6.2.3 Hydraulic Oil System

6.2.3 Hydraulic Oil System
6.2.3.1 General
The Gas Turbine Combined Hydraulic and Lift Oil system functions to provide fluid power required for operating control components and to provide lift at the Gas Turbine and Generator bearings. The control components include the Gas Valves (hydraulically actuated servo valves) and the Inlet Guide Vanes-IGV’s (positioned by a hydraulic cylinder located on the turbine base), and on Dual Fuel Gas Turbine units.

The Liquid Fuel Valve (hydraulically actuated servo valve). The major components of the system include the pumps and motors, accumulator, filters, and valves contained in the manifold assembly as shown in Figure (6.5). This document will describe how the system normally operates.

6.2.3.2 Pump Inlet and Discharge

Pressure regulated, filtered, and cooled lubrication oil from the main lube oil header in the A160 is used as the hydraulic/lift oil, high-pressure fluid. The system is designed with two redundant parallel flow paths. Under normal operation, only one circuit on the system is in use. Isolation valves are used to isolate either of the circuits so that maintenance can be performed on or off line.
 On the inlet to the system, pressure switches (63HQ-6A, 63HQ-6B) signal an alarm, which prevents the hydraulic/lift pump motors (88HQ-1, 88HQ-2) from starting should there be insufficient inlet pressure. This feature will prevent the pumps from cavitations. High-pressure fluid is then pumped to the supply manifold by one of the two pressure-compensated, variable displacement pumps (PH1-1, PH2-1).

 Each pump is driven by its own AC electric motor. The turbine operator controls the lead-lag sequence on the pumps. The pumps are constant pressure, variable positive displacement axial piston pumps with built in dual pressure compensators (VPR3-1, VPR3-2). The compensators act by varying the stroke of the pistons to maintain a set pump discharge. Each pump has a high and low-pressure compensator setting. The high-pressure setting is used when lift oil supply to the rotor bearings is needed. The low pressure setting is used when actuation of the gas valves and IGV’s is required.

 Each pump/motor contains a heater, (23HQ-1, 23HQ-2), which prevents condensation and freezing while the motors are not running. Air bleed valves are located immediately downstream of the pump discharge to ensure rapid pressurization of the supply fluid. Each circuit contains an oil filter (FH2-1, FH2-2) with integral differential pressure switches (63HF-1, 63HF-2). Hydraulic/lift oil supply pressure relief valves (VR21-1, VR22-1) provide pressure relief in order to prevent component failure due to over-pressurization, in the event that one of the pressure compensators fail or are inadvertently set wrong.

 6.2.3.3 Lift Oil Supply
Bearing lift oil is used to raise the turbine-generator rotor onto a thin, static oil film at each journal bearing to minimize rotation friction losses the gas turbine starting means or turning   gear must overcome. Lift oil supply isolation valve (20QB-1) is a solenoid-operated valve. When energized, high-pressure oil is allowed to flow to each of the turbine-generator bearings. Each bearing is equipped with a flow-regulating valve to keep lift oil supply flow rate constant.

In addition, the lift oil supply lines at the bearings contain check valves to prevent bearing feed oil from back flowing into lift oil supply lines. 20QB-1 has a manual override to be used if the solenoid fails. There is also a sensing line connected from downstream of the solenoid to the compensator block.

When the solenoid is open, the sensing line is pressurized, thus selecting the high-pressure setting. Bearing Lift Oil Supply Pressure Switch (63QB-1) provides an alarm in the turbine control system if lift oil supply pressure is low, and will prevent the turning gear motor from starting should there be insufficient pressure.

 6.2.3.4 Hydraulic Oil Supply
 Hydraulic Supply pressure is required to actuate the gas valves, IGV’s, and liquid fuel valve (for Dual Fuel units only). Each pump circuit contains a Hydraulic Oil Supply Pressure Regulating Valve (VPR4-3, VPR4-4). These pressure-regulating valves maintain hydraulic pressure to hydraulic actuated components during normal operation, regardless of whether the pump is operating at lift pressure or hydraulic pressure.

 Hydraulic Discharge Oil Supply Pressure Switches (63HQ-1A, 63HQ-1B) are used to indicate if the lead pump is not supplying enough pressure to the system. Should this be the case, the lag pump will be activated. Hydraulic Supply Low Pressure Relief Valve (VR23-2) is provided to prevent over-pressurization of hydraulic supply components in the event pressure regulating valves fail or are set incorrectly.

 Off of the hydraulic oil supply header is a single Accumulator (AH1-1) that stores hydraulic fluid for use in transient's condition (e.g. valve actuation). The accumulator is in-service regardless of which pump is in operation. The accumulator contains an isolation valve and flow control valve to control recharge rate as shown in Figure (6.6). A Manual Bypass Valve allows the operator to quickly depressurize and drain hydraulic oil supply header. This is useful when resetting pump compensators, relief valves, or pressure regulators. The bypass valve also serves as an accumulator drain valve.

6.2.2.2 Heat Exchanger and Filters

6.2.2.2 Heat Exchanger and Filters
The lubricant oil heat exchangers (LOHX-1 and LOHX-2) connect to the parallel lubricant filters (LF3-1 and LF3-2). This design is provided so that filters not in service can be changed (or heat exchangers cleaned) without taking the turbine out of service. Filter housings and heat exchangers are self-venting. A sight glass is located in the vent line from the filter and heat exchanger.

When the heat exchanger and filter housing are full, oil will be visible in this sight glass. By means of the manually-operated three-way transfer valve, one filter can be put into service as the Second is taken out, without interrupting the oil flow to the main lube oil header. The transfer of operation from one filter to the other should be accomplished as follows:
1. Open the filler valve and fill the standby filter until a solid oil flow can be seen in the flow sight in the filter vent pipe. This will indicate a “filled” condition.
2. Operate the transfer valve to bring the standby filter into service.
3. Close the filler valve. This procedure simultaneously brings the reserve heat exchanger into service.

6.2.2.3 Pressure Protection Devices
Two pressure switches (63QA-1A and -1B) mounted on the main pump discharge header sense lube oil pressure. If either of these senses low lubricant oil pressure, an alarm is sounded and the lag pump is automatically started. Pressure switches 63QT-2A and -2B in combination with alarm switches 63QA-1A and 63QA-1B trip the unit and start the emergency DC motor-driven pump (88QE-1) when they sense low pressure.

This will occur if AC power is lost. For a trip, one of the two 63QT switches and one of the two 63QA switches must signal. This voting logic prevents a trip due to a false signal. The DC Emergency Pump is designed to provide adequate lube oil circulation for coast down following a trip. Once the unit is at rest, the DC pump should only operate a few minutes per hour, in order to remove heat, but conserve battery life. If the bearing metal temperature is above 250F, the DC pump is run.

Continuously, the emergency pump is sized to clear the trip pressure switches (63QT-2A, - 2B), but will not clear the alarm pressure level (63QA-1A, -1B). On dual fuel units with a single atomizing air compressor pressure switch (63QA-3) is provided at the oil supply to the air compressor gearbox. Two Pressure switches (63QA-3 and 4) are provided on dual fuel units with two atomizing air compressors. These pressure switches will alarm if low pressure is sensed at those points but they will not start the Lag pump.

The operation of the 63QA and 63QT switches can be verified by shutting off the normally open valve between the switch and the oil system. When the normally closed valve to the oil drain is opened, the Oil in the switch lines will drain, the proper warning signal will annunciate and proper lag/emergency pump start-up should occur.

6.2.2 Functional Description

6.2.2 Functional Description
6.2.2.1 Lubricant Reservoir and Piping
The oil reservoir is a 2600 gallon (9843 liter) tank which is integral with the module. The interior of the tank is coated with an oil resistant protective coating. The top of the tank is the base on which components such as the pumps, and heat exchangers are mounted. Under normal operating conditions oil is provided to the system by one of two main AC motor driven centrifugal pumps (PQ1-1 and PQ1-2). The selection of lead and lag pumps is made by the operator through the turbine control system prior to startup.

By alternating the lead/lag pump selection, the operating hours can be equalized. Each AC motor includes a motor space heater (23QA-1 and -2) to prevent condensation in the motor. All pumps have a check valve on the discharge line so that oil does not flow into the tank through a pump, which is not in service. Two pressure switches (63QA-1A and -1B) are mounted in the common header just downstream of the main pumps to ensure proper pump operation. If either of these senses low pressure, an alarm is sounded and the lag pump is automatically started.

If this occurs, the operator must manually shut off one pump and check that system pressure is stable. The oil is first pumped through one of the two parallel proper bearing header temperatures. The maximum allowable bearing header temperature under normal operating conditions is 160F (71.1̊C). The oil then flows through one of the two full flow parallel filters (LF3-1 and LF3-2). A three-way transfer valve controls selection of which set of heat exchanger/filter is in use. The lubricant oil filters have removable filter elements.

A differential pressure gauge provides visual indication of the dP over the filter. Pressure switches (63QQ-21, -22) provide a high differential pressure alarm signal across each filter. Filter elements should be replaced near or at the alarm set point as shown in Figure (6.4). Taps (OS), (OR-1) and (OLT-1), which are located downstream of the filters, supply lube oil to the generator bearing seals,hydraulic/lift oil system and trip oil system respectively. Pressure regulating valve (VPR2-1) then controls the oil pressure to the turbine and generator bearings and the turning gear.

The system is ventilated through a mist eliminator mounted on top of the lube oil reservoir. A slight negative pressure is maintained in the system by redundant motor driven fans (88QV-1A and 88QV-1B) pulling air through the mist eliminator. This negative pressure draws sealing air through the gas turbine bearing seals.

Each AC motor includes a motor space heater (23QV-2A and 23QV-2B) to prevent condensation in the motor. The motor driven fans have no DC backup motors and are not required to run in the emergency situation, when the DC pumps has taken over. The fans are set up to run in a lead/lag configuration and are designed to run one at a time.

The selection of lead and lag fans is made by the operator through the turbine control system prior to startup. The lag fan takes over whenever the lead fan has failed to run, has been overloaded or if there is insufficient vacuum in the lube oil reservoir. If the lag fan is started automatically by the control system due to insufficient tank vacuum level, the lead fan will be automatically shut off. Pressure switch (63QV-1) provides a low differential pressure alarm signal when there is insufficient vacuum in the lube oil reservoir. A regulating valve is downstream of each fan, and is adjusted to regulate tank vacuum level.

A level alarm device (float operated) is mounted on the top or side of the lube reservoir. The float mechanism operates two level switches (71QH-1 and 71QL-1). The switches are connected into the alarm circuit of the turbine control panel to initiate an alarm if the liquid level rises above, or fall below, the levels shown on the Schematic Piping Diagram. The oil level is visually indicated by a gauge on the side of the tank. An oil drain connection is located on the side of the accessory module to drain the reservoir.

6.2 LUBRICATION SYSTEM

6.2 LUBRICATION SYSTEM
6.2.1 Lube Oil System
The lubricating and hydraulic oil requirements for the gas turbine power plant are furnished by a separate, enclosed, forced-feed lubrication module. This lubrication module, complete with tank, pumps, coolers, filters, valves and various control and protection devices, furnishes oil to the gas turbine bearings, generator bearings (absorbing the heat rejection load), starting means, load gear and on dual fuel units the atomizing air/purge compressors as shown in Figure. (6.2) and Figure. (6.3). This module is also used to supply oil for the lift oil system, trip oil system and the hydrogen seals on the generator.

Additionally, a portion of the pressurized fluid is diverted and filtered again for use by hydraulic control devices as control fluid. Refer to “Lubricating Oil Recommendations for Gas Turbines with Bearing Ambient above 500F (260C)” in the fluid specifications section of this manual for the lubricating oil requirements. The lubrication system is designed to supply filtered lubricant at the proper temperature and pressure for operation of the turbine and its associated equipment. Refer to the Lube Oil Schematic Piping Diagram in this section. Major system components include:

1. Lubricant oil reservoir which serves as a base for the accessory module.
2. Two centrifugal pumps (PQ1-1 and PQ1-2) each driven by an AC electrical motor (88QA-1 and 88QA-2). Each AC motor includes a motor space heater (23QA-1 and -2) to prevent condensation in the motor.
3. Emergency oil pump (PQ2-1) with DC motor (88QE-1)
4. Main Seal oil pump (PQ3-1) driven by AC motor (88QS-1). AC motor includes motor space heater (23QS-1).
5. Emergency seal oil pump driven by DC motor (88ES-1). Note, in most instances PQ3-1 is a “piggy-back” AC/DC motor driving one pump. If the Customer has opted to purchase separate AC and DC seal oil pumps, the separate DC pump will be named PQ3-2.
6. Dual lubricating oil heat exchangers in parallel (LOHX-1 and LOHX-2).
7. Two full flow lubricating oil filters in parallel (LF3-1 and LF3-2).
8. Bearing header pressure regulator (VPR2-1).
9. Mist eliminator with redundant fan/motor (88QV-1A and 88QV-1B) and motor space heaters (23QV-2A and 23QV-2B).
10. Pressure Protection Switches (63QA-1A, 63QA-1B, 63QE-1, 63QT-2A and 63QT-2B and on units with liquid fuel 63QA-3).
11. Tank temperature switches (26QL-1, 26QN-1) or tank temperature thermocouples (LT-OT-4A, LT-OT-5A) for pump start permissive and immersion heater control.
12. Lube oil header thermocouples (LT-TH-1A, 1B, 2A, 2B, 3A, 3B).
13. Lube oil drain thermocouples (LT-B1D-1A/1B, LT-B2D-1A/1B, LT-G1D-1A/1B and LT-B2D-1A/1B). Note that LT-B1D-1A/1B and LT-B2D–1A/1B may be single thermocouples named LT-B1D-1 and LT-B2D-1 on some units. The lube oil is circulated by a redundant set of AC pumps. A DC pump is provided in case AC power to the site is interrupted. These pumps are the first of the auxiliary equipment to be energized during a startup.

Sequence. Following shutdown of the unit, these pumps continue to run throughout the extensive cool down period and are the last of the auxiliary equipment to be stopped. The lube oil system is self-contained. After Lubricating and removing heat from the rotating equipment, oil is returned to the lube oil tank. It is cooled by oil-to-water heat exchangers as it is pumped from the tank and re-circulated. Various sensing devices are Included in the design to ensure adequate oil level in the tank, oil pressure, and oil temperature.

All pumps have a check valve on the pump discharge line so that oil does not flow into the tank through a pump, which is not in service. Oil tank temperature is indicated by a thermometer on the side of the tank. Thermocouples connected to the control panel indicate lube oil temperature in the bearing header. Thermocouples in the bearing drains are also wired to the turbine control panel for monitoring. A bearing header oil sampling port is located upstream of VPR2-1.

For turbine starting, a maximum oil viscosity of 800 SUS (173 centistokes) is specified for reliable operation of the control system and for bearing lubrication. Temperature switch 26QN-1 or LT-OT-4A prevents turbine startup if the temperature of the lubricant decreases to a point where oil viscosity exceeds 800 SUS (173 centistokes).

6.1.3 Purge Air System

As stated above, the bypass valve is open and atomizing air is recalculating. After a short time delay, to allow the pressure ratio (regulated by valve YPR54) of the atomizing air system to reach a lower level, solenoid valve 20PL-l is energized. The system then starts operating to purge the oil passages in the oil fuel nozzles during gas fuel operation. The purge air system is necessary to prevent of oil fuel in the nozzle oil passages, during gas fuel operation, and in of the nozzles as a result of oil fuel cooking. The purge system minimizes such fouling, and keeps the oil fuel nozzle clean and ready for operation when oil fuel operation is resumed.

The small flow of air through the atomizing air passages of the oil fuel nozzle also prevents entry of any combustion products that could foul this section of the oil fuel nozzle. When solenoid valve 20PL-1 is energized, operating air (from the turbine compressor discharge) is admitted to the diaphragm of purge air valve VA19 which opens, allowing purge air (atomized) to flow into the purge air manifold on the turbine. A porous filter (FA3) is installed in the purge air check valves, one for each nozzle, are connected into the oil feed lines to the nozzles.

These purge air check valves prevent oil fuel from entering the purge air system when the machine is operating on oil fuel. Similarly, the oil fuel check valves installed in the oil piping to the oil fuel, it will drain out of the purge air manifold through the normally open port of the three-way purge air valve. “Tell-Tale” leak-off piping connected to the purge valve vent port provides a visual means for determining the general condition of the check valves. There should not be any leakage.

6.1.4 Water Wash Provisions
When water washing the gas turbine’s compressor section and turbine sections it is important to keep water out of the atomizing air system. To keep water out of the atomizing air system, the system inlet and discharge are equipped with vent valves and an isolation valve or full area spacers. The vent valves are used to avoid completely throttling or deadening the compressors, and the drain valves to remove any leakage past the isolation valves.

During normal operation of the gas turbine, the vent and drain valves must be closed and the isolation valves or spacers must be opened. Before initiating water wash, the isolation valve must be closed or blank area spacers installed to keep water out of the atomizing air system. The vent valves must be opened to allow air to pass through the atomizing air compressors.

CHAPTER 6 AUXILIARY SYSTEM AND LUBRICATION SYSTEM OF GAS TURBINE UNIT

6.1AUXILIARY SYSTEM
6.1.1 Atomizing Air System
Atomizing air systems provide sufficient pressure in the air atomizing chamber of the fuel nozzle body to maintain the ratio of atomizing air pressure to compressor discharge pressure at approximately 60% speed or greater over the full operating range of the turbine. Since the output of the main atomizing air compressor, driven by the accessory gear, is low at turbine firing speed, a starting atomizing air compressor provides a similar pressure ratio during the firing and warm-up period of the starting cycle, and during operation of the accelerating cycle.

Major system components include the main atomizing air compressor, starting atomizing air compressor and atomizing air heat exchanger. Refer to the Atomizing Air Schematic Piping Diagram in the Reference Drawings section of the Inspection and Maintenance volume as shown in Figure (6.1).

6.1.2 Functional Description
When liquid fuel oil is sprayed into the turbine combustion chambers it forms large droplets as it leaves the fuel nozzles. The droplets will not burn completely in the chambers and many could go out of the exhaust stack in this state. A low pressure atomizing air system is used to provide atomizing air through supplementary orifices in the fuel nozzle which directs the air to impinge upon the fuel jet discharging from each nozzle.

This stream of atomizing air breaks the fuel jet up into a fine mist, permitting ignition and combustion with significantly increased efficiency and a decrease of combustion particles discharging through the exhaust into the atmosphere. It is necessary, therefore, that the air atomizing system be operative from the time of ignition firing through acceleration, and through operation of the turbine.

Air taken from the atomizing air extraction manifold of the compressor discharge casing passes through the air-to water heat exchanger (pre cooler) HXl to reduce the temperature of the air sufficiently to maintain a uniform air inlet temperature to the atomizing air compressor.
The atomizing air pre cooler heat exchanger Located in the turbine base under the inlet plenum uses water from the turbine cooling water system as the cooling medium to dissipate the heat. Switch 26AA:"1 is an adjustable heat sensitive thermo switch provided to sound an alarm when the temperature of the air from the atomizing air pre cooler entering the main atomizing air compressor is excessive.

When the atomizing air reaches the temperature setting of this switch, the alarm is activated. Improper control of the temperature may be due to failure of the sensor, the pre cooler or insufficient cooling water flow. Continued operation above 275 F should not be permitted for any significant length of time since it may result in failure of the main atomizing air compressor or in insufficient atomizing air to provide proper combustion Compressor discharge air, now cleaned and cooled reaches the main atomizing air compressor. This is a single stage, flange mounted, centrifugal compressor driven by an inboard shaft of the turbine accessory gear.

It contains a single impeller mounted on the pinion shaft of the integral input speed-increasing gearbox driven directly by the accessory gear. Output of the main compressor provides sufficient air for atomizing and combustion when the turbine is at approximately 60-percent speed.

Differential pressure switch 63AD-1, located in a bypass around the compressor, monitors the air pressure and annunciates an alarm if the pressure rise across the compressor should drop to a level inadequate for proper atomization of the fuel. Air, now identified as atomizing air, valves the compressor and is piped to the atomizing air manifold with “pigtail” piping providing equal pressure distribution of atomizing air to the 10 individual fuel nozzles.
When the turbine is first fired, the accessory gear is not rotating at full speed and the main atomizing air compressor is not outputting sufficient air for proper fuel atomization. During this period, the starting (booster) atomizing air
compressor, driven by an electric motor is in operation supplying the necessary atomizing air. The starting atomizing air compressor at this time has a high-pressure ratio and is discharging through the main atomizing air compressor, which has a low-pressure ratio.

The main atomizing air compressor pressure ratio increases with increasing turbine speed and at approximately 60% speed the flow demand of the main atomizing air compressor approximates the maximum flow capability of the starting atomizing air compressor. The check valve in the air input line to the main compressor begins to open allowing air to be supplied to the main compressor simultaneously from both the main airline and the starting air compressor.

The pressure ratio of the starting atomizing air compressor decreases one, and when the turbine becomes self sustaining, the starting compressor is shut down at approximately 95 percent speed 4HS pickup. Now all of the air being supplied to the main compressor is directly from the pre cooler through the check valve, bypassing the starting air compressor completely. At this time the (20AB) solenoid is energized and the isolation valve (VA22) is closed preventing any air getting to the booster compressor.

5.4 FIRED SHUTDOWN

A normal shutdown is initiated by clicking on the “STOP” target (L1STOP) and “EXECUTE”; this will produce the L94X signal. If the generator breaker is closed when the stop signal is initiated, the Turbine Speed Reference (TNR) counts down to reduce load at the normal loading rate until the reverse power relay operates to open the generator breaker; TNR then continues to count down to reduce speed. When the STOP signal is given, shutdown Fuel Stroke Reference FSRSD is set equal to FSR.

When the generator breaker opens, FSRSD ramps from existing FSR down to a value equal to FSRMIN, the minimum fuel required to keep the turbine fired. FSRSD latches onto FSRMIN and de-creases with corrected speed. When turbine speed drops below a defined there's a hold (Control Constant K60RB) FSRSD ramp to a blowout of one flame detector.

The sequencing logic remembers which flame detectors were functional when the breaker opened. When any of the functional flame detectors senses a loss of flame, FSRMIN/FSRSD decreases at a higher rate until flame–out occurs, after which fuel flow is stopped. Fired shut down is an improvement over the former fuel shut off at L14HS drop out. By maintaining flame down to a lower speed there is significant reduction in the strain developed on the hot gas path parts at the time of fuel shut off.

5.5 SPEED CONTROL
The Speed Control System controls the speed and load of the gas turbine generator in response to the actual turbine speed signal and the called–for speed reference. While on speed control the control mode message “SPEED CTRL” will be displayed.

5.5.1 Speed Signal
Three magnetic sensors are used to measure the speed of the turbine. These magnetic pickup sensors (77NH–1,–2,–3) are high output devices consisting of a permanent magnet surrounded by a hermetically sealed case. The pickups are mounted in a ring around a 60–toothed wheel on the gas turbine compressor rotor. With the 60–tooth wheel, the frequency of the voltage output in Hertz is exactly equal to the speed of the turbine in revolutions per minute.

The voltage output is affected by the clearance between the teeth of the wheel and the tip of the magnetic pickup. Clearance between the outside diameter of the toothed wheel and the tip of the magnetic pickup should be kept within the limits specified in the Control Specifications (approx. 0.05 inch or 1.27 mm). If the clearance is not maintained within the specified limits, the pulse signal can be distorted. Turbine speed control would then operate in response to the incorrect speed feedback signal. The signal from the magnetic pickups is brought into the Mark VI panel, one mag pickup to each controller <RST>, where it is monitored by the speed control software.

5.5.2 Speed/Load Reference

speed control software will change FSR in proportion to the difference between the actual turbine–generator speed (TNH) and the called–for speed reference (TNR). The called–for–speed, TNR, determines the load of the turbine. The range for generator drive turbines is normally from 95% (min.) to 107% (max.) speed. The start–up speed reference is 100.3% and is preset when a “START” signal is given.

The turbine follows to 100.3% TNH for synchronization. At this point the operator can raise or lower TNR, in turn raising or lowering TNH, via the 70R4CS switch on the generator control panel or by clicking on the targets on the <HMI>, if required. Refer to Figure (5.4). Once the generator breaker is closed onto the power grid, the speed is held constant by the grid frequency. Fuel flow in excess of that necessary to maintain full speed no load will result in increased power produced by the generator. Thus the speed control loop becomes a load control loop and the speed reference is a convenient control of the desired amount of load to be applied to the turbine–generator unit.

Droop speed control is a proportional control, changing FSR in proportion to the difference between actual turbine speed and the speed reference. Any change in actual speed (grid frequency) will cause a proportional change in unit load. This proportionality is adjustable to the desired regulation or “Droop”. The speed vs. FSR relationship is shown on Figure (5.5). If the entire grid system tends to be overloaded, grid frequency (or speed) will decrease and cause an FSR increase in proportion to the droop setting. If all units have the same droop, all will share a load increase equally. Load sharing and system stability are the main advantages of this method of speed control.

Normally 4% droop is selected and the set point is calibrated such that 104% set point will generate a speed reference which will produce an FSR resulting 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 is sufficient to maintain full speed with no generator load. By closing the generator breaker and raising TNR via raise/lower, the error between speed and reference is increased. This error is multiplied by a gain constant dependent on the desired droop setting and added to the FSNL FSR setting to produce the required FSR to take more load and thus assist in holding the system frequency. Refer to Figures (5.5) and (5.6).


The minimum FSR limit (FSRMIN) in the SPEEDTRONIC Mark VI system prevents the speed control circuits from driving the FSR below the value which would cause flameout during a transient condition. For example, with a sudden rejection of load on the turbine, the speed control system loop would want to drive the FSR signal to zero, but the minimum FSR setting establishes the minimum fuel level that prevents a flameout. Temperature and/or start–up control can drive FSR to zero and are not influenced by FSRMIN.

 5.6 ACCELERATION CONTROL
 Acceleration control compares the present value of the speed signal with the value at the last sample time. The difference between these two numbers is a measure of the acceleration. If the actual acceleration is greater than the acceleration reference, FSRACC is reduced, which will reduce FSR, and consequently the fuel to the gas turbine. During start–up the acceleration reference is a function of turbine speed; acceleration control usually takes over from speed control shortly after the warm–up period and brings the unit to speed. At “Complete Sequence”, which is normally 14HS pick–up, the acceleration reference is a Control Constant, normally 1% speed/second. After the unit has reached 100% TNH, acceleration control usually serves only to contain the unit‟s speed if the generator breaker should open while under load.