Dual Shaft Turbines

GE Power Systems

TWO–SHAFT GAS TURBINE CONTROL C ONTROL & OPERATION OPERATION in load or ambient conditions, the control system modulates the flow of fuel to the gas turbine. For example, if the exhaust temperature is exceeding its allowable value for a given operating condition, the temperature control system reduces the fuel supplied to the turbine and thereby limits the exhaust temperature.

Basic Concept Control of the gas turbine is done by the start–up, low–pressure shaft speed, exhaust temperature, low–pressure shaft acceleration, high–pressure shaft acceleration and manual control functions illustrated in Figure 1. Sensors monitor turbine speed, exhaust temperature, compressor discharge pressure and other parameters to determine the operating conditions of the unit. When it is necessary to alter the turbine operating conditions because of changes

Operating conditions of the turbine are monitored by various sensors and utilized as feedback signals to the SPEEDTRONIC™ control system. There are

TO CRT DISPLAY

FSRN

FUEL

LP SPEED FSR ACL

MANUAL

FSR MAN

FSR ACC

MINIMUM VALUE SELECT GATE FSR

TO CRT DISPLAY

START UP

FUEL SYSTEM

FSRSU

TO CRT DISPLAY

TEMPERATURE

TO TURBINE

FSRT

TTRX HP SPEED

TNH

TSRNZ

SECOND STAGE NOZZLE

MIN SEL

TTXM

Figure 1 Simplified Two–Shaft Control Schematic A00150

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TWO–SHAFT GAS TURBINE CONTROL & OPERATION

GE Power Systems three major control loops – startup, speed and temperature – which may be in control during turbine operation. The output of these control loops is connected to a minimum value gate circuit as shown in Figure 1. The secondary control modes of acceleration and manual FSR control operate in a similar manner.

Two–shaft gas turbines have two mechanically independent 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–presl ow–pressure (LP) turbine, drives the load. The use of two mechanically–separate turbines allows the two shafts to operate at different speeds to meet the varying load requirements of the driven equipment while allowing the high–pressure shaft to run at the design speed of the axial–flow compressor.

Fuel Stroke Reference (FSR) is the command signal for fuel flow. The minimum value select gate connects the output signals of the six control modes to the FSR controller; the lowest FSR output of the six control loops is allowed to pass through the gate to the fuel control system as the controlling FSR. The controlling FSR will establish the fuel input to the turbine at the rate required by the system which is in control. Only one control loop will be in control at any particular time and the control loop which is controlling FSR will be displayed at the operator interface.

A variable–area second–stage nozzle separates the high–pressure and low–pressure turbines. The total energy level/fuel flow is established by the load demands on the low–pressure shaft, while the energy split between the high pressure and low pressure turbines is determined by the pressure drop across the respective turbines. Opening the variable–area second–stage nozzle decreases the back pressure on

FUEL

SECOND STAGE NOZZLE

TURB 1st STG

COMPRESSOR

HP SET

LOAD

LP SET

FIRE

16%

ACCELERATE

45%

SELF SUSTAINING

80%

GOVERNING MIN

80%

MAX

TURB 2nd STG

BREAKAWAY DEPENDS UPON LOAD GOVERNING MIN 50%  MAX 105%

100%

Figure 2 Two–Shaft Turbine TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems PROCESS SETPOINT

FUEL

DISPLAY STATUS LOAD SETPOINT

LP SPEED

AUTO/ MANUAL SELECT

DISPLAY STATUS

MINIMUM VALUE SELECT GATE

FSR FUEL SYSTEM

SECOND STAGE NOZZLE

START UP DISPLAY STATUS

TURB 1st STG

COMPRESSOR

DISPLAY STATUS

HP SPEED

MINIMUM SELECT

TURB 2nd STG

LOAD

TSRNZ

NOZZLE TEMPERATURE EXHAUST TEMPERATURE

Figure 3 Gas Turbine Two–Shaft Control Schematic

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

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

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

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

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GE Power Systems pending on measured exhaust temperature and IGV Temperature Control selection (On or Off).

proper sequencing of command signals to the accessories, starting device and fuel control system. Since a safe and successful start–up depends on proper functioning of the gas turbine equipment, it is important to verify the state of selected devices in the sequence. Much of the control logic circuitry is associated not only with actuating control devices, but enabling protective circuits and obtaining permissive conditions before proceeding.

Start–up Control The start–up control loop is an open–loop control and is connected through the minimum value select gate to the fuel control system along with the other control loops. Start–up control is designed to safely bring the gas turbine from zero speed to operating speed by providing the proper amount of fuel to establish flame and accelerate the turbine rotors to their minimum operating speeds. This is done in a manner as to minimize the low cycle fatigue of the hot gas path parts during the sequence. This involves MANUAL

START

FSRMAN

FSRSU

UP

TNL

LP ACC.

TNL

LP SPEED

TTXM

TEMP.

General values for control settings are given in this description to help in the understanding of the operating system. Actual values for control settings for a particular machine are given in the Control Specifications.

FSRACL

FSR1

FUEL PUMP

MINIMUM VALUE SELECT GATE

FSR

FSRN

SPLITTER

FSRT

GAS CONTROL VALVE

FSR2 P 2

CPD

TNH

HP ACC.

FSRACC GAS RATIO

FPRG

GAS RATIO VALVE

TNH NOZZLE HP SPEED EXH TEMP

TSRNZ

Figure 4

SECOND STAGE NOZZLES

Two–Shaft Turbine Simplified Control Loops TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems During the starting mode, some minor differences are found in the sequencing checks for various devices. For example, in compressor drive applications, the station and compressor valves must be sequenced before the load compressor is permitted to turn. These checks are usually required as a permissive to energize the starting means.

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

Speed Detectors

Full–speed detector logic L14HS pick–up indicates the high–pressure turbine is approaching its minimum operating speed and that the accelerating sequence is almost complete. This signal provides the logic for various control sequences such as stopping auxiliary lube oil and hydraulic oil pumps.

Accelerating speed detector logic L14HA pick–up indicates when the HP turbine has reached approximately 50 percent speed; this indicates that turbine start–up is progressing and keys certain protective features.

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 the start–up and shutdown of the gas turbine. The following speed sensors and speed relays are used on two–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 on diesel or motor started units)

•

14ST Starting Turbine Full Speed (Not used on diesel or motor started units; also certain turbine started units)

Zero–speed detector logic L14LR indicates when the low–pressure shaft starts or stops rotating. Speed level detector logic L14LS indicates the low–pressure turbine is approaching its minimum operating speed and indicates that the unit is ready for loading. Two detectors are used for the starting turbine sequencing (if applicable): 14SR to show that the starting turbine has stopped which is a permissive to start and 14ST to indicate the starting turbine has reached its maximum operating speed. The start–up control operates as an open loop control using preset levels of the fuel command signal FSR. The levels are: “ZERO”, “FIRE”, “WARM– UP”, “ACCELERATE” and “MAX”. The Control Specifications provide proper settings calculated for the fuel anticipated at the site. The FSR levels are set as Control Constants in the SPEEDTRONIC control start–up loop.

Zero–speed detector logic L14HR indicates when the 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 permissive logic initiates ratchet operation during the automatic start–up/cooldown sequence of the turbine. The ‘L’ in L14HR represents ‘logic’; logic signals, or pseudo–relays, are contained in the sequencing software and are either a 1, ‘picked–up’, or a 0, ‘dropped–out’.

The fuel command signals are sequenced by the SPEEDTRONIC control start–up software. The start signal energizes the Master Control and Protection circuit (the “L4” circuit) and starts the necessary auxiliary equipment. The “L4” circuit permits pressurization of the trip oil system and engages the starting clutch. With the “L4” circuit permissive and the starting clutch engaged, the starting device starts turning.

Minimum speed detector logic L14HM indicates that the high–pressure turbine has reached the minimum firing speed and initiates the purge cycle prior to the introduction of fuel and ignition. The dropout A00150

When the HP rotor ‘breaks away’ (starts to rotate), the L14HR signal de–energizes starting clutch solenoid 20CS and shuts down the hydraulic ratchet. 5


GE Power Systems The clutch then requires torque from the starting device to maintain engagement. Turbine speed relay logic L14HM indicates that the turbine is turning at the speed required for proper purging and ignition in the combustors; purge timer L2TV is initiated with the L14HM signal. The purge time is set to allow three to four changes of air through the unit and exhaust system to ensure that any combustible mixture has been purged from the system. Units which have extensive exhaust systems may have a fairly long purge timer, but simple–cycle units usually only require two minutes time.

Other control modes are also able to reduce and modulate FSR to perform their control functions during the start–up phase. In the acceleration phase of start–up, it is possible to reach the temperature control limit or shaft acceleration rate limits. The speed and temperature control systems will not permit these limits to be exceeded by controlling FSR as required. The Operator Interface always displays which mode is in control. When the turbine is started, the second–stage nozzle requires maximum nozzle area because speed and temperature are below their set points. The nozzle will open to place maximum energy into the high–pressure set. This sequence can be followed in Figure 5.

The completion of the purge cycle (L2TVX) ‘enables’ fuel flow, ignition, sets firing level FSR and initiates the firing timer (L2F). When the flame detector output signals indicate flame has been established in the combustors (L28FD), the warm–up timer (L2W) starts and the fuel command signal is reduced to the “WARM–UP” FSR level. The warm– up time is provided to minimize the thermal stresses of the hot gas path parts during the initial part of the start–up.

As the HP rotor speed increases, the axial–flow compressor pumps more air and exhaust temperature will stop rising and start decreasing. At approximately 60–65% TNH, the turbine will pull away from the starting device, disengaging the clutch and causing the starting device to shutdown. Acceleration control detects whether the high–pressure or low–pressure turbine rotors attempt to increase in speed faster than the allowable acceleration rate of 1% per second and will reduce FSR to hold that rate. In Figure 3A–5 observe how FSR is cut back as the HP and LP become more efficient towards the end of the start–up. The acceleration phase of start–up ends when both HP and LP rotors are at their low speed stop speeds and Complete Sequence is reached. At this phase, the second–stage nozzle maintains high– pressure shaft speed at its low speed stop point and the auxiliary pumps shut down. The LP turbine goes on speed control and is ready for loading.

If flame is not established by the time the firing timer times out, typically 60 seconds, fuel flow is halted. The unit will remain on CRANK and can be given another start signal, but firing will be delayed by the L2TV timer to avoid fuel accumulation in successive attempts. At the completion of the warm–up period (L2WX), the start–up control ramps FSR at a predetermined rate to the setting for “ACCELERATE LIMIT”. The start–up cycle has been designed to moderate the highest firing temperature produced during acceleration. This is done by programming a slow rise i n FSR. As fuel is increased, the turbine begins the acceleration phase of start–up. The clutch is held in as long as the starting device provides torque to the gas turbine. When the turbine overruns the starting device the clutch will disengage, shutting down the starting device. Speed relay logic L14HA indicates the turbine is accelerating. The low–pressure turbine will break away and start accelerating sometime during this phase; the actual point at which it breaks away depends on the load. TWO–SHAFT GAS TURBINE CONTROL & OPERATION

Acceleration Control Acceleration control compares the present value of rotor speed with the last sampled value. The difference between these two numbers is a measure of the acceleration and is done for both the HP and LP rotors. Both rotors use an allowable acceleration reference of 1% increase in rotor speed per second. If the actual acceleration of the HP rotor is greater than the acceleration reference, FSRACC is reduced; if the actual acceleration of the LP rotor is greater than the 6

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GE Power Systems

NOZZLE ANGLE (TSRNZ)

HP

TURBINE SPEED

TURBINE SPEED

REF. (TNR)

(TNH)

FUEL

EXHAUST TEMP

STROKE REF (FSR)

(TTXC)

LP TURBINE SPEED (TNL)

Figure 5 Two–Shaft Turbine Start–up Curve

acceleration reference, FSRACL is reduced. Either of these may reduce total fuel flow to the gas turbine (FSR). If, during start–up, the low–pressure rotor accelerates too quickly, FSRACL will cut back on fuel and the unit may not come up to operating speed. A00150

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 variable area second–stage nozzle. 7


GE Power Systems these signals, FSR will be changed to eliminate that error. This is an isochronous–type governor control.

LP Speed Signal – TNL

Clearance between the outside diameter of the toothed wheel and the tip of the magnetic pickup should be kept within the limits specified on the Control Specifications. If the clearance is not maintained within the specified limits, an erroneous speed signal could be generated and the turbine speed control will then operate in response to the incorrect speed feedback signal.

Low–pressure turbine speed is measured by magnetic sensors 77NL–1, 77NL–2 and 77NL–3. These magnetic pick–up sensors are high output devices consisting of a permanent magnet surrounded by a coil in an hermetically sealed case. The pickups are mounted in a ring around a 60–tooth wheel on the low–pressure rotor. With the 60–tooth wheel, the frequency of the voltage output in hertz is equal to the speed of the turbine in revolutions per minute. For example, 6000 rpm divided by 60 seconds in a minute is 100 revolutions per second. One hundred revolutions per second times 60 teeth per revolution is 6000 cycles per second. This frequency signal is fed into a pulse rate to digital converter. See Figure 6. The signal then is compared to the LP turbine speed reference (TNR). If there is any error between

Turbine Speed Reference – TNR The Turbine Speed Reference (TNR) signal represents the reference point for the LP speed control loop. See Figure 6. Thus, by changing the speed reference, or ‘called–for speed’, the actual speed of the LP turbine and thus load, can be changed. The operator can control this raising and lowering of TNR via

FSRMAX

MEDIAN SELECT

FSRMIN

–

TNL

FSRN

+

TNR

FSKNG

Figure 6 LP Speed Control TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems raise/lower commands at the operator interface. Since load is a function of speed on a pumping compressor, the TNR may also be considered roughly proportional to load.

limit is automatically raised during mechanical overspeed testing, but must be changed for electronic overspeed testing.

A block diagram of the Turbine Speed Reference is shown in Figure 7. TNR comes from a Median Select Gate whose inputs are:

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

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

3. Raise/Lower signal which adds or subtracts TNKR1_n to the last sampled value of TNR.

A

TNR A=B

MIN

B

A

TNKR3

TNR A=B

TNKR4

MAX MEDIAN SELECT

MIN

RAISE

TNR

L83DJn RATE SELECT TNKR1_n

–

+ +

LAST SAMPLE

TNKR7

START PRESET

Figure 7 Turbine Speed Reference A00150

9

AT MAX

B

LOWER

AT


GE Power Systems This is how the operator or automatic load control changes TNR. The ‘n’ value of L83JD_n decides which rate to be used. A typical rate of change of TNR would be 9.2%/minute; during Auto–Load, TNKR1_4, which would equal 9.2%/min, would be enabled. The Manual or Fast–Load rate, TNKR_3, is typically 10.0%/minute.

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

During start–up, TNR is pre–set to the minimum governing speed (50–85%) which is stored in constant TNKR4. For auto loading from a station process or remote panel, a signal is brought into the raise/lower logic to automatically increase or decrease TNR which in turn changes the LP speed/ load.

The two–shaft exhaust temperature reference (TTRXB) is determined by output TTR of a minimum select gate whose inputs are a CPD–biased curve, an FSR–biased curve and the Base Isothermal. Figure 9 shows these curves. There is an additional signal generated to bias TTRXB that is a function of high–pressure rotor speed TNH. This signal biases the temperature control reference to a higher value in a linear relationship with TNH and changes the temperature reference only when the high–pressure shaft is operating at less than 100% rated speed. This type of control is significant only on regenerative– or combined–cycle turbines. The bias sets the exhaust temperature fuel control limit higher than normal at low high–pressure rotor speeds to attain high exhaust temperature at low load. The exhaust temperature is higher than it would normally be at low loads because the compressor is turning at less than design speed and moving less air. As the exhaust temperature increases beyond a certain point, HP rotor speed setpoint TNRH is increased from its low speed stop to a maximum of 100% (high speed stop). The bias sets the exhaust temperature fuel control limit higher at low HP turbine speed condition to provide increased load pick up capability and reduce interaction between exhaust temperature control of fuel (FSR) and exhaust temperature control of the second–stage nozzle. As a result of this signal, the temperature set point is biased to a higher value in a linear relationship with TNH, Figure 10. When subjected to a sudden load increase at partial load, the temperature (fuel) control limit must have sufficient fuel margin to permit acceleration of the HP turbine. The slope is determined by the constant TTKRX4 which is typically 8°F/%TNH.

Temperature Control The Exhaust Temperature Control System will limit fuel flow to the gas turbine to maintain the internal operating temperatures within the design limitations of the turbine hot gas path parts. The highest temperature in the gas turbine occurs in the flame zone of the combustion chambers. The combustion gas in that zone is diluted by cooling air and flows into the turbine section through the first stage nozzle. The temperature of that gas as it exits the first stage nozzle is known as the ‘firing temperature’ of the gas turbine; it is that temperature that must be limited by the control system. From thermodynamic relationships and gas turbine cycle performance calculations, firing temperature can be determined as a function of exhaust temperature and pressure ratio across the turbine; the latter is determined by the measured compressor discharge pressure (CPD). The temperature control system is designed to measure and control turbine exhaust temperature rather than firing temperature because it is impractical to measure temperatures at the turbine inlet. This indirect control of turbine firing temperature is made practical by utilizing known gas turbine aero– and thermo–dynamic characteristics to bias the exhaust temperature signal, since the exhaust temperature alone is not a true indication of firing temperature. TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems

FSR

+

–

–

+

CORNER SLOPE MIN SELECT

ISOTHERMAL

TTR

SLOPE CORNER

CPD

LAST SAMPLE

MEDIAN SELECT

TTKRXR1 TTKRXR2

100% TNH

TNH

MEDIAN SELECT

TTRXB TEMP. REF

MIN BIAS

TTKRX4

Figure 8 Two–Shaft Turbine Temperature Control With Speed Bias A00150

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GE Power Systems

ISOTHERMAL

ISOTHERMAL

) x T (

) x T (

E R U T A R E P M E T T U S A H X E

E R U T A R E P M E T T U S A H X E

FUEL STROKE REFERENCE (FSR)

COMPRESSOR DISCHARGE PRESSURE (CPD)

Exhaust Temperature vs. Fuel Control Command Signal

Exhaust Temperature vs. Compressor Discharge Pressure

Figure 9 Temperature Control Relationships Constants TTKRXR1 and TTKRXR2 are used to limit the rate of change of TTRXB in either the positive or negative direction. Typical values are an increase of 1.5 °F/second and a decrease of 1.0°F/second.

allotted to the high–pressure turbine. Conversely, in the full–closed position, maximum power is diverted to the low–pressure turbine for driving the load. Axial–flow compressors are sensitive to speed, going into surge if not operated in the correct speed range. Also, the output of the gas turbine is a function of mass airflow and pressure ratio. Therefore the compressor has to be maintained in a predetermined speed range (typically 92–100% TNH) which means that some power has to be made available for the high pressure turbine. The control system modulates the second–stage nozzle angle (or area) to maintain the HP rotor at the correct speed.

Second–stage Nozzle Control A variable area second–stage nozzle is provided on two–shaft gas turbines and located between the high–pressure stage and low–pressure stage of the turbine. The high–pressure turbine provides power to drive the gas turbine axial–flow compressor and the low–pressure turbine is coupled to and drives the load.

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

Division of power between the low–pressure and high–pressure turbine sections is accomplished through modulation of the second–stage nozzle area. In the full–open position, maximum power is TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems

TTRXB

REFERENCE EXHAUST TEMPERATURE

8°F/%N

40°F DELTA–T

TTRX

TTRXB = TTRX @ 100%TNH

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

TNH

92%

100%

Figure 10 Two–Shaft Turbine Second–Stage Nozzle Control mum and, for a given load output and site condition, the exhaust temperature is at a minimum.

lowing increased air flow to maintain an acceptable exhaust temperature. The net effect of this nozzle control scheme is a higher exhaust temperature at part–load operation, maximizing the effect of the combined– or regenerative–cycle.

For a combined–cycle (or regenerative–cycle) gas turbine application, the control system for the second–stage nozzle is calibrated to run the high pressure turbine at reduced speed during part load operations; the typical speed range is 92–100% TNH. As a consequence of this lower speed operation, the resultant low–load exhaust temperature is higher due to reduced air flow. It follows also that exhaust temperature will increase more rapidly at reduced air flow as more output is demanded.

Operation The second–stage nozzle control system is designed to regulate two parameters: high–pressure turbine speed and exhaust temperature. Figure 11 and 12 show the essential elements in the SPEEDTRONIC control circuit to control high–pressure speed (TNH). TSRNZ is the reference signal for the nozzle actuator and TNRH is the reference signal for HP rotor speed.

The high–pressure shaft speed will remain at the l ow speed stop until the exhaust temperature reaches a certain point. With further loading beyond this point, high–pressure turbine speed is increased, alA00150

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GE Power Systems

MAX. ANGLE

MEDIAN SELECT

MIN. ANGLE

TSRNZ

GAIN

TNRH LAST SAMPLE

TNH

GAIN

FSR

FSR MIN MEDIAN SELECT

TSKRNZGM 00

Figure 11 Second–Stage Nozzle Stroke Reference

Reference signal TSRNZ will adjust the second– stage nozzle angle to make the HP rotor speed equal HP rotor speed reference TNRH. If the actual speed starts to exceed the reference, the difference will cause nozzle reference TSRNZ to decrease, closing the nozzle and depriving the high–pressure turbine of pressure drop until speed decreases. If TNH drops below TNRH, TSRNZ will increase (open the nozzle) to develop more pressure drop across the high–pressure turbine until TNH increases to the proper value. TWO–SHAFT GAS TURBINE CONTROL & OPERATION

For combined– or regenerative–cycle operation, it is desirable to maximize exhaust temperature and speed setpoint TNRH will vary according to exhaust temperature. During start–up and at low loads, TNRH is at its Low Speed Stop, TNKRHLSS. As the the unit is loaded and exhaust temperature increases, the increasing exhaust temperature causes TNRH to increase, eventually reaching its High Speed Stop, TNKRHHSS. TNRH is low at low loads to maintain a minimum amount of airflow through the unit, maximizing exhaust temperature. 14

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GE Power Systems

TNH

A

TNR RAISE

A
A

B A

B

A

B

B TNKRHHSS

TNKRHLSS

CNCF

A MAX SELECT

B

TNRH MIN SELECT

TTRX

TTXM

TNKRHO

LAST SAMPLE

TNKRHG

Figure 12 HP Turbine Speed Reference As load and exhaust temperature increase, TNRH is raised to increase HP rotor speed and increase airflow through the unit to maintain an acceptable firing temperature. To accomplish this, another algorithm is used as shown in Figure 12.

To generate this second signal, exhaust temperature feedback signal TTXM is compared to exhaust temperature limit reference TTRX. The temperature reference is basically the same as that generated by the temperature control algorithm. To ensure nozzle control of exhaust temperature before fuel control of exhaust temperature, temperature reference TTRX may be offset by a constant (TNKRHO); this value is typically 0 F. See Figure 13.

The low speed stop is typically 92% TNH and this control constant (TNKRHLSS) is modified by a correction factor, CNCF, which accounts for different air densities at different ambient conditions. The corrected signal enters a gate which selects the maximum of the corrected low speed stop and a second signal derived from the temperature control reference TTRX. A00150

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The difference between the allowable exhaust temperature (reference temperature) and the actual exhaust temperature generates an error signal which is added to the last sampled value of TNRH and enters 15


GE Power Systems the maximum select gate. The maximum select gate selects the higher of the ambient temperature–corrected low speed stop speed or the exhaust temperature–corrected speed setpoint TNRH. Normally, the low speed stop speed will be selected until measured exhaust temperature TTXM reaches reference temperature TTRX. The output of the max select gate is input to a minimum select gate where it is compared to the high speed stop speed (control constant TNKRHHSS). This is the maximum allowable speed for the HP rotor. Once the high speed stop is

reached, TNRH will not go any higher. The high pressure turbine speed reference (TNRH) is then compared to TNH and if there is any error, the nozzle is modulated to control the speed as previously discussed. See Figure 11. In the event FSR should be a value less than ‘minimum blow–out fuel’ FSRMIN, the nozzle is made to go to maximum angle (15 ) to prevent the axial– flow compressor from surging. Under normal operating conditions, when actual HP rotor speed TNH _

TTKO_S

TTKO_C

TTKO_I TNKRHO

TTXM TEMPERATURE CONTROL NOZZLE

FUEL LIMIT

CONTROL

CPD Figure 13 Nozzle–Temperature Control Curve TWO–SHAFT GAS TURBINE CONTROL & OPERATION

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GE Power Systems

EXHAUST T.C’S.

TSRNZBAK TTXM TNRH LSS

HSS TSRNZ

HP SHAFT

TANZ TO SERVO

D/A

TNH

77NH–123

LEGEND ELEC. CONN. HYD. PIPING

65NV

NOZZLE DUMP VALVE NOZZLE OLT

DUMP VALVE

96NC–1

NOZZLE CONTROL RING

OH

CLOSE

OPEN

Figure 14 Nozzle Control Schematic equals reference speed TNRH, nozzle reference angle TSRNZ should be zero.

is any error, a new signal is sent to servovalve 65NV which will reposition the hydraulic actuator to open or close the nozzle as required. A hydraulic dump valve is provided in case of a trip to ensure the nozzle goes to the full open position to minimize power flow to the LP rotor.

Output TSRNZ is converted to an analog signal (see Figure 14) and compared to the position feedback coming from two LVDTs (96NC–1 and –2). If there A00150

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GE Power Systems respond to the Turbine Speed Reference (TNR) signal.

Two–shaft Operating Characteristics

Figure 15 is intended to show the direction of parameter change during the load cycle only. Actual magnitudes are dependent on load compressor characteristics, pipeline pressure and flows and site conditions.

Loading Loading may be automatically controlled by a process signal or manually increased using the Raise/ Lower switches. In either case, it is turbine speed control setpoint TNR which changes, creating an error signal which causes FSR to increase or decrease to maintain called–for speed.

An increase in the TNR setpoint, either automatically or manually, results in a difference between the actual low–pressure rotor speed and the low–pressure rotor speed setpoint. Fuel command FSR is increased to accelerate the low–pressure turbine to the set–point speed. The increased fuel flow causes the high–pressure rotor speed to increase, but the second–stage nozzle operates to decrease the pressure drop across the high–pressure turbine and maintain high–pressure turbine speed at a constant level.

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

Simple Cycle

The second–stage nozzle will continue to hold the high–pressure turbine speed constant as fuel flow increases until exhaust temperature limit TTRX is reached. The nozzle will then operate to increase the high–pressure turbine speed from the low speed stop speed to the high speed stop speed to hold exhaust temperature at this level; this is “Nozzle Temperature Control”. This point will be somewhat below the limit for exhaust temperature control of fuel. After the HP rotor has reached full speed, continued loading will then cause exhaust temperature to reach temperature control limit TTRXB and fuel flow will be stopped from increasing. The exhaust temperature control circuit will then modulate FSR to maintain an acceptable exhaust temperature. Once on exhaust temperature fuel control, the TNR signal can no longer act to increase FSR. Theoretically, the exhaust temperature control point for fuel will be reached as the high pressure rotor attains 100% design speed.

If the turbine is expected to run without a regenerator or waste–heat recovery equipment, it will be set up for simple cycle operation with the high–pressure turbine always running at design speed, even at part load. This is justified mainly because of reduced firing temperature at part load. There is no significant difference in part load efficiency operating with variable high–pressure set speed. Essentially constant speed control is accomplished by setting the HP rotor low speed stop at 99% and high speed stop at 100%. The load characteristic for a simple cycle unit is shown in Figure 15. Startup will have brought the high–pressure set to the low speed stop. During the loading phase of operation, the speed of the low–pressure turbine and load compressor and the power output of the turbine are increased according to the speed–load characteristic of the driven load. The gas turbine uses the low–pressure turbine speed as an indicator of load; low–pressure speed setpoint TNR may be increased or decreased automatically by the station process signal or manually at the turbine control panel. As more load compressor output is required, the low–pressure speed setpoint is increased. The other turbine parameters TWO–SHAFT GAS TURBINE CONTROL & OPERATION

The same sequence occurs, in reverse, during unloading. See Figure 18.

Combined Cycle For a combined– or regenerative–cycle gas turbine application, the control system for the second–stage 18

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GE Power Systems NOZZLE ANGLE (TSRNZ)

TNH

TNL

TNR

TTXC FSR

Figure 15 Two–Shaft Turbine Simple–Cycle Loading nozzle is calibrated to run the high pressure turbine at reduced speed during part load operations; the typical speed range is 92–100% TNH. As a consequence, for a given low–load output and site condition, the resultant exhaust temperature is higher due to reduced air flow.

an acceptable exhaust temperature. The net effect of this nozzle control scheme is a higher exhaust temperature at part–load operation, maximizing the effect of the regenerative– or combined–cycle. Turbine loading begins as it did in the simple–cycle case as shown in Figure 15. The nozzle will close when necessary to hold the HP rotor at the low speed stop. When the exhaust temperature reaches the nozzle temperature control setpoint, the nozzle will modulate HP rotor speed to avoid exceeding that temperature setpoint. With continued loading, HP

The high–pressure shaft speed will remain at the l ow speed stop until the exhaust temperature reaches a certain point. With further loading beyond this point, the compressor speed or high–pressure turbine speed is allowed to increase air flow to maintain A00150

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GE Power Systems rotor speed increases to keep the exhaust temperature from increasing; this requires directing slightly more power to the HP turbine. Eventually, the nozzle control runs HP rotor speed setpoint TNRH to its high speed stop and the HP rotor will be operating at design speed (100% TNH). As load continues to increase after the HP rotor is at design speed, exhaust temperature will increase until the “Fuel Temperature Control” exhaust temperature limit is reached. At the point, FSR will not increase further.

haust temperature by itself. Both the variable inlet guide vanes and the variable second–stage nozzle are controlled to maintain maximum exhaust temperature for part–load combined– cycle efficiency. A typical start–up curve is shown in Figure 16. As can be seen, it is very similar to the previously discussed simple cycle start–up. The VIGVs will modulate open from the full closed angle of 42 to minimum full speed angle of 56 . The rate of change is based on the corrected H.P. speed curve. This is to avoid compressor pulsation. Once loading begins as shown in Figure 3A–17 the exhaust temperature (TTXC) will reach the IGV temperature setpoint (usually 30 F/16.6 C below fuel temperature setpoint). The IGVs will then start to modulate to maintain this exhaust temperature. When the IGVs reach their limit of 85 angle, they stop modulating. If the unit continues to load, the exhaust temperature then will continue to increase to the nozzle temperature control point. The nozzle will then start to open to maintain this temperature. This allows the HP rotor speed to increase and, if loading continues, to reach the high speed stop; the nozzle will then modulate to maintain 100% TNH. As loading continues, the exhaust temperature will reach the fuel temperature control setpoint and loading will end. _

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Variable Inlet Guide Vanes The variable inlet guide vane control loop is added to the SPEEDTRONIC control system to provide two functions:

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1. Avoid compressor pulsation by closing the inlet guide vanes to the low–flow position during start–up.

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


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TNH

TNL FSR

TNR

TTXC

TSRNZ FSR

Figure 16 Two–Shaft Turbine Simple–Cycle Unloading

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GE Power Systems

TSRNZ

TNH TNL

TTXC

IGV FSR

TNL

ANGLE

Figure 17 Two–Shaft Turbine Combined–Cycle Start–up


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TNH

TSRNZ

TTXC

TNL

IGV ANGLE

FSR

Figure 18 Two–Shaft Turbine Combined–Cycle Loading

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GE Power Systems

GE Power Systems Training General Electric Company One River Road Schenectady, NY 12345


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Dual Shaft Turbines

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