IEEE Transaction on Power Apparatus and Systems, Vol. PAS-103, No. 8, August 1984

AC AUXILIARY POWER SYSTEM DESIGN FOR THE SHOUBRAH EL-KHEIMA POWER STATION

Dr. Emad El-SharkawiEgyptian Electricity AuthorityCairo, Egypt

Eng. Hassan ShaffieEqyptian Electricity AuthorityCairo, Egypt

Denis J. King, Member, IEEEBechtel Power CorporationGaithersburg, Maryland

Abstract - This paper describes the design of theac auxiliary power distribution system of a largemulti-unit thermal power station located near Cairo,Egypt. The design features unitized auxiliary sys-tems, shared start-up sources, and a combustionturbine-generator capable of feeding into each unit'selectrical auxiliary system. The combustion tur-bine-generator is utilized both for black start of thestation and to supply peaking power when required bythe utility system. The auxiliary system configurationand the interrelationship of the design between unitsprovide flexibility for operations and sufficient re-dundancy to ensure reliable sources of power for plantauxiliaries.

INTRODUCTION

Egypt is a developing country which faces manychallenges, including a tremendous need for electricalpower. Most of the people live on 4 percent of theland, and one-fourth of the population lives in andaround Cairo. The country's largest generatingstation is located at the Aswan High Dam in UpperEgypt, approximately 790 km south of the major loadcenter in Cairo.

The electrical power generated by the hydrostations at both the Aswan Dam and the High Dam istransmitted to Cairo at 500 kV where it ties into the220 kV transmission system. Generation by the HighDam is limited when low water occurs from Decemberthrough February.

To achieve the hopes of reclaiming the desert,strengthening the agricultural basis of the economy,and meeting the needs of the people in crowded popu-lation centers, a reliable electrical power supply isessential. The largest thermal power station in Egyptis currently under construction at Shoubrah El-Kheimanear Cairo. The power station consists of three315 MW steam turbine generators, associated naturalgas or oil-fired boilers, and one combustion turbinegenerator with a base rating of 30.8 MW at siteconditions. The ac auxiliary system of the station was

designed to be reliable and highly flexible foroperations.

The design of the ac auxiliary system for theShoubrah El-Kheima Power Station combines manyelements common in the designs of modern-day powerstations and utility systems. These elements includeunitized design, shared start-up sources, onsiteemergency power generation, and peaking power.

84 WM 231-7 A paper recommended and approvedby the IEEE Power Generation Committee of the IEEEPower Engineering Society for presentation at theIEEE/PES 1984 Winter Meeting, Dallas, Texas,January 29 - February 3, 1984. Manuscript submit-ted September 7, 1983; made available for printingNovember 18, 1983.

This paper describes the configuration of the ac

auxiliary system buses and power sources, majorcriteria for system equipment sizing, and systemoperation including bus transfer.

System Configuration

The major system components are arranged andconnected as shown on the single-line diagram inFigure 1. Each unit's generator is connected to a

main power transformer, which feeds power to the220 kV switchyard, and to a unit auxiliary trans-former, which supplies power to the auxiliary systembuses. Two start-up transformers are used for thethree generating units. A combustion turbine gener-ator is connected through its transformer to theemergency power system buses, which are connected tothe auxiliary system buses of each unit.

The medium voltage auxiliary system of each unithas two load groups fed by separate lineups of 6.3 kVswitchgear. Each load group is connected to the unitauxiliary transformer and to the start-up transformer.During normal operation, each 6.3 kV bus is fed fromits associated secondary winding of the unit auxiliarytransformer. During start-up or when the main gen-erating unit trips, load is fed from the associatedsecondary of the start-up transformer. This is a

fairly common design for power plants with unitizedauxiliary systems and was used for each of the threeunits at the Shoubrah El-Kheima Power Station.

However, in lieu of three separate start-uptransformers, two start-up transformers are used andshared among three units. This saves not only thecost of the third transformer but also the cost of a

switchyard breaker position and tie line. Two start-up transformers are shared among three units bybifurcating the leads on one of the split secondarywindings of each transformer.

There is one emergency power system for thestation; it is utilized as a link between the three unitauxiliary systems. The system consists of a combus-tion turbine generator as the onsite power source, an

emergency power transformer with a split secondarywinding, and two separate 6.3 kV emergency buses.Each 6.3 kV emergency bus is connected to one of thetransformer secondary windings and consists of a mainbreaker and three distribution breakers. The dis-tribution breakers connect the emergency power systeminto the auxiliary system buses associated with theindividual units.

Equipment Sizing

Equipment used on the ac auxiliary system issized to permit flexibility of operation and to providesufficient redundancy to back up vital equipment thatmay be out of service.

The main power transformer is an outdoor, oil-filled unit rated at 247.0/328.5/410 MVA, OA/FA/FOA,with a turns ratio of 19 kV to 220 kV. Since the sitelocation has a generally warm climate with a mean of

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maximum ambient temperatures at 42°C and a maximumtemperature of 47°C, the transformer was rated at a550C temperature rise above the 420C ambient toenhance reliability for the life of the plant. Also, dueto spare parts availability and lead times for carryingout repairs, the transformer was sized to maintain100 percent of its rated output at a 55°C temperaturerise, with one of the multiple oil coolers out of ser-vice. This contributes to high reliability of the plantwithout derating.

The main power transformer is sized to deliverthe full output of the turbine generator with valveswide open and a 5 percent boiler overpressure. Sincetransformers are output rated devices, the specifiedrating took into account transformer losses. Thetransformer is sized to deliver all available power tothe switchyard if a unit auxiliary transformer is out ofservice or if the combustion turbine generator is inthe peaking mode.

The unit auxiliary transformers, start-up trans-formers, and emergency power transformer are out-door, oil-filled units with ratings based on a 550Ctemperature rise. Each of these transformer's sec-ondary windings is split into two windings that arebalanced, symmetrical, and of equal voltage, imped-ance, and MVA rating. The secondary winding is splitto reduce short-circuit contribution from motor feed-back.

The impedance of each transformer was calcu-lated, as a minimum, to limit the short-circuit level atthe 6.3 kV bus to 95 percent of the momentary andinterrupting ratings of the 500 MVA switchgear. Theother criterion that established the maximum value ofthe impedance was the requirement that the largestmotor be started, with other auxiliary load running, atnot less than 80 percent of the 6000 V rated motorvoltage.

The unit auxiliary transformer is rated24/32 MVA, OA/FA, with a 20 kV-6.3 kV/6.3 kV turnsratio and a primary to secondary impedance of8.73 percent at the rated 24 MVA base. It is sized tofeed the auxiliary loads connected to each bus, in-cluding the start-up boiler feed pump. The normalrunning load is 9.3 MVA on each bus. In addition toits starting duties, the motor-driven start-up boilerfeed pump is used when one of the normal runningturbine-driven feed pumps is out of service . Thisincreases the loading on bus B2 to 15.3 MVA. It wasthis loading that established the forced air rating ofthe unit auxiliary transformer. Since the normalrunning condition of the plant is expected to be withboth turbine-driven feed pumps operating, the normalload on the unit auxiliary transformer is well within itsself-cooled rating.

The start-up transformer is rated at 30/40/50MVA, OA/FA/FA, with a turns ratio of 220 kV-6.3 kV/6.3 kV and a primary to secondary impedance of12.5 percent at the rated 30 MVA base. It is sized tobe capable of starting up one unit while carrying thenormal running auxiliary load of a second unit. Thestart-up loads are 7.0 MVA on bus Bi and 11.3 MVAon bus B2. The start-up transformer is also equippedwith a load tap changer on the primary winding toaccommodate the wide voltage variations on the 220 kVtransmission system. System voltage at the 220 kVlevel ranges from 198 kV to 242 kV.

The emergency power transformer is rated at24/32/40 MVA, OA/FA/FA, with a turns ratio of11 kV-6.3 kV/6.3 kV.

Standard medium voltage levels utilized on 50 Hzsystems in Egypt include 3.3 kV, 6.3 kV, and11.0 kV. The 6.3 kV voltage level was selected aseconomically meeting the design requirement forfeeding the -maximum bus loading during plantoperation. This includes starting the largest motorconnected to the bus, which is the 7500 hp start-upfeed pump. Other large motors range in size from 300hp to 4000 hp.

To use the combustion turbine generator as ablack start emergency power source, the continuousrating is required to carry the start-up load of oneunit, which is approximately 18 MW. However, a com-bustion turbine generator rating of 18 MW would notbe capable of starting the boiler feed pump motor atthe required 80 percent rated motor voltage. Thenotable feature of the auxiliary system design for theShoubrah El-Kheima Power Station is the combined useof the combustion turbine generator as a black startmachine as well as a peaking unit. Therefore, alarger machine would fulfill the requirement of startingthe feed pump motor as well as contributing additionalpeaking power.

The final size determination of the combustionturbine generator yielded a machine with a 30.8 MWgenerator output at site conditions of 420C. Thegenerator is rated at 38.5 MVA. Site conditions arespecified because combustion turbine generator outputis dependent on ambient air temperature and type offuel. A machine rated 30.8 MW at site conditions of42°C using distillate fuel is capable of producingapproximately 37 MW at the standard rating tempera-ture of 15°C using the same fuel. Output ratings fornatural gas are slightly higher. This allows themachine to be capable of feeding the auxiliary loads ofat least two of the units during cooler times -of theyear, since the auxiliary load on each unit isapproximately 18.6 MVA under normal operating condi-tions.

The start-up sequence was reviewed and revisedto allow the motor-driven feed pump to start beforeother start-up loads were running. Since very fewother loads, such as general plant lighting and ser-vices, would be connected when starting up after ablackout, the voltage of the generator could be raisedto 1.05 per unit in order to more easily meet the80 percent rated voltage requirement at the motorterminals.

A typical curve of generator voltage versus timeis shown in Figure 2. As can be seen from the curve,the voltage dips immediately upon application of thelarge motor load. The excitation system responds andrestores the voltage level within 2 to 3 seconds. Thecurve shows a starting voltage of 1.1 pu at the6.3 kV bus because the generator, which is rated at11. 5 kV, is set at 1. 05 pu and the emergency powertransformer turns ratio is 11 kV-6 .3 kV, which gainsanother 0.05 pu voltage.

System Operation

For normal start-up of a unit, the auxiliarysystem is connected to the start-up transformer byclosing the appropriate breakers. After start-up iscomplete and the main generating unit is on line, theauxiliary buses are transferred to the unit auxiliarytransformer by momentarily paralleling the two sourcesand then tripping the breaker connecting the start-uptransformer. This manual transfer is supervised by asynchronism check relay.

When the unit trips, auxiliary load is auto-matically transferred to the start-up transformer by a

2021auxiliary buses of each unit can be connected to thoseof another unit through the emergency power system.

When all three units trip and a station blackoutoccurs, emergency power generation is required tostart up one of the units and get the station back online. The combustion turbine generator has its ownblack start capability which is automatically initiated onloss of switchyard voltage, indicating the station isblacked out and separated from the system. Once themachine is running and ready to accept load, generalplant services and lighting can be energized. Theunit to be started first is then selected and theauxiliary buses are aligned with the combustion turbinegenerator through the emergency power transformerand buses. The generator voltage is raised to 1.05per unit and the 7,500 hp start-up boiler feed pump isstarted first. The normal start-up sequence for theunit is then followed. After the main unit is runningand on line, auxiliary load can be transferred from thecombustion turbine generator to the unit auxiliarytransformer.

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FIGURE 2TYPICAL VOLTAGE RESPONSE FOR A LARGE MOTORSTARTING ON A SYSTEM WITH LIMITED GENERATION

fast bus transfer scheme. It is important that thistransfer be made as fast as possible, to prevent thedecay of the internal voltages of running motors. Ifthe transfer is made too slowly, motors may bedamaged from excessive current and torque that wouldoccur upon reapplication of bus voltage to the motorswith out-of-phase residual voltage. The fast bustransfer scheme is designed using a "b" contact of thetripping breaker -to initiate closing of the breakerconnected to the start-up transformer. It is a commonpractice in the U. S. to use an early "b" contact infast transfer schemes. This limits the dead bus timeto 2.5 to 3.0 cycles. However, switchgear for theShoubrah El-Kheima power station was not furnishedby a U. S. manufacturer, and the early "b" contactwas not available. The fast transfer time was testedand the dead bus time was found to be 4.2 to 4. 5cycles without arcing. This is sufficient to permit afast transfer without significant decay of the internalvoltage of running motors. The fast bus transfer ismonitored by a synchronism check relay which blocksclosing when there is a significant difference involtage or phase angle between the bus and thesource. Transfer is also prevented under the condi-tions of a bus fault.

If one unit auxiliary transformer is out of ser-vice, the auxiliary loads can be fed by the start-uptransformer. The main power transformer is largeenough to deliver the full output of the turbine gen-erator so that power normally fed to the auxiliarytransformer can be delivered to the switchyard andthen back to the auxiliary system through the start-uptransformer. Additional losses do occur because theauxiliary power goes through two levels of transforma-tion.

The system configuration is such that the stationcould also continue to operate all three units if twoauxiliary transformers were out of service, or if anycombination of one auxiliary transformer and one

start-up transformer were out of service. This opera-tional flexibility is achieved because the start-uptransformers are sized to feed the auxiliary load of oneunit and the start-up load of a second unit. Also, the

This is accomplished by bringing the combustionturbine generator into synchronism with the main unitgenerator, paralleling the sources through the auxil-iary transformer and then tripping the feed from theemergency power transformer.

Combustion turbine generators used as peakingunits are usually connected to the grid through atransformer that steps the voltage up to the trans-mission system voltage, which requires an additionalswitchyard breaker position and a more costly trans-former due to the higher primary winding insulationlevel. Furthermore, the voltage drop on attempting tostart the start-up feed pump under blackout conditionswould be even greater through two levels of transfor-mation, i. e., step-up transformer and start-up trans-former.

When the combustion turbine generator atShoubrah is used to contribute peaking power, load istransferred from the unit auxiliary transformer to theemergency power transformer after the two sources aresynchronized and paralleled. The auxiliary trans-former is then tripped and the combustion turbinegenerator and the auxiliary load it feeds run inde-pendent and separated from the rest of the system.

This allows peaking power to be fed into thesystem at the 220 kV level by the main unit generatorsthrough the main power transformers. The amount ofpeak power is sufficient to feed three auxiliary busesduring warmer times of the year. When the tempera-ture is less, the peak capability increases and fourauxiliary system buses can be fed by the combustionturbine generator. The normal ambient temperatureduring January and February is approximately 70C,which is also the time of low water at the Aswan HighDam.

CONCLUSION

The system configuration and sizing of the equip-ment make the ac auxiliary system at the ShoubrahEl-Kheima Power Station flexible for operation withstart-up or auxiliary transformers out of service.Combining peaking and black start duties of the com-bustion turbine generator at the power station sitemakes full use of installed equipment.

Considering the location of the power station, thesystem it serves, and the full use of the auxiliarysystem equipment, the design provides flexibility foroperations and redundancy of power sources for thepower station auxiliaries.

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RELATED BIBLIOGRAPHY

[1] Alexander Borst, "Standby and Emergency PowerSupply of German Nuclear Power Plants," IEEETransactions on Power Apparatus and Systems,vol. PAS-94, pp. 1098, July/August 1975.

[2] R. M. Damar and J. P. Henschel, "An AuxiliaryPower System for a 500 to 600 MW Coal PowerPlant," IEEE Transactions on Power Apparatusand Systems, vol. PAS-100, pp. 4571-4577,November 1981.

[3] H. E. Head and Umesh Rao, "Excitation Controlof Peaking Generator Connected to Large Power

System," IEEE Transactions on Power Apparatusand Systems, Vol. PAS-86, pp. 1504-1508,December 1967.

[4] W. S. Morgan, "Muskingum Unit No. 5 AuxiliarySystem," IEEE Transactions on Power Apparatusand Systems, vol. PAS-86, pp. 1500-1504,December 1967.

[5] G. R. Reed and D. R. Webster, "Design of A. C.Auxiliary Power Distribution Systems for LargeTVA Thermal Power Generating Plants," IEEETransactions on Power Apparatus and Systems,vol. PAS-94, pp. 1755-1762, September/October 1975.