CHAPTER Gas Turbine Applications in Power Stations, Gas ... · PDF filein Power Stations, Gas Turbine Protective Systems, ... controlled over the full ... fossil-fuel stations when

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CHAPTER 1Gas Turbine Applications

in Power Stations, Gas Turbine Protective

Systems, and Tests

1.1 IntroductionGas turbines are used to supply power for the following purposes:

• Peak shaving: This situation occurs when the load demand is high and the steam power stations are unable to generate additional power.

• Startingapowerstationwhenthegridsupplyisnotavailable.

• Followingthetrippingofapowerstationfromloadwhenthegridsupplyisnotavailable.Gasturbinesprovidepowerinthissituationtoprovidelubricationandcoolingtohotstationequipmenttopreventdamage.

The following are the requirements for the gas turbine plant in power stations:

• Outputhigherthan10MW.Thisisthepowerrequiredtostartalargegenerator(morethan600MW)

• Highavailability(morethan99%insomeapplications)

• Highflexibility

• Minimummaintenancecost

Twoorthreeaeroderivativeenginesareusedforthisgasturbineplant.Theoutputofeachengine isbetween10and25MW.Someapplicationsusegasgenerators(theportionofthegasturbinewhichgeneratesthehotgases—seeFig.1.1)thatdischargetheir gases into a turbine. This turbine is known as “free power turbine.” The generator is coupled directly to this turbine.

Moderngasturbinepowerstationsuseasingle70-MWelectricgenerator(Fig.1.2).This electric generator is coupled to two free power turbines at each end. Each free

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powerturbineisdrivenbyagasgeneratorsimilartotheoneshowninFig.1.1.Thisconfigurationallowstheelectricgeneratortooperateatpartload.Thisisdonebyvary-ing the following:

• Numberofengines(gasgenerators)inservice

• Thegasflowgeneratedbyeachengine

1.2 Working CycleFigure1.3illustratestheworkingcycleofthegasturbine.Thegasvolume(orvelocity)increasesinthecombustionchambers(knownascombustors).However,thecombus-tion occurs at constant pressure (there is a slight decrease in pressure inside the com-bustorsduetofriction).Thisallowsfortheuseoflightweightcombustorsandlow-octanefuels.

1.2.1 StartingThegasgeneratorisstartedbyastartermotor.ThismotorrotatestheHPcompressor.ThisinducesairintotheLPcompressor.TheinducedairforcestheLPcompressortorotate.TheignitersareenergizedwhentheHPcompressorspeedreaches800rpm.Thefuelboostpumpisstarted2secondslaterandthefuelservo-valveopens.Thecompres-sormustbeabletodevelopsufficientairpressuretoprovideairforcombustionandcoolingthecombustionchambersandothercomponentsatlight-up.Thisoccursataspeedof1000rpm.Fuelisadmittedatthisspeed.

Thecompressorwillreachaspeedof1000to2000rpmwithin15to20seconds.Thefuel is admitted with the igniters energized. All combustion chambers must be lit within 10to20seconds.Theengineexhausttemperatureincreasesrapidlyatthisstage.Theexhaustgastemperaturemustexceed200°Cwithin45secondsfromthebeginningofthestartsequence.Otherwise,theenginewillbeshutdown.Thisisdonetoprotecttheplant from the hazards of unburnt fuel. The gas flow increases with the speed. The unit is synchronized at the operating speed.

Theengineexhausttemperatureincreasessignificantlyduringloading.Thistem-perature isbetween450and490°C at full load. The loading rate is restricted by the rapid rise in this temperature. The time required to reach full load varies from 2 to 5minutes.

Thefuel/airratiomustbeaccuratelycontrolledoverthefullrangeofoperation.Thisisessential tomaintainoptimumefficiencyandacceptableenvironmentalconditions.Microprocessor-basedenginemanagementsystemisusedforthispurpose.Thissystemisprogrammedtomatchthecharacteristicsoffueldeliverysystemsandvariationsincompressorperformance.Moderncontrolsystemsalsoprovidethefollowing:

• Plantconditionmonitoring

• Alarmscanning

• Dataprocessing

• Faultdiagnosis


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1.2.2 ShutdownThe shutdown process is normally the reverse of the starting process. However, acontrolled cooling period is required for the free power turbine in some designs. This period will reduce the thermal shock when the engine is tripped. The hot ducting and stack will generate a natural draught following a trip. This will draw cold air through the engine and power turbine for a period of time. The cold air flow will produce high thermalstressesinthefreepowerturbine.Theunitshouldbeoperatedfor5minutesat2or3MW.Thiswillreducethegastemperaturesenteringthepowerturbineto300to325°C. This will reduce the thermal shock on the power turbine significantly. Alterna-tively, theengineair-intakeshouldbeclosedfollowinga tripbyclosingadamper.Steam turbines have a period of shaft barring following shutdown to prevent thethermal stresses.

1.3 ProtectionThe high-speed shut-off cock(HSSOC)islatchedopenatthestartofarun.Thisspring-operateddeviceadmitshigh-pressurefueltothecombustors.Thisdeviceisequivalenttothesteamturbineemergencystopvalveonanaero-engine.Thetrippingofthisdeviceinitiates a shutdown under controlled or emergency conditions.

The low-pressure cock(LPC)isasecondfuelshut-offvalve.Thisdeviceispositionedbeforetheenginefuelpumpsandregulatorvalve.TheLPCoperatesalwayswiththeHSSOC.However,theLPCismanuallyoperatedforprimingtheenginefuelsystemfollowing maintenance.

Gas turbines are housed in acoustic enclosures. These enclosures are strengthened toprotectpersonnelfrompossibleengineexplosions.Theenginesareprotectedbyafiredetectionsystem.Carbondioxide(CO2)andhalon1301havebeentraditionallyusedasextinguishingagents for the turbine,auxiliarycomponents,andgenerator.However,preactionwaterspraysystemshavebeenusedforturbineunitsinbuildings.Caremustbeexercisedtopreventwaterimpingementonhotturbineparts.Oneormorefiredetec-torsmustactuate forpreactionwater spraysystems todischarge.Heatmustalsobepresenttomeltthefusiblelinksinthenozzles.Thus,thesesystemsarelesssusceptibleto inadvertent discharge. The National Fire Protection Association (NFPA) providesstandards for the design and installation requirements for fire protection systems.

GasturbineoperatorshavepreferredhalonoverCO2 for many years. This is because halon is not an asphyxiant. However, halon breaks down into corrosive and toxicbyproducts(hydrogenfluoride,hydrogenbromide,andfreebromine)afterithasactedunderfireorwhenitisexposedtosurfacetemperaturesabove482°C(900°F).

Watermisthasprovedtobeahighlypromisingagentfortheprotectionofgastur-bines.Theextinguishingmechanismsofwatermistareheatremoval,oxygendepletion,andsteamexpansion.Watermisthasthefollowingadvantages:

• Itissafeforoccupiedareas.

• Providesbettercoolingeffectsthangaseoussuppressionagents.

• Watermistsystemsarepre-engineeredandapprovedforacompartmentofcertainmaximumvolume(gaseoussystemsmustbeengineeredforaspecificapplication).

• Watermistsystemsarenotassensitivetoenclosuretightnessasgaseoussystems.



NFPA 750, Watermist Fire Protection Systems, governs the design, installation,maintenance,andtestingofthesesystems.Severalgasturbineinstallationshaveusedwatermist systems successfully. The operation of the fire detectors will trip the unit and closethefirevalves.Thesevalvesarepositionedinthefuelsupplyline.

The precautions taken during the starting phase of the gas turbine to preventfuel ignition following a flame-out must continue throughout normal operation.Flame-detectiondevices(knownasflamescanners)haveprovedtobeunreliableatdetectingflame-out.Theengineexhausttemperaturehasprovedtobethemostreli-ablemethodfordetecting flame-out.Theexit-gas temperature isaround250°C for maximum fuel settings. Thus, the engine is assumed to have flame-out when thetemperaturedropsbelow200°C. The protection system should trip the engine when theexit-gastemperaturedropsbelow200°C. All engine bearings are sealed with air bled from the compressor. The engine will be tripped if a fault occurs in the sealing system. The engine will also trip on low oil pressure in the bearings and low fuel pressure in the supply line.

1.4 Black StartBlack start is a tern used to describe starting a gas turbine from a battery bank when the normalACelectricalsupplyhasbeenlost.Theunitmustbeabletooperateatfull-loadfor sufficient time to reestablish normal AC electrical supplies. The battery bank must havesufficientenergytoperformthefollowingfunctions:

• Operate the starting device (e.g., DC motor) to drive the engine to the self-sustainingspeed(thisspeedatwhichtheturbinecandrivethecompressortotheoperatingspeedwithouttheaidofthestartingdevice).

• Operatefueligniters,instruments,andcontrolequipment.

• Operate the turbine-generator lubricating oil pumps and engine fuel boostpumps when fitted.

• 110-VDCbatteryisnormallyusedforthestartingdevice,control,andinstru-mentation.A28-Vtappingfromthisbatteryisprovidedfortheigniters.Thefuelpumpsandlubricatingoilpumpsaresuppliedfroma240-VDCstationbattery.

The Black start is initiated in the following cases:

• Lossofbusbarvoltage

• Dropinfrequencybelowapredetenninedvalue

Theenginewillnotbeabletosynchronizewiththebusbarwhenthebusbarvoltageis lost (the circuit-breaker will not be able to close when the synchronous speed isreached).Thus,thesynchronizinginterlocksmustbeoverriddenifitisessentialtoclosethecircuit-breakerautomatically.Thisisthecaseinnuclearpowerplants.

Thecontrolsareperformedmanually infossil-fuelstationswhenthegasturbinereaches synchronous speed. Station instructions are followed at this stage to restoreelectrical power. These instructions are consistent with the capacity of the gas turbines



and the anticipated requirements of the plant. The following are the highest priority power supplies of these instructions:

• Restoringbattery-charger

• Replenishmentofgasturbinefuelstocks

• Turbine-generatorlubricatingoilpumps

• Generatorhydrogensealoilpump

• Turbineturninggear(barringmotor)

• Boilercirculationpumps

• Boilerforced-draftandinduced-draftfans(toremoveexplosivegasesfromtheboilers)

1.5 Routine TestsMostgasturbineenginesoperatinginpowerstationsaredesignedtooperateatleast2000servicehoursbetweenmajoroverhauls.Theoperatingregimeinpowerstationsisdifferentfromtheaviationindustry.Thefollowingarethemaindifferences:

• Thegasturbinesoperatecontinuouslyatgroundlevelinpowerstations.Theairdensity is higher at high altitudes. This increases the stress on the engine components when operating at full power. The life of these components will be reducedduetothisincreaseinstress.Mostaeroderivativeenginesarederatedwhen operating at ground level. This is done to reduce the stress on thesecomponents.Thisderatingisbetween20and25%oftheengineratedcapacity.

• Thenumberofenginestartsishigheratground-leveloperation.

The engine condition is reflected in the following two areas:

1. Bearingvibrationlevels

2. Debrisfoundinthebearinglubricatingoilscavengefilters

Thefollowinginspectionsshouldbeperformedat500-hourintervals:

• Air-intakeareas

• Compressorbladeconditions

• Fueldrainsystems

Theprotectionsystemsshouldbetestedroutinelyatintervalsrangingfrom1to 6 months. The state of the compressor blades is determined by the following two conditions:

1. Levelofimpurities(sand,dust,organicmaterials,etc.)intheambientairaroundthe engine

2. Filtereffectiveness



The compressor-blade cleaning should be performed before significant degrada-tion.Thefollowingprocedureisrecommendedforcompressor-bladecleaning:

• Injectanapprovedsolventintothecompressorintakewhiletheengineisrunningat cranking speed. This will loosen the dirt and grease stuck to the compressor blades.

• Allowthesolventtosoakforafewhours.

• Spraydemineralizedwateratcrankingspeedtoflushthecompressor.

• Runtheengineatidlespeedforashortperiodoftime.Thiswilldrytheengine.

Reliabilityoftheseunitsisveryimportant.Thereliabilitytargetforsomestationsis99%.Thisindicatesthatthegasturbinesystem(2or3gasturbines)mustbeavailabletosupplypowertothestationmorethan99%ofthetime.Eachgasturbineenginecandeliverthestationpowerrequirement.Theredundancyinthesystemisknownas2or3×100%.Eachunitisstartedweekly.Thisisdoneiftheenginedidnotoperateduringthis period.

1.6 BibliographyBritishElectricity International,Modern Power Station Practice—Station Operation and

Maintenance,Vol.G,3rded.PergamonPress,Oxford,UnitedKingdom,1991.



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CHAPTER Gas Turbine Applications in Power Stations, Gas ... · PDF filein Power Stations, Gas Turbine Protective Systems, ... controlled over the full ... fossil-fuel stations when