Generator Auxiliary Systems

8/22/2019 Generator Auxiliary Systems

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c)a)b)

PI 3Q.23-2Electrical Equipment - Course PI 30.2

G E N E ~ T O R AUXILIARY SYSTEMS

OBJECTIVESOn completion of this module the student will be able to:

I . State the purpose of the generator stator cooling system.2. List and briefly explain five operational reqUirements of the generator

stator cooling system.3. Given a simplified diagram of a typical stator cooling system:

Label each flagged component;Identify the flow direction of the demineralized cooling waterus"; ng arrows.Briefly explain the function of each component in a stator coolingsystem.4. State the purpose of the generator hydrogen seal.5. State three operational reqUirements of the generator hydrogen seal.6. Given a cross-sectional diagram of a typical generator hydrogen seal:

a) Briefly explain its operation;


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PI 30,23212. Given a simplified diagram of a large AC generator:

a) Label the components related to , or cooled by the hydrogen coolingsystem;b) Briefly explain the function of each of the components youidentified in (a).c) Indicate with arrows, the flow paths of the hydrogen gas.13. List and briefly explain six precautions with respect to thegenerator auxiliary systems discussed in objectives 1 to 13 inclusive.14. aJ

bJList and briefly explain five advantages of choosing hydrogenrather than air as a cooling medium for large generators;List and briefly explain two disadvantages of choosing hydrogenrather than air as a cooling medium for large generators.


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PI 30.23-2

1.0 INTRODUCTIONThis module will introduce the trainee to:(a) The generator stator cooling system.(b) The generator hydrogen seal.(c) The generator hydrogen seal oil system.(d) The generator hydrogen cooling system.(e) Precautions relating to generator cooling systems.(f) The advantages and disadvantages of hydrogen, compared to air, as acoolant.

2.0 THE GENERATOR STATOR COOLING SYSIEM2.1 Puroose of The Cenerator Stator Cooling System

The purpose of the generator stator cooling system is to maintainthe copper stator bars and the end core magnetic screen plateswithin their proper operating temperature range under all operatingconditions, by passing cooled, demineralized water through them.2.2 Operational Requirements of the Generator Stator Cooling System

To ensure safe operation of the generator, five operationalrequirements must be met. These are:(i) To provide demineralized cooling water to the generatorstator windings and the end core magnetic screen plates, at a


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PI 30.23-2

2.3 A TYPical Stator Cooling SystemFigure 2 is a simplified diagram of a typical stator cooling system,showing typical system components and the direction of the coolantflow. The follOWing are brief explanations of the functions of themajor components of the system:(i ) ac Pumps - PM I, PM 2

Two 100% duty pumps, operating on Class IV ac power, areprovided. Either of these pumps can prOVide 100% of therequired flow. Therefore, one pump is in service and theother is on standby.

( i i ) Emergency pump - PM 3The emergency pump is a 50% duty pump, meaning it is capableof supplying only 50% of the required full power flow. It ispowered from Class I and starts automatically if both acpumps fail. Some stations may not have an emergency pump.

(iii) Check Valves - NV24 and OthersVarious check valves are provided to prevent reverse rotationof the pumps and to ensure correct flow direction of thestator coolant.

(iv) Stator Water Coolers - HXI, HX2In order to minimize demineralized water cost and to preventingress of impurities, the demineralized water ;srecirculated through the stator conductors and the coolingsystem in a closed loop. Two 100% duty heat exchangers,


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PI30.23-2

COOI.ANfHEADERTANKVENTS VENTS

GENERATOR." STATOR ./f'" ".() J.pts , ~ RESIST,ANCE.Jf' ," " "COlUMNS"".. ..,-, -"., 0 ., -, - .,-,-. .. ."". .. _.".... .... .. ...,

ENOCOREMAGNETICSCREEN

STATORCOOI.ANfSTRAINER

COOLWATER

HX2

VENT

.,.,,.,i,,.,,.,,.,

:'l.I'l.''!.''l.,/:t-'-l>


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PI 30.Z3-Z

(vi) Deexygeoating Unjt and IX CQlumn - IXIIn Qrder to prQvide the required insulating prQperties, thestatQr water cQnductivity must be held belQw preset limits.A typical Qperating cQnductivity is inthe 0.5 ~ s / c m range.Since cQnductivity will tend tQ rise during QperatiQn, an IXcQlumn and deQxygenating unit are prQvided tQ scrub thedemineralized water circulating in the stator cQQling system.

(vii) VentsVents are prQvided as shQwn, at variQus high pQints in thestatQr cQQling system, tQ permit bleeding Qff Qf any gases(OZ, NZ, HZ) that might accumulate and cause an "air-1Qck" tQfQrm.

(viii) CQQlant Header TaokThe cQQlant header tank hQlds a supply Qf demineralized waterat a relatively cQnstant head pressure fQr the stator cQQlantsystem. It is impQrtant tQ keep the statQr cQQling systemfilled with demineralized water in Qrder tQ minimize thecQrrQsiQn and Qther fQrms Qf contaminatiQn that will arise ifthe statQr cQQling system is repeatedly Qpened tQ theatmQsphere.

(ix) Resistance ColymnsThe resistance columns carry the stator cooling water into orQut Qf the generatQr while electrically iSQlating the statQrcQQling system frQm the statQr cQnductQrs. StatQr cQnductQrvQltages are as high as Z4 kV.


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(i)

PI 30.23-2

3.0 THE GENERATOR HYDROGEN SEAL3.1 purpose of the Generator Hydrogen Seal

Hydrogen seals are prOVided at each end of the generator to ensurethat there is a minimum of hydrogen leakage between the rotatinggenerator shaft and the stationary end cover of the stator. Thisrequires maintaining a continuous seal for operating periods of ayear or more, at hydrogen working pressures of 300 - 400 kPa(g), atgenerator rotor speeds ranging from stationary to I SOO RPM.3.2 Operational Regyirements of a Generator Hydrogen Seal

Three operational requirements of a generator hydrogen seal are:It must provide a seal between the generator rotor and thestationary end cover of the generator.

(ii) It must accommodate significant axial movement of the rotorshaft with respect to the generator end cover.(iii) It must minimize the ingress of oil and/or air to thegenerator cavity.

3.3 ATypical Generator Hydrogen SealFigure 3 is a simplified sectional diagram of a typical generatorhydrogen seal. The cool, clean seal oil is supplied to thestationary oil feed chamber at a pressure somewhat greater than thehydrogen pressure in the generator. A preset spring loading, aidedslightly by the oil pressure provides an axial force whichcontinuously holds the seal ring toward the shaft ring face. On thefront of the seal ring is the soft metal continuous seal face. The


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PI30.23-2

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PI 30.23-2

3.4 Radial HydrQgen SealsThe radial Qil seal shQwn in Figure 4 is a newer fQrm Qf hydrQgenseal having nQ SQft metal seal faces. Oil is pumped tQward theshaft frQm two sets Qf hQles in a statiQnery ring surrQunding thegeneratQr shaft, fQrming tWQ rings Qf Qil between the mQving shaftand the statiQnary ring. Acentral vacuum ring extracts the QilfrQm the seal area. These rings Qf Qil accQmmQdate bQth axial andradial shaft mQvement while cQntinuQusly sealing the hydrQgen withinthe generator. VACUUM-

OIL OUT ---::: '" PRESSURE .....+__/ ,OIL INSEALING'RING I t I SEALING RINGHOUSING, + " -+ - HOUSING

QILSEAL H2 PRESSURE AIR OUTSIDE GENERATORINlice_ Q E _ N _ E R _ ' _ T G R _ - - ! . / . : . . ~ " " OIL SEAL

GENERATOR SHAFT------ . . ~ i i - -END VIEW SIDE VIEW

Figure 4; ATypical. Simplified. Radial HydrQgen Oil Seal4.0 GENERATOR HYDROGEN SEAL OIL SYSIEM


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(i v)

(i i i )

PI 30.23-2

To maintain the seal oil at the correct operating temperatureunder all operating conditions.To provide filtration to remove any particulates which couldscore the soft metal seal face.

(v) To remove entrained hydrogen from the oil and vent thehydrogen safely to atmosphere.(vi) To provide an emergency oil supply in the event of failure ofthe main seal oil pumps.

4.3 ATypical Generator Hydrogen Seal Oil SystemFigure 5, is a simplified diagram of a typical generator hydrogenseal oil system, showing the system components and the direction ofthe oil flow. The following is a brief description of the functionof the major components:4.3.1 Oil From Main Turbine Oil System

The seal oil is normally supplied from the main turbinelubricating oil system via a turbine shaft driven pump andpressure relief valve (PRY).4.3.2 AC Seal Oil Pump

If the turbine shaft driven pump is unable to providesuitable seal oil pressure, a Class IY ac pump is used. Thecombination of shaft driven and/or ac pumps will vary fromstation to station.4.3.3 QC Seal Oil Pump


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PI30.23-2

TURBINEEND SEAL SUPRINGEND SEAL

GENERATOR

ERVICEWATER-A

PRV

SEALOIL

I < l - - OIL FROM

SHAFT DRIVENPUMP

FILTERFR1

SEALOL


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PI 30.23-2

4.3.5 Seal Oil Filters and CoolersThe seal oil filters are provided to remove any particulatesfrom the oil before it is supplied to the generator hydrogenseals. The filters are instrumented for high AP across theinput/output lines. The coolers cool the oil before it flowsthrough the seals, thereby maintaining the soft-metal sealfaces within their operating temperature range. The seal oiltemperature is controlled via the seal oil coolers which aresupplied by manually operated service water valves.Thermocouples embedded in the seal face are used to monitorfor high seal face temperatures.4.3.6 Detrainjng TankThe major portion of the seal oil flow drains to the bearingsumps and then to the turbine and seal oil tank.The small amount of seal oil that flows inwards to thehydrogen side then drains down to the hydrogen detrainingtank via a sight glass and 10 cm diameter pressure balanceline. In the detraining tank, the entrained hydrogenseparates from the oil. The oil is forced up to the hydrogentrap by the generator hydrogen pressure.

4.3.7 Hydrogen Trap and Extraction FanAny hydrogen remaining in the oil is removed in the hydrogentrap and is safely vented to atmosphere by an extraction fan.The seal oil flows by gravity down to the turbine and sealoil tank.

4.3.8 Turbine and Seal Oil Tank


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PI 30,23-2

5.0 GENERATOR HYDROGEN COOLING SYSTEM5,1 purpQse Qf the GeneratQr HYdrpgen CQQling System

The purpose of the generator hydrogen cooling system is to maintainthe generatQr rQtQr and the statQr irQn within their prQperQperating temperature ranges under all Qperating cQnditiQns,5,2 OperatiQDal Requirements Qf the GeDeratQr HydrQgen CQQling System

As was indicated in PI30.23-1 heat is remQved frQm the generatQrrQtQr and the statQr irQn by cQntinuQusly passing hydrQgen gasthrQugh them. Seven QperatiQnal requirements Qf the generatQrhydrQgen cQQling system are:(a)(b)(c)(d)(eJ( f )(g)

TQ cQntinuQusly recirculate the hydrQgen gas within thegenerator.TQ CQQl the hydrQgen tQ the required temperature,TQ dry the hydrQgen tQ the required dewpQint.TQ maintain the CQrrect hydrQgen gas pressure in thegeneratQr by prQviding make-up hydrQgen tQ cQmpensate fQr1eaks,TQ prQvide an alarm fQr liquid Qil Qr water within thegeneratQr cavity,TQ mQnitQr the hydrQgen gas purity,TQ prQvide C02, Air and H2. fQr purging and charging thegenerator.

5.3 ATypical GeneratQr HydrQgen CpQ]ing SystemFigure 6 is a simplified diagram Qf a typical, large generatQrshQwing the directiQns Qf the hydrQgen flQW within the generatQr,The fQllQwing are brief explanatiQns Qf the functiQns Qf the majQr


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H2DRYERS

BULK H2SUPPLY &PRESSURECONTROLI

H2 GASPURITYANALYSERS

. . ...H2COOLERS I IQ9

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+++. --+....ffif-.+--+----.- - - - -

HOT ;-l:" .,~ ~ . ~r. ''I,r::-=tfi.....LhhI.c:=. ...... .........--

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I r-,-oN'"Z- '"-< 0."oo- Figure 6: A Typical Generator Hydrogen Cooling System


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PI 30.Z3-Z

5.3.Z Hydrogen CoolersThe hydrogen coolers are long, finned, U-tube units mountedaxially in compartments located in the generator yoke.(Also, see Figures 5 and 6 of PI30.Z3-!). Service water iscirculated through the cooler tubes. The hot hydrogen passesover the finned tubes, loses its heat to the service waterand then flows on to cool the stator iron and rotorconductors. The hydrogen temperature is controlled byautomatically regulating the flow of service water to thecoolers, using RTDs within the stator to measure the hydrogentemperature.

5.3.3 Gas Supplies - HZ, COZ and AirHydrogen from a bulk storage system is fed to the generatorvia a pressure regulating valve. The HZ pressure within thegenerator is held relatively constant by the hydrogen make-upsystem.COZ from portable bottles is used to purge hydrogen or airfrom the generator when required. The COZ is then displacedby clean, dry air if the generator is to be opened formaintenance.

5.3.4 Hydrogen DryerTypically, the hydrogen dryer will be a twin tower type usingbeds of activated alumina. Cycle times are adjusted to suitthe drying load. Refrigeration type driers are used in somestations.


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PI 30.23-2

6.0 PRECAUTIONSThere are six major precautions related to the generator cooling systemsdiscussed in this lesson. This section is a review and consolidation ofthe previous material.(a) Stator Cooling Water Conductivity

Since the large generators used in NGS operate at 18 000 volts ac orabove, it is absolutely essential that the stator cooling waterconductivity be kept low enough to provide adequate electricalinsulation. This is both a personnel and an equipment concern.(b) Hydrogen/Ajr ConcentrationsThe hydrogen/air concentration must be kept outside the explosionrange to avoid serious damage to equipment and possible fire injuryto personnel.

(c) Hydrogen to Seal Oil Differential pressureThe seal oil pressure must be greater than the hydrogen pressure toprevent leakage of hydrogen from the generator. Again the concernis for personnel and hardware.

(d) Hydrogen to Stator Water Differential pressureTo prevent leakage of liquid water from the stator system into thehydrogen, the hydrogen pressure must be greater than the statorcooling water pressure. Liquid water inside the generatorrepresents a physical impact hazard to the spinning rotor and isalso potentially an electrical short circuit hazard if i t picks upimpurities.


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PI 30.232

7.0 HYDROGEN GAS AS A COOLANTFive advantages relating to the choice of hydrogen rather than air as acooling medium in large NGS generators are discussed very briefly below:(aJ Density

Hydrogen gas has a lower density than air, so windage losses areless and less fan power is required for circulation. This lowdensity permits higher working pressures thereby increasing heatremoval capability.(bJ Soecific Heat Caoacity

Hydrogen has approximately seven times the specific heat capacity ofair.(cJ Mass Flow

The cooling capability and, hence, the output of the machine issignificantly increased, without a corresponding increase in windagelosses, by pressurizing the hydrogen.(dJ Insylation Life

When a machine is hermetically sealed and kept free of oxygen, theinterior is less subject to contaminants. This prolongs insulationlife.(el Fire

A,fire hazard jDsjde the generator is eliminated because the pure


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PI 30.23-2

ASSIGNMENT

(Il With respect to the generator stator cooling system shown in the attacheddiagram:(a) State its purpose.(b) List and briefly explain five operational requirements of the statorcooling system.(c) Identify the numbered components.(d) Briefly explain the function of each component identified in (c).(e) Using arrows, identify the flow direction of the stator coolingwater. '

(2) With respect to the generator hydrogen seal shown in the attacheddiagram:(a) State its purpose.(b) State three operational requirements of the seal.(c) Briefly explain its operation.(d) State the flow directions of the seal oil.

(3) With respect to the generator hydrogen seal oil system shown in theattached diagram:(a) State its purpose.


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PI 30.23-2

(4) With respect to the generator hydrogen cooling system shown in theattached diagram:(a) State its purpose.(b) List and briefly explain seven operational requirements of thesystem.(c) Identify the numbered components.(d) Briefly explain the function of each component identified in (c).(e) Using arrows, identify the flow paths of the hydrogen gas.

(5) List and briefly explain six precautions related to the generator coolingsystems used with the large turbo-generators in NGS.(6) List and briefly describe five advantages and two disadvantages relatedto the use of hydrogen as a coolant in large generators in NGS.


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QUESTION nIo

PI 30.23-2

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QUESTION #2

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PI 30,23-2

INSULATION

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PI 30.23-2

QUESTIQN #3

ERVICEWATER-ROM

AFT DRIVENPUMP

lURBlNE SUPRINGEND SEAL END SEALGENERATOR ~

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I

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Generator Auxiliary Systems