dual shaft turbines

GE Power Systems

1 TWO–SHAFT GAS TURBINECONTROL & OPERATION

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TWO–SHAFT GAS TURBINE CONTROL & OPERATION

Basic Concept

Control of the gas turbine is done by the start–up,low–pressure shaft speed, exhaust temperature,low–pressure shaft acceleration, high–pressureshaft acceleration and manual control functions il-lustrated in Figure 1. Sensors monitor turbine speed,exhaust temperature, compressor discharge pres-sure and other parameters to determine the operatingconditions of the unit. When it is necessary to alterthe turbine operating conditions because of changes

in load or ambient conditions, the control systemmodulates the flow of fuel to the gas turbine. For ex-ample, if the exhaust temperature is exceeding its al-lowable value for a given operating condition, thetemperature control system reduces the fuel sup-plied to the turbine and thereby limits the exhausttemperature.

Operating conditions of the turbine are monitoredby various sensors and utilized as feedback signalsto the SPEEDTRONIC control system. There are

TNH

MANUALFSR MAN

NOZZLESTAGE

SECOND

TSRNZ

TTXM

TTRX

HPSPEED SEL

MIN

FSR

SYSTEMFUEL

FUEL

TURBINETO

TEMPERATURE

UPSTART FSRSU

TO CRT DISPLAY

TO CRT DISPLAY

FSRT

FSR ACC

FSR ACL

SPEEDLP

FSRN

TO CRT DISPLAY

GATESELECTVALUE

MINIMUM

Simplified Two–Shaft Control Schematic

Figure 1

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2TWO–SHAFT GAS TURBINECONTROL & OPERATION

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three major control loops – startup, speed and tem-perature – which may be in control during turbineoperation. The output of these control loops is con-nected to a minimum value gate circuit as shown inFigure 1. The secondary control modes of accelera-tion and manual FSR control operate in a similarmanner.

Fuel Stroke Reference (FSR) is the command signalfor fuel flow. The minimum value select gate con-nects the output signals of the six control modes tothe FSR controller; the lowest FSR output of the sixcontrol loops is allowed to pass through the gate tothe fuel control system as the controlling FSR. Thecontrolling FSR will establish the fuel input to theturbine at the rate required by the system which is incontrol. Only one control loop will be in control atany particular time and the control loop which iscontrolling FSR will be displayed at the operator in-terface.

Two–shaft gas turbines have two mechanically in-dependent turbines and rotors. Refer to Figure 2.The first–stage, or high–pressure (HP) turbine,drives the axial–flow compressor and the shaft–driven accessories; the second–stage, or low–pres-sure (LP) turbine, drives the load. The use of twomechanically–separate turbines allows the twoshafts to operate at different speeds to meet the vary-ing load requirements of the driven equipment whileallowing the high–pressure shaft to run at the designspeed of the axial–flow compressor.

A variable–area second–stage nozzle separates thehigh–pressure and low–pressure turbines. The totalenergy level/fuel flow is established by the load de-mands on the low–pressure shaft, while the energysplit between the high pressure and low pressure tur-bines is determined by the pressure drop across therespective turbines. Opening the variable–area se-cond–stage nozzle decreases the back pressure on

LP SETHP SET

BREAKAWAY DEPENDS UPON LOADGOVERNING MIN 50% � MAX 105%

MAX 100%

GOVERNING MIN 80%

SELF SUSTAINING 80%

ACCELERATE 45%

16%FIRE

FUEL

SECOND STAGE NOZZLE

LOADSTG2nd

TURBCOMPRESSOR

STG1st

TURB

Two–Shaft Turbine

Figure 2

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SECOND STAGE NOZZLE

EXHAUST TEMPERATURE

PROCESS SETPOINT

LOAD

STG2nd

TURBCOMPRESSOR

STG1st

TURB

STATUSDISPLAY

TEMPERATURE

NOZZLESPEEDHP

TSRNZMINIMUMSELECT

DISPLAYSTATUS

DISPLAYSTATUS

SETPOINTLOAD

SELECTMANUALAUTO/

SPEEDLP FSR

SYSTEMFUEL

FUEL

UPSTART

STATUSDISPLAY

GATESELECTVALUE

MINIMUM

Gas Turbine Two–Shaft Control SchematicFigure 3

the high–pressure turbine, resulting in greater pres-sure drop and more torque being generated by thehigh–pressure turbine. This is the manner in whichthe speed of the high–pressure shaft is controlled.Refer to Figure 3 which shows the relation of thecontrol modes to the gas turbines.

GE two–shaft turbines incorporate another controlloop to control high–pressure (HP) shaft speed. Thisis done by the second–stage turbine nozzle control.The second–stage nozzle control loop modulates se-

cond–stage nozzle angle to maintain HP shaft speedat or above a minimum value (the Low Speed Stopspeed), increasing HP rotor speed as exhaust tem-perature increases until the rotor is at full speed(High Speed Stop speed).

The Inlet Guide Vane control loop (not shown) mod-ulates the inlet guide vanes between their minimumfull speed angle, a nominal 56 degree angle and theirfull open position, a nominal 85 degree angle, de-

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pending on measured exhaust temperature and IGVTemperature Control selection (On or Off).

Start–up Control

The start–up control loop is an open–loop controland is connected through the minimum value selectgate to the fuel control system along with the othercontrol loops. Start–up control is designed to safelybring the gas turbine from zero speed to operatingspeed by providing the proper amount of fuel to es-tablish flame and accelerate the turbine rotors totheir minimum operating speeds. This is done in amanner as to minimize the low cycle fatigue of thehot gas path parts during the sequence. This involves

proper sequencing of command signals to the acces-sories, starting device and fuel control system. Sincea safe and successful start–up depends on properfunctioning of the gas turbine equipment, it is im-portant to verify the state of selected devices in thesequence. Much of the control logic circuitry isassociated not only with actuating control devices,but enabling protective circuits and obtaining per-missive conditions before proceeding.

General values for control settings are given in thisdescription to help in the understanding of the oper-ating system. Actual values for control settings for aparticular machine are given in the Control Specifi-cations.

NOZZLESSTAGESECOND

VALVERATIOGAS

VALVECONTROLGAS

TSRNZ

FPRG

2P

PUMPFUEL

FSR2

FSR1

EXH TEMP

FSR

GASRATIO

NOZZLEHP SPEED

SPLITTER

FSRACC

FSRT

FSRN

FSRACL

FSRSU

FSRMAN

TNH

TNH

TTXM

TNL

TNL

HPACC.

CPD

TEMP.

SPEEDLP

ACC.LP

UP

START

MANUAL

Two–Shaft Turbine Simplified Control Loops

GATESELECTVALUE

MINIMUM

Figure 4

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During the starting mode, some minor differencesare found in the sequencing checks for various de-vices. For example, in compressor drive applica-tions, the station and compressor valves must besequenced before the load compressor is permittedto turn. These checks are usually required as a per-missive to energize the starting means.

Speed Detectors

An important part of the starting sequence in start–up control of the turbine is proper speed sensing.This is necessary for the logic sequences during thestart–up and shutdown of the gas turbine. The fol-lowing speed sensors and speed relays are used ontwo–shaft turbines:

• 14HR Zero Speed – HP rotor

• 14HM Minimum Firing Speed – HP rotor

• 14HA Accelerating Speed – HP rotor

• 14HS Full Speed – HP rotor

• 14LR Zero Speed – LP rotor

• 14LS Full Speed – LP rotor

• 14SR Starting Turbine Zero Speed (Not used ondiesel or motor started units)

• 14ST Starting Turbine Full Speed (Not used ondiesel or motor started units; also certain turbinestarted units)

Zero–speed detector logic L14HR indicates whenthe high–pressure shaft starts or stops rotating.When the shaft speed is below 14HR, or at zero–speed, L14HR picks–up (fail safe) and the permis-sive logic initiates ratchet operation during theautomatic start–up/cooldown sequence of the tur-bine. The ‘L’ in L14HR represents ‘logic’; logic sig-nals, or pseudo–relays, are contained in thesequencing software and are either a 1, ‘picked–up’,or a 0, ‘dropped–out’.

Minimum speed detector logic L14HM indicatesthat the high–pressure turbine has reached the mini-mum firing speed and initiates the purge cycle priorto the introduction of fuel and ignition. The dropout

of L14HM provides several permissive functions inthe restarting of the gas turbine after shutdown.

Accelerating speed detector logic L14HA pick–upindicates when the HP turbine has reached approxi-mately 50 percent speed; this indicates that turbinestart–up is progressing and keys certain protectivefeatures.

Full–speed detector logic L14HS pick–up indicatesthe high–pressure turbine is approaching its mini-mum operating speed and that the accelerating se-quence is almost complete. This signal provides thelogic for various control sequences such as stoppingauxiliary lube oil and hydraulic oil pumps.

Zero–speed detector logic L14LR indicates whenthe low–pressure shaft starts or stops rotating. Speedlevel detector logic L14LS indicates the low–pres-sure turbine is approaching its minimum operatingspeed and indicates that the unit is ready for loading.

Two detectors are used for the starting turbine se-quencing (if applicable): 14SR to show that the start-ing turbine has stopped which is a permissive to startand 14ST to indicate the starting turbine has reachedits maximum operating speed.

The start–up control operates as an open loop con-trol using preset levels of the fuel command signalFSR. The levels are: “ZERO”, “FIRE”, “WARM–UP”, “ACCELERATE” and “MAX”. The ControlSpecifications provide proper settings calculated forthe fuel anticipated at the site. The FSR levels are setas Control Constants in the SPEEDTRONIC controlstart–up loop.

The fuel command signals are sequenced by theSPEEDTRONIC control start–up software. Thestart signal energizes the Master Control and Protec-tion circuit (the “L4” circuit) and starts the necessaryauxiliary equipment. The “L4” circuit permits pres-surization of the trip oil system and engages thestarting clutch. With the “L4” circuit permissive andthe starting clutch engaged, the starting device startsturning.

When the HP rotor ‘breaks away’ (starts to rotate),the L14HR signal de–energizes starting clutch sole-noid 20CS and shuts down the hydraulic ratchet.

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The clutch then requires torque from the starting de-vice to maintain engagement. Turbine speed relaylogic L14HM indicates that the turbine is turning atthe speed required for proper purging and ignition inthe combustors; purge timer L2TV is initiated withthe L14HM signal. The purge time is set to allowthree to four changes of air through the unit and ex-haust system to ensure that any combustible mixturehas been purged from the system. Units which haveextensive exhaust systems may have a fairly longpurge timer, but simple–cycle units usually only re-quire two minutes time.

The completion of the purge cycle (L2TVX) ‘en-ables’ fuel flow, ignition, sets firing level FSR andinitiates the firing timer (L2F). When the flame de-tector output signals indicate flame has been estab-lished in the combustors (L28FD), the warm–uptimer (L2W) starts and the fuel command signal isreduced to the “WARM–UP” FSR level. The warm–up time is provided to minimize the thermal stressesof the hot gas path parts during the initial part of thestart–up.

If flame is not established by the time the firing timertimes out, typically 60 seconds, fuel flow is halted.The unit will remain on CRANK and can be givenanother start signal, but firing will be delayed by theL2TV timer to avoid fuel accumulation in succes-sive attempts.

At the completion of the warm–up period (L2WX),the start–up control ramps FSR at a predeterminedrate to the setting for “ACCELERATE LIMIT”. Thestart–up cycle has been designed to moderate thehighest firing temperature produced during accel-eration. This is done by programming a slow rise inFSR. As fuel is increased, the turbine begins the ac-celeration phase of start–up. The clutch is held in aslong as the starting device provides torque to the gasturbine. When the turbine overruns the starting de-vice the clutch will disengage, shutting down thestarting device. Speed relay logic L14HA indicatesthe turbine is accelerating. The low–pressure turbinewill break away and start accelerating sometimeduring this phase; the actual point at which it breaksaway depends on the load.

Other control modes are also able to reduce andmodulate FSR to perform their control functionsduring the start–up phase. In the acceleration phaseof start–up, it is possible to reach the temperaturecontrol limit or shaft acceleration rate limits. Thespeed and temperature control systems will not per-mit these limits to be exceeded by controlling FSRas required. The Operator Interface always displayswhich mode is in control. When the turbine isstarted, the second–stage nozzle requires maximumnozzle area because speed and temperature are be-low their set points. The nozzle will open to placemaximum energy into the high–pressure set. Thissequence can be followed in Figure 5.

As the HP rotor speed increases, the axial–flowcompressor pumps more air and exhaust tempera-ture will stop rising and start decreasing. At approxi-mately 60–65% TNH, the turbine will pull awayfrom the starting device, disengaging the clutch andcausing the starting device to shutdown. Accelera-tion control detects whether the high–pressure orlow–pressure turbine rotors attempt to increase inspeed faster than the allowable acceleration rate of1% per second and will reduce FSR to hold that rate.In Figure 3A–5 observe how FSR is cut back as theHP and LP become more efficient towards the end ofthe start–up. The acceleration phase of start–up endswhen both HP and LP rotors are at their low speedstop speeds and Complete Sequence is reached. Atthis phase, the second–stage nozzle maintains high–pressure shaft speed at its low speed stop point andthe auxiliary pumps shut down. The LP turbine goeson speed control and is ready for loading.

Acceleration Control

Acceleration control compares the present value ofrotor speed with the last sampled value. The differ-ence between these two numbers is a measure of theacceleration and is done for both the HP and LP ro-tors. Both rotors use an allowable acceleration refer-ence of 1% increase in rotor speed per second. If theactual acceleration of the HP rotor is greater than theacceleration reference, FSRACC is reduced; if theactual acceleration of the LP rotor is greater than the

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(FSR)REFSTROKEFUEL

(TTXC)TEMPEXHAUST

(TNL)SPEEDTURBINELP

(TNR)REF.SPEEDTURBINE

(TNH)SPEEDTURBINEHP

(TSRNZ)ANGLENOZZLE

Two–Shaft Turbine Start–up Curve

Figure 5

acceleration reference, FSRACL is reduced. Eitherof these may reduce total fuel flow to the gas turbine(FSR). If, during start–up, the low–pressure rotoraccelerates too quickly, FSRACL will cut back onfuel and the unit may not come up to operatingspeed.

Speed Control

Two–shaft speed control regulates fuel flow (FSR)to maintain LP rotor speed at the desired setpoint;high–pressure shaft speed is controlled by the vari-able area second–stage nozzle.

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LP Speed Signal – TNL

Low–pressure turbine speed is measured by mag-netic sensors 77NL–1, 77NL–2 and 77NL–3. Thesemagnetic pick–up sensors are high output devicesconsisting of a permanent magnet surrounded by acoil in an hermetically sealed case. The pickups aremounted in a ring around a 60–tooth wheel on thelow–pressure rotor. With the 60–tooth wheel, thefrequency of the voltage output in hertz is equal tothe speed of the turbine in revolutions per minute.For example, 6000 rpm divided by 60 seconds in aminute is 100 revolutions per second. One hundredrevolutions per second times 60 teeth per revolutionis 6000 cycles per second. This frequency signal isfed into a pulse rate to digital converter. See Figure6. The signal then is compared to the LP turbinespeed reference (TNR). If there is any error between

these signals, FSR will be changed to eliminate thaterror. This is an isochronous–type governor control.

Clearance between the outside diameter of thetoothed wheel and the tip of the magnetic pickupshould be kept within the limits specified on theControl Specifications. If the clearance is not main-tained within the specified limits, an erroneousspeed signal could be generated and the turbinespeed control will then operate in response to the in-correct speed feedback signal.

Turbine Speed Reference – TNR

The Turbine Speed Reference (TNR) signal repre-sents the reference point for the LP speed controlloop. See Figure 6. Thus, by changing the speed ref-erence, or ‘called–for speed’, the actual speed of theLP turbine and thus load, can be changed. The opera-tor can control this raising and lowering of TNR via

FSKNG

TNR

–+

FSRMINFSRN

SELECTMEDIAN

TNL

FSRMAX

LP Speed ControlFigure 6

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raise/lower commands at the operator interface.Since load is a function of speed on a pumping com-pressor, the TNR may also be considered roughlyproportional to load.

A block diagram of the Turbine Speed Reference isshown in Figure 7. TNR comes from a Median Se-lect Gate whose inputs are:

1. Maximum limit control constant TNKR3 whichis normally 105% (high speed stop speed). This

limit is automatically raised during mechanicaloverspeed testing, but must be changed for elec-tronic overspeed testing.

2. Minimum limit control constant TNKR4 (lowspeed stop speed) which can be between 50%and 85% depending on the type and require-ments of the driven load.

3. Raise/Lower signal which adds or subtractsTNKR1_n to the last sampled value of TNR.

MIN

MAX

TNR

TNR

AT

MAX

MIN

AT

TNR

STARTPRESET

RATE SELECTL83DJn

RAISE

LOWER

SAMPLELAST

+

A=B

A

B

B

A

A=B

TNKR1_n

TNKR7

–+

TNKR4

SELECTMEDIAN

TNKR3

Turbine Speed ReferenceFigure 7

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This is how the operator or automatic load con-trol changes TNR. The ‘n’ value of L83JD_n de-cides which rate to be used. A typical rate ofchange of TNR would be 9.2%/minute; duringAuto–Load, TNKR1_4, which would equal9.2%/min, would be enabled. The Manual orFast–Load rate, TNKR_3, is typically10.0%/minute.

During start–up, TNR is pre–set to the minimumgoverning speed (50–85%) which is stored inconstant TNKR4. For auto loading from a stationprocess or remote panel, a signal is brought into theraise/lower logic to automatically increase or de-crease TNR which in turn changes the LP speed/load.

Temperature Control

The Exhaust Temperature Control System will limitfuel flow to the gas turbine to maintain the internaloperating temperatures within the design limitationsof the turbine hot gas path parts. The highest temper-ature in the gas turbine occurs in the flame zone ofthe combustion chambers. The combustion gas inthat zone is diluted by cooling air and flows into theturbine section through the first stage nozzle. Thetemperature of that gas as it exits the first stagenozzle is known as the ‘firing temperature’ of the gasturbine; it is that temperature that must be limited bythe control system. From thermodynamic relation-ships and gas turbine cycle performance calcula-tions, firing temperature can be determined as afunction of exhaust temperature and pressure ratioacross the turbine; the latter is determined by themeasured compressor discharge pressure (CPD).The temperature control system is designed to mea-sure and control turbine exhaust temperature ratherthan firing temperature because it is impractical tomeasure temperatures at the turbine inlet. This indi-rect control of turbine firing temperature is madepractical by utilizing known gas turbine aero– andthermo–dynamic characteristics to bias the exhausttemperature signal, since the exhaust temperaturealone is not a true indication of firing temperature.

Firing temperature can also be approximated as afunction of exhaust temperature and fuel flow(FSR); an FSR–biased exhaust temperature controlcurve is used as back–up to the primary CPD–biasedtemperature control curve. The two–shaft turbinetemperature control system is equipped with anadditional input, the high–pressure rotor speed(TNH) feedback, as shown in a simplified block dia-gram, Figure 8.

The two–shaft exhaust temperature reference(TTRXB) is determined by output TTR of a mini-mum select gate whose inputs are a CPD–biasedcurve, an FSR–biased curve and the Base Isother-mal. Figure 9 shows these curves. There is an addi-tional signal generated to bias TTRXB that is afunction of high–pressure rotor speed TNH. Thissignal biases the temperature control reference to ahigher value in a linear relationship with TNH andchanges the temperature reference only when thehigh–pressure shaft is operating at less than 100%rated speed. This type of control is significant onlyon regenerative– or combined–cycle turbines. Thebias sets the exhaust temperature fuel control limithigher than normal at low high–pressure rotorspeeds to attain high exhaust temperature at lowload. The exhaust temperature is higher than itwould normally be at low loads because the com-pressor is turning at less than design speed and mov-ing less air. As the exhaust temperature increasesbeyond a certain point, HP rotor speed setpointTNRH is increased from its low speed stop to a max-imum of 100% (high speed stop). The bias sets theexhaust temperature fuel control limit higher at lowHP turbine speed condition to provide increasedload pick up capability and reduce interaction be-tween exhaust temperature control of fuel (FSR) andexhaust temperature control of the second–stagenozzle. As a result of this signal, the temperature setpoint is biased to a higher value in a linear relation-ship with TNH, Figure 10. When subjected to a sud-den load increase at partial load, the temperature(fuel) control limit must have sufficient fuel marginto permit acceleration of the HP turbine. The slope isdetermined by the constant TTKRX4 which is typi-cally 8°F/%TNH.

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100% TNH

TTR

REFTEMP.

TTRXB

TTKRX4

TNH

MIN BIAS

SELECTMEDIAN

TTKRXR2

TTKRXR1

CPD

SLOPE

CORNER

ISOTHERMAL

FSR+

––

+

SAMPLE

LAST

SLOPE

SELECTMIN

CORNER

Two–Shaft Turbine Temperature Control With Speed Bias

Figure 8

SELECTMEDIAN

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EX

HA

SU

T T

EM

PE

RA

TU

RE

(T

x)

COMPRESSOR DISCHARGE PRESSURE (CPD)

ISOTHERMAL

Exhaust Temperature vs.Compressor Discharge Pressure

FUEL STROKE REFERENCE (FSR)

EX

HA

SU

T T

EM

PE

RA

TU

RE

(T

x)

ISOTHERMAL

Exhaust Temperature vs.Fuel Control Command Signal

Figure 9

Temperature Control Relationships

Constants TTKRXR1 and TTKRXR2 are used tolimit the rate of change of TTRXB in either the posi-tive or negative direction. Typical values are an in-crease of 1.5°F/second and a decrease of1.0°F/second.

Second–stage Nozzle Control

A variable area second–stage nozzle is provided ontwo–shaft gas turbines and located between thehigh–pressure stage and low–pressure stage of theturbine. The high–pressure turbine provides powerto drive the gas turbine axial–flow compressor andthe low–pressure turbine is coupled to and drives theload.

Division of power between the low–pressure andhigh–pressure turbine sections is accomplishedthrough modulation of the second–stage nozzlearea. In the full–open position, maximum power is

allotted to the high–pressure turbine. Conversely, inthe full–closed position, maximum power is di-verted to the low–pressure turbine for driving theload. Axial–flow compressors are sensitive tospeed, going into surge if not operated in the correctspeed range. Also, the output of the gas turbine is afunction of mass airflow and pressure ratio. There-fore the compressor has to be maintained in a prede-termined speed range (typically 92–100% TNH)which means that some power has to be made avail-able for the high pressure turbine. The control sys-tem modulates the second–stage nozzle angle (orarea) to maintain the HP rotor at the correct speed.

There are two exhaust system applications normallyused on heavy duty gas turbines – Simple Cycle orCombined Cycle. For simple–cycle gas turbine ap-plications, the control system is calibrated to modu-late the second–stage nozzle to hold 99 to 100%high–pressure turbine speed (TNH). In such ap-plications, the gas turbine air flow is at or near maxi-

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TEMPERATUREEXHAUST

Two–Shaft Turbine Second–Stage Nozzle ControlFigure 10

REFERENCE

TNH92% 100%

40°FDELTA–T

TTRX

TTRXB

8°F/%N

TTRXB = TTRX @ 100%TNH

TTRXB controls fuel flow (FSR)TTRX controls CSRGV and TNRH

mum and, for a given load output and site condition,the exhaust temperature is at a minimum.

For a combined–cycle (or regenerative–cycle) gasturbine application, the control system for the se-cond–stage nozzle is calibrated to run the high pres-sure turbine at reduced speed during part loadoperations; the typical speed range is 92–100%TNH. As a consequence of this lower speed opera-tion, the resultant low–load exhaust temperature ishigher due to reduced air flow. It follows also thatexhaust temperature will increase more rapidly at re-duced air flow as more output is demanded.

The high–pressure shaft speed will remain at the lowspeed stop until the exhaust temperature reaches acertain point. With further loading beyond thispoint, high–pressure turbine speed is increased, al-

lowing increased air flow to maintain an acceptableexhaust temperature. The net effect of this nozzlecontrol scheme is a higher exhaust temperature atpart–load operation, maximizing the effect of thecombined– or regenerative–cycle.

Operation

The second–stage nozzle control system is designedto regulate two parameters: high–pressure turbinespeed and exhaust temperature. Figure 11 and 12show the essential elements in the SPEEDTRONICcontrol circuit to control high–pressure speed(TNH). TSRNZ is the reference signal for the nozzleactuator and TNRH is the reference signal for HP ro-tor speed.

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TSKRNZGM

FSR MIN

GAIN

FSR

MEDIANSELECT

MAX. ANGLE

TNRH

TSRNZ

TNH

GAIN

MIN. ANGLE

SAMPLELAST

SELECTMEDIAN

Second–Stage Nozzle Stroke Reference

Figure 11

00

Reference signal TSRNZ will adjust the second–stage nozzle angle to make the HP rotor speed equalHP rotor speed reference TNRH. If the actual speedstarts to exceed the reference, the difference willcause nozzle reference TSRNZ to decrease, closingthe nozzle and depriving the high–pressure turbineof pressure drop until speed decreases. If TNH dropsbelow TNRH, TSRNZ will increase (open thenozzle) to develop more pressure drop across thehigh–pressure turbine until TNH increases to theproper value.

For combined– or regenerative–cycle operation, it isdesirable to maximize exhaust temperature andspeed setpoint TNRH will vary according to exhausttemperature. During start–up and at low loads,TNRH is at its Low Speed Stop, TNKRHLSS. Asthe the unit is loaded and exhaust temperature in-creases, the increasing exhaust temperature causesTNRH to increase, eventually reaching its HighSpeed Stop, TNKRHHSS. TNRH is low at lowloads to maintain a minimum amount of airflowthrough the unit, maximizing exhaust temperature.

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TNRH

RAISETNR

TNKRHG

TNKRHO

TTXM

TTRX

MINSELECT

A<BA

B

CNCF

A B

A

B

TNKRHLSS

B

A

BA

TNKRHLSL

TNH

TNKRHHSS

SAMPLELAST

SELECTMAX

HP Turbine Speed ReferenceFigure 12

As load and exhaust temperature increase, TNRH israised to increase HP rotor speed and increase air-flow through the unit to maintain an acceptable fir-ing temperature. To accomplish this, anotheralgorithm is used as shown in Figure 12.

The low speed stop is typically 92% TNH and thiscontrol constant (TNKRHLSS) is modified by acorrection factor, CNCF, which accounts for differ-ent air densities at different ambient conditions. Thecorrected signal enters a gate which selects the max-imum of the corrected low speed stop and a secondsignal derived from the temperature control refer-ence TTRX.

To generate this second signal, exhaust temperaturefeedback signal TTXM is compared to exhaust tem-perature limit reference TTRX. The temperature ref-erence is basically the same as that generated by thetemperature control algorithm. To ensure nozzlecontrol of exhaust temperature before fuel control ofexhaust temperature, temperature reference TTRXmay be offset by a constant (TNKRHO); this value istypically 0�F. See Figure 13.

The difference between the allowable exhaust tem-perature (reference temperature) and the actual ex-haust temperature generates an error signal which isadded to the last sampled value of TNRH and enters

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the maximum select gate. The maximum select gateselects the higher of the ambient temperature–cor-rected low speed stop speed or the exhaust tempera-ture–corrected speed setpoint TNRH. Normally, thelow speed stop speed will be selected until measuredexhaust temperature TTXM reaches reference tem-perature TTRX. The output of the max select gate isinput to a minimum select gate where it is comparedto the high speed stop speed (control constantTNKRHHSS). This is the maximum allowablespeed for the HP rotor. Once the high speed stop is

reached, TNRH will not go any higher. The highpressure turbine speed reference (TNRH) is thencompared to TNH and if there is any error, the nozzleis modulated to control the speed as previously dis-cussed. See Figure 11.

In the event FSR should be a value less than ‘mini-mum blow–out fuel’ FSRMIN, the nozzle is made togo to maximum angle (15�) to prevent the axial–flow compressor from surging. Under normal oper-ating conditions, when actual HP rotor speed TNH

CPD

FUEL LIMIT

CONTROL

TEMPERATURE

TTXM

CONTROL

NOZZLE

TNKRHO

TTKO_I

TTKO_C

TTKO_S

Nozzle–Temperature Control CurveFigure 13

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HYD. PIPING

ELEC. CONN.

LEGEND

OH

96NC–1

CLOSE OPEN

NOZZLE CONTROL RING

65NV

VALVE

DUMP

NOZZLEOLT

NOZZLE DUMP VALVE

<I/O>

SERVOTO

TANZTSRNZ

TSRNZBAK

<RST>

D/A

<RST>

77NH–123

TNH

TNRHLSS

HSS

TTXM

SHAFTHP

T.C’S.

EXHAUST

Nozzle Control SchematicFigure 14

equals reference speed TNRH, nozzle referenceangle TSRNZ should be zero.

Output TSRNZ is converted to an analog signal (seeFigure 14) and compared to the position feedbackcoming from two LVDTs (96NC–1 and –2). If there

is any error, a new signal is sent to servovalve 65NVwhich will reposition the hydraulic actuator to openor close the nozzle as required. A hydraulic dumpvalve is provided in case of a trip to ensure the nozzlegoes to the full open position to minimize powerflow to the LP rotor.

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Two–shaft Operating Characteristics

Loading

Loading may be automatically controlled by a pro-cess signal or manually increased using the Raise/Lower switches. In either case, it is turbine speedcontrol setpoint TNR which changes, creating an er-ror signal which causes FSR to increase or decreaseto maintain called–for speed.

Load will increase in accordance with the speed–load characteristics of the driven compressor. De-pending on the way nozzle control is programmed,the unit’s high pressure turbine will operate at eitherconstant or variable speed during loading.

Simple Cycle

If the turbine is expected to run without a regeneratoror waste–heat recovery equipment, it will be set upfor simple cycle operation with the high–pressureturbine always running at design speed, even at partload. This is justified mainly because of reduced fir-ing temperature at part load. There is no significantdifference in part load efficiency operating withvariable high–pressure set speed.

Essentially constant speed control is accomplishedby setting the HP rotor low speed stop at 99% andhigh speed stop at 100%. The load characteristic fora simple cycle unit is shown in Figure 15. Startupwill have brought the high–pressure set to the lowspeed stop.

During the loading phase of operation, the speed ofthe low–pressure turbine and load compressor andthe power output of the turbine are increased accord-ing to the speed–load characteristic of the drivenload. The gas turbine uses the low–pressure turbinespeed as an indicator of load; low–pressure speedsetpoint TNR may be increased or decreased auto-matically by the station process signal or manuallyat the turbine control panel. As more load compres-sor output is required, the low–pressure speed set-point is increased. The other turbine parameters

respond to the Turbine Speed Reference (TNR) sig-nal.

Figure 15 is intended to show the direction of param-eter change during the load cycle only. Actual mag-nitudes are dependent on load compressorcharacteristics, pipeline pressure and flows and siteconditions.

An increase in the TNR setpoint, either automatical-ly or manually, results in a difference between theactual low–pressure rotor speed and the low–pres-sure rotor speed setpoint. Fuel command FSR is in-creased to accelerate the low–pressure turbine to theset–point speed. The increased fuel flow causes thehigh–pressure rotor speed to increase, but the se-cond–stage nozzle operates to decrease the pressuredrop across the high–pressure turbine and maintainhigh–pressure turbine speed at a constant level.

The second–stage nozzle will continue to hold thehigh–pressure turbine speed constant as fuel flow in-creases until exhaust temperature limit TTRX isreached. The nozzle will then operate to increase thehigh–pressure turbine speed from the low speed stopspeed to the high speed stop speed to hold exhausttemperature at this level; this is “Nozzle Tempera-ture Control”. This point will be somewhat belowthe limit for exhaust temperature control of fuel. Af-ter the HP rotor has reached full speed, continuedloading will then cause exhaust temperature to reachtemperature control limit TTRXB and fuel flow willbe stopped from increasing. The exhaust tempera-ture control circuit will then modulate FSR to main-tain an acceptable exhaust temperature. Once onexhaust temperature fuel control, the TNR signalcan no longer act to increase FSR. Theoretically, theexhaust temperature control point for fuel will bereached as the high pressure rotor attains 100% de-sign speed.

The same sequence occurs, in reverse, during un-loading. See Figure 18.

Combined Cycle

For a combined– or regenerative–cycle gas turbineapplication, the control system for the second–stage

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FSRTTXC

TNL

TNR

TNH

(TSRNZ)ANGLENOZZLE

Two–Shaft Turbine Simple–Cycle LoadingFigure 15

nozzle is calibrated to run the high pressure turbineat reduced speed during part load operations; thetypical speed range is 92–100% TNH. As a conse-quence, for a given low–load output and site condi-tion, the resultant exhaust temperature is higher dueto reduced air flow.

The high–pressure shaft speed will remain at the lowspeed stop until the exhaust temperature reaches acertain point. With further loading beyond thispoint, the compressor speed or high–pressure tur-bine speed is allowed to increase air flow to maintain

an acceptable exhaust temperature. The net effect ofthis nozzle control scheme is a higher exhaust tem-perature at part–load operation, maximizing the ef-fect of the regenerative– or combined–cycle.

Turbine loading begins as it did in the simple–cyclecase as shown in Figure 15. The nozzle will closewhen necessary to hold the HP rotor at the low speedstop. When the exhaust temperature reaches thenozzle temperature control setpoint, the nozzle willmodulate HP rotor speed to avoid exceeding thattemperature setpoint. With continued loading, HP

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rotor speed increases to keep the exhaust tempera-ture from increasing; this requires directing slightlymore power to the HP turbine. Eventually, thenozzle control runs HP rotor speed setpoint TNRHto its high speed stop and the HP rotor will be operat-ing at design speed (100% TNH). As load continuesto increase after the HP rotor is at design speed, ex-haust temperature will increase until the “Fuel Tem-perature Control” exhaust temperature limit isreached. At the point, FSR will not increase further.

Variable Inlet Guide Vanes

The variable inlet guide vane control loop is addedto the SPEEDTRONIC control system to providetwo functions:

1. Avoid compressor pulsation by closing the inletguide vanes to the low–flow position duringstart–up.

2. Increase the part–load thermal efficiency of theunit by modulating air flow to the axial–flowcompressor to maximize exhaust temperature.This is used only for combined– or regenera-tive–cycle operation. The 92% minimum speedof the high–pressure shaft limits the ability ofthe nozzle control to maximize part–load ex-

haust temperature by itself. Both the variable in-let guide vanes and the variable second–stagenozzle are controlled to maintain maximum ex-haust temperature for part–load combined–cycle efficiency.

A typical start–up curve is shown in Figure 16. Ascan be seen, it is very similar to the previously dis-cussed simple cycle start–up. The VIGVs will mod-ulate open from the full closed angle of 42� tominimum full speed angle of 56�. The rate of changeis based on the corrected H.P. speed curve. This is toavoid compressor pulsation. Once loading begins asshown in Figure 3A–17 the exhaust temperature(TTXC) will reach the IGV temperature setpoint(usually 30�F/16.6�C below fuel temperature set-point). The IGVs will then start to modulate to main-tain this exhaust temperature. When the IGVs reachtheir limit of 85� angle, they stop modulating. If theunit continues to load, the exhaust temperature thenwill continue to increase to the nozzle temperaturecontrol point. The nozzle will then start to open tomaintain this temperature. This allows the HP rotorspeed to increase and, if loading continues, to reachthe high speed stop; the nozzle will then modulate tomaintain 100% TNH. As loading continues, the ex-haust temperature will reach the fuel temperaturecontrol setpoint and loading will end.

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FSR

FSR

TTXC

TNL

TNR

TNH

TSRNZ

Two–Shaft Turbine Simple–Cycle UnloadingFigure 16

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TNL

ANGLEFSR

TTXC

TNL

IGV

TNH

TSRNZ

Two–Shaft Turbine Combined–Cycle Start–upFigure 17

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ANGLE

FSR

TTXC

TNL

IGV

TNH

TSRNZ

Two–Shaft Turbine Combined–Cycle LoadingFigure 18

General Electric CompanyOne River RoadSchenectady, NY 12345

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dual shaft turbines

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