Design of the electrical power systemJ. Wilson. C.Eng.. F.I.E.E.. and G. Tuft. B.Sc. C.Eng., M.I.E.E.

Indexing term: Power systems and plant

Abstract: The objectives of the power system design and choice of equipment are to ensure maximum securityof supplies with operational flexibility at an acceptable cost. The paper describes the development of the design,from the conceptual pretender stage to the completion of manufacture of equipment ready for shipment to thesite. In the development of the design a number of problems are identified, relating to the constraints of faultlevels and voltage regulation on power systems with large induction-motor loads. Alternative solutions areexamined, and reasons given for the solution adopted. The importance of computer modelling is emphasised inthe analysis of power system design, not only for the solution of technical problems, but for optimisation torealise any cost advantages available. Finally, the authors indicate their views on future trends in system designfor large power stations, and conclude that the objectives of supply security, operational flexibility and accept-able cost have been achieved at Castle Peak B.

1 Introduction

This paper discusses the design of the electrical powersystem for Castle Peak B in three stages. The first stage isthe preliminary design work, which was carried out duringthe initial conceptual design. The second stage is the devel-opment of the design, which has been done during thepostcontract engineering phase of the project up to thepresent time. The final stage will be the use of equipmentworks test results to confirm that the final design meets allthe project requirements.

2 Power system design objectives and constraints

Owing to the load growth and the necessity to operatewith a small generating capacity margin, a high degree ofplant reliability and availability are prime considerations.The objectives of the system design and choice ofequipment are to ensure maximum security of supplieswith operational flexibility, at an acceptable cost.

To achieve these objectives, it was understood from theoutset that the equipment being supplied would be of atype which had been extensively field-proven in service andwould be compliant with British Standard specifications,UK power station practice, and, in selected areas, UKElectricity Supply Industry specifications.

In addition to the complex matrix of technical problemsassociated with such large projects, a principal constraintidentified on Castle Peak was the short time available inthe project design programme before it became necessaryto place manufacturing orders. These orders had to beplaced at an early stage, to enable civil design to be final-ised for the issue of civil subcontracts and to meet thehardware delivery dates.

To meet the required programme requires parallel oper-ation of the civil, mechanical and electrical design pro-grammes, with the consequent effect that any design delaysin one technology immediately affect the others and theoverall project schedule.

It was necessary at the conceptual design stage to estab-lish not only the fundamental design concept, but alsosome of the detail design parameters. Any proposedchanges to the preliminary design have to be carefully con-sidered before implementation, because of the impact onthe overall* project schedule.

Paper 3372C, first read before IEE Power Division Professional Group P10, 18thMay 1983Mr. Wilson is with GEC Turbine Generators Ltd., Power Station Projects Division,PO Box 131, Trafford Park, Manchester M60 1AF, England, and Mr. Tuft is withChina Light & Power Company Limited, Argyle Street, Kowloon, Hong Kong

In the situation described it is essential that the client orhis agent and the system designers work together closely,to minimise approval times and avoid delays caused bydifferences in opinion as to the most appropriate technicalsolution to any problem. China Light & Power and GEChave established a close relationship, and through a highdegree of consultation have rapidly resolved design issuesto allow the project to proceed without delay.

Some of the design decisions were conditioned by theexistence of A station and the existing facilities. Theseinclude the following:

(a) The 400 kV substation building, which was designedto accommodate an extension of the A station 400 kV SF6

gas insulated metalclad switchgear to connect to the Bstation units

(b) The 132 kV substation, which can supply the stationtransformers and has connections to the existing gasturbine generators for black-start purposes

(c) The coal-handling plant, which requires extension forB station use.

3 400 kV switchgear

This is an extension of the Castle Peak A 1̂ switch bussystem shown in Fig. 1. It is intended to operate A and Bstations as a 'solid' system, but facilities exist for runningas two 'split' systems. The use of a compact design ofswitchgear enabled GEC HV Switchgear to offer the com-plete 400 kV substation equipment at a significantlyreduced cost, compared with the A station arrangement,while meeting the requirement of proven experience.

The significant difference in cost is achieved by the useof cast aluminium alloy housings for disconnector/earthswitches, CT chambers and 'T' chambers, rather than thefabricated assemblies used on A station.

Substantial savings in space and complexity are madeby the use of motor-gearbox drive units for disconnectorsand earth switches. These drive units mount directly ontothe disconnector chamber, so that mechanical linkages tooperating cubicles and walkways to give access to suchcubicles are not required. The motor-gearbox units operateremotely from ground-level by an electrical control andinterlock system.

The GEC 420 kV SF6 circuit breakers, equipped withfour series-connected interrupter units per phase, are iden-tical to those supplied on A station, and operate at 7.5 bar(gauge) gas pressure. The 'back portion' chambers, trunk-ing etc., operate at 3.8 bar (gauge) pressure.

The complete equipment design is the same as that pro-vided by GEC on the CEGB system at City Road,London.

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4 Auxiliary power system

4.1 Design conceptWith the existence of a 132 kV substation on site, a con-ventional unit and station auxiliary power system was anatural choice. This allows the use of station transformerssupplied at 132 kV for startup of the four units.

The specification requires that each of the two stationtransformers should be capable of supplying the full-loadauxiliary power of a running unit, the starting load of asecond unit and a proportion of the station common ser-vices load. It is also specified that busbar voltages shouldbe maintained within 96.5% to 106% during all steady-state operational conditions. During transient conditions,for example motor starting, the motor terminal voltagemust not be less than 80% of the nominal voltage.

Preliminary information at the conceptual design stageof the project indicated that approximately 52 MVA ofauxiliary power was required for coal-fired units of thissize. This includes three 50% electrically driven boiler feedpumps, which were chosen after comparison with otherdrive combinations. Unit transformers of 60 MVA BSrating were considered necessary for this application.Voltage levels of 11 kV, 3.3 kV and 415 V were selected forthe power system, on the basis of proven plant of UKmanufacture for the size of loads and ratings of switchgear.

The loading requirements for two station transformersresulted in an estimated 11 kV rating of 120 MVA each,which is above the current rating of one 11 kV circuitbreaker. The alternatives considered were four 67 MVAdouble-wound transformers and two 134/67/67 MVAthree-winding transformers by BS rating. The three-winding transformers offered substantial cost savings andwere selected as the configuration for the station trans-formers.

The major problem to be overcome was to design apower system to supply these loads during any practicaloperational condition without exceeding permissible faultlevels, and at the same time to allow starting of the largestmotor. With large drives, such as the 10.5 MW boiler feedpump motors direct on-line starting, the voltage regulationon starting is severe. Motors of this size also make a sig-nificant fault contribution.

The problems of exceeding permissible fault levels arisewhen the station and unit transformers are connected inparallel. Three possible solutions were considered:

(a) a fast bus-transfer scheme(b) segregation of station and unit 11 kV busbars into

smaller groups with separate transformers(c) a conventional unit and station 11 kV system with

interbus reactors.

In view of the arduous conditions imposed on auxiliaryplant during reswitching, and the complex switching logicwhich potentially may reduce system reliability, the bus-transfer scheme was not developed.

The bus-segregation scheme involved two separate unitswitchboards, which could be supplied from separate unittransformers or from a single three-winding unit trans-former, shown in Fig. 2. The fault-level problems could beavoided by paralleling each unit board with a stationboard sequentially to transfer load. The design margins aresmall, however, and a more detailed design study would benecessary to confirm that all the requirements of the spe-cification could be met.

The conventional system is similar to A station design,except for reactors, and is shown in Fig. 3. Owing to thefault levels when the unit and station transformers are in

parallel, interbus reactors were considered necessary, tolimit the fault levels in this condition. Starting a unit isachieved by supplying the unit auxiliaries from the 11 kVstation board via a solid interconnector. After synchro-nising the generator, the interbus reactor is connectedbefore closing the unit transformer 11 kV circuit breaker.The interbus reactor is then disconnected, leaving the unitauxiliaries supplied from the unit transformer. Shutdown isa reversal of the startup procedure.

The advantage of this system is that transformer reac-tances can be reduced to give a greater margin on motorstarting and voltage regulation, and the fault-level situ-ation is contained by a suitable choice of reactor. The pre-liminary study indicated a reactor of 30 MVA. 11%reactance would be suitable. A cost comparison betweenthe bus-segregation and the interbus-reactor schemeshowed very little difference.

On the basis of the information available at the tenderand conceptual design stage, it was agreed that a schemeutilising interbus reactors would provide all the necessarydesign margins to meet the specification requirements. Adecision was made to base equipment details on thisarrangement and to make provision for reactors in thecivil design. However, it was recognised that when adetailed system design study was completed using moreaccurate design data, it might be possible to eliminate thereactors. It was decided, therefore, not to order reactorsuntil the system study confirmed that they were essential.

The voltage levels and fault ratings specified are(i) 11 kV; 900 MVA make, 750 MVA break(ii) 3.3 kV; 150 MVA make and break(iii) 415 V; 31 MVA make and break

In addition, lighting and small-power supplies of 240 Vand 110 V AC are derived from the 415 V system. Uninter-ruptable supplies include battery-backed 240 V and 110 Vinverter-fed instrument supplies, 240 V DC emergencypower supplies, and 110 V and 50 V DC control supplies.

4.2 Type and arrangemen t of s witchboardsIn view of the equipment supplied on A station, and in theinterests of standardisation, air-break circuit breakers ofthe same type were chosen for the 11 kV, 3.3 kV and 415 Vswitchgear. Air-break was chosen for A station after com-parison with vacuum and oil-break. Although vacuumcircuit breakers offered important advantages in cost andmaintenance requirements, they were considered to haveinsufficient proven field experience at that time. Air-breakwas chosen over oil-break on economic grounds, takinginto consideration the additional support systems for oil-handling and fire protection.

One significant change on the B station was the choiceof vacuum contactors for all the 3.3 kV motor drives toreplace the air-break fuse backed contactors used on Astation. There is now considerable service experience withvacuum contactors in onerous operating conditions, suchas oil-rigs, steel mills, mining etc. This service experience,together with the capital and maintenance cost savings ofvacuum contactors, was considered sufficient to make thechange of equipment from A station acceptable. As thereare a large number of 3.3 kV contactor drives, comparedwith the number of circuit breakers, this represented a sig-nificant cost reduction.

The auxiliary systems are segregated into unit andstation systems with varying degrees of standby facilities,as required. Drives up to 200 kW are supplied at 415 V,Drives between 200 kW and 1000 kW are supplied at3.3 kV and all drives above 1000 kW are supplied at

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11 kV. All switchboards are provided with facilities foralternative supply sources, in the event of the loss of theprimary source, for that board as shown in Fig. 3.

All the 3.3 kV switchboards are supplied from identical12.5 MVA transformers, which are sized for the largestload and are interchangeable. Each pair of station aux-iliary boards is interconnected, to allow one 12.5 MVAtransformer to supply two boards in the event of a failureof the normal source of supply. A 3.3 kV station standbyboard is provided, with interconnectors to each unit aux-iliary board, to give an alternative source of supply forthese boards.

The 415 V switchboards with loadings up to 2 MVA aresupplied from dry-type transformers. Each transformer islocated, together with incoming and interconnector circuitbreakers, in cubicles at the end of, and connected to, amotor-control centre, to form a single flush-frontedassembly. The motor-control centres are equipped withmotor-control gear and switch fuses to supply all the415 V auxiliary loads. With the exception of workshopsupplies, which have a single 800 kVA transformer feeder,all the dry-type transformers are rated at 2 MVA for inter-changeability between transformers of the same voltageratio. All essential 415 V drives are duplicated withsupplies from separate switchboards. Failure of one driveresults in automatic startup of the stand-by drive.

The precipitator switchboards are supplied from2.5 MVA oil-filled transformers, located outdoors adjacentto the precipitator switchrooms, and connected by cables.Oil-filled transformers are used for this application becauseof insufficient service experience with dry-type trans-formers of this rating. Alternative supplies are provided forall switchboards of 2 MVA and above by means of inter-connectors, so that two boards may be paralleled and sup-plied from one transformer. The transformers are rated forthis duty.

4.3 Postcontract-commencement of designdevelopment

It was agreed that during the detail design studies, effortswould be made to optimise the system design, with a viewto elimination of the interbus reactors, in the interests ofimproved operational flexibility.

The preliminary design studies had been carried out onthe basis of manual calculations, using cumulative adversetolerances of transformer reactances, and ignoring cablereactance for fault-level calculations. It was also recognisedthat in motor-starting calculations, the dynamic effect ofrunning motors cannot be included in manual calculations.These factors produce pessimistic results, leading to theconclusion that interbus reactors were required. To opti-mise the system design it is necessary to carry out detailedstudies using computer modelling. This is being done byGEC using the facility available at the University of Man-chester Institute of Science and Technology. This inter-active computer system combines a suite of power-systemanalysis programs, and provides full network diagramsand graphics facilities in a software package, named Inter-active Power System Analysis (IPSA).

In discussions between GEC and CLP concerning elimi-nation of reactors, it was agreed that as the fault-level diffi-culties arise when unit and station transformers areparalleled, it would be reasonable to consider the practicaloperational situation, and not necessarily the absoluteworst-case theoretical loading, which could occur if all themotors connected to the unit and station boards wererunning at the time of paralleling. The paralleling occurswhen a unit is being run up and loaded or shut down, at

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which time only one boiler feed pump would be oper-ational. It was decided to include the fault contributionfrom only one boiler feed pump and one soot blower com-pressor in the fault studies, together with the other nor-mally running auxiliaries. It was also agreed that anattempt would be made to obtain boiler feed pump motorswith a reduced starting current, and transformers withreduced tolerance on the specified reactance.

The motor supplier agreed to supply boiler feed pumpmotors with a starting current demand of 4.5 times thefull-load current, including tolerance. It was necessary todefine a transformer impedance envelope, before dis-cussions could take place with the transformer supplier.

The load-flow studies confirmed that transformerratings were satisfactory. The introduction of cable imped-ances reduced the fault levels by approximately 14%, com-pared with the previous manual calculations. Thecomputer studies also demonstrated that the schemewithout reactors was satisfactory if transformer nominalreactances were considered, instead of the cumulativemaximum adverse tolerances used in the manual calcu-lations. The unit and station transformer reactances have amajor influence on the operational performance of thesystem, particularly at the 11 kV level. If maximum nega-tive tolerances are considered, the prospective fault levelon the 11 kV switchboards exceeds the permissible limits.If maximum positive tolerances are considered, the voltagedip on motor-starting is unacceptable. BS 171 allows+ 10% tolerance on impedance at middle tap, and addi-tional tolerance limits are specified over the range of taps.

Various study cases were considered to determine thepermissible tolerance limits of impedance for the unit andstation transformers, while maintaining the normal ± 10%tolerance of the other transformers. These study casessimulated all the normal operational conditions, in addi-tion to a number of abnormal conditions, such as a unittransformer out of service with a station transformer sup-plying the auxiliary load and starting a second unit.Various fault conditions were also studied to demonstratepostfault system recovery. These included faults on theexternal 400 kV and 132 kV systems. By this means it waspossible to define impedance envelopes for the station andunit transformers and to determine tap settings for alltransformers. The required impedances are:

(a) Station transformer; 14.7% at nominal tap, with±6.1% tolerance on the nominal over the majority of thetap range, and ± 7.2% tolerance at the maximum positivetap position

(b) Unit transformer; 15.85% at nominal tap, with±6% tolerance over the whole of the tap range.

Discussions with the transformer manufacturer indicatedthat although they would normally expect to achieve thesereduced tolerances, it would not be possible to guaranteethis without special measures being adopted. The trans-former reactance is largely determined by the distancebetween the HV and LV windings. The reactance cantherefore be modified by changing the diameter of the HVwinding. To guarantee the impedance envelope, it wasdecided to order material for two HV windings on the firstunit and station transformers. One winding for each trans-former will be manufactured, and the transformer imped-ances will be measured as early as possible in themanufacturing process. If the impedance is outside therequired tolerance, the spare winding material will be usedto manufacture a new winding of modified dimensions, toprovide the necessary impedance envelope.

The system studies demonstrate that the performance of

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the auxiliary power system will be satisfactory under allthe operational conditions specified. However, the condi-tion of a unit transformer out of service requires certainswitching operations to maintain busbar voltages withinspecified limits. The voltage variations arise because of thethree-winding station transformer concept. The on-loadtap changer is on the primary winding, and it is necessaryto have the majority of reactance associated with the sec-ondary windings, because of fault-level considerations.This makes the voltage regulation load-dependent, withthe result that when one secondary winding is loaded withthe auxiliary load of a running unit, the other lightlyloaded winding will have a higher voltage than nominal.The 3.3 kV and 415 kV boards supplied from this lightlyloaded winding will have voltages in excess of the specifiedlimits.

To avoid this condition there are two possible solutions.To remove a unit transformer from service requires thatthe unit is shut down. During the shutdown, the off-loadtaps of the lower-voltage transformers connected to thelightly loaded station transformer winding can be changed,to avoid the over-voltage condition when the unit is putback on load.

Alternatively, the lightly loaded station board may betransferred to the other station transformer by use of theinterconnector between station boards. This will involveparalleling one LV winding of each station transformer.This can be done without exceeding permissible faultlevels. In this way, the voltage levels can be controlledindependently by the tap changers of the two station trans-formers. It is considered that either of these alternative sol-utions is justified, in the unlikely event of a unittransformer being out of service, in comparison to theadditional cost of four separate station transformers oron-load tap changers on the secondary windings of thethree-winding transformers.

An additional operating regime identified by the powerstation operations group is to transfer the auxiliary load ofa running machine onto the station transformer, with themachine at full load. The operations group advised thatthere are occasions when a machine is vulnerable to a unittrip, for example, if there was instability of boiler firing. Onsuch occasions, if the auxiliary load was transferred to thestation transformer, it would be possible to restart andbring the unit back on-line faster following a unit trip. Itwas agreed that, although this operation was not envis-aged in the original design concept, it was desirable toachieve it if possible.

The lightly loaded station board can be transferred tothe other station transformer as described above. However,there may not be sufficient time to effect this transfer inpreparation for the auxiliary load transfer. It may thereforebe necessary to transfer the unit auxiliaries first. The pro-cedure would be to parallel the unit and station boards,followed by adjustment of the station transformer tap to apredetermined value, so that when the unit load is trans-ferred to the station system, the voltage level of the unitauxiliary system is satisfactory. This transfer operation willproduce an over-voltage condition on the second windingof this station transformer, which is lightly loaded.However, this condition will only be for a short time, untilthe lightly loaded station board is transferred to thesecond station transformer by another paralleling oper-ation. While this operation presents no problems, theparalleling unit/station board can only be effected undercertain conditions, if the prospective fault level is to bekept within the specified limits. The computer studies showthat with the system fault levels on the 400 kV and 132 kV

systems equivalent to the maximum fault rating of theswitchgear, and using cumulative adverse tolerances on thereactance values of unit and station transformers, theprospective fault level at the 11 kV bus is above the speci-fied limit during paralleling. Further work is being carriedout, using realistic external system fault-level values, toestablish the permissible negative tolerance on impedancefor the station and unit transformers, so that the unit/station transformers can be paralleled with the unit at fullload. Should the measured impedances be outside thesevalues, the replacement HV windings will be used toachieve the required impedance envelopes, and permit thisauxiliary load transfer under full-load conditions.

The final stage of the system design work has yet to becompleted. The equipment has all been ordered and is inmanufacture. When works test data are available, the mostonerous case studies will be repeated using this informa-tion, to demonstrate that the final design meets all thespecified performance requirements. It is confidentlyexpected that this will be achieved.

5 Future trends in system design

5.1 Design tolerancesThe British Standards allow substantial tolerances ontransformer and motor reactances and motor-startingcurrent. With the increasing use of computer-aided design,it should be possible to predict these parameters moreaccurately, and therefore minimise a major constraint inpower system design. The system designer cannot useworks test values until much later in a project, when astudy is carried out to confirm that the system design issatisfactory. At the conceptual design stage prior to sub-mitting a tender, the designer must ensure sufficient per-formance margin in the design to ensure that possiblechanges in the postcontract stage can be accommodated,and to establish firm costs. This requires the use of cumu-lative adverse tolerances for fault-level and motor-startingstudies.

5.2 SwitchgearAs the present limits of 11 kV switchgear are beingreached, consideration should be given to the design ofequipment for even higher ratings, or, alternatively, forswitchgear and equipment for higher voltage levels. Thereis no 40kA and 3000A service proven switchgear currentlyavailable in the UK between 11 kV and 33 kV. A voltagerating of, say, 15 kV would allow sufficient margin for eco-nomic system design of future large units for the foresee-able future.

It is likely that vacuum switchgear will replace air-breakequipment for power station applications, on the groundsof reduced capital and maintenance costs.

5.3 Energy reco very drivesConsiderable development is taking place in the field ofenergy recovery schemes for large drives. The most prom-ising of these is the static frequency convertor, using abrushless supersynchronous motor for boiler feed pumps.When it can be positively shown that the high capitalinvestment is more than offset by the saving in fuel costs, itis anticipated that these schemes will gain widespreadacceptance. The static frequency convertor drive has amost useful advantage from the power system design pointof view, although this may be difficult to quantify in a costevaluation of different types of drives. The starting currentcan be as low as 1.5 times the full-load current, and themotor fault contribution is zero, due to the ability of the

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frequency convertor to cut off reverse power flow. In apower system design which is approaching the limits ofequipment fault-rating, the use of such drives could show acost advantage in the overall power system, in addition tothe energy recovery from the drive.

5.4 ConclusionsThe choice of a power system configuration requires issuessuch as security of supplies, flexibility of operation andacceptability of costs to be optimised. The authors believethat the optimum arrangement has been achieved for theparticular requirements at Castle Peak, and full advantagetaken of cost reduction without programme penalties.

The computer design studies have shown that manualcalculations at the conceptual design stage are not suffi-ciently accurate to allow detailed project design and pro-curement to proceed, and that such methods produceresults which are too pessimistic. While this may be usefulas a preliminary step, more detailed studies are necessarybefore equipment can be ordered and cost advantagesachieved.

The studies have also shown that, on large coal-firedunits, the limits of commercially available distributionswitchgear are being reached.

6 Discussion

6.1 Guest Editor's comments on paperThe paper demonstrates very well the complexity of thedesign problems and their solution, especially as related tothe auxiliary power system. The solution of the electricalproblems has to be considered as part of the design of themechanical plant and its operating requirements, withinthe context of a construction programme which requiresearly civil engineering design information, and of coursethere are also economic and commercial restraints. Ofgreat interest is the description of the postcontract-commencement design studies, to verify or modify theinitial decisions, and also the authors' views on futuretrends in system design.

6.2 At meetingJ. Preece (CEGB) asked, in view of the tight limits on faultlevels and switchgear short-circuit rating, what is the

ability of the transformer manufacturers to tightly controlthe impedances of the transformers? Mr. Wilson repliedthat special measures had to be taken to ensure close toler-ances. The distance between the HV and LV winding on atransformer is the critical factor in the transformer imped-ance. For this reason, the transformer design was capableof accepting alternative HV windings with different diam-eters, so that the oil gap between the two windings couldbe adjusted if the impedance tests revealed this to benecessary. Furthermore, the necessary materials for analternative HV winding were made available.

H. Heimer (Kennedy & Donkin) asked whether, in thedesign of the auxiliary power system, the possible use of ahigh-speed transfer of loads from one power source toanother had been considered. Wilson stated that GEChave experience of both high-speed transfer and parallel-transfer systems, in switching the auxiliaries from thestation transformer supply to the unit transformer supply,and vice versa. The high-speed system has the disadvan-tage that it could introduce shock loads to the rotatingplant, while the parallel system has the disadvantage thatit involves the risk of short-duration paralleling of the twosupply transformers. The appropriate system is chosen tosuit the circumstances, and in the case of Castle Peak B,the customer's preference was to choose the parallelsystem, for reasons of reliability.

E. Curtis (Monenco Associates) raised the question ofwhether consideration has been given to providing anopen-circuit transfer of the unit boards to the station ser-vices' source of power, in the event of a loss of voltage onthe unit boards during normal full-load operation of theboiler and turbine. This would allow some of the boilerand turbine auxiliaries to remain in service. Fault relaysand interlocks would prevent an 'unhealthy' unit systembeing transferred to the healthy station services system.

Finally, the issue was raised of whether some trip-inhibiting interlocks during parallel transfer had beeninvestigated, so that if a fault occurred during live transfer,the auxiliary power system circuit breaker was not over-stressed. For example, the 132 kV circuit-breaker supply-ing the station transformer could be tripped under thesecircumstances. Mr. Wilson replied that neither of thesealternatives had been investigated, because they introducedcomplications which might reduce the reliability of thesystem.

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