed with 10 mm coarse aggregate in the proportion of 1:1 byweight. The gravel particle size should not exceed 14 mm and the proportion of crushed material shouldnot exceed 50%.

Bitumen-Bound Sand - The cohesion of selected sand is improved by the addition of a bitumen/fluxingoil mixture to 6% of total weight. The mixture was made from fluxing oil and bitumen in the proportionof 2:1 by weight. Although present on some older cable installations bitumen-bound sand is no longerused.

The choice of stabilised backfill will be influenced by their cost, availability and mechanical behaviourand the methods available to mix them. Because cement-bound sand offers mechanical restraint to thecable it is often chosen. It does however suffer from the serious disadvantage that its removal is oftendifficult and unless special care is taken damage to the cable may result. Gravel and sand is almost aseffective as cement-bound sand but bitumen-bound sand behaves mechanically as poorly as sand.

10 CABLES IN AIR

These may take the form of cables passing down shafts before going underwater, small tunnels underwater, or larger tunnels used for other purposes, for example road and rail tunnels. For any particularcable the rating is determined by the air temperature and in short tunnels and shafts the naturalventilation may be sufficient. However in long tunnels it may be necessary to provide some form ofadditional cooling or ventilation to maintain the required temperature. Considerably higher ratings maybe obtained with cables in shafts or tunnels compared to those installed in the ground. However the costof such installations will generally be considerable.

11 ARTIFICIAL COOLING OF CABLES

11.1 External Pipe Cooling

When the rating of a cable is required to match that of an overhead line then it may be necessary toinstall cables to operate in parallel. The obvious example is the Goring Gap circuits where there aretwo cable circuits with two cables per phase for each overhead line circuit. This naturally increases thecost of the installation and the width of the reserve of land through which they pass. In suchcircumstances it may be necessary to consider increasing the rating of the cable circuit by artificiallycooling its thermal environment. Various methods to achieve this are available. One such method isto pass water through plastic pipes suitably positioned with the cables. With this technique the normalthermal resistance of the cable environment is shunted by the thermal resistance into the water.The pipes are usually arranged as go and return circuits.


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These go and return circuits start from cooling stations situated along the route, the cold inlet waterextracts heat and then warm water returns along the same section to the cooling station.

11.2 Surface Cooling

Cables may be more intensively cooled by placing each one in a rigid plastic pipe through which wateris pumped. This is more expensive than external pipe cooling and problems may occur due tomovement of the cable within the pipe caused by electro-mechanical forces. This method has beenapplied to the cables in the Severn tunnel. For these and cost reasons the external pipe scheme islargely preferred.

An alternative to water-filled pipes is to place cables in water-filled troughs in tunnels. The joints of suchinstallations are usually above the water level and they are cooled by longitudinal heat flow to theimmersed cable. Such water-filled trough arrangements may have weirs situated along their length tocontrol the flow of water along any natural incline. They have cooling stations where the water leavingthe end of the route is cooled before being returned to the beginning to repeat its cooling cycle.

11.3 Cable Accessories

It is generally assumed that all cable accessories meet the rating requirement of the cable - this includessealing ends, stop joints and straight joints plus the associated fluid expansion tanks. It is taken that asjoints tend to be larger than the normal cable cross-section they may be assumed to have greatercontinuous and short-term ratings than the cable. The joint generally has more copper, lower electricstresses and a greater area for heat transfer to the adjacent ground or cooling air flow, also any hydraulicor mechanical limit arising from the joints will be small compared with the overall length of the circuit.

One major exception to this assumption is where the cable is water cooled and the joints are not. Eitherspecial provision for the joint cooling has to be made or the rating of the joints will become limiting.On the South London Ring the joints are considered to be limiting and to preserve their integrity the ratedoperating temperature for the cable conductor has been lowered to 70oC to be an equivalent restrictionto the joint rating.

Various design innovations have been introduced for joint cooling by manufacturers. Oscillation of thefluid in the cable fluid duct has enabled the heat from the joint region to be transferred to the adjacentlengths of conductor so that the joint hot spot is reduced in magnitude. This approach was adopted forthe cables in the Thames tunnel and on the Beddington-Chessington cable circuits. Water cooling ofjoints may be required to be more intensive than just relying on separate pipes and water-cooled jacketshave been applied to the joint boxes. In the Severn tunnel cables the joints has turbulators fitted whichwould have increased heat transfer within the joints if an uprating using fluid circulation through the fluidduct were used - this enhancement has not been implemented.

12 CABLE RATING METHODS

The continuous ratings of underground power cables are calculated using the method described inIEC 287. This method analytically solves a thermal resistance ladder network to give the current atwhich the maximum conductor temperature is reached. IEC 853-2 presents a similar method for thecalculation of cyclic and overload currents for overload periods greater than 10 minutes. The use ofthese methods requires a large amount of data to be known about the cable and its environment.The continuous ratings quoted in Schedule D of the tender documents are based on IEC 287.

NGTS 2.5 provides a method of calculation of overload ratings from the minimum amount of input data.The short-term overload ratings now quoted in the Schedule D of present tender documents are basedon NGTS 2.5. The previous tender documents for the majority of NGC cables did not require suchinformation.

The introduction of computers meant that it was possible to solve ladder networks numerically, enablingthe use of more accurate models of power cables. The GIMLI program uses a numerical method tocalculate cable ratings. The program can include the effects of forced water cooling, sheath losses andair-cooling to calculate continuous, cyclic and overload ratings.


A version of GIMLI is used in the Cable System Monitor which gives on-line cable ratings to the variouscontrol rooms. A full review of all the cable rating software is given in TR(T)238.

The Critical Unit Program (CUP) produces circuit rating schedules for use in control rooms. Essentiallythe program identifies the limiting item of plant in the circuit for continuous operation and for variousoverload periods at various pre-fault loadings. A circuit rating sheet is then produced which shows therating of the circuit in each circumstance. CUP therefore needs to know the continuous and overloadratings of each item of plant in the circuit. For lengths of cable, overload ratings can either be calculatedautomatically by CUP using the NGTS 2.5 method or they can be manually entered if a GIMLI study hasbeen performed. A User Guide to CUP is being written, TGN(T)113, and a TGN on the application ofcircuit thermal ratings will be available shortly, TGN(T)98.

12.1 NGTS 2.5

NGTS 2.5 uses a straightforward thermal model to calculate the overload needed to raise the conductortemperature to its limit after a given time from a given pre-fault loading. The model assumes that all theheat generated by the overload current remains in the cable and hence it is a safe method to use in theabsence of detailed information about the cable circuit.

The simple model used by the NGTS 2.5 method generally loses its validity for overload times in excessof an hour. It eventually gives overload ratings lower than the continuous rating of the cable for longperiods. In these cases the cable is usually assigned an overload rating equal to the continuous ratingor another method to calculate the rating must be used.

The information needed to perform NGTS 2.5 calculations is listed in Appendix A.

12.2 GIMLI

GIMLI is a computer program which numerically solves a thermal model to give continuous, cyclic andoverload ratings as requested. This model needs to be provided with a large amount of material data,thermal parameters and physical dimensions and these may be difficult to locate. A detailed search ofthe cable route records is required to identify potentially limiting sections of cable such as:-

(i) Deeply buried sections

(ii) Closely spaced sections

(iii) Surface troughs

(iv) Air-cooled sections

(v) Naturally-cooled sections in a forced-cooled cable circuit

(vi) Sections in close proximity to other heat sources, for example electricity cables or hot water pipes

(vii) Ducted sections under roads

(viii) Tunnel sections

Changes to the cable route may have occurred after installation and these may sometimes not havebeen recorded on the route records. Such changes affect the cables thermal behaviour. Visualinspection of the cable route may be needed to look for undocumented changes to the cable route whichaffect the cables thermal behaviour, for example new buildings near the cable route, new electricitycables or ground surfaces that have been raised by new roads. When the potentially limiting sectionshave been found then GIMLI input files can be created for each section. GIMLI will then calculate thedesired ratings and the limiting section can be found for each circumstance. The information neededto perform GIMLI calculations is listed in Appendix B and the material properties are given in AppendixC.


August 1996

13 RATING SEASONS

The rating seasons for overhead lines, transformers and switchgear are given in TGN(T)26, TGN(T)29,TGN(T)68 and TGN(T)109 respectively and all use the following seasons:-

Winter December - February.Spring/Autumn March, April and September-November.Summer May-August.

Until recently cables had the following seasons:-

Winter November - April All cables.Summer May - October Directly buried cables.Summer - Normal May, September and October Cables in troughs.Summer - Hot June - August Cables in troughs.

Additionally cables in air or in tunnels were rated according to the overhead line rating seasons above.

The CUP rating sheets were designed to cope with three season plant and two season cable ratings.There has never been any intention of increasing the number of rating seasons to separate May fromthe rest of the summer season although some individual rating sheets were produced to help GridSystem Management. Instead it was recognised that cables in troughs could also be modelled as threeseason plant and for the commissioning of the CSM all cables were provided with two or three seasonratings, as appropriate. So the next revision of NGTS 2.5 will delete any reference to a Summer Hotseason.

However, this decision meant that there were some periods of the year when cables might lose ratingdue to the change of modelling assumptions, even if the corresponding gains at other periods were atleast as large. To avoid this eventuality and to give the maximum flexibility consistent with the CUPrating sheet layout and with existing seasons used by Grid System Management it has been decidedto rate cables for four seasons and the next revision of NGTS 2.5 will contain this change. This requiressome development within CUP although it does not change the layout of the rating sheets. GSM alreadydescribe seasons within ESCORT as CC, NC, NH and HH and this nomenclature is adapted below toavoid ambiguity.

CC Cold-Cold Winter December - February

NC Normal-Cold Spring/Autumn March, April and November

NH Normal-Hot Autumn September - October

HH Hot-Hot Summer May - August

This retains all the assumptions about seasons in existing rating sheets and merely divides theSpring/Autumn period into two parts for cables alone. The reasoning behind the rating parameters forcables is dealt with in greater detail in the next section and the revised values are listed in Appendix D.This Document uses the values in Appendix D for the NH season to represent the NC season also. ieThe whole Spring/Autumn season defined for overhead lines and other plant had one set of designvalues for cables. In future it will be possible to enhance cable ratings in the NC season by choosingthe lower values in Appendix D.

14 RATING PARAMETERS

The main characteristics of the environment affecting cable ratings are its temperature, thermalresistivity and thermal capacity. The latter two factors are dependent upon the moisture content.IEC 287, 1982 edition, quotes typical values of soil thermal resistivity used by various countries - onlyFrance is quoted as distinguishing between Winter and Summer although the similar UK practice isincluded in the table below:-


Soil ThermalResistivity (Km/W)

Average Value Minimum Maximum

UK 1.2 (S) 1.05 (W) - 2.7*/3.0*

France 1.2 (S) 0.85 (W) - -

Australia 1.2 - -

Austria 1.0 0.7 1.2

Finland 1.0 0.4+ -

Germany 1.0 - 2.5*

Italy 1.0 - -

Japan 1.0 0.4+ 1.2

Norway 1.0 - -

Sweden 1.0 - -

Switzerland 1.0 - 1.3

Canada 0.9 0.6 1.2

USA 0.9 - -

Netherlands 0.8 0.5+ -

+ Wet *Dried Out

Where there is no information it was recommended in the early versions of IEC 287 that the followingfigures were used. However, better information is now available and TSD have suitable equipment formeasuring ground thermal resistivity.

Thermal Resistivity (Km/W) Soil and Climate Conditions

0.7 Very moist soil and continuously wet climate.

1.0 Moist soil and regular rainfall.

2.0 Dry soil and seldom rains.

3.0 Very dry soil and little or no rain.

The UK figures which are shown in more detail in Appendix D and in NGTS 2.5 are at the high end ofinternational comparisons. TSD is intending to start new investigations of soil conditions shortly. Thefigures for thermal resistivity assume a moist soil in Winter (From November to April) with a value of1.05 Km/W and a drier soil in Summer (from May to October) with a value of 1.2 Km/W. The seasonslag the climate seasons by one month because of the thermal lag in the ground temperature at about1 m depth and the presumed equivalent delay in rewetting of the soil. The thermal capacity of the soilis taken as 1.7 MJ/m3K which corresponds to a dry density of 1795 kg/m 3 and a moisture content ofabout 3 % by dry weight. The figure is taken to be constant all year round, although in practice it is likelyto increase considerably with moisture content during the Winter period. This will also be the subject offurther investigation.

Ground temperatures depend on geographic latitude and local climate and on depth below the surface.For the purposes of rating calculations the ambient temperature is the temperature that would exist atthe position of the cable if the cable was not producing any heat. When UK cable ratings were firstconsidered the main need was to cover directly buried cables at about 1 m depth and the seasonaltemperatures where taken with the same periods as for resistivity, namely Winter (from November toApril) at 10bC and Summer (from May to October) at 15bC.


August 1996

Since all plant ratings need to be configured into circuit rating sheets the cable rating sheets need to becompatible with overhead line and substation plant seasons as described in the previous section. Cableratings have been defined for two and three seasons and will shortly be defined for four seasons.All these calculations use the same methods but with different assumed seasonal conditions. All canbe incorporated in the CUP method of producing circuit rating sheets. Examples of how cable ratingsmight be used are given below. All are compatible with standard seasons since only cables as a plantitem differ from the standard overhead line seasons.

Two season cable - directly buried cable CC/NC and NH/HH apply.Three season cable - cable in air CC, NC/NH and HH apply.Four season cable - cable in filled trough CC, NC, NH and HH apply.

The values given in Appendix D and in NGTS 2.5 are those applying to new circuits. There is a greaterpossibility of dry out in old cable circuits laid in selected sand backfill. However, it is likely that anypotential dry out regions would be investigated on site with fibre optic probes to measure temperaturebefore any de-rating was considered. Where there is a need new stabilised backfill material with lowerthermal resistivity can be used. Weak-mix cement with a graded sand as the main component has beenused on the Birkenhead-Lister Drive Circuit to give higher performance and a thermal resistivity of0.8 Km/W was quoted. Wax-filled sand backfills have also been developed with a thermal resistivity of0.7 Km/W although they have not yet been used on a cable circuit.

Sometimes when cable ratings have been investigated it has been possible to re-evaluate the basis ofrating calculation to help enhance the rating of an important circuit. This has usually been on the basisof site specific circumstances and site investigations. This fits in with the manufacturers practice ofavoiding potential limits on short sections of cable by considering the effects of longitudinal alleviationand the relatively small effect of expansion on the overall cable section.

Section 20 lists some of the special circumstances which were incorporated in the modelling during theinvestigation and commissioning of the CSM.

14.1 Sources of Information

The main sources for cable information are:-

(i) AMIS records.

(ii) Operating Diagram and Technical Data Sheets.

(iii) Route records including Schedule D.

(iv) Operating and Maintenance Instructions

(v) Transmission Area records

(vi) Cable company records

(vii) Local knowledge of cable circuits

(viii) Current rating sheets

The Schedule D document is provided by the cable contractor during tendering. It contains all theinformation needed to calculate overload ratings to NGTS 2.5 and most of the information needed forGIMLI calculations. Schedule D for newer cables also contains overload ratings calculated inaccordance with NGTS 2.5, for various pre-fault continuous loads as percentages of the maximumseasonal continuous cable rating.

Route records include information on the make-up of the cable, pilot cables and water pipes (whereappropriate). The location of the cable and its joints are shown, with cross-sections and profiles of theroute. The route records are the main source of information when looking for limiting sections to modelwith GIMLI. However, route records generally only describe the route of the cable as it was installed.Additional cables installed more recently which cross or run parallel to the original cable may affect the


rating of this cable and yet may not be shown on its route records.

The existing circuit rating sheets should give enough information to calculate short-term ratings toNGTS 2.5. Where the ratings of circuits which incorporate cables are calculated using CUP a table ofshort-term overloads is automatically generated for the composite circuit and separately for each itemof plant.

The cable company should be able to provide details of the cable's construction and electrical propertiesif the Schedule D document is not available within NGC. This information, combined with inspection ofthe route records, will enable GIMLI calculation of cable ratings.

15 RATINGS OF CABLES AS PART OF A CIRCUIT

The foregoing sections have served to briefly illustrate the nature of cable ratings and the manyvariables which may be involved. Ideally the transmission system designer would like to be able todetermine the size of cable and type of installation necessary to match the specific parameters of thecircuit he is designing. The high costs of cable circuits means that the cable designer is required tooptimise his design ensuring that the size of the cable and its installation including the civil and othercosts are balanced to provide the most economic cost solution. It is also necessary to be able toaccurately assess the ratings of cables under various loading conditions in order that they be moreeconomically utilized.

A cable is usually rated by its continuous ratings for the various seasons or parameters specified andthis is often referred to as its design rating. This is the continuous rating that the cable will carry in theprescribed environment without the conductor exceeding the maximum temperature for which, theinsulation and installation have been designed. In practice however it is very rare for the cable to beoperating continuously at its rated value and therefore it is worthwhile economically to be able to defineother ratings to correspond to these other types of load. A daily load curve may fluctuate according toa regular and predictable pattern and, since a buried cable has a slow thermal response it may bepossible to load the cable to a daily peak which is 110% or 120% of the continuous rating of the cablewithout exceeding the maximum allowable conductor temperature. The cyclic rating is the peak currentwhich the cable will carry without exceeding its maximum design temperature when subjected to arepetitive daily load cycle over a long period.

It follows from this that when a cable is loaded below its maximum rating on either a continuous or cyclicbasis then it is possible for it to carry a short duration overload without it exceeding its designtemperature. This may be required of a cable under certain circuit outage conditions either as a resultof fault or of planned maintenance.

Consequently, unlike other items of transmission plant, it has been the practice to specify the actualrating requirements in cable tenders and not the standardised values used for example for switchgearor transformers. The purchaser therefore relies upon the tenderers to effectively design the most costeffective solution to the specified rating required. However the techniques used to assess the ratingsof the various designs of cable installation are well known and NGC works closely with the cablecompanies when required to assess and agree their installation designs.

Nevertheless the circuit designer needs to appreciate the thermal and rating behaviour of a cable whenit forms part of a composite circuit in order that he should correctly specify the rating required of it.Foremost in this appreciation must be the knowledge of the rating seasons applicable to cablescompared to other plant. In order to present this as simply and concisely as possible this is given intabular form in Appendix D. Also the designer has to be aware of the way in which cable systemsrespond thermally to changes in continuous, cyclic and transient loading. Each of these loading regimesis described below.


August 1996

15.1 Detrimental Effects On Installation

In applying a continuous load to a cable which is equal to its installation design rating for that particularseason it follows that it may attain full design conductor temperature if all the parameters of environmentthermal resistivity and temperature are met. This may not be the case if the installation conditions whichdetermined the assigned current rating have changed. Some of the factors which could adversely affectthe rating and hence temperature of the cable are:-

(i) Increase of ground cover over a directly buried cable

(ii) Reinstatement of the backfill of the cable with a thermally unsuitable material, for example usingordinary sand instead of a selected sand.

(iii) Placing a second loaded circuit adjacent to the circuit which was designed to be thermallyindependent of other sources of heat such as cables or hot pipes.

(iv) Restricting the air flow over a ventilated trough installation

(v) Placing unfilled spare ducts near a buried cable

(vi) Allowing natural vegetation and shrubs to absorb the moisture from the surround of a directly buriedcable

(vii) Allowing silt to build up around the cable in water-filled troughs

(viii) Decreasing the phase spacing of cables from that of the original design

(ix) Alterations or defects in the bonding system which could cause or increase the sheath circulatingcurrent heating effects on the cable

(x) Abnormal cable seasons, such as periods of very high air temperatures or solar radiation for cablesin troughs or extended periods of drought which would promote soil dry out for directly buriedinstallations

(xi) Failure or maloperation of cable cooling systems

It should also be noted that the hydraulic design of a cable system is carried out using the permittedmaximum design temperature of the conductor. If the cable is allowed to operate above this designvalue then the ability of the hydraulic system to cater for the increased volumetric change of the cablefluid may be insufficient and damage could result. In a similar manner any restriction imposed on thehydraulic capacity of a fluid-filled cable by for example the isolation of cable fluid tanks could result insimilar damage. 15.2 Cyclic Loading

Cyclic loading is the term used to describe a load which is not continuous with time and usually variesthroughout a 24 hour period. A suitably installed cable subjected to such loading patterns will rely onits thermal capacity and that of its environment, which is usually considerable, to materially reduce theeffects of load peaks on its temperature rise. The thermal response of the cable and environment isslow so that the temperature rise of the cable during such cycles tends to be smoothed. Over manyyears methods of determining cyclic ratings were improved to the present where the uncertainties in thecalculation method are much less than in the values of the environmental thermal parameters.

15.3 Short-term Ratings

These ratings utilize the slow temperature response of a buried cable and if it is initially loaded belowits rated current then it is obviously at a lower temperature and a further temperature rise is availablebefore it meets its maximum design temperature. Thus it has the ability to carry a higher than ratedcurrent for a short duration before reaching its maximum design temperature.


15.4 Combinations of Loading Regimes

The complexity of the thermal performance of cable systems is such that due care has to be exercisedin using the rating information given or specified for cables. For example a cyclic rating defined for adomestic type of load cycle may not be applicable if the load changes to that of an industrial cycle. Alsothe short-term ratings are dependent on the nature and duration and magnitude of the previous loadingof the cable. When any such ratings are calculated it should be appreciated that they are particular tothat cable, installation and loading regime. Also unless specifically calculated the repeat application ofa particular short-term overload is not advised unless a sufficient time is allowed between eachapplication to enable the cable to return to its initial temperature. Normally repeated use of short-termratings would be possible after twenty-four hours. TSD will advise on any particular requirements.

16 DERATING OF CABLES

In Section 15.1 some of the factors which may be prejudicial to maintaining the declared cable ratingwithout exceeding the conductor design temperature were outlined. It is therefore important thatengineers are aware of these and where possible endeavour to ensure that they do not occur. Wherethey do occur then the relevant facts should be drawn to the attention of TSD in order that appropriateadvice may be given. The majority of the factors listed, especially those relating to installation conditionsare easily discerned by the engineer responsible for the cable. In many cases even though a deficiencymay appear prejudicial to the cable rating mitigating factors may reduce the perceived and actual risks.There should be no automatic de-rating of cable circuits except where water cooling has failed or whereforced air cooling has failed.

For example, it is unusual for a cable circuit to carry a high and sustained load and if it does theoperation of the transmission system usually requires this to be for a relatively short period of perhapsa few days. The load is rarely sustained at the maximum declared cable rating for more than a fewhours and the cyclic nature of the load helps in maintaining temperatures below their maximum designvalue. In addition the thermal environment of the cable may be better than that allowed for in the design.The thermal resistivities of buried cable backfills may be lower than thought. The ground temperaturesmay be less than those assumed. The cable may be installed below the natural water table and otherfactors may also be relevant.

With regard to the Special Bonding Systems employed on the majority of NGC circuits, these may becompromised by defects in the cable oversheath which are generally detected by oversheath testscarried out during maintenance. However, for the Specially Bonded Systems to allow the passage ofcirculating currents along the sheath the earth resistance of the serving fault and that of the earthing ofthe bonding system would have to be low. The susceptibility of present designs of link box to floodingin adverse conditions may also pose a problem. The ingress of water into such link boxes may causedeterioration of the Synthetic Resin Bonded Paper (SRBP) link pillar support plate and if a sufficientquantity of water is present the isolation of the bonding links from each other and earth may be affected.

Both of these and similar mechanisms require a low impedance path to be created in the bondingsystem which might then allow circulating currents of sufficient magnitude to adversely affect thedeclared rating of the cable. The factors which determine the resistance of such link box and oversheathearth faults are currently under investigation by TSD. Until such time that this work is completed it isconsidered that such defects when considered along with other ameliorating factors do not require thecable system to be derated. However, such a decision should be taken in the knowledge that the natureof the defect needs to be investigated and account taken of the circumstances of load and conditionsin which the cable may be expected to operate in the period before such defects are corrected. It shouldalso be appreciated that the performance of flooded link boxes and sheath voltage limiters will beimpaired under fault conditions.


August 1996

17 GUIDANCE IN SPECIFYING CABLE RATINGS

It will be appreciated that specification of a cable to meet defined ratings requires the cable systemdesigner to have full knowledge of the purchasers requirements. It is essential that the maximumcontinuous ratings for both the Winter and Summer cable (CC and HH) seasons applicable to thedesired type of installation be quoted. Spring and Autumn ratings (more correctly NC and NH seasonratings) are also applicable to cables in troughs. If the cable is required to have a particular cyclic orshort-term capability then this together with the necessary cyclic load curves or preloads should bespecified.

17.1 Cables Associated with Transformer Circuits

TGN(T)29 shows that autotransformers with taps have 1.20 pu, 1.13 pu and 1.06 pu cyclic ratings inWinter, Spring/Autumn and Summer respectively and that other transformers have similar cycliccapability. These assessments have been based on a standard load curve as shown in Figure 4.Unless the expected load curve is expected to be materially different from this then the cable tendershould include this curve and the tenderer should be required to match the cable to the cyclic rating ofthe transformer. For example a buried cable must match the Spring/Autumn transformer rating in theSummer cable season. Where such load curves do differ from the standard then a separate study ofthe likely capability of the transformer should be initiated and the cable requirements matchedaccordingly. TSD Transformers and Cables Group should be requested to assist in such studies. Thepossibility of providing three or four season ratings for cables in troughs will clearly help in matching thetransformer rating. It will also be necessary to liaise with TSD Transformers and Cables Group over thedefinitions of transformer ratings because there is a move to assign seasonal continuous ratings whererequired.

17.2 Cables Associated with Overhead Line Circuits

TGN(T)26 provides guidance on the current ratings applicable to overhead lines. It provides both pre-fault and post-fault ratings for various overhead line configurations and operating temperatures. Sincethe thermal time constants of cable systems are considerably longer those of the overhead lineconductors, it is usual that any cable required to match the pre-fault continuous rating of an overheadline will have short-term overloads in excess of those of the line. Furthermore the current issue ofTGN(T)26 recommends that the post-fault rating of an overhead line should not be used for a periodgreater than 24 hours - that it can be accepted in a post-fault situation until the pre-fault requirementscan be restored. This will normally be for a period well within twenty four hours. Dependent upon thetype of cable installation and its pre-fault loading it may be possible for a cable which matches theoverhead line pre-fault rating to also carry the post-fault rating of the overhead line for the requiredperiod of up to 24 hours. Care should therefore be exercised when determining the required ratings ofcables to match overhead lines and TSD Transformers and Cables Group should be requested to advisein specific instances.

17.3 Cables Associated with Switchgear

TGN(T)68 and TGN(T)109 provide guidance on the continuous and short-term ratings applicable to themajority of the switchgear installed on the NGC system. The continuous ratings are generallysignificantly higher than the nameplate rating and the short-term ratings are much higher still. Again ifthe circuit capability is required to match these then any associated cable specification should detail theswitchgear and the loadings that are required to be matched.

17.4 Seasonal Dependence of Ratings

Figure 5 shows how the continuous ratings for different plant types vary with season. For overhead linesthese are the lowest ratios of Spring/Autumn or Summer rating to Winter rating from TGN(T)26 Issue2 Table B3. For transformers, the lowest six hour ratings, which are seasonal and independent of pre-load, from TGN(T)29 were used - these are effectively limited period (ie days or weeks rather than manyyears) continuous ratings. For switchgear the lowest ratio of Spring/Autumn or Summer rating to Winterrating from TGN(T)68 was used. Within each plant type there is some variation but it can be seen thatcables have different seasonal dependence to other plant items which arises from the designparameters chosen.


To ensure that a cable is not limiting in a circuit it is clear from Figure 5 that:-

(i) For a directly buried cable the Summer cable rating should meet the Spring/Autumn rating for otherplant items - season NH (September and October).

(ii) For a cable in a filled trough the Summer cable rating should meet the Summer rating for other plantitems - season HH (May to August).

If the directly buried cable has to match an overhead line in NH season it is probably economic if thecable continuous rating is chosen to match the pre-fault continuous rating of the overhead line.In Section 18 it is shown that directly buried cables have six hour ratings at least 22% more thancontinuous rating for 85% pre-load and cyclic ratings at least 21% more than continuous rating for thestandard load cycle and the examples shown in Table 1. The overhead line can carry 19% more thanits pre-fault continuous rating (1/0.84) for up to 24 hours.

If the cable in a filled trough has to match an overhead line in HH season it may also be sufficient tomatch the pre-fault continuous rating. In Section 18 it can be seen that the six hour ratings from 85%preload are between 10% and 14% greater than continuous ratings and the cyclic ratings are from 14%and 17% greater. This is not as great as the 19% increase (1/0.84) available to the overhead line butsince it depends critically on the cable design parameters chosen and the cable depth cases need tobe studied individually once the system requirement is known to give the most economic outcome.

If the directly buried cable has to match a transformer in NH season then the cyclic rating of thetransformer will be at least 1.23 pu for a grid supply transformer or 1.13 pu for a grid supply transformer.The cyclic rating of directly buried cables in Section 18 are shown to be at least 21% above continuousrating. This suggests that the Summer cable continuous rating should be chosen to equal thetransformer nameplate rating.

If the cable in a filled trough has to match a transformer in HH season it may also be sufficient to matchthe Summer cable continuous rating with the transformer nameplate rating. The cable cyclic rating isat least 14 % greater than the continuous rating and the transformer cyclic rating for the HH season willbe at least 1.16 pu for an interconnection transformer or 1.06 pu for a grid supply transformer.

TSD Transformers and Cables Group will be able to advise on the appropriate matching of ratings oncethe system requirement is clear.

17.5 Guaranteed Ratings from Manufacturers

When cables are installed and commissioned they given ratings by the manufacturer as required by theoriginal contract. These ratings will generally include continuous ratings for the appropriate cableseasons (In the past generally Winter and Summer or Winter, Summer Normal and Summer Hot forcables in a trough or Winter, Spring/Autumn and Summer for cables required to match overhead lineseasons) and cyclic ratings for specified load curve and appropriate seasons.

There is no check on rating performance as laid and the rating guarantee period will usually be for oneyear with some elements of the contract guaranteed for up to six years. There has generally been goodcooperation with manufacturers on all aspects of operational performance over the guaranteedmaintenance period.

The use of fibre optic temperature sensing which is generally supported for all new contacts will lead tothe possibility of checking rating performance. This has already happened on the new cable tails forThorpe Marsh SGT1 where the environmental thermal resistivity appears to be higher than the standardvalue of 1.2 Km/W according to fibre optic temperature measurements during a period of loading up toand beyond the nameplate rating of 750 MVA for the transformer.

When there is better evidence on laying conditions and environmental parameters than was availableat the time of placing the contract it is in NGCs interests to use newly assessed ratings rather than theguaranteed ratings provided by the manufacturer. It is presumed that most of the rating changes willbe upwards although there may be occasions when re-assessing the cable ratings highlights apreviously unknown shortcoming that may need to be addressed.


August 1996

18 RATINGS OF TYPICAL CABLE SYSTEMS

18.1 Ratings from GIMLI

The program GIMLI2F has been used to provide rating sheets for 52 different cable systems asexamples covering the main range of configurations used on the system. The configurations aresummarised in Table 1 which covers five conductor sizes, three dielectric types, two sheath types andfourteen laying arrangements. The laying arrangements include four normally buried designs withdifferent laying depth and cable separation, two filled trough designs with different cable separations twoventilated trough designs with different cable separations, two designs with cables in air in flat formationand in trefoil formation, and two water cooled designs, normally buried and in troughs with cooling onand off.

Normally buried cables have two seasons and all other cables have three seasons. Table 2 gives thecontinuous seasonal ratings for all 52 cable systems - these range from 1255A to 2555A (869 MVA to1770 MVA) at 400 kV for one cable per phase, from 1165A to 2534A (555 MVA to 1207 MVA) at 275 kVsimilarly and from 979A to 2049A (224 MVA to 468 MVA) at 132 kV similarly.

Table 3 gives the cyclic ratings for all 52 cable systems and Table 4 gives the cyclic rating factorssimilarly.

The continuous and short-term seasonal ratings for all 52 cable systems are given in Tables 5 to 56.The short-term ratings of cables tend to be very large with the three minute rating being up to 20700A,21170A and 9900A for 400 kV, 275 kV and 132 kV cable system respectively. These values are usuallyfar greater than those of other circuit items which then set the limit. However, there is a need to defineratings for all periods and as the period increases the cable rating is progressively more likely to be thelimit in a circuit.

The six hour rating of a cable has a real potential value for circuits as the enhanced value over thecontinuous rating may enable the circuit to carry load in a post-fault scenario long enough to meetnormal requirements for re-securing the system as the load peak drops over the period or as freshgeneration is despatched from a cold start. The six hour rating is dependent on the thermal capacity ofthe cable system and its environment - normally buried cables have the largest ratio of six hour ratingto continuous rating and cables in air have the least value. Intensively cooled cable circuits using watercooling come in between with an enhancement of about 10% for the typical arrangements covered inTables 16, 18, 33 and 35. There are effectively 28 cases of normally buried cable systems, 6 cases ofcable systems with filled troughs and 12 cases of cable systems in air covered in Tables 5 to 56. Theratio of six hour rating to continuous rating is greater than the following percentages for the range ofcases covered. The ratio of cyclic rating to continuous rating has a very similar set of values.

Minimum six hour ratingenhancement over continuousrating at 85% preload (%)

Minimum cyclic ratingenhancement over continuousrating for standard cycle (%)

Cable voltage (kV) 400 275 132 400 275 132

Directly buried 26 22 22 23 21 21

Filled trough 14 13 10 17 16 14

Cables in air 2 1 0 5 3 0 18.2 Ratings from Cable Manufacturer

Further examples of cable system ratings are given in Table 57 for fluid-filled cables at 400 kV and275 kV in Table 58 for fluid-filled cables at 132 kV and 66 kV and in Table 59 for XLPE dielectric cablesat 275 kV, 132 kV and 66 kV. These ratings were obtained from Pirelli Cables and are for generalindication only. However, they are very useful in that they cover a range of laying conditions and manyconductor sizes.


It was decided that in Tables 57, 58 and 59 it would be better to restrict the entries to conductor sizesgenerally greater than 500 mm2 and to use only the Winter and Summer ratings provided. Theprovision of a dried-out rating in Summer, when material inside the 50FC isotherm which is not stabilisedbackfill is modelled as dried out material, has not been included from the Pirelli Cables data becauseit will depend on the choice of backfilled region. More importantly, consideration is being given tomodifying NGTS 2.5 to make clear that if an appropriate amount of stabilised backfill is included thenthe rating will be determined from the standard Summer value for soil thermal resistivity.

The ratings are given for five laying arrangements, all except the last are normally buried:

(i) Solidly bonded trefoil cables(ii) Single point or cross-bonded trefoil cables(iii) Single point or cross-bonded flat spaced cables at twice diameter centres(iv) Single point or cross-bonded flat spaced cables at 300 mm centres(v) Single point or cross-bonded isolated cables in air

The ratings are based on a 90FC conductor temperature and 900 mm soil cover to top of cables fornormally buried arrangements. The ratings for the fluid-filled cables very from 567A to 2888A at 400 kV(393 MVA to 2001 MVA) from 603A to 3112A at 275 kV (287 MVA to 1482 MVA) and from 578A to2949A at 132 kV (132 MVA to 674 MVA).

The ratings of the XLPE dielectric cables in Table 59 can be compared with the fluid-filled cables inTables 57 and 58. For a 275 kV cable with 500 mm2 copper conductor and aluminium sheath the XLPEcable has a continuous rating about 3% greater than that for an fluid-filled cable. However, as theconductor size increases the continuous rating of the XLPE dielectric cable drops relative to the fluid-filled cable. For cables at 132 kV the relative ratings of the XLPE cables compared with the fluid-filledcables are 6% lower on average than at 275 kV and show the same dependence on conductor size.

However, the relative advantages of the choice between the two dielectric system is much wider thenratings and covers many technical, economic and environmental factors.

19 CHARACTERISTICS OF CABLE CIRCUITS

A comparison of the continuous ratings of cables and overhead lines are shown in Table 60.The overhead line ratings are taken from TGN(T)26 Issue 2, Table B3 which are for 75bC ratedtemperature. The ratings are for the highest rated twin conductor circuit at 400 kV, 275 kV and 132 kV.They utilise twin Redwood 850 mm2, All Aluminium Alloy Conductors (AAAC) with 3.05 cm resistivityat 20bC at 400 kV and 275 kV and twin Zebra 400 mm2 Aluminium Conductor Steel Reinforced (ACSR)at 132 kV.

The cable ratings are calculated by GIMLI for the largest conductor sizes in Tables 6, 8, 23, 25 and 44,directly buried and in filled troughs, and from Tables 57 and 58 for the largest conductors sizes obtainedfrom Pirelli Cables.

It can be seen that to match overhead line ratings with one cable per phase is not possible even at132 kV. In addition where overhead lines are rated at temperatures up to 90bC in TGN(T)26 this raisesthe ratings by a further 10% in Winter and 13% in Summer. However, all circuits are not installed withthe ultimate load carrying capacity in mind so that it will usually be possible for the circuit designer toemploy one cable per phase and the ratings in Tables 2 to 59 are guidance as to what rating capacityis possible for different conductor sizes and laying configurations.

It can be seen from Table 60 that the continuous ratings of directly buried cables are lower than forcables in filled troughs but that the summer six hour ratings of directly buried cables are higher than forcables in filled troughs. This is true for actual preloads as well as for percentage preloads shown inTable 60. However, it depends critically on the design assumptions built into the ratings and TSD willbe investigating ways of increasing cable ratings by varying the assumptions built into Appendix D. Moresite investigations, fibre optic temperature measurement exercises and use of data obtained from theCSM will form the main elements of the work.


August 1996

Figure 5 shows the main length of conductor size characteristics for 206 cable circuits at 275 kV and400 kV which comprise the vast majority of circuits at these voltages with a total length of 476 circuit-km.Half the circuits are less than about 300 m long and act as cable tails for transformers or links withinsubstations.

20 CIRCUIT THERMAL MONITOR RATINGS

The Circuit Thermal Monitor (CTM) is the real-time system used by NGC to provide ratings based onmeasured values of load and coolant temperatures rather than on standard pre-fault loads at seasonalcoolant conditions. It amalgamates the CSM and the Transformer Thermal Monitor (TTM) in a singledisplay system based on three main processors at Cumberland House, Becca Hall and Durley Park.Various attached workstations calculate ratings in various forms for a total of sixty circuits. The real-timemeasurements cover eighteen transformers, ten with cable tails, and forty two cable circuits. In eachcase the complete circuit thermal rating is given, as the performance of the other plant in the circuit isincluded as a so called Truncation Rating Sheet. The system has been described by OShea et al andby Larsen et al but in summary it provides:

(i) Overview pages of circuit load and rating.

(ii) Circuit Monitor page with 24 hours history.

(iii) Ratings page with circuit 3 minute to continuous rating.

(iv) Predictor page for 24 hours ahead.

(v) Raw data page.

The main features of the cable modelling are summarised in Table 61 which lists the circuits in the orderin which they appear on the overview pages of the three systems. Table 62 lists the same informationin circuit NASAP code order. The information in Columns A to E relates to the cable modellingcharacteristics in the GIMLI mesh file for the section. In summary these are:-

53 circuits with cable sections modelled.

141 cable sections modelled.

56 directly buried cable sections. (Depth >0.5 m, B)51 cable in trough sections (Depth @0.5 m, T)34 cable in air sections (A)

38 sections with water cooling (W)35 sections with mutual heating (M)20 sections with a limit temperature of 70bC (7)24 sections with a limit temperature of 90bC (9).

The abbreviations shown in brackets above relate to Tables 61 & 62. The sections with 90bC limittemperature are still operating to an 85bC limit within the CSM. There is a change request for this inplace.

20.1 Cable Mesh Files

The 141 cable sections modelled have either two GIMLI meshes corresponding to Winter and Summer,seasonal thermal resistivities (90 cases) or one GIMLI mesh where the conditions are taken to beconstant through the years (51 cases). Within the cable mesh file data there are design value for themain environment thermal resistivity and for the dried-out part of the environment. There were a totalof 231 mesh files, of which 34 dealt with cables in air. The thermal resistivity values for the remaining197 mesh files were:-


21 cases 0.50 - 1.01 Km/W Mostly modelling air-filled troughs62 cases 1.05 Km/W6 cases 1.10 - 1.15 Km/W92 cases 1.20 Km/W16 cases 1.50 - 2.20 Km/W Modelling cables with 70bC limit.

Secondary thermal resistivity values allowing for soil dry out and modelling complications of cables andwater pipes in ducts required by GIMLI were:-

10 cases 0.80 - 1.00 Km/W5 cases 1.05 Km/W2 cases 1.20 Km/W15 cases 1.50 - 1.89 Km/W3 cases 2.70 Km/W2 cases 2.75 Km/W12 cases 3.00 Km/W1 case 4.00 Km/W

20.2 Cable Boundary Temperatures

The boundary temperatures for the cable meshes in the CSM can be either real-time values or valuesset on a monthly value within the database. If the real-time value becomes unavailable then a singledefault value is defined in the database although manually set values could be used as an alternativewhich would tend to provide higher ratings since the default value will be a year round conservativefigure.

Real-time values are provided as cooling water or cooling air temperatures or as ground temperaturesfrom dummy troughs. Dummy troughs were installed at the following locations with single platinumresistance temperature detectors (PRTD) set at appropriate depths for the local circuits as shown below.

The dummy troughs have been extended in use to cover additional circuits and where the sensor depthis inappropriate the use of additional sensors is being considered.

Dummy Trough Sensor Depth (mm)

St Johns WoodHurstLewindon Wood(Goring Gap on Bramley-Didcot circuits)Lister DrivePitsmoorSkelton Grange

230210

200 and 1016

400305

190 and 360

The St Johns Wood dummy trough was initially installed with two thermocouples, these were replacedwith one PRTD. Due to high water levels this was subject to moisture ingress and a higher specificationsensor was fitted. The mesh files for the St Johns Wood-Tottenham circuits have thermal resistivityvalues of 1.0 Km/W for the main environment and 0.8 Km/W for the trough contents throughout the yearto model the wet conditions and hence produce higher ratings.

The Lewindon Wood trough has two PRTDs at trough depth and two at directly buried depth. At presentthese are being used to provide trough ratings but not directly buried cable ratings which are calculatedon fixed Kew temperatures, see below. There will be benefit in using the Lewindon Wood troughtemperatures more effectively and TSD has already suggested work in this area.

The Skelton Grange trough has PRTDs at two depths for different circuits. The Lister Drive trough hasone PRTD at 400 mm depth which is a greater depth than for most cables in troughs - to cater for thePenwortham transformer cable tails it is likely that an additional PRTD will be fitted at Lister Drive at 220mm depth.

Directly buried cables are modelled in the CSM with fixed boundary monthly temperatures derived fromfrom surveys at Kew over many years.


August 1996

Month J F M A M J J A S O N D

Ground Temperature (bC) 8 7 8 10 12 15 16 17 16 15 12 10

Apart from July, August, September and November these figures are lower than the standar

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