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.
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.
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