49793809-fund-mk-vi

GE Power Systems

1 FUNDAMENTALS OF SPEEDTRONICMARK VI CONTROL SYSTEM

Fund_Mk_VI

FUNDAMENTALS OF SPEEDTRONICMARK VI CONTROL SYSTEM

SPEEDTRONIC Mark VI Control contains anumber of control, protection and sequencing sys-tems designed for reliable and safe operation of thegas turbine. It is the objective of this chapter to de-scribe how the gas turbine control requirements aremet, using simplified block diagrams and one–linediagrams of the SPEEDTRONIC Mark VI control,protection, and sequencing systems. A generatordrive gas turbine is used as the reference.

CONTROL SYSTEM

Basic Design

Control of the gas turbine is done by the startup, ac-

celeration, speed, temperature, shutdown, andmanual control functions illustrated in Figure 1.Sensors monitor turbine speed, exhaust tempera-ture, compressor discharge pressure, and other pa-rameters to determine the operating conditions ofthe unit. When it is necessary to alter the turbine op-erating conditions because of changes in load or am-bient conditions, the control modulates the flow offuel to the gas turbine. For example, if the exhausttemperature tends to exceed its allowable value for agiven operating condition, the temperature controlsystem reduces the fuel supplied to the turbine andthereby limits the exhaust tempera-ture.

TEMPERATURE

SPEED

TO CRT DISPLAY

FUEL

TO TURBINE

FSR

FUELSYSTEMMINIMUM

ACCELERATIONRATE

STARTUP

SHUTDOWN

MANUAL

TO CRTDISPLAY

TO CRT DISPLAY

VALUESELECTLOGIC

Figure 1 Simplified Control Schematic

id0043

Operating conditions of the turbine are sensed andutilized as feedback signals to the SPEEDTRONICcontrol system. There are three major control loops –startup, speed, and temperature – which may be incontrol during turbine operation. The output of thesecontrol loops is connected to a minimum value gatecircuit as shown in Figure 1. The secondary control

modes of acceleration, manual FSR, and shutdownoperate in a similar manner.

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 six

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2FUNDAMENTALS OF SPEEDTRONICMARK VI CONTROL SYSTEM

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Figure 2 Block Diagram – Control Schematic

TTXM

TTRX

FSRSU FSR

MIN

FSRACC

FSRMAN

FSRSD

FSRN

FSRT

TNRI

TNR

FSRSU

FSR

TNH

TNHAR

FSRMIN

LOGIC

CQTC

FSRACC

LOGIC

FSRC

FSR

FSRMIN

FSRSD

FSRMANLOGIC

FSRC

TNHAR

FSRMIN

FSRN

LOGIC

TNH

TNHCOR

CQTC

<R><S><T>START-UPCONTROL

<R><S><T>ACCELERATIONCONTROL

<R><S><T>MANUAL FSR

<R><S><T>SHUTDOWNCONTROL

FSR

GATE

SPEED CONTROL <R><S><T>LOGIC

LOGIC

LOGIC TNRI

PR/D

TEMPERATURE CONTROL

LOGIC

<R><S><T>

<R><S><T>

FSRT

<R><S><T>LOGIC

FSR

TTXM

TTRX

TTXD

FSR

TTXD

96CD

TNH

TNR

MEDIAN

id0038V

ISOCHRONOUSONLY

77NH

TTURVTUR

A/D

A/D

TBTCVTCC

TBAIVAIC

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control 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 on the <HMI>.

Figure 2 shows a more detailed schematic of thecontrol loops. This can be referenced during the ex-planation of each loop to show the interfacing.

Start–up/Shutdown Sequence and Control

Start–up control brings the gas turbine from zerospeed up to operating speed safely by providingproper fuel to establish flame, accelerate the turbine,and to do it in such a manner as to minimize the lowcycle fatigue of the hot gas path parts during the se-quence. This involves proper sequencing of com-mand signals to the accessories, starting device andfuel control system. Since a safe and successfulstart–up depends on proper functioning of the gasturbine equipment, it is important to verify the stateof selected devices in the sequence. Much of thecontrol logic circuitry is associated not only with ac-tuating control devices, but enabling protective cir-cuits and obtaining permissive conditions beforeproceeding.

The gas turbine uses a static start system wherebythe generator serves as a starting motor. A turninggear is used for rotor breakaway.

General values for control settings are given in thisdescription to help in the understanding of the oper-ating system. Actual values for control settings aregiven in the Control Specifications for a particularmachine.

Speed Detectors

An important part of the start–up/shutdown se-quence control of the gas turbine is proper speedsensing. Turbine speed is measured by magneticpickups and will be discussed under speed control.

The following speed detectors and speed relays aretypically used:

–L14HR Zero–Speed (approx. 0% speed)

–L14HM Minimum Speed (approx. 16%speed)

–L14HA Accelerating Speed (approx. 50%speed)

–L14HS Operating Speed (approx. 95%speed)

The zero–speed detector, L14HR, provides the sig-nal when the turbine 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 turning gear or slow–rolloperation during the automatic start–up sequence ofthe turbine.

The minimum speed detector L14HM indicates thatthe turbine has reached the minimum firing speedand initiates the purge cycle prior to the introductionof fuel and ignition. The dropout of the L14HMminimum speed relay provides several permissivefunctions in the restarting of the gas turbine aftershutdown.

The accelerating speed relay L14HA pickup indi-cates when the turbine has reached approximately50 percent speed; this indicates that turbine start–upis progressing and keys certain protective features.

The high–speed sensor L14HS pickup indicateswhen the turbine is at speed and that the acceleratingsequence is almost complete. This signal providesthe logic for various control sequences such as stop-ping auxiliary lube oil pumps and starting turbineshell/exhaust frame blowers.

Should the turbine and generator slow during an un-derfrequency situation, L14HS will drop out at theunder–frequency speed setting. After L14HS dropsout the generator breaker will trip open and the Tur-bine Speed Reference (TNR) will be reset to100.3%. As the turbine accelerates, L14HS willagain pick up; the turbine will then require anotherstart signal before the generator will attempt to auto–synchronize to the system again.

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The actual settings of the speed relays are listed inthe Control Specification and are programmed in the<RST> processors as EEPROM control constants.

START–UP CONTROL

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 MarkVI start–up control.

Start–up control FSR signals operate through theminimum value gate to ensure that other controlfunctions can limit FSR as required.

The fuel command signals are generated by theSPEEDTRONIC control start–up software. In addi-tion to the three active start–up levels, the softwaresets maximum and minimum FSR and provides formanual control of FSR. Clicking on the targets for“MAN FSR CONTROL” and “FSR GAG RAISE

OR LOWER” allows manual adjustment of FSRsetting between FSRMIN and FSRMAX.

While the turbine is at rest, electronic checks aremade of the fuel system stop and control valves, theaccessories, and the voltage supplies. At this time,“SHUTDOWN STATUS” will be displayed on the<HMI>. Activating the Master Operation Switch(L43) from “OFF” to an operating mode will acti-vate the ready circuit. If all protective circuits andtrip latches are reset, the “STARTUP STATUS” and“READY TO START” messages will be displayed,indicating that the turbine will accept a start signal.Clicking on the “START” Master Control Switch(L1S) and “EXECUTE” will introduce the start sig-nal to the logic sequence.

The start signal energizes the Master Control andProtection circuit (the “L4” circuit) and starts thenecessary auxiliary equipment. The “L4” circuitpermits pressurization of the trip oil system. Withthe “L4” circuit permissive and starting clutch auto-matically engaged, the starting device starts turning.Startup status message “STARTING” will be dis-played on the <HMI>. See point “A” on the TypicalStart–up Curve Figure3.

100

80

60

40

20

0

APPROXIMATE TIME – MINUTES

IGNITION &CROSSFIRE

STARTAUXILIARIES &

DIESEL WARMUP

PURGE COAST

DOWN

WARMUP

1 MIN

ACCELERATE

SPEED – %

IGV – DEGREES

FSR – %

Tx – °F/10

Figure 3 Mark VI Start-up Curve

id0093A B

C

D

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The starting clutch is a positive tooth type overrun-ning clutch which is self–engagifng in the break-away mode and overruns whenever the turbine rotorexceeds the turning gear speed.

When the turbine ‘breaks away’ the turning gear willrotate the turbine rotor from 5 to 7 rpm. As the staticstarter begins it’s sequence, and accelerates the rotorthe starting clutch will automatically disengage theturning gear from the turbine rotor. The turbinespeed relay L14HM indicates that the turbine is turn-ing at the speed required for proper purging and igni-tion in the combustors. Gas fired units that haveexhaust configurations which can trap gas leakage(i.e., boilers) have a purge timer, L2TV, which is ini-tiated with the L14HM signal. The purge time is setto allow three to four changes of air through the unitto ensure that any combustible mixture has beenpurged from the system. The starting means willhold speed until L2TV has completed its cycle.Units which do not have extensive exhaust systemsmay not have a purge timer, but rely on the startingcycle and natural draft to purge the system.

The L14HM signal or completion of the purge cycle(L2TVX) ‘enables’ fuel flow, ignition, sets firinglevel FSR, and initiates the firing timer L2F. Seepoint “B” on Figure 3. When the flame detector out-put signals indicate flame has been established in thecombustors (L28FD), the warm–up timer L2Wstarts and the fuel command signal is reduced to the“WARM–UP” FSR level. The warm–up time is pro-vided to minimize the thermal stresses of the hot gaspath parts during the initial part of the start–up.

If flame is not established by the time the L2F timertimes out, typically 60 seconds, fuel flow is halted.The unit can be given another start signal, but firingwill be delayed by the L2TV timer to avoid fuel ac-cumulation in successive attempts. This sequenceoccurs even on units not requiring initial L2TVpurge.

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. See point “C” on Figure 3. As fuel is increased,the turbine begins the acceleration phase of start–up.The clutch is held in as long as the turning gear pro-vides torque to the gas turbine. When the turbineoverruns the turning gear, the clutch will disengage,shutting down the turning gear. Speed relay L14HAindicates the turbine is accelerating.

The start–up phase ends when the unit attains full–speed–no–load (see point “D” on Figure 3). FSR isthen controlled by the speed loop and the auxiliarysystems are automatically shut down.

The start–up control software establishes the maxi-mum allowable levels of FSR signals during start–up. As stated before, other control circuits are able toreduce and modulate FSR to perform their controlfunctions. In the acceleration phase of the start–up,FSR control usually passes to acceleration control,which monitors the rate of rotor acceleration. It ispossible, but not normal, to reach the temperaturecontrol limit. The <HMI> display will show whichparameter is limiting or controlling FSR.

Fired Shutdown

A normal shutdown is initiated by clicking on the“STOP” target (L1STOP) and “EXECUTE”; thiswill produce the L94X signal. If the generator break-er is closed when the stop signal is initiated, the Tur-bine Speed Reference (TNR) counts down to reduceload at the normal loading rate until the reverse pow-er relay operates to open the generator breaker; TNRthen continues to count down to reduce speed. Whenthe STOP signal is given, shutdown Fuel Stroke Ref-erence FSRSD is set equal to FSR.

When the generator breaker opens, FSRSD rampsfrom existing FSR down to a value equal toFSRMIN, the minimum fuel required to keep theturbine fired. FSRSD latches onto FSRMIN and de-creases with corrected speed. When turbine speeddrops below a defined threshold (Control ConstantK60RB) FSRSD ramps to a blowout of one flamedetector. The sequencing logic remembers whichflame detectors were functional when the breakeropened. When any of the functional flame detectors

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senses a loss of flame, FSRMIN/FSRSD decreasesat a higher rate until flame–out occurs, after whichfuel flow is stopped.

Fired shut down is an improvement over the formerfuel shut off at L14HS drop out. By maintainingflame down to a lower speed there is significant re-duction in the strain developed on the hot gas pathparts at the time of fuel shut off.

SPEED CONTROL

The Speed Control System controls the speed andload of the gas turbine generator in response to theactual turbine speed signal and the called–for speedreference. While on speed control the control modemessage “SPEED CTRL”will be displayed.

Speed Signal

Three magnetic sensors are used to measure thespeed of the turbine. These magnetic pickup sensors(77NH–1,–2,–3) are high output devices consistingof a permanent magnet surrounded by a hermeticallysealed case. The pickups are mounted in a ringaround a 60–toothed wheel on the gas turbine com-pressor rotor. With the 60–tooth wheel, the frequen-cy of the voltage output in Hertz is exactly equal tothe speed of the turbine in revolutions per minute.

The voltage output is affected by the clearance be-tween the teeth of the wheel and the tip of the mag-netic pickup. Clearance between the outsidediameter of the toothed wheel and the tip of the mag-netic pickup should be kept within the limits speci-fied in the Control Specifications (approx. 0.05 inchor 1.27 mm). If the clearance is not maintained with-in the specified limits, the pulse signal can be dis-torted. Turbine speed control would then operate inresponse to the incorrect speed feedback signal.

The signal from the magnetic pickups is brought intothe Mark VI panel, one mag pickup to each control-ler <RST>, where it is monitored by the speed con-trol software.

Speed/Load Reference

The speed control software will change FSR in pro-portion to the difference between the actual turbine–generator speed (TNH) and the called–for speedreference (TNR).

The called–for–speed, TNR, determines the load ofthe turbine. The range for generator drive turbines isnormally from 95% (min.) to 107% (max.) speed.The start–up speed reference is 100.3% and is presetwhen a “START” signal is given.

FU

LL

SP

EE

D N

O L

OA

D F

SR

MIN

IMU

M F

SR

MA

X F

SR

RA

TE

D F

SR

LOW SPEED STOP

“FSNL”

SP

EE

DR

EF

ER

EN

CE

% (

TN

R)

104

100

95

FUEL STROKE REFERENCE (LOAD)(FSR)

HIGH SPEED STOP

TNR MIN.

TNR MAX.

Figure 4 Droop Control Curve

107

id0044

The turbine follows to 100.3% TNH for synchro-nization. At this point the operator can raise or lowerTNR, in turn raising or lowering TNH, via the70R4CS switch on the generator control panel or byclicking on the targets on the <HMI>, if required.Refer to Figure 4. Once the generator breaker isclosed onto the power grid, the speed is heldconstant by the grid frequency. Fuel flow in excessof that necessary to maintain full speed no load willresult in increased power produced by the generator.Thus the speed control loop becomes a load controlloop and the speed reference is a convenient control

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of the desired amount of load to be applied to the tur-bine–generator unit.

Droop speed control is a proportional control,changing FSR in proportion to the difference be-tween actual turbine speed and the speed reference.Any change in actual speed (grid frequency) willcause a proportional change in unit load. This pro-portionality is adjustable to the desired regulation or“Droop”. The speed vs. FSR relationship is shownon Figure 4.

If the entire grid system tends to be overloaded, gridfrequency (or speed) will decrease and cause an FSRincrease in proportion to the droop setting. If all

units have the same droop, all will share a load in-crease equally. Load sharing and system stability arethe main advantages of this method of speed control.

Normally 4% droop is selected and the setpoint iscalibrated such that 104% setpoint will generate aspeed reference which will produce an FSR result-ing in base load at design ambient temperature.

When operating on droop control, the full–speed–no–load FSR setting calls for a fuel flow which issufficient to maintain full speed with no generatorload. By closing the generator breaker and raisingTNR via raise/lower, the error between speed andreference is increased. This error is multiplied by a

Figure 5 Speed Control Schematic

FSNL

TNRSPEEDREFERENCE

TNHSPEED

DROOP

ERRORSIGNAL

SPEED CONTROL

<RST>

FSRN+

–

SPEED CHANGER LOAD SET POINT

MEDIANSELECT

TNR

SPEEDREFERENCE

MIN.

MAX. LIMIT

PRESET

OPERATING

<RST>

L83SDRATE

L70RRAISE

L70LLOWER

L83PRESPRESETLOGIC

START-UP

OR SHUTDOWN

L83TNROPMIN. SELECT LOGIC

++

id0040

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gain constant dependent on the desired droop settingand added to the FSNL FSR setting to produce therequired FSR to take more load and thus assist inholding the system frequency. Refer to Figures 4 and5.

The minimum FSR limit (FSRMIN) in the SPEED-TRONIC Mark VI system prevents the speed con-trol circuits from driving the FSR below the valuewhich would cause flameout during a transientcondition. For example, with a sudden rejection ofload on the turbine, the speed control system loopwould want to drive the FSR signal to zero, but theminimum FSR setting establishes the minimum fuellevel that prevents a flameout. Temperature and/or

start–up control can drive FSR to zero and are not in-fluenced by FSRMIN.

Synchronizing

Automatic synchronizing is accomplished usingsynchronizing algorithms programmed into <RST>and <VPRO> software. Bus and generator voltagesignals are input to the <VPRO> core which con-tains isolation transformers, and are then paralleledto <RST>. <RST> software drives the synch checkand synch permissive relays, while <VPRO> pro-vides the actual breaker close command. See Figure6.

<RST>

<XYZ>

AUTO SYNCH

AND

L25

BREAKERCLOSE

AND

AUTO SYNCHPERMISSIVE

L83ASAUTO SYNCHPERMISSIVE

A

B

A>B

A

B

A>B

REF

REF

GEN VOLTS

LINE VOLTS

Figure 6 Synchronizing Control Schematic

id0048V

CALCULATED PHASE WITHIN LIMITS

CALCULATED SLIP WITHIN LIMITS

CALCULATED ACCELERATION

CALCULATED BREAKER LEAD TIME

There are three basic synchronizing modes. Thesemay be selected from external contacts, i.e., genera-tor panel selector switch, or from the SPEEDTRON-IC Mark VI <HMI>.

1. OFF – Breaker will not be closed by SPEED-TRONIC Mark VI control

2. MANUAL – Operator initiated breaker closurewhen permissive synch check relay 25X is satis-fied

3. AUTO – System will automatically match volt-age and speed and then close the breaker at theappropriate time to hit top dead center on thesynchroscope

For synchronizing, the unit is brought to 100.3%speed to keep the generator “faster” than the grid, as-suring load pick–up upon breaker closure. If the sys-tem frequency has varied enough to cause anunacceptable slip frequency (difference betweengenerator frequency and grid frequency), the speedmatching circuit adjusts TNR to maintain turbinespeed 0.20% to 0.40% faster than the grid to assurethe correct slip frequency and permit synchronizing.

For added protection a synchronizing check relay isprovided in the generator panel. It is used in serieswith both the auto synchronizing relay and themanual breaker close switch to prevent large out–of–phase breaker closures.

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ACCELERATION CONTROL

Acceleration control compares the present value ofthe speed signal with the value at the last sampletime. The difference between these two numbers is ameasure of the acceleration. If the actual accelera-tion is greater than the acceleration reference,FSRACC is reduced, which will reduce FSR, andconsequently the fuel to the gas turbine. Duringstart–up the acceleration reference is a function ofturbine speed; acceleration control usually takesover from speed control shortly after the warm–upperiod and brings the unit to speed. At “CompleteSequence”, which is normally 14HS pick–up, theacceleration reference is a Control Constant, nor-mally 1% speed/second. After the unit has reached100% TNH, acceleration control usually serves onlyto contain the unit’s speed if the generator breakershould open while under load.

EX

HA

SU

T T

EM

PE

RA

TU

RE

(T

x)

COMPRESSOR DISCHARGE PRESSURE (CPD)

ISOTHERMAL

Figure 7 Exhaust Temperature vs.Compressor Discharge Pressure

id0045

TEMPERATURE CONTROL

The Temperature Control System will limit fuelflow to the gas turbine to maintain internal operatingtemperatures within design limitations of turbinehot gas path parts. The highest temperature in the gas

turbine occurs in the flame zone of the combustionchambers. The combustion gas in that zone is di-luted by cooling air and flows into the turbine sec-tion through the first stage nozzle. The temperatureof that gas as it exits the first stage nozzle is known asthe “firing temperature” of the gas turbine; it is thistemperature that must be limited by the control sys-tem. From thermodynamic relationships, gas tur-bine cycle performance calculations, and known siteconditions, firing temperature can be determined asa function of exhaust temperature and the pressureratio across the turbine; the latter is determined fromthe measured 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 directly in the combustionchambers or at the turbine inlet. This indirect controlof turbine firing temperature is made practical byutilizing known gas turbine aero– and thermo–dy-namic characteristics and using those to bias the ex-haust temperature signal, since the exhausttemperature alone is not a true indication of firingtemperature.

Firing temperature can also be approximated as afunction of exhaust temperature and fuel flow (FSR)and as a function of exhaust temperature and genera-tor output (DWATT). Either FSR or megawatt ex-haust temperature control curves are used asback–up to the primary CPD–biased temperaturecontrol curve.

These relationships are shown on Figures 7 and 8.The lines of constant firing temperature are used inthe control system to limit gas turbine operatingtemperatures, while the constant exhaust tempera-ture limit protects the exhaust system during start–up.

Exhaust Temperature Control Hardware

Chromel–Alumel exhaust temperature thermocou-ples are used and, typically 27 in number. Thesethermocouples circumferentially inside the exhaustdiffuser. They have individual radiation shields thatallow the radial outward diffuser flow to pass over

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FUEL STROKE REFERENCE (FSR)

EX

HA

SU

T T

EM

PE

RA

TU

RE

(T

x)

ISOTHERMAL

Figure 8 Exhaust Temperature vs. FuelControl Command Signal

id0046

these 1/16” diameter (1.6mm) stainless steelsheathed thermocouples at high velocity, minimiz-ing the cooling effect of the longer time constant,cooler plenum walls. The signals from these individ-ual, ungrounded detectors are sent to the SPEED-TRONIC Mark VI control panel through shieldedthermocouple cables and are divided amongst con-trollers <RST>.

Exhaust Temperature Control Software

The software contains a series of application pro-grams written to perform the exhaust temperaturecontrol and monitoring functions such as digital andanalog input scan. A major function is the exhausttemperature control, which consists of the followingprograms:

1. Temperature control command

2. Temperature control bias calculations

3. Temperature reference selection

The temperature control software determines thecold junction compensated thermocouple readings,selects the temperature control setpoint, calculatesthe control setpoint value, calculates the representa-

tive exhaust temperature value, compares this valuewith the setpoint, and then generates a fuel com-mand signal to the analog control system to limit ex-haust temperature.

Temperature Control Command Program

The temperature control command programcompares the exhaust temperature control setpointwith the measured gas turbine exhaust temperatureas obtained from the thermocouples mounted in theexhaust plenum; these thermocouples are scannedand cold junction corrected by programs describedlater. These signals are accessed by <RST>. Thetemperature control command program in <RST>(Figure 9) reads the exhaust thermocouple tempera-ture values and sorts them from the highest to thelowest. This array (TTXD2) is used in the combus-tion monitor program as well as in the TemperatureControl Program. In the Temperature Control Pro-gram all exhaust thermocouple inputs are monitoredand if any are reading too low as compared to aconstant, they will be rejected. The highest and low-est values are then rejected and the remaining valuesare averaged, that average being the TTXM signal.

If a Controller should fail, this program will ignorethe readings from the failed Controller. The TTXMsignal will be based on the remaining Controllers’thermocouples and an alarm will be generated.

The TTXM value is used as the feedback for the ex-haust temperature comparator because the value isnot affected by extremes that may be the result offaulty instrumentation. The temperature–control–command program in <RST> compares the exhausttemperature control setpoint (calculated in the tem-perature–control–bias program and stored in thecomputer memory) TTRXB to the TTXM value todetermine the temperature error. The software pro-gram converts the temperature error to a fuel strokereference signal, FSRT.

Temperature Control Bias Program

Gas turbine firing temperature is determined by themeasured parameters of exhaust temperature and

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SORTHIGHEST

TOLOWEST

AVERAGEREMAINING

REJECTHIGHANDLOW

REJECTLOWTC’s

TTXDR

TTXDS

TTXDT

TTXD2

TTXM

QUANTITY

<RST>

TOCOMBUSTIONMONITOR

OF TC’s USED

TEMPERATURE CONTROL

MEDIANSELECT

FSRMIN

FSRMAX

TTRXB

TTXM

GAIN

FSR

<RST>

FSRT

Figure 9 Temperature Control Schematic

id0032V

++

.

TEMPERATURECONTROL

REFERENCE

MINSELECT

CORNER

CPD

SLOPE

ISOTHERMAL

FSR

SLOPE

CORNER

<RST>

compressor discharge pressure (CPD) or exhausttemperature and fuel consumption (proportional toFSR). In the computer, firing temperature is limitedby a linearized function of exhaust temperature andCPD backed up by a linearized function of exhausttemperature and FSR (See Figure 8). The tempera-ture control bias program (Figure 10) calculates theexhaust temperature control setpoint TTRXB basedon the CPD data stored in computer memory andconstants from the selected temperature–referencetable. The program calculates another setpoint basedon FSR and constants from another temperature–reference table.

Figure 11 is a graphical illustration of the control set-points. The constants TTKn_C (CPD bias corner)and TTKn_S (CPD bias slope) are used with theCPD data to determine the CPD bias exhaust tem-

DIGITALINPUTDATA

SELECTEDTEMPERATURE

REFERENCETABLE

CONSTANTSTORAGE

COMPUTERMEMORY

TEMPERATURECONTROL

BIASPROGRAM

COMPUTERMEMORY

Figure 10 Temperature Control Bias

id0023

perature setpoint. The constants TTKn_K (FSR biascorner) and TTKn_M (FSR bias slope) are used withthe FSR data to determine the FSR bias exhaust tem-perature setpoint. The values for these constants are

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given in the Control Specifications–Control SystemSettings drawing. The temperature–control–biasprogram also selects the isothermal setpointTTKn_I. The program selects the minimum of thethree setpoints, CPD bias, FSR bias, or isothermalfor the final exhaust temperature control reference.During normal operation with gas or light distillatefuels, this selection results in a CPD bias controlwith an isothermal limit, as shown by the heavy lineson Figure 11. The CPD bias setpoint is comparedwith the FSR bias setpoint by the program and analarm occurs when the CPD setpoint is higher. Forunits operating with heavy fuel, FSR bias controlwill be selected to minimize the effect of turbinenozzle plugging on firing temperature. The FSR biassetpoint will then be compared with the CPD biassetpoint and an alarm will occur when the FSR set-point exceeds the CPD setpoint. A ramp function isprovided in the program to limit the rate at which thesetpoint can change. The maximum and minimumchange in ramp rates (slope) are programmed inconstants TTKRXR1 and TTKRXR2. Consult theControl Sequence Program (CSP) and the ControlSpecifications drawing for the block diagram il-lustration of this function and the value of theconstants. Typical rate change limit is 1.5°F per se-cond. The output of the ramp function is the exhausttemperature control setpoint which is stored in thecomputer memory.

Figure 11 Exhaust Temperature Control Setpoints

EX

HA

US

T T

EM

PE

RA

TU

RE

CPDFSR

TTKn_C

ISOTHERMALTTKn_K

TTKn_I

id0054

Temperature Reference Select Program

The exhaust temperature control function selectscontrol setpoints to allow gas turbine operation atvarious firing temperatures. The temperature–refer-ence–select program (Figure 12) determines the op-erational level for control setpoints based on digitalinput information representing temperature controlrequirements. Three digital input signals are de-coded to select one set of constants which define thecontrol setpoints necessary to meet those require-ments. A typical digital signal is “BASE SELECT”,selected by clicking on the appropriate target on theoperator interface <HMI>.

FUEL CONTROL SYSTEM

The gas turbine fuel control system will change fuelflow to the combustors in response to the fuel strokereference signal (FSR). FSR actually consists of twoseparate signals added together, FSR1 being thecalled–for liquid fuel flow and FSR2 being thecalled–for gas fuel flow; normally, FSR1 + FSR2 =FSR. Standard fuel systems are designed for opera-tion with liquid fuel and/or gas fuel. This chapterwill describe a dual fuel system. It starts with the ser-vo drive system, where the setpoint is comparedwith the feedback signal and converted to a valveposition. It will describe liquid, gas and dual fuel op-eration and how the FSR from the control systemspreviously described is conditioned and sent as a setpoint to the servo system.

DIGITALINPUT DATA

CONSTANTSTORAGE

TEMPERATUREREFERENCE

SELECT

SELECTEDTEMPERATURE

Figure 12 Temperature Reference Select Program

id0106

REFERENCETABLE

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Fund_Mk_VI

Servo Drive System

The heart of the fuel system is a three coil electro–hydraulic servovalve (servo) as shown in Figure 13.The servovalve is the interface between the electri-cal and mechanical systems and controls the direc-tion and rate of motion of a hydraulic actuator basedon the input current to the servo.

Â

3-COIL TORQUE MOTOR

TORQUE

FORCEFEEDBACKSPRING

SPOOL VALVE

1350 PSI

HYDRAULICACTUATOR

TO <RST> LVDT

DRAIN PS

TORQUEMOTOR

JET TUBE

FAILSAFEBIASSPRING

MOTORARMATURE

P

1 2

N N

S S

R P

id0029

FILTER

��

Figure 13 Electrohydraulic Servovalve

The servovalve contains three electrically isolatedcoils on the torque motor. Each coil is connected toone of the three Controllers <RST>. This providesredundancy should one of the Controllers or coilsfail. There is a null–bias spring which positions theservo so that the actuator will go to the fail safe posi-tion should ALL power and/or control signals belost.

If the hydraulic actuator is a double–action piston,the control signal positions the servovalve so that itports high–pressure oil to either side of the hydraulic

actuator. If the hydraulic actuator has spring return,hydraulic oil will be ported to one side of the cylin-der and the other to drain. A feedback signal pro-vided by a linear variable differential transformer(LVDT, Figure 13) will tell the control whether ornot it is in the required position. The LVDT outputsan AC voltage which is proportional to the positionof the core of the LVDT. This core in turn is con-nected to the valve whose position is being con-trolled; as the valve moves, the feedback voltagechanges. The LVDT requires an exciter voltagewhich is provided by the VSVO card.

Figure 14 shows the major components of the servopositioning loops. The digital (microprocessor sig-nal) to analog conversion is done on the VSVO card;this represents called–for fuel flow. The called–forfuel flow signal is then compared to a feedback rep-resenting actual fuel flow. The difference is ampli-fied on the VSVO card and sent through the TSVOcard to the servo. This output to the servos is moni-tored and there will be an alarm on loss of any one ofthe three signals from <RST>.

Liquid Fuel Control

The liquid fuel system consists of fuel handlingcomponents and electrical control components.Some of the fuel handling components are: primaryfuel oil filter, fuel oil stop valve, three fuel pumps,fuel bypass valve, fuel pump pressure relief valve,flow divider, combined selector valve/pressuregauge assembly, false start drain valve, fuel lines,and fuel nozzles. The electrical control componentsare: liquid fuel pressure switch (upstream) 63FL–2,fuel oil stop valve limit switch 33FL, liquid fuelpump bypass valve servovalve 65FP, flow dividermagnetic speed pickups 77FD–1, –2, –3 andSPEEDTRONIC control cards TSVO and VSVO. Adiagram of the system showing major components isshown in Figure 15.

The fuel bypass valve is a hydraulically actuatedvalve with a linear flow characteristic. Located

GE

Po

wer S

ystems

14F

UN

DA

ME

NT

AL

S OF

SPE

ED

TR

ON

IC

MA

RK

VI C

ON

TR

OL

SYST

EM

Fu

nd

_Mk_V

I

Fig

ure 14 S

ervo P

ositio

nin

g L

oo

ps

TSVO

POSTION FEEDBACK

FUEL

HYDRAULICACTUATOR

HIGHPRESSURE

OIL

TORQUEMOTOR

EXCITATION

SERVOVALVE

LVDT

LVDT

EXCITATION

POSTION FEEDBACK

<R>

<S>

<T>

REF

REF

REF

D/A

D/A

D/A

3.2KHZ

3.2KHZ

TSVO

id0026

VSVO

VSVO

VSVO

3.2KHZ

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Fund_Mk_VI

between the inlet (low pressure) and discharge (highpressure) sides of the fuel pump, this valve bypassesexcess fuel delivered by the fuel pump back to thefuel pump inlet, delivering to the flow divider the

fuel necessary to meet the control system fuel de-mand. It is positioned by servo valve 65FP, whichreceives its signal from the controllers.

63FL-2

Figure 15 Liquid Fuel Control Schematicid0031V

DIFFERENTIALPRESSURE GUAGE

COMBUSTIONCHAMBER

FLOWDIVIDER

FUEL PUMP

FQROUT

BY-PASS VALVE ASM.

TYPICALFUEL NOZZLES

OFV

FSR1

TNHL4L20FLX

OHHYDRAULIC

SUPPLY

FUELSTOPVALVE VR4

OLT-CONTROL

OIL

FALSE STARTDRAIN VALVE

CHAMBER OFD

TO DRAIN

FQ1 <RST>

<RST>

OF

P R 65FP

33FL

PR/A

<RST>

40µ

77FD-3

AD

77FD-1

77FD-2

VSVO

TSVO

M

(QTY 3)

The flow divider divides the single stream of fuelfrom the pump into several streams, one for eachcombustor. It consists of a number of matched highvolumetric efficiency positive displacement gearpumps, again one per combustor. The flow divider isdriven by the small pressure differential between theinlet and outlet. The gear pumps are mechanicallyconnected so that they all run at the same speed,making the discharge flow from each pump equal.Fuel flow is represented by the output from the flowdivider magnetic pickups (77FD–1, –2 & –3). Theseare non–contacting magnetic pickups, giving apulse signal frequency proportional to flow dividerspeed, which is proportional to the fuel flow deliv-ered to the combustion chambers.

The TSVO card receives the pulse rate signals from77FD–1, –2, and –3 and outputs an analog signalwhich is proportional to the pulse rate input. The

VSVO card modulates servovalve 65FP based oninputs of turbine speed, FSR1 (called–for liquid fuelflow), and flow divider speed (FQ1).

Fuel Oil Control – Software

When the turbine is run on liquid fuel oil, the controlsystem checks the permissives L4 and L20FLX anddoes not allow FSR1 to close the bypass valve unlessthey are ‘true’ (closing the bypass valve sends fuel tothe combustors). The L4 permissive comes from theMaster Protective System (to be discussed later) andL20FLX becomes ‘true’ after the turbine vent timertimes out. These signals control the opening andclosing of the fuel oil stop valve.

The FSR signal from the controlling system goesthrough the fuel splitter where the liquid fuel re-quirement becomes FSR1. The FSR1 signal is mul-tiplied by TNH, so fuel flow becomes a function of

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Fund_Mk_VI

speed – an important feature, particularly while theunit is starting. This enables the system to have bet-ter resolution at the lower, more critical speedswhere air flow is very low. This produces theFQROUT signal, which is the digital liquid fuelflow command. At full speed TNH does not change,therefore FQROUT is directly proportional to FSR.

FQROUT then goes to the VSVO card where it ischanged to an analog signal to be compared to thefeedback signal from the flow divider. As the fuelflows into the turbine, speed sensors 77FD–1, –2,and –3 send a signal to the TSVO card, which in turnoutputs the fuel flow rate signal (FQ1) to the VSVOcard. When the fuel flow rate is equal to the called–for rate (FQ1 = FSR1), the servovalve 65FP ismoved to the null position and the bypass valve re-mains “stationary” until some input to the systemchanges. If the feedback is in error with FQROUT,the operational amplifier on the VSVO card willchange the signal to servovalve 65FP to drive the by-pass valve in a direction to decrease the error.

The flow divider feedback signal is also used forsystem checks. This analog signal is converted todigital counts and is used in the controller’s softwareto compare to certain limits as well as to display fuelflow on the <HMI>. The checks made are as fol-lows:

L60FFLH:Excessive fuel flow on start–up

L3LFLT1:Loss of LVDT position feedback

L3LFBSQ:Bypass valve is not fully open whenthe stop valve is closed.

L3LFBSC:Servo current is detected when thestop valve is closed.

L3LFT:Loss of flow divider feedback

If L60FFLH is true for a specified time period (nom-inally 2 seconds), the unit will trip; if L3LFLT1through L3LFT are true, these faults will trip the unitduring start–up and require manual reset.

Gas Fuel Control

The dry low NOx II (DLN–2) control system regu-lates the distribution of gas fuel to a multi–nozzlecombustor arrangement. The fuel flow distributionto each fuel nozzle assembly is a function of com-bustion reference temperature (TTRF1) and IGVtemperature control mode. By a combination of fuelstaging and shifting of combustion modes from dif-fusion at ignition through premix at higher loads,low nitrous oxide (NOx) emissions are achieved.

Fuel gas is controlled by the gas stop/speed ratiovalve (SRV), the primary, secondary and quaternarygas control valves (GCV) , and the premix splittervalve (PMSV). The premix splitter valve controlsthe split between secondary and tertiary gas flow.All valves are servo controlled by signals from theSPEEDTRONIC control panel (Figure 16).

It is the gas control valve which controls the desiredgas fuel flow in response to the command signalFSR. To enable it to do this in a predictable manner,the speed ratio valve is designed to maintain a prede-termined pressure (P2) at the inlet of the gas controlvalve as a function of gas turbine speed.

There are three main DLN–2 combustion modes:Primary, Lean–Lean, and Premix.

Primary mode exists from light off to 81% correctedspeed, fuel flow to primary nozzles only. Lean–Lean is from 81% corrected speed to a preselectedcombustion reference temperature, with fuel to theprimary and tertiary nozzles. In Premix operationfuel is directed to secondary, tertiary and quaternarynozzles. Minimum load for this operation is set bycombustion reference temperature and IGV posi-tion.

The fuel gas control system consists primarily of thefollowing components: gas strainer, gas supplypressure switch 63FG, stop/speed ratio valve assem-bly, fuel gas pressure transducer(s) 96FG, gas fuelvent solenoid valve 20VG, control valve assembly,LVDT’s 96GC–1, –2, –3, –4, –5, –6, 96SR–1, –2, 96PS–1, –2, electro–hydraulic servovalves 90SR,65GC and 65PS, dump valve(s) VH–5, three pres-sure gauges, gas manifold with ‘pigtails’ to respec-

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Fund_Mk_VI

tive fuel nozzles, and SPEEDTRONIC control cardsTBQB and TCQC. The components are shownschematically in Figure 17. A functional explana-

tion is contained in subsequent para-graphs.

Figure 16 DLN–2 Gas Fuel System

TURBINE COMPARTMENTGAS SKID

SINGLEBURNINGZONE

5 BURNERS

SRV

SGCV

PGCV

QGCV

PMSV

*

T

S

P

Q

DLN–2 GAS FUEL SYSTEM

SRV SPEED/RATIO VALVE

PGCV GAS CONTROL, PRIMARY

SGCV GAS CONTROL, SECONDARY

QGCV GAS CONTROL, QUATERNARY

PMSV PREMIX SPLITTER VALVE

T TERTIARY MANIFOLD, 1 NOZ. PREMIX ONLY

S SECONDARY MANIFOLD, 4 NOZ. PREMIX INJ.

P PRIMARY MANIFOLD, 4 NOZ. DIFFUSION INJ.

Q QUAT MANIFOLD, CASING. PREMIX ONLY

PURGE AIR (PCD AIR SUPPLY)*

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Fund_Mk_VI

96FG-2A

96FG-2B

96FG-2C

id0059V

96SR-1,2 96GC-1,2

LVDT’S

GASMANIFOLD

COMBUSTIONCHAMBER

HYDRAULICSUPPLY

GAS

STOP/RATIOVALVE

SPEED RATIOVALVE CONTROL

GAS CONTROLVALVE SERVO

20VG

VENT

GAS CONTROLVALVE POSITION

FEEDBACK

GASCONTROL

VALVE

TRANSDUCERS

POS1

FSR2

FPG

63FG-3

LVDT’S

FPRG

Figure 17 Gas Fuel Control System

P2

VH5-1 DUMPRELAY

TRIP

90SR SERVO65GC SERVO

ElectricalConnection HydraulicPiping

Gas Piping

POS2

VSVOTSVO VSVO TSVO

TBAIVAIC

TSVO

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Fund_Mk_VI

Gas Control Valves

The position of the gas control valve plug is intendedto be proportional to FSR2 which represents called–for gas fuel flow. Actuation of the spring–loaded gascontrol valve is by a hydraulic cylinder controlled byan electro–hydraulic servovalve.

When the turbine is to run on gas fuel the permis-sives L4, L20FGX and L2TVX (turbine purge com-plete) must be ‘true’, similar to the liquid system.This allows the Gas Control Valve to open. Thestroke of the valve will be proportional to FSR.

FSR goes through the fuel splitter (to be discussed inthe dual fuel section) where the gas fuel requirementbecomes FSR2, which is then conditioned for offsetand gain. This signal, FSROUT, goes to the VSVOcard where it is converted to an analog signal and

then output to the servo valve through the TSVOcard. The gas control valve stem position is sensedby the output of a linear variable differential trans-former (LVDT) and fed back through the TSVO cardto an operational amplifier on the VSVO card whereit is compared to the FSROUT input signal at a sum-ming junction. There are two LVDTs providingfeedback ; two of the three controllers are dedicatedto one LVDT each, while the third selects the highestfeedback through a high–select diode gate. If thefeedback is in error with FSROUT, the operationalamplifier on the VSVO card will change the signalto the hydraulic servovalve to drive the gas controlvalve in a direction to decrease the error. In this waythe desired relationship between position and FSR2is maintained and the control valve correctly metersthe gas fuel. See Figure 18.

OFFSET

GAIN

<RST>

FSR2

L4

L3GCVFSROUT

ANALOGI/O

GAS CONTROL VALVE

SERVOVALVE

GAS CONTROL VALVEPOSITION LOOPCALIBRATION

PO

SIT

ION

LVD

T

FSR

LVDT’S96GC-1, -2

<RST>

GASP2

++

id0027V

HIGHSELECT

Figure 18 Gas Control Valve Control Schematic

ELECTRICAL CONNECTION

GAS PIPING

HYDRAULIC PIPING

ÎÎÎÎÎÎ

TBQC

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Fund_Mk_VI

GAIN

<RST>

TNH

LVDT’S

<RST>

Figure 19 Stop/Speed Ratio Valve Control Schematic

TRIP OIL

OFFSET

ÎÎÎÎÎÎÎÎÎ

++

ELECTRICALCONNECTION

GAS PIPING

HYDRAULICPIPING

DIGITAL

LEGEND

OPERATINGCYLINDER

PISTON

SPEED RATIO VALVE

GAS

POS2

FPRG

AD

HIGHSELECT

HYDRAULICOIL

TNH

L4

L3GRV

96SR-1,2

SERVOVALVE

DUMPRELAY

FPG

P2 or PRESSURE

CONTROL VOLTAGE

Speed Ratio Valve Pressure Calibrationid0058V

96FG-2A

96FG-2B

96FG-2C

VAIC

VSVO

TSVO

TBAI

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Fund_Mk_VI

The plug in the gas control valve is contoured to pro-vide the proper flow area in relation to valve stroke.The gas control valve uses a skirted valve disc andventuri seat to obtain adequate pressure recovery.High pressure recovery occurs at overall valve pres-sure ratios substantially less than the critical pres-sure ratio. The net result is that flow through thecontrol valve is independent of valve pressure drop.Gas flow then is a function of valve inlet pressure P2and valve area only.

As before, an open or a short circuit in one of the ser-vo coils or in the signal to one coil does not cause atrip. Each GCV has two LVDTs and can run correct-ly on one.

Stop/Speed Ratio Valve

The speed ratio/stop valve is a dual function valve. Itserves as a pressure regulating valve to hold a de-sired fuel gas pressure ahead of the gas control valveand it also serves as a stop valve. As a stop valve it isan integral part of the protection system. Any emer-gency trip or normal shutdown will move the valveto its closed position shutting off gas fuel flow to theturbine. This is done either by dumping hydraulic oilfrom the Stop/Speed Ratio Valve VH–5 hydraulictrip relay or driving the position control closed elec-trically.

The stop/speed ratio valve has two control loops.There is a position loop similar to that for the gascontrol valve and there is a pressure control loop.See Figure 19. Fuel gas pressure P2 at the inlet to thegas control valve is controlled by the pressure loopas a function of turbine speed. This is done by pro-portioning it to turbine speed signal TNH, with anoffset and gain, which then becomes Gas Fuel Pres-sure Reference FPRG. FPRG then goes to theVSVO card to be converted to an analog signal. P2pressure is measured by 96FG which outputs a volt-age proportional to P2 pressure. This P2 signal(FPG) is compared to the FPRG and the error signal(if any) is in turn compared with the 96SR LVDTfeedback to reposition the valve as in the GCV loop.

The stop/speed ratio valve provides a positive stopto fuel gas flow when required by a normal shut–down, emergency trip, or a no–run condition. Hy-draulic trip dump valve VH–5 is located between theelectro–hydraulic servovalve 90SR and the hydrau-lic actuating cylinder. This dump valve is operatedby the low pressure control oil trip system. If permis-sives L4 and L3GRV are ‘true’ the trip oil (OLT) is atnormal pressure and the dump valve is maintained ina position that allows servovalve 90SR to control thecylinder position. When the trip oil pressure is low(as in the case of normal or emergency shutdown),the dump valve spring shifts a spool valve to a posi-tion which dumps the high pressure hydraulic oil(OH) in the speed ratio/stop valve actuating cylinderto the lube oil reservoir. The closing spring atop thevalve plug instantly shuts the valve, thereby shuttingoff fuel flow to the combustors.

In addition to being displayed, the feedback signalsand the control signals of both valves are comparedto normal operating limits, and if they go outside ofthese limits there will be an alarm. The following aretypical alarms:

L60FSGH: Excessive fuel flow on start–up

L3GRVFB: Loss of LVDT feedback on the SRV

L3GRVO: SRV open prior to permissive to open

L3GRVSC: Servo current to SRV detected priorto permissive to open

L3GCVFB: Loss of LVDT feedback on theGCV

L3GCVO: GCV open prior to permissive toopen

L3GCVSC: Servo current to GCV detectedprior to permissive to open

L3GFIVP: Intervalve (P2) pressure low

The servovalves are furnished with a mechanicalnull offset bias to cause the gas control valve orspeed ratio valve to go to the zero stroke position(fail safe condition) should the servovalve signals orpower be lost. During a trip or no–run condition, apositive voltage bias is placed on the servo coilsholding them in the ‘valve closed’ position.

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Fund_Mk_VI

Premix Splitter Valve

The Premix splitter valve (PMSV) regulates the splitof secondary/tertiary gas fuel flow between the sec-ondary and tertiary gas fuel manifolds. The valve isreferenced to the secondary fuel passages, i.e. 0%valve stroke corresponds to 0% secondary fuel flow.Unlike the SRV and GCV’s the flow through thesplitter valve is not linear with valve position.Thecontrol system linearizes the fuel split setpoint andthe resulting valve position command FSRXPOUTis used as the position reference.

Dual Fuel Control

Turbines that are designed to operate on both liquidand gaseous fuel are equipped with controls to pro-vide the following features:

1.Transfer from one fuel to the other on com-mand.

2. Allow time for filling the lines with the type offuel to which turbine operation is being trans-ferred.

3. Operation of liquid fuel nozzle purge whenoperating totally on gas fuel.

4. Operation of gas fuel nozzle purge when oper-ating totally on liquid fuel.

The software diagram for the fuel splitter is shown inFigure 20.

Fuel Splitter

As stated before FSR is divided into two signals,FSR1 and FSR2, to provide dual fuel operation. SeeFigure 20.

FSR is multiplied by the liquid fuel fraction FX1 toproduce the FSR1 signal. FSR1 is then subtractedfrom the FSR signal resulting in FSR2, the controlsignal for the secondary fuel.

Figure 20 Fuel Splitter Schematic

RAMP

L84TGTOTAL GASL84TLTOTAL LIQUID

MEDIANSELECT

MAX. LIMIT

L83FZPERMISSIVES

L83FGGAS SELECTL83FLLIQUID SELECT

FSR

FUEL SPLITTER<RST>

A=B

MIN. LIMIT

FSR1LIQUID REF.

FSR2GAS REF.

A=B

RATE

id0034

Fuel Transfer – Liquid to Gas

If the unit is running on liquid fuel (FSR1) and the“GAS” target on the <HMI> screen is selected thefollowing sequence of events will take place, pro-viding the transfer and fuel gas permissives are true(refer to Figure 21):

FSR1 will remain at its initial value, but FSR2 willstep to a value slightly greater than zero, usually0.5%. This will open the gas control valve slightly tobleed down the intervalve volume. This is done incase a high pressure has been entrained. The pres-ence of a higher pressure than that required by thespeed/ratio controller would cause slow response ininitiating gas flow.

After a typical time delay of thirty seconds to bleeddown the P2 pressure and fill the gas supply line, thesoftware program ramps the fuel commands, FSR2to increase and FSR1 to decrease, at a programmedrate through the median select gate. This is completein thirty seconds.

When the transfer is complete logic signal L84TG(Total Gas) will de–energize the liquid fuel forward-ing pump, close the fuel oil stop valve by de–ener-gizing the liquid fuel dump valve 20FL, and initiatethe purge sequence.

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Fund_Mk_VI

Transfer from Full Gas to Full Distillate

Transfer from Full Distillate to Full Gas

Transfer from Full Distillate to Mixture

UN

ITS

FSR2

FSR1

PURGETIME

SELECT DISTILLATE

SELECT GAS

SELECT GAS SELECT MIX

FSR1

FSR2

PURGE

FSR1

FSR2

PURGE

TIME

TIME

UN

ITS

UN

ITS

id0033

Figure 21 Fuel Transfer

Liquid Fuel Purge

To prevent coking of the liquid fuel nozzles whileoperating on gas fuel, some atomizing air is divertedthrough the liquid fuel nozzles. The following se-quence of events occurs when transfer from liquid togas is complete.

Air from the atomizing air system flows through acooler (HX4–1), through the fuel oil purge valve(VA19–3) and through check valve VCK2 to eachfuel nozzle.

The fuel oil purge valve is controlled by the positionof a solenoid valve 20PL–2 . When this valve is en-ergized , actuating air pressure opens the purge oilcheck valve, allowing air flow to the fuel oil nozzlepurge check valves.

Fuel Transfer – Gas to Liquid

Transfer from gas to liquid is essentially the same se-quence as previously described, except that gas andliquid fuel command signals are interchanged. Forinstance, at the beginning of a transfer, FSR2 re-mains at its initial value, but FSR1 steps to a valueslightly greater than zero. This will command asmall liquid fuel flow. If there has been any fuel leak-age out past the check valves, this will fill the liquidfuel piping and avoid any delay in delivery at the be-ginning of the FSR1 increase.

The rest of the sequence is the same as liquid–to–gas, except that there is usually no purging se-quence.

Gas Fuel Purge

Primary gas fuel purge is required during premixsteady state and liquid fuel operation. This systeminvolves a double block and bleed arrangement,wherby two purge valves (VA13–1, –2) are shutwhen primary gas is flowing and intervalve vent so-lenoid (20VG–2) is open to bleed any leakage acrossthe valves. The purge valves are air operated throughsolenoid valves 20PG–1, –2. When there is no pri-mary gas flow, the purge valves open and allow com-pressor discharge air to flow through the primaryfuel nozzle passages. Secondary purge is requiredfor the secondary and tertiary nozzles when second-ary and tertiary fuel flow is reduced to zero and whenoperating on liquid fuel. This is a block and bleed ar-rangement similar to the primary purge with twopurge valves (VA13–3, –4), intervalve vent solenoid(20VG–3), and solenoid valves 20PG–3, –4.

MODULATED INLET GUIDE VANESYSTEM

The Inlet Guide Vanes (IGVs) modulate during theacceleration of the gas turbine to rated speed, load-ing and unloading of the generator, and decelerationof the gas turbine. This IGV modulation maintainsproper flows and pressures, and thus stresses, in the

GE Power Systems


Fund_Mk_VI

compressor, maintains a minimum pressure dropacross the fuel nozzles, and, when used in a com-

bined cycle application, maintains high exhausttemperatures at low loads.

<RST>

CSRGVD/A

HIGHSELECT

TSVO

CLOSE

OPENHYD.SUPPLY

IN OUTFH6–1

<RST>

R P

2 1

HM3-1

96TV-1,2

D

OD

ORIFICES (2)

90TV-1

VH3-1

A

OLT-1TRIP OILC1

C2

Figure 23 Modulating Inlet Guide Vane Control Schematic

id0030

CSRGV

CSRGVOUTIGV REF

VSVO

Guide Vane Actuation

The modulated inlet guide vane actuating system iscomprised of the following components: servovalve90TV, LVDT position sensors 96TV–1 and96TV–2, and, in some instances, solenoid valve20TV and hydraulic dump valve VH3. Control of90TV will port hydraulic pressure to operate thevariable inlet guide vane actuator. If used, 20TV andVH3 can prevent hydraulic oil pressure from flow-ing to 90TV. See Figure 23.

Operation

During start–up, the inlet guide vanes are held fullyclosed, a nominal 27 degree angle, from zero to83.5% corrected speed. Turbine speed is correctedto reflect air conditions at 27° C (80° F); this com-pensates for changes in air density as ambient condi-tions change. At ambient temperatures greater than80° F, corrected speed TNHCOR is less than actualspeed TNH; at ambients less than 27° C (80° F),TNHCOR is greater than TNH. After attaining aspeed of approximately 83.5%, the guide vanes will

GE Power Systems


Fund_Mk_VI

modulate open at about 6.7 degrees per percent in-crease in corrected speed. When the guide vanesreach the minimum full speed angle, nominally 54°,they stop opening; this is usually at approximately91% TNH. By not allowing the guide vanes to closeto an angle less than the minimum full speed angle at100% TNH, a minimum pressure drop is maintainedacross the fuel nozzles, thereby lessening combus-tion system resonance. Solenoid valve 20CB is usu-ally opened when the generator breaker is closed;this in turn closes the compressor bleed valves.

As the unit is loaded and exhaust temperature in-creases, the inlet guide vanes will go to the full openposition when the exhaust temperature reaches oneof two points, depending on the operation mode se-lected. For simple cycle operation, the IGVs move tothe full open position at a pre–selected exhaust tem-perature, usually 371° C (700° F). For combinedcycle operation, the IGVs begin to move to the fullopen position as exhaust temperature approachesthe temperature control reference temperature; nor-mally, the IGVs begin to open when exhaust temper-ature is within 17° C (30° F) of the temperaturecontrol reference.

During a normal shutdown, as the exhaust tempera-ture decreases the IGVs move to the minimum fullspeed angle; as the turbine decelerates from 100%TNH, the inlet guide vanes are modulated to the ful-ly closed position. When the generator breakeropens, the compressor bleed valves will be opened.

In the event of a turbine trip, the compressor bleedvalves are opened and the inlet guide vanes go to thefully closed position. The inlet guide vanes remainfully closed as the turbine continues to coast down.

For underspeed operation, if TNHCOR decreasesbelow approximately 91%, the inlet guide vanesmodulate closed at 6.7 degrees per percent decreasein corrected speed. In most cases, if the actual speeddecreases below 95% TNH, the generator breakerwill open and the turbine speed setpoint will be resetto 100.3%. The IGVs will then go to the minimumfull speed angle. See Figure 24.

IGV

AN

GLE

– D

EG

RE

ES

(C

SR

GV

)

FULL OPEN (MAX ANGLE)

MINIMUM FULL SPEED ANGLE

REGION OF NEGATIVE5TH STAGE EXTRACTIONPRESSURE

ROTATINGSTALL

REGION

FULL CLOSED(MIN ANGLE)

0CORRECTED SPEED–%

100

0

FSNLEXHAUST TEMPERATURE

BASE LOAD

100LOAD–%

STARTUPPROGRAM

SIMPLE CYCLE(CSKGVSSR)

COMBINEDCYCLE

(TTRX)

Figure 24 Variable Inlet Guide Vane Schedule

id0037

(TNHCOR)

PROTECTION SYSTEMS

The gas turbine protection system is comprised of anumber of sub–systems, several of which operateduring each normal start–up and shutdown. The oth-er systems and components function strictly duringemergency and abnormal operating conditions. Themost common kind of failure on a gas turbine is thefailure of a sensor or sensor wiring; the protectionsystems are set up to detect and alarm such a failure.If the condition is serious enough to disable theprotection completely, the turbine will be tripped.

Protective systems respond to the simple trip signalssuch as pressure switches used for low lube oil pres-sure, high gas compressor discharge pressure, orsimilar indications. They also respond to more com-plex parameters such as overspeed, overtempera-ture, high vibration, combustion monitor, and loss offlame. To do this, some of these protection systemsand their components operate through the mastercontrol and protection circuit in the SPEEDTRON-IC control system, while other totally mechanicalsystems operate directly on the components of theturbine. In each case there are two essentially inde-pendent paths for stopping fuel flow, making use ofboth the fuel control valve (FCV) and the fuel stopvalve (FSV). Each protective system is designed in-dependent of the control system to avoid the possi-

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bility of a control system failure disabling theprotective devices. See Figure 25.

VIBRATION

OVERSPEED

OVERTEMP

COMBUSTIONMONITOR

MASTERPROTECTION GAS FUEL

CONTROL VALVE

20FG

CIRCUIT<RST>

MASTERPROTECTION

CIRCUIT<XYZ>

GAS FUELSPEED RATIO/STOP VALVE

FUELPUMP

Figure 25 Protective Systems Schematic

id0036V

LIQUIDFUEL STOPVALVE

RELAY

MODULEVOTING

RELAY

MODULEVOTING 20FL

SRVSERVOVALVE

GCVSERVOVALVE

SERVOVALVE

BYPASSVALVE

PRIMARY

OVERSPEEDSECONDARY

FLAME

LOSSof

Trip Oil

A hydraulic trip system called Trip Oil is the primaryprotection interface between the turbine control andprotection system and the components on the tur-bine which admit, or shut–off, fuel. The system con-tains devices which are electrically operated bySPEEDTRONIC control signals as well as some to-tally mechanical devices.

Besides the tripping functions, trip oil also providesa hydraulic signal to the fuel stop valves for normalstart–up and shutdown sequences. On gas turbinesequipped for dual fuel (gas and oil) operation thesystem is used to selectively isolate the fuel systemnot required.

Significant components of the Hydraulic Trip Cir-cuit are described below.

Inlet Orifice

An orifice is located in the line running from thebearing header supply to the trip oil system. This ori-fice is sized to limit the flow of oil from the lube oilsystem into the trip oil system. It must ensure ade-quate capacity for all tripping devices, yet preventreduction of lube oil flow to the gas turbine and otherequipment when the trip system is in the trippedstate.

Dump Valve

Each individual fuel branch in the trip oil system hasa solenoid dump valve (20FL for liquid, 20FG forgas). This device is a solenoid–operated spring–re-turn spool valve which will relieve trip oil pressureonly in the branch that it controls. These valves arenormally energized–to–run, deenergized–to–trip.This philosophy protects the turbine during all nor-

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mal situations as well as that time when loss of dcpower occurs.

PROTECTIVESIGNALS

MASTERPROTECTION

L4CIRCUITS

INLET ORIFICE

LIQUIDFUEL

LIQUID FUELSTOP VALVE

OH

20FG 20FL

GAS FUELSPEED RATIO/GAS

FUEL

GAS FUELDUMP RELAY

VALVE

WIRING

PIPING

ORIFICE ANDCHECK VALVE

NETWORK

63HG

63HL

Figure 26 Trip Oil Schematic – Dual Fuel

id0056

STOP VALVE

Check Valve & Orifice Network

At the inlet of each individual fuel branch is a checkvalve and orifice network which limits flow out ofthat branch. This network limits flow into eachbranch, thus allowing individual fuel control with-out total system pressure decay. However, when oneof the trip devices located in the main artery of thesystem, e.g., the overspeed trip, is actuated, thecheck valve will open and result in decay of all trippressures.

Pressure Switches

Each individual fuel branch contains pressureswitches (63HL–1,–2,–3 for liquid, 63HG–1,–2,–3for gas) which will ensure tripping of the turbine ifthe trip oil pressure becomes too low for reliable op-eration while operating on that fuel.

Operation

The tripping devices which cause unit shutdown orselective fuel system shutdown do so by dumpingthe low pressure trip oil (OLT). See Figure 26. An in-

dividual fuel stop valve may be selectively closed bydumping the flow of trip oil going to it. Solenoidvalve 20FL can cause the trip valve on the liquid fuelstop valve to go to the trip state, which permits clo-sure of the liquid fuel stop valve by its spring returnmechanism. Solenoid valve 20FG can cause the tripvalve on the gas fuel speed ratio/stop valve to go tothe trip state, permitting its spring–returned closure.The orifice in the check valve and orifice networkpermits independent dumping of each fuel branch ofthe trip oil system without affecting the otherbranch. Tripping all devices other than the individu-al dump valves will result in dumping the total tripoil system, which will shut the unit down.

During start–up or fuel transfer, the SPEEDTRON-IC control system will close the appropriate dumpvalve to activate the desired fuel system(s). Bothdump valves will be closed only during fuel transferor mixed fuel operation.

The dump valves are de–energized on a “2–out–of–3 voted” trip signal from the relay module. Thishelps prevent trips caused by faulty sensors or thefailure of one controller.

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The signal to the fuel system servovalves will alsobe a “close” command should a trip occur. This isdone by clamping FSR to zero. Should one control-ler fail, the FSR from that controller will be zero.The output of the other two controllers is sufficientto continue to control the servovalve.

By pushing the Emergency Trip Button, 5E P/B, theP28 vdc power supply is cut off to the relays control-ling solenoid valves 20FL and 20FG, thus de–ener-gizing the dump valves.

Overspeed Protection

The SPEEDTRONIC Mark VI overspeed system isdesigned to protect the gas turbine against possibledamage caused by overspeeding the turbine rotor.Under normal operation, the speed of the rotor iscontrolled by speed control. The overspeed systemwould not be called on except after the failure of oth-er systems.

The overspeed protection system consists of a pri-mary and secondary electronic overspeed system.The primary electronic overspeed protection systemresides in the <RST> controllers. The secondaryelectronic overspeed protection system resides inthe <XYZ> controllers (in <VPRO>). Both systemsconsist of magnetic pickups to sense turbine speed,speed detection software, and associated logic cir-cuits and are set to trip the unit at 110% rated speed.

Electronic Overspeed Protection System

The electronic overspeed protection function is per-formed in both <RST> and <XYZ> as shown in Fig-ure 27. The turbine speed signal (TNH) derived fromthe magnetic pickup sensors (77NH–1,–2, and –3) iscompared to an overspeed setpoint (TNKHOS).When TNH exceeds the setpoint, the overspeed tripsignal (L12H) is transmitted to the master protectivecircuit to trip the turbine and the “OVERSPEEDTRIP” message will be displayed on the <HMI>.This trip will latch and must be reset by the masterreset signal L86MR.

TNKHOSSETAND

LATCH

RESET

HIGH PRESSURE OVERSPEED TRIP

HP SPEEDTNHA

A>BB

<RST> <XYZ>

Figure 27 Electronic Overspeed Trip

TNKHOST

LH3HOST

L86MR1

TRIP SETPOINT

TEST

TESTPERMISSIVE

MASTER RESET

SAMPLING RATE = 0.25 SEC

L12H TO MASTERPROTECTIONAND ALARMMESSAGE

id0060

Overtemperature Protection

The overtemperature system protects the gas turbineagainst possible damage caused by overfiring. It is abackup system, operating only after the failure of thetemperature control system.

Figure 29 Overtemperature Protection

id0053

TTKOT1 TRIP

TRIP MARGINTTKOT2

ALARM MARGINTTKOT3

EX

H T

EM

P

CPD/FSR

TTRX

Under normal operating conditions, the exhausttemperature control system acts to control fuel flowwhen the firing temperature limit is reached. In cer-tain failure modes however, exhaust temperatureand fuel flow can exceed control limits. Under suchcircumstances the overtemperature protection sys-tem provides an overtemperature alarm about 14° C(25° F) above the temperature control reference. Toavoid further temperature increase, it starts unload-ing the gas turbine. If the temperature should in-crease further to a point about 22° C (40° F) abovethe temperature control reference, the gas turbine istripped. For the actual alarm and trip overtempera-

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ture setpoints refer to the Control Specifications.See Figure 29.

Overtemperature trip and alarm setpoints are deter-mined from the temperature control setpointsderived by the Exhaust Temperature Control soft-ware. See Figure 30.

TTKOT3

TTKOT2

TTKOT1TRIP ISOTHERMAL SET

ANDLATCH

RESET

TO ALARMMESSAGE

AND SPEEDSETPOINT

LOWER

OR

L30TXA

L86TXT

TRIPTO MASTER

PROTECTIONAND ALARMMESSAGE

ALARM

OVERTEMPERATURETRIP AND ALARM

SAMPLING RATE: 0.25 SEC.

TTXM

TTRXB

L86MR1

AA>B

B

AA>B

B

AA>B

B

<RST>

id0055

ALARM

Figure 30 Overtemperature Trip and Alarm

Overtemperature Protection Software

Overtemperature Alarm (L30TXA)

The representative value of the exhaust temperaturethermocouples (TTXM) is compared with alarm andtrip temperature setpoints. The “EXHAUST TEM-PERATURE HIGH” alarm message will be dis-played when the exhaust temperature (TTXM)exceeds the temperature control reference (TTRXB)plus the alarm margin (TTKOT3) programmed as aControl Constant in the software. The alarm will au-tomatically reset if the temperature decreases belowthe setpoint.

Overtemperature Trip (L86TXT)

An overtemperature trip will occur if the exhausttemperature (TTXM) exceeds the temperature con-trol reference (TTRXB) plus the trip margin(TTKOT2), or if it exceeds the isothermal trip set-point (TTKOT1). The overtemperature trip willlatch, the “EXHAUST OVERTEMPERATURETRIP” message will be displayed, and the turbine

will be tripped through the master protection circuit.The trip function will be latched in and the master re-set signal L86MR1 must be true to reset and unlatchthe trip.

Flame Detection and Protection System

The SPEEDTRONIC Mark VI flame detectors per-form two functions, one in the sequencing systemand the other in the protective system. During a nor-mal start–up the flame detectors indicate when aflame has been established in the combustion cham-bers and allow the start–up sequence to continue.Most units have four flame detectors, some havetwo, and a very few have eight. Generally speaking,if half of the flame detectors indicate flame and half(or less) indicate no–flame, there will be an alarmbut the unit will continue to run. If more than half in-dicate loss–of–flame, the unit will trip on “LOSS OFFLAME.” This avoids possible accumulation of anexplosive mixture in the turbine and any exhaustheat recovery equipment which may be installed.The flame detector system used with the SPEED-TRONIC Mark VI system detects flame by sensingultraviolet (UV) radiation. Such radiation resultsfrom the combustion of hydrocarbon fuels and ismore reliably detected than visible light, which va-ries in color and intensity.

The flame sensor is a copper cathode detector de-signed to detect the presence of ultraviolet radiation.The SPEEDTRONIC control will furnish +24Vdcto drive the ultraviolet detector tube. In the presenceof ultraviolet radiation, the gas in the detector tubeionizes and conducts current. The strength of thecurrent feedback (4 – 20 mA) to the panel is a pro-portional indication of the strength of the ultravioletradiation present. If the feedback current exceeds athreshold value the SPEEDTRONIC generates alogic signal to indicate ”FLAME DETECTED” bythe sensor.

The flame detector system is similar to other protec-tive systems, in that it is self–monitoring. For exam-ple, when the gas turbine is below L14HM allchannels must indicate “NO FLAME.” If this condi-tion is not met, the condition is annunciated as a

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“FLAME DETECTOR TROUBLE” alarm and theturbine cannot be started. After firing speed has beenreached and fuel introduced to the machine, if atleast half the flame detectors see flame the startingsequence is allowed to proceed. A failure of one de-tector will be annunciated as “FLAME DETECTORTROUBLE” when complete sequence is reached

and the turbine will continue to run. More than halfthe flame detectors must indicate “NO FLAME” inorder to trip the turbine.

Note that a short–circuited or open–circuited detec-tor tube will result in a “NO FLAME” signal.

28FDUV Scanner

TurbineProtection

Logic

FlameDetection

Logic

TurbineControlLogic

AnalogI/O

<HMI>Display

SPEEDTRONIC Mk VI Flame Detection

NOTE: Excitation for the sensors and signal processing isperformed by SPEEDTRONIC Mk VI circuits

28FDUV Scanner

28FDUV Scanner

28FDUV Scanner

ido115Figure 31 SPEEDTRONIC Mk VI Flame Detection

TBAIVAIC

Vibration Protection

The vibration protection system of a gas turbine unitis composed of several independent vibration chan-nels. Each channel detects excessive vibration bymeans of a seismic pickup mounted on a bearinghousing or similar location of the gas turbine and thedriven load. If a predetermined vibration level is ex-

ceeded, the vibration protection system trips the tur-bine and annunciates to indicate the cause of the trip.

Each channel includes one vibration pickup (veloc-ity type) and a SPEEDTRONIC Mark VI amplifiercircuit. The vibration detectors generate a relativelylow voltage by the relative motion of a permanentmagnet suspended in a coil and therefore no excita-tion is necessary. A twisted–pair shielded cable is

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used to connect the detector to the analog input/out-put module.

The pickup signal from the analog I/O module is in-putted to the computer software where it iscompared with the alarm and trip levels pro-grammed as Control Constants. See Figure 32.When the vibration amplitude reaches the pro-grammed trip set point, the channel will trigger a tripsignal, the circuit will latch, and a “HIGH VIBRA-TION TRIP” message will be displayed. Removalof the latched trip condition can be accomplishedonly by depressing the master reset button(L86MR1) when vibration is not excessive.

FAULT

AA<B

B

ALARM

AA>B

B

TRIP

AA>B

B

OR

ANDSETAND

LATCH

RESET

VF

VA

VT

TRIP

AUTO OR MANUAL RESETL86AMR

FAULT

<RST>

39V

ALARM

L39VF

L39VA

TRIPL39VT

Figure 32 Vibration Protection

id0057

L39TEST

When the “VIBRATION TRANSDUCER FAULT”message is displayed and machine operation is notinterrupted, either an open or shorted condition maybe the cause. This message indicates that mainte-nance or replacement action is required. With the<HMI> display, it is possible to monitor vibrationlevels of each channel while the turbine is runningwithout interrupting operation.

Combustion Monitoring

The primary function of the combustion monitor isto reduce the likelihood of extensive damage to thegas turbine if the combustion system deteriorates.The monitor does this by examining the exhausttemperature thermocouples and compressor dis-charge temperature thermocouples. From changesthat may occur in the pattern of the thermocouplereadings, warning and protective signals are gener-ated by the combustion monitor software to alarmand/or trip the gas turbine.

This means of detecting abnormalities in the com-bustion system is effective only when there is in-complete mixing as the gases pass through theturbine; an uneven turbine inlet pattern will cause anuneven exhaust pattern. The uneven inlet patterncould be caused by loss of fuel or flame in a combus-tor, a rupture in a transition piece, or some othercombustion malfunction.

The usefulness and reliability of the combustionmonitor depends on the condition of the exhaustthermocouples. It is important that each of the ther-mocouples is in good working condition.

Combustion Monitoring Software

The controllers contain a series of programs writtento perform the monitoring tasks (See CombustionMonitoring Schematic Figure 33). The main moni-tor program is written to analyze the thermocouplereadings and make appropriate decisions. Severaldifferent algorithms have been developed for thisdepending on the turbine model series and the typeof thermocouples used. The significant programconstants used with each algorithm are specified inthe Control Specification for each unit.

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CALCULATEALLOWABLE

SPREAD

CALCULATEACTUAL

SPREADS

MEDIANSELECT

COMBUSTION MONITOR ALGORITHM

MEDIANSELECT

TTXSPL

L60SP1

L60SP2

L60SP3

L60SP4

CTDA

TTKSPL1MAX

MIN

TTXM

TTKSPL2

TTKSPL5

TTKSPL7

CONSTANTS

MAX

MIN

TTXD2

A

BA>B

<RST>

id0049

A

BA>B

A

BA<B

A

BA<B

Figure 33 Combustion Monitoring Function Algorithm (Schematic)

The most advanced algorithm, which is standard forgas turbines with redundant sensors, makes use ofthe temperature spread and adjacency tests to differ-entiate between actual combustion problems andthermocouple failures. The behavior is summarizedby the Venn diagram (Figure 34) where:

TRIP IF S1 & S2OR S2 & S3

ARE ADJACENT

TC ALARMMONITORALARM

TRIP IF S1 & S2ARE ADJACENT

K3

K1 K2

VENN DIAGRAM

S2Sallow

S1Sallow

� K1

COMMUNICATIONSFAILURE

TYPICAL K1 = 1.0K2 = 5.0K3 = 0.8

S1Sallow

ALSO TRIP IF:

Figure 34 Exhaust Temperature Spread Limits

id0050

Sallow is the “Allowable Spread”, based on aver-age exhaust temperature and compressor dis-charge temperature.

S1, S2 and S3 are defined as follows:

a. SPREAD #1 (S1): The difference between thehighest and the lowest thermocouple reading

b. SPREAD #2 (S2): The difference between thehighest and the 2nd lowest thermocouplereading

c. SPREAD #3 (S3): The difference between thehighest and the 3rd lowest thermocouplereading

The allowable spread will be between the limitsTTKSPL7 and TTKSPL6, usually 17° C ⟨30° F) and53° C (125° F). The values of the combustion moni-tor program constants are listed in the Control Speci-fications.

The various controller processor outputs to the<HMI> cause alarm message displays as well as ap-propriate control action. The combustion monitoroutputs are:

Exhaust Thermocouple Trouble Alarm(L30SPTA)

If any thermocouple value causes the largest spreadto exceed a constant (usually 5 times the allowable

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spread), a thermocouple alarm (L30SPTA) is pro-duced. If this condition persists for four seconds, thealarm message “EXHAUST THERMOCOUPLETROUBLE” will be displayed and will remain onuntil acknowledged and reset. This usually indicatesa failed thermocouple, i.e., open circuit.

Combustion Trouble Alarm (L30SPA)

A combustion alarm can occur if a thermocouplevalue causes the largest spread to exceed a constant(usually the allowable spread). If this condition per-sists for three seconds, the alarm message “COM-BUSTION TROUBLE” will be displayed and willremain on until it is acknowledged and reset.

High Exhaust Temperature Spread Trip(L30SPT)

A high exhaust temperature spread trip can occur if:

“COMBUSTION TROUBLE” alarm exists, thesecond largest spread exceeds a constant (usual-ly 0.8 times the allowable spread), and the low-est and second lowest outputs are from adjacentthermocouples

“EXHAUST THERMOCOUPLE TROUBLE”alarm exists, the second largest spread exceeds aconstant (usually 0.8 times the allowablespread), and the second and third lowest outputsare from adjacent thermocouples

the third largest spread exceeds a constant (usu-ally the allowable spread) for a period of fiveminutes

If any of the trip conditions exist for 9 seconds, thetrip will latch and “HIGH EXHAUST TEMPERA-TURE SPREAD TRIP” message will be displayed.The turbine will be tripped through the master pro-tective circuit. The alarm and trip signals will be dis-played until they are acknowledged and reset.

Monitor Enable (L83SPM)

The protective function of the monitor is enabledwhen the turbine is above 14HS and a shutdown sig-nal has not been given. The purpose of the “enable”signal (L83SPM) is to prevent false action duringnormal start–up and shutdown transient conditions.When the monitor is not enabled, no new protectiveactions are taken. The combustion monitor will alsobe disabled during a high rate of change of FSR. Thisprevents false alarms and trips during large fuel andload transients.

The two main sources of alarm and trip signals beinggenerated by the combustion monitor are failed ther-mocouples and combustion system problems. Othercauses include poor fuel distribution due to pluggedor worn fuel nozzles and combustor flameout due,for instance, to water injection.

The tests for combustion alarm and trip action havebeen designed to minimize false actions due to failedthermocouples. Should a controller fail, the thermo-couples from the failed controller will be ignored(similar to temperature control) so as not to give afalse trip.

General Electric CompanyOne River RoadSchenectady, NY 12345

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