Protection, control, alarm andauxiliary equipment for EHVsubstationsFaults on EHV systems can prove to be expensive for bothsupply authority and user. Careful selection of protectivedevices and ancillary equipment will minimise damage toplant and outage times

by W. D. Goodwin

This article is intended to complement thatentitled The design of outdoor open-typesubstations' published in the March 1987 issueof Power Engineering Journal However, thecontents are generally applicable to all EHVsubstations, whether open-terminal ormetalclad, indoor or outdoor.

The cost of protection control and auxiliaryequipment represents a fraction of the cost ofthe main electrical plant in a substation, but itsrole is essential to the safe and successfuloperation of that plant. Hence it is vital thatthe greatest possible care is taken in both itsdesign and manufacture, particularly as muchof it will stand idle for the greater part of its lifeyet must be in perfect working order whencalled upon to operate.

ProtectionWhen a fault occurs the function of a

circuitbreaker is to automatically isolate thefaulty circuit or plant from the rest of thesystem. Protection equipment, in conjunctionwith current and voltage transformers, providesthe means of detecting the fault and initiatingtripping of the appropriate circuitbreakers.

Over the years, protection philosophy andequipment has been developed to match theneeds of expanding transmission systems. Withthe ever-increasing size of turbine generatorsand the total interconnection of all generatingplant in any given country, fault currents in thehigher voltage range of 380-550 kV canexceed 60 kA.

Quite apart from prospective fault damage,system stability demands the immediateisolation of faulty sections. To achieve this, themaximum time from fault occurrence to circuitclearance may be limited to 100 ms. Of this,40 ms is circuitbreaker 'operating time', leavingonly 60 ms for 'relay and associated signalling

POWER ENGINEERING JOURNAL JULY 1987

time'. The development of solid-state relayscoupled with faster operating circuitbreakershas allowed this level of timing to be achieved.

A belt, braces and safety-pin approach isadopted by protection engineers. Mainprotection is duplicated using alternativesystems or designs together with a furtherbackup but slower-operating system.

The protective systems associated with EHVsubstations are detailed below.

FeedersDistance systems: By measuring both currentand voltage, distance systems determine theimpedance, which is proportional to thedistance between the measuring point and thefault location. This determines whether thefault lies within the protected zone or not.Because of the difficulties of determiningaccurately the actual impedance of a line andthe inherent errors in relays and transformers,the practice is to have a zones scheme.

Zone 1 has a directional characteristic,normally approximating to 80-90% of a feederlength. Zone 2, also directional and with a timedelay, covers 120%. Zone 3, with a non-directional characteristic and longer time delay,is arranged to 'look forward' into a feeder, and'look back' into the busbars. Separatemeasuring elements are used for each phasefor both phase and earth faults and for eachzone.

More economical forms of distanceprotection with slightly reduced performance/facilities are available using common measuringrelays for all three phases and also for one ormore zones. These are known as switcheddistance schemes and are generally limited to36-145 kV systems.

Because Zone 2 times may be unacceptablylong end-to-end signalling is introduced so that

191

1 Relay panels at Tornesspower station, SSEB

relays 'seeing' a Zone 2 fault are reset to Zone1 time by a signal from the Zone 1 relays atthe remote end.

A further development has been theintroduction of schemes which only permittripping if the fault has been identified asinternal from both ends, giving them thecharacteristic of unit protection. These fall intotwo categories: 'permissive' and 'blocking'. Inthe 'permissive' scheme, tripping will only takeplace if the fault is detected at one end and aconfirmatory signal of an internal fault isreceived from the other. 'Permissive' schemesmay be either 'under-reach', i.e. the normal80% of feeder length, or 'over-reach', i.e. 120%.The latter are used in connection with auto-reclose schemes or for very short lines when itis not possible to obtain relay settingscompatible with the line impedance.

In the blocking schemes, discrimination isobtained by the use of a reverse directionalZone 1 element which, when the Zone 2 relayhas not operated (indicating an external fault),sends a blocking signal which inhibits trippingto the remote end.

Signalling systems: For long overhead linefeeders use is made of HF signals in the band30-500 kHz transmitted over the linesthemselves. Briefly, DC signals from theprotection relays actuate an audiofrequencysignalling unit, which in turn modulates a high-frequency carrier generated in a single-sideband radio transmitter. Output from thetransmitter is connected via coaxial cable tothe overhead line via the capacitance ofcapacitor-type voltage transformers or linecouplers. Line traps inserted at both ends ofthe line confine the signal to the line. At theremote end the signal is detected in the local

192

radio receiver and an audiofrequency output isapplied to the signalling unit and hence to theprotection relays.

Power line carrier systems can also beextended to incorporate telephone, telex anddata link systems. Transmission can be phase-to-phase or phase-to-earth or a combination ofboth. Depending upon the security required, asingle transmitter can be used to send one ormore signals using both lower and uppersidebands. For any given line, the operatingfrequency has to be chosen so as not tointerfere with, or be interfered with by, otherradio signals over the route length.

Where a direct point-to-point line-of-siteroute is practical microwave radio links canprovide the signalling channel. A typicalexample would be transmission across water,where submarine cable links have beeninstalled.

With the great advances in the developmentof fibre-optics it is now possible to use thisnew technology for end-to-end signalling,particularly for the shorter distances. Theadvantages of such systems are that they areunaffected by external electrical interferenceand that they can carry many independentsignals over a single fibre-optic tube. Foroverhead lines, the fibre-optic tube can beincorporated in the earth wire.

Phase-comparison systems: Phase-comparison systems are of the unit type inthat they compare the phase angle of thecurrent at each end of a feeder. To enable thiscomparison to be carried out, several methodsof end-to-end signalling have been devised.Where pilot wires are available, a 50 Hz signalfrom the protection relays modulates a voice-frequency signal (1800 Hz) generated in a localtransmitter which is transmitted over theinterconnecting pilots. The local signal and thatreceived from the remote end are compared.Depending upon whether the fault is internalor external, tripping is initiated or inhibited.

Alternatively, the 50 Hz protection signalsare used to modulate a built-in high-frequencytransmitter (30-500 kHz) which is coupled tothe overhead line. A third alternative is to havevoice-frequency channels incorporated inpower-line carrier signalling equipment. Owingto the effects of capacitance, phase-comparison systems are limited to about 100miles of overhead line or 20 miles of cable.

A further alternative for short distances is amicrowave radio link.

Choice of systems: Distance protectionrequires three voltage transformers at each endtogether with the additional equipment forend-to-end signalling. Phase comparisonprotection requires only two voltagetransformers or line couplers at each end, ornone at all if pilot wire schemes are used andthe signalling equipment is integral with it. Inthis last case less panel or rack space isrequired. It is, however, limited to use onrelatively short lines or cables.

Current practice is to duplicate mainprotection, either distance with phasecomparison or double distance, usingequipment of different manufacturers.

POWER ENGINEERING JOURNAL JULY 1987

Pilot wire protection: For the lower-voltagelines and cables in the range 66-170 kV wherethe distances are short and pilot wires availableone of the many end-to-end differentialsystems based on the original Merz Priceprinciples can be used. Some form ofsupervision of the pilots may be incorporated.

TransformersUnit protection of the circulating current

type is applied across the HV and LV terminalsof transformers. For generator transformerswith teed-off unit transformers it is necessaryto include the connections to the unittransformer within the protected zone. As unitprotection will not normally give low enoughsettings for earth faults, separate restrictedearth fault and standby earth fault schemesare incorporated for both HV and LV windings.

For large transformers, it is usual to duplicatethe main protection.

Although these systems will cater for phaseand earth faults, they will not detect intertumfaults which cause heating and generation ofgas. These are detected by Buchholz deviceswhich are actuated by gas (alarm) or surges ofoil to the conservator (trip). In addition, windingtemperature trip and alarm relays are activatedby thermocouples embedded in the windings.

Unit protection is normally backed up byovercurrent and earth fault relays incorporatinghigh set elements.

Generator transformers are particularlysusceptible to overfluxing during run-up of amachine, which, if allowed to persist, isharmful. This situation can be detected by arelay, the input of which is proportional to v/fand on initiation can be arranged to reduceexcitation of the associated generator. Wheretransformers are physically located somedistance from the station, the HV connectionsto it will be separately protected by single orduplicate circulating current systems.

Busbar zoneA fault in the busbar system of a substation

can have the most widespread effects,including the complete shutdown of thestation. To prevent such an occurrence busbarsystems are divided up into sections by meansof bus coupler and bus section circuitbreakers,such that any one section which hasdeveloped a fault can be isolated and shutdown leaving the remaining healthy sections inservice.

Unit protection is applied to each of thebusbar zones. The current transformers areplaced on the remote sides of each circuit toensure that the extent of individual zoneprotected is at a maximum. As the integrity ofthe current transformer circuit is vital,supervision is incorporated. Duplicate systemsare applied to each zone so that both systemsmust be activated before tripping can takeplace.

For mesh-connected stations, a zone coverseach mesh corner, embracing twocircuitbreakers, a feeder and a transformer.

In breaker-and-a-half configurations, inaddition to bus zone protection, it is usual to

POWER ENGINEERING JOURNAL JULY 1987

provide separate unit protection across eachfeeder 'end' embracing two circuitbreakers inany one diameter and the connections to theoutgoing circuit. The actual feeder ortransformer protection is arranged to overlapthis 'end' protection.

CircuitbreakersEHV circuitbreakers stand in a closed

position for the greater part of their life but arerequired to operate instantaneously in theevent of protective relays initiating tripping. Toguard against the possibility of a circuitbreakerfailing to trip, generally known as 'stuckbreaker' or 'breaker fail' an additionalprotective device is fitted.

This comprises a time-lagged current-checkrelay, the operating circuit of which is closedwhen the tripping circuit of the circuitbreakeris activated. If fault current continues to flowafter a predetermined time (i.e. thecircuitbreaker has failed to operate) the'breaker fail' relay initiates tripping ofcircuitbreakers on either side of the failedbreaker to ensure fault clearance.

With single-phase mechanisms on a three-phase breaker, it may be possible for thephases not to operate together. This situationcan be detected by a 'phases not together'relay.

Auto-redosingFeeder faults are frequently of a transient

nature caused for example by the clashing ofthe phase wires of overhead lines in windyconditions, foreign material blown across theline which subsequently releases itself or aflashover due to switching or lightning surgeswhich becomes self-extinguishing. It is usualtherefore to incorporate auto-reclose relays onfeeder circuits such that, following a trip on

2 Relay racks at Tornesspower station, SSEB

193

fault, the circuitbreaker is automaticallyreclosed after a predetermined time. Shouldthe circuitbreaker trip again it is presumed thatthe fault is permanent and the circuitbreaker islocked out.

Circuitbreakers may be designed for three-phase auto-reclosing. If single phase reclosingis required, then individual trip/closemechanisms and their associated relays mustbe fitted to each phase of the circuitbreaker.

Advances in computer technology have nowmade it possible to build a comprehensiveauto-reclose system for a complete station intoa single 'black box' and to provide the softwareso that all possible fault conditions can beidentified and the appropriate switching actiontaken. This can include the necessary switchingto quench ferro-resonance.

Protective relaysWith the advances in solid-state equipment,

particularly the introduction of microprocessortechnology, the electromagnetic measuringrelay is fast disappearing. Relays today arenormally flush-mounting and of thewithdrawable type. For the comprehensivesystem such as distance or phase comparison,printed circuit cards slot into withdrawablemodules.

Past practice was to mount protective relayson floor-mounted cubicles with rear doors foraccess. In the event of subsequent protectionchanges, panels had to be drilled and blankingplates fitted. Rack-mounted relays with openterminals at the rear were in vogue for a shorttime, giving ready access and makingmodification easier. The solid-state relay withwithdrawable modules has encouraged the useof Post Office-type racks giving easy access formaintenance and modification. Comprehensivemodular systems are now available.

Ideally, protective relays should be mountedas close as possible to their associated currentand voltage transformers and circuitbreakers.This keeps the lengths of interconnectingmulticore cables to a minimum, reducing leadlength burden, voltage drop and exposure tophysical damage. To achieve this relay roomsmay be built within the confines of thesubstation, either in block houses adjacent toeach circuit or in a single building housing bothcontrol and protective equipment located nextto the switchyard. For indoor substations, asingle building accommodating both the mainplant and control/protective equipment isusual.

For a substation associated with agenerating station it may be considereddesirable to accommodate all control andprotective equipment in the main stationbuilding. This will necessitate multicore cableinterconnections of considerable length andcareful consideration must be given to thesizing of the cables and their routing.

Ancillary equipmentFault locators typically measure the

inductance to the fault and compare this withthe line inductance, so giving the distancereading from the substation to the fault

194

position. They are particularly useful on longlines and where lines are not readily accessible.

The modern fault recorder is a compactsolid-state device which has the facility to givea printed output of currents and voltages in allphases during a set period before and after afault. They facilitate investigation of the causesof a fault and also assist in fault location.

Control equipmentEHV substations, in addition to having local

control adjacent to the plant, will have remotecontrol from a master control room. This maybe in a separate building on the substation site,in the main control room of the associatedgeneration plant, or at a central control roomcovering a number of EHV substations. In thelatter case a 'standby control board' is locatedat the actual substation.

PanelsThe standard control panel comprises a suite

of floor-mounting cubicles, each with rearaccess doors, generally one cubicle per circuit.On each panel will be mounted the controlswitches for the circuitbreakers, disconnectorsand earth switches associated with the circuit,and the remote/local selector switches topermit transfer of control to the plant itself.

The control board will incorporate a mimicdiagram usually painted on the panels,indicating to the operator the mainconnections and plant within the substation.Circuitbreakers, disconnectors and earthswitches are represented by semaphoreindicators. These latter may be of theelectromagnetic type which automaticallyindicate the open/close position of the plant,or of the hand-dressed discrepancy patternwhereby a lamp is illuminated if there is adiscrepancy between actual and indicatedposition. Alternatively, discrepancy-type controlswitches which combine the functions ofcontrol switch and position indicator may beused.

The control panel will also incorporateindicating instruments associated with thecircuit, ammeter, voltmeter, wattmeter andvarmeter. Practice today is to use flush-mounting instruments with extended scaleoperated through transducers.

An alternative to the standard control boardis the mosaic diagram which, although moreexpensive, makes the updating of the controlsas main plant is added or removed easier. Themosaic panel comprises a set of modules, eachfronted by a plastic tile on which is engraved acircuit symbol. Thus a complete diagram canbe built up to suit the initial installation andlater modified as the station is extended.

Although the trip and closing coils ofcircuitbreakers etc. are normally designed for110 V or 220 V operation, interconnectionbetween plant and control board is generally ata lower voltage, i.e. 50 V through the mediumof interposing relays. This permits batterypower requirements to be reduced.

DesksFor the ocupied substation an alternative to

POWER ENGINEERING JOURNAL JULY 1987

control panels is the use of control desks atwhich the operator is permanently seated. It isusual to use these in conjunction with amosaic diagram. Desks are designed specificallyfor a station in either wood or metal and mayincorporate master controls, alarms,communication equipment and event loggers.

Very sophisticated event logger equipment isnow available. Both digital and analoguetransient information from remote positionscan be received and processed to give visualpresentation on VDUs or permanent printouts.Waveforms can be displayed on VDUs.

SCADAIn the event of a number of remote

substations being controlled from one maincontrol room, an overall control scheme has tobe installed. Computer technology now allowsthe state of all plant position indicators,measuring instruments, protective relays,alarms etc. to be continuously monitored andthe data stored and subsequently transmittedto the control station using pulse-code-modulated transmission. Such systems, knownas SCADA (system control and dataacquisition) permit complete control from asingle location.

Tap changeTap change equipment associated with

power transformers is mounted on separatepanels located in the control room. Thisequipment will comprise raise/lower switches,tap position indicators, automatic voltageregulators and line-drop compensators.

SynchronisingEHV substations not associated with a

generating plant will be equipped with check

synchronising facilities. These comprise relayswhich monitor the phase angle of both'running' and 'incoming' circuits and inhibitclosing if the phase angle difference is outsidepredetermined limits. These relays are usuallymounted within the control panels.

Where generation plant is involved, then fullsynchronising facilities are required. Although itis still usual to provide hand-operatedequipment comprising running voltmeter,incoming voltmeter, synchroscope, frequencymeter etc., the modern station will incorporatefully automatic synchronising equipment.

MeteringIntegrating and summation metering

equipment, although accommodated in thecontrol room, is normally segregated in acontrolled environment. Summators are of thesolid-state type receiving impulses fromoptoelectronic transmitters mounted on theintegrating instruments. Outputs can be fed toprinters, VDUs, data loggers etc.

AlarmsA comprehensive alarm system, both visual

and audible, is incorporated into the controlroom. This will cater for alarms associated withthe plant itself, i.e. low air, low gas pressure,Buchholz devices and winding temperatureindicators, alarms associated with monitoringcircuits (secondary voltage and currenttransformer supervision) and a vast array ofalarms associated with the many protectivedevices.

The alarm indicators themselves, whichconsist of solid-state monitoring relaysoperating at 48 V and arranged to activate aflashing light behind an engraved window, aregrouped together in facias of 6-144 ways.

3 Control panels and deskat St John's Woodsubstation, CEGB

POWER ENGINEERING JOURNAL JULY 1987 195

Pushbutton facilities are provided to 'accept'the alarm, i.e. steady the light andsubsequently to cancel. For urgent alarms, anaudible warning, either bell or klaxon, is alsoinitiated.

Alarms associated with a particular circuitare mounted on the circuit control panel.Alarms of a general nature associated with thewhole plant are grouped on a general alarmspanel.

Auxiliary equipmentCompressed air

Compressed air is required for two purposesin EHV substations: for circuit interruption andconditioning of internal insulation for air blastcircuit breakers, and for operatingcircuit breakers having pneumatic mechanisms.In either case, the basic requirement is for anadequate supply of clean dry air under allcircumstances. However, for air-blastswitchgear, the quantity required is very muchlarger.

The standard method of ensuring that onlydry air is admitted to a circuitbreaker and itsoperating mechanism is to use the two-pressure system. Air is compressed and storedat a high pressure and then reduced to therequired operating pressure. The consequentincrease in volume reduces the humidity andensures that the dewpoint is never reachedand so no water precipitated. Alternatively,single pressure schemes with air-dryers can beused.

Two basic schemes are in use: centralisedcompressor plant which supplies all thecircuitbreakers in a substation and unitcompressor plant providing a supply to anindividual circuitbreaker.

In the centralised scheme, high-pressure hotair from the compressor is passed throughafter-coolers and filters to reduce moisturecontent and impurities, and stored in main airreceivers. These receivers are usually mountedoutdoors to give additional cooling. Standardpractice is to have a completely duplicatedscheme interconnected on both the high- andlow-pressure sides such that one compressor/main air receiver unit can supply all therequirements of the station while the other isout of service for overhaul. The output fromthe main air receivers is passed throughpressure-reducing valves and then fed via aring main pipework system to local air receiversmounted on the individual circuitbreakers.

Stop valves are provided on each side of thetee-offs to the circuitbreakers, so allowingmaintenance of the air equipment associatedwith the circuitbreaker while maintainingsupplies to the rest of the substation. Theinstallation is protected by safety valves,pressure gauges with alarm contacts andfusible plugs in the air receivers. Compressorsare started and stopped automatically bypressure detection devices.

The ring main pipework is usually located inmulticore cable trenches, supported on specialclamps which allow expansion. For long runsexpansion bends are incorporated. Fig. 4 showsthe arrangement schematically.

In the unit compressor system a compressoris located in the vicinity of each circuitbreaker,feeding the local air receiver via filters anddryers.

For the larger substations, the centralisedscheme is the most economical, but withstations having up to four circuitbreakers theunit scheme can be considered. To cater forthe failure of a unit compressor, they can beconnected together to give an alternativesource of supply.

In designing an overall scheme, the followingfactors need to be taken into account:

• the duty cycle of the circuitbreaker• the effects of multiple tripping• the overall leakage from the pipework

system.

Duty cycles for circuitbreakers are laid down inthe various international specifications.I EC. 56-2 proposes 0-0.3 s-CO-3 min-CO tocater for high-speed auto-reclose schemes.Effectively, this means the local air receivershould have the capacity of two open andclose operations without replenishment fromthe main air receiver. Additionally, the refillingtime from the main air receiver should be lessthan 3 min. For non-reclose schemes, a dutycycle of 0-3 min-CO-3 min-CO might bespecified. In this case, a local air receiver with acapacity for one open/close operation wouldbe adequate.

Bus zone protection schemes involve themultiple tripping of circuitbreakers and dueregard must be taken of the need tosubsequently reclose a number ofcircuitbreakers as quickly as possible after suchan event. Main air receivers must be sized totake this into account assuming that thecompressors are out of service due to electricalsupply failure.

Although valves, coupling etc. are specificallydesigned for the high pressures involved, it isimpractical to make an air system completelyleak-free. Compressors therefore must not onlybe sized to provide a reasonable running timefor replenishing main air receivers aftercircuitbreaker operation, but the daily runningtime for making up losses due to leakageshould not be excessive.

Four hours per day is regarded assatisfactory for plant which is normallyunattended. This will give a reasonable lengthof service between overhauls.

Compressor plant is designed for installationeither indoors or outdoors with suitablecladding. Main air receivers should beprotected from the effects of excessivetemperature, and at the other extreme heatersshould be provided to prevent the freezing ofmoisture in the high-pressure system. Theusual precautions should also be taken toprotect against the deleterious effects of hightemperature and humidity on electricalequipment and the ingress of dust, termitesetc.

BatteriesBatteries perform an absolutely key role in

the operation of an EHV substation out of all

196 POWER ENGINEERING JOURNAL JULY 1987

proportion to their cost relative to the rest ofthe plant. It is vital therefore that very carefulconsideration is given to their specification andselection. Duties come under the followingmain headings:

• tripping and closing of circuitbreakers• general control, indication and alarms• operation of signalling and

telecommunication equipment• emergency lighting.

Research into batteries continues to produce

new types of cell. However, the high-performance sealed lead/acid battery continuesto prove to be the most economical andtechnically satisfactory design for electricalsubstations.

The nickel/cadmium design with alkalineelectrolyte has certain advantages. It can beneglected for long periods without permanentdamage and, when supplied in steel cases, isvery robust. It is, however, more expensive asmore cells are required for a given output andit has a higher internal resistance.

Standard operating voltages are 220 V and4 Central compressorsand ring main

compressor system

safetyvalve

^

safetyvalve

r reducing valve

airreceiverworkingpressure30 bar

n on-returnvalve

—w—

air filterand dryer

compressor A30 bar1.98 litre/s

II

A

handunloader

automaticblowdown/unloader

to trench

to trench

Lairreceiverworkingpressure30 bar

handunloader

reducing valveHO-

non-returnvalve

b a r

automaticblowdown/unloader

air filterand dryer

2vsafetyvalve

compessor' B'30 bar1.98 litre/s

Pressure gauges

Location Ref Function Operation Scale

CompressorA

PGS6 compressor control

PGS7 LP system low airPGS8 HP system low airPGS9 compressor fail

low contacts make on falling pressure of 23 barhigh contacts break on rising pressure of 30 barcontacts make on falling pressure of 12.4 barcontacts make on falling pressure of 16 barcontacts make on falling pressure of 14 bar

40 bar

25 bar40 bar40 bar

CompressorB

PGS6 compressor control

PGS7 LP system low airPGS8 HP system low airPGS11 compressor controlPGS9 compressor fail

low contacts make on falling pressure of 22 barhigh contacts break on rising pressure of 30 barcontacts make on falling pressure of 12.4 barcontacts make on falling pressure of 16 barcontacts make on falling pressure of 24 barcontacts make on falling pressure of 14 bar

40 bar

25 bar

40 bar

40 bar

POWER ENGINEERING JOURNAL JULY 1987 197

110 V for tripping, closing, control andindication, with 48 V for alarms and forprotection, signalling and telecommunicationsequipment, although individual users have theirpreferences.

The principal reason for using batteries forthe essential duties of tripping, closing, controletc. is to provide continuity of service in theevent of the loss of low-voltage AC supplies.The period for which a battery is required toperform without recharge from the mains isknown as the standby period. Individual userswill have their own requirements in thisrespect, but 6 h is generally consideredacceptable.

Obviously, the longer the standby periodspecified, the more costly will be the batteryinstallation, the larger its physical size and thecost of accommodation higher. If alternativestandby arrangements such as dieselgenerators are installed, then considerationmight be given to reducing this period.

In sizing a battery, therefore, considerationmust be given to its ability to supply acontinuous standby load, e.g. lamps,emergency lighting, for a period of up to 6 hwithout recharge and then still be capable ofproviding the high-current short-period duty ofsay multiple tripping associated with a buszone fault. Manufacturers offer their batteriesfor a given ampere hour capacity related to aspecific discharge rate, usually 10 h. Havingestablished the ampere hours required for allduties, these should be converted to thestandard discharge rate using conversion tablesand then the appropriate battery selected.

For a battery to operate satisfactorily, it isessential that it be kept in a charged condition.Chargers are designed to give both trickle andboost charging. In selecting charge rates, dueaccount must be taken of the maximumpermissible voltage on the terminals ofconnected equipment. This is generally fixed byinternational or customer specifications, but inany case must not be sufficiently high to causedamage.

Equally, the battery voltage in the dischargestate must be considered in respect of theminimum permitted voltage and the voltagedrop in the connecting cables. If necessary, thecross-section of cables should be increased.

Another factor to be considered in selectingbattery capacity is the effect of temperature.As the temperature falls the battery internalresistance increases, and consequently thevoltage regulation increases under high loadssuch as tripping operations. Battery roomtemperatures should therefore be kept at notless than 5°C.

It is usual to connect DC loads via adistribution board located next to the batterycharger unit. All connections should be kept toa minimum length.

Most faults associated with batteries arerelated to their chargers. For the moreimportant stations, particularly where feedershave first and second main protectionschemes, consideration should be given toduplication of chargers or, alternatively,battery/charger combinations. Although the

ring main system of cabling provides aneconomic means of securing duplicate suppliesto each bay, voltage drop considerations maydictate direct duplicate radial feeds.

Very little gas is emitted from the sealed leadacid battery when on boost charge. It cantherefore be safely accommodated within themanufacturer's specially designed andventilated cabinets in the same room asprotection, control and telecommunicationsequipment. Adequate ventilation should,however, be provided to ensure that the smallquantities of gas emitted are absorbed by theatmosphere.

LV electrical suppliesThe source of LV AC supplies for a

substation will depend upon its location. Ifassociated with a power station, these will bedrawn from the station auxiliary system. If thesubstation is in an established developed areawith an existing interconnected HV and LVnetwork, the local supply may be brought onto the site.

In situations where a firm and secure localLV supply is not available, then the gridtransformer may be equipped with a deltatertiary winding from which an LV supply canbe drawn. Where it is necessary to install anearthing transformer, these can be wound withstar/delta windings, the star winding beingused for earthing purposes and the delta as asource of site supply.

LV AC supplies are required for a multitudeof purposes in an EHV substation site, amongwhich may be numbered the following:

• Transformers: Tap changers, oil coolingpumps, cooling fan motors, oil handlingplant.

• Switchgear: Battery chargers, operatingmechanism motors, gas and oil handlingplant, LCC (local-control cubicle) andmarshalling kiosk heaters.

• Control/relay panels: Cubicle heaters andlighting.

• Auxiliary buildings and site: Heating, lighting,air conditioning, socket outlets.

It is necessary to marshall these in an orderlyfashion and provide means of protecting boththe terminal equipment and the connectingcables. Where an auxiliary building exists, it isusual to provide indoor switchgear, generally acombination of three-phase air circuitbreakersand three-phase and single-phase fuseswitches. Alternatively, or where auxiliarybuildings are not available or are too fardistant, weatherproof outdoor equipmentnormally comprising fuse switches can beused.

A common practice is to segregate essentialand nonessential services and provide twointerconnected switchboards, one for eachservice. Whatever the source of LV power,duplicate connections must be brought to theLV AC switchboards, preferably either side of abus section circuitbreaker or disconnector.

Where essential services are segregated anystandby diesel generator should be connectedto this board. In the event of loss of both main

198 POWER ENGINEERING JOURNAL JULY 1987

supplies, these circuits will be isolated and thediesel generator automatically started andswitched to the busbars. On the restoration ofthe supply, the main feeders should bereconnected and the diesel generator isolatedand shut down.

Gas-handling plantThe introduction of SF6 gas as both an

insulating and interrupting medium inswitchgear and associated equipment hasnecessitated the provision of auxiliary plant onsite.

SF6 circuitbreakers are normally despatchedto the site carrying a small charge of SF6 gas,the purpose of which is to maintain the interiorin a clean dry condition. Alternatively, they maybe despatched to site with a small charge ofnitrogen. Fully charging with gas beforedespatch is not possible due to transportregulations. Before commissioning on sitetherefore, the circuitbreaker must be filled withgas up to the designed operating pressure.

Although major maintenance on SF6

interrupters is unlikely to be frequent, it doesnecessitate the removal of all gas from the unitbefore work can commence and subsequentrefilling prior to recommissioning.

The quantity of gas contained in a singlelive-tank circuitbreaker is relatively smallcompared with the equivalent dead-tankarrangement. For substations with live-tankcircuitbreakers, it is generally accepted that SF6,being non-toxic, should be extracted and thendumped to the atmosphere, replenishmentbeing made from gas stored in bottles on site.Various simple trolley arrangementsincorporating a vacuum pump, filter, storagefor bottles and flexible interconnectingstainless-steel pipework and valves have beendeveloped.

For installations where the quantity of gas islarge, e.g. those incorporating dead-tankcircuitbreakers, or for metal-enclosedsubstations, it is considered economicallyworthwhile to refurbish used gas, store andrecycle. Mobile gas carts accommodatingextraction pumps, filtering equipment,refrigeration, storage tanks etc. are available,the gas being stored in liquid form. The cartcan be coupled by stainless-steel flexible pipesto the service valves in the equipment andsubsequently removed. An alternative is toprovide static equipment in the auxiliarybuildings permanently connected to standpipeslocated at convenient points around theswitchyard, although this is losing favour thesedays.

Multicore cablingThe multicore cabling requirements in an

EHV substation are fairly substantial and needcareful consideration to keep overall costs to aminimum. The scope usually includes the low-voltage AC and DC distribution system cablesand the incoming three-phase power cablesfrom the sources of power.

Standard practice is to lay cables in trenchesarranged in a ring around the site with branchconnections to the individual bays. Trenches

may be lined with concrete or earthenwaretiles and topped with concrete slabs.

Although many authorities will accept cableslaid directly on the trench bottom, others insiston locating them on trays attached to the sideof the trench. This is particularly necessarywhere trenches are liable to flooding or attackfrom vermin, termites etc. The cable trenchalso provides a convenient place for layingcompressed air pipework and branchconnections of the earthing system.

In each bay of equipment, cables from thecircuitbreaker, disconnectors, earth switches,current and voltage transformers, surgearresters and line coupling equipment are ledinto the branch cable trenches and thenmarshalled at a common point adjacent to themain ring trench. Usually a separate cable-marshalling kiosk is provided for this purpose,but it is possible to use the circuitbreaker localcontrol cubicle if the number of cores islimited.

From these marshalling kiosks,interconnecting cables carry common servicesto the kiosks in adjacent bays. Trunk cablescarry the control and protection circuitsthrough to the associated auxiliary buildings orpower station control room as the case maybe.

Care must be taken in the segregation ofservices. AC and DC low-voltage supplies are,of necessity, large section separate cables.These may be run in a ring around the site oras duplicate radial feeders to each bay. Firstand second main protections must be run inseparate cables, bus zone protection cables arealways run separately and terminated insegregated shielded sections within themarshalling kiosks. Power line carrierequipment is coupled by coaxial cable from theline couplers to the HF transmitter unit.Telecommunications multipair cables should bearmoured not only for mechanical protectionbut also to provide electrical screening.

The PVC cable has today largely supersededthe paper-insulated lead-covered (PILC) cable,greatly reducing terminating costs. Armouringprovides protection not only againstmechanical damage during laying but alsofrom vermin and termite attack during thelifetime of the cable. There is an increasingtendency to specify fire-retardant cable butthis considerably increases the capital cost.

AcknowledgmentsThe author wishes to acknowledge the

assistance of colleagues at NEI Reyrolle andNEI Electronics in preparing this article and thefacilities provided by the company.

Reference1 GOODWIN, W. D. and WILLS, A. S.: The design

of outdoor open-type EHV substations' PowerEngineering Journal, 1987,1, 75-83

© IEE: 1987

Dave Goodwin is UK Sales Manager of ReyrollePower Switchgear, Hebburn-on-Tyne, Tyne andWear NE31 1UR UK. He is an IEE Fellow

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