Gas Turbine Starting System

Turbine compressor strings require a large electric motor and variable frequency drive (VFD) capable of starting the string (Starter Mode), adding torque at any operating speed point (Helper Mode), and returning excess GT torque available under specific conditions as electrical power to the QGII electrical distribution system (Generator Mode).

Furthermore, the motor shall drive the GT in order to perform Normal Turning at 450 rpm (or Emergency Turning at 300 rpm) and Off-line water wash at crank speed (300 rpm).

In order to manage this function, specific logics and regulations have been implemented in Mark VI.

This paragraph describes these logics and regulators.

Two major analog signals are generated into MARK VI: torque request to VFD and Power available to Power Management System.

From a regulation point of view, a split range as a secondary cascade of a maximum selection scheme is implemented.

As shown in the following scheme there, are several regulators that are maximum selected:

The first 50% of TSP_MO1X is effected as a decrease of power available for electric generation and the second 50% is effected as a torque request to the helper motor. Both actions perform a decrease of the turbine total load without interacting with the plant request.

TSP_MO1X= 50% or THEORETIC POWER AVAILABLE FOR GENERATION =0 is the crossing condition from generator to motor way of functioning.

Before the end of the start sequence, the minimum value of regulators is 50%, so the Output is activated as a torque request; in this phase the TSP_TNH, speed regulator via motor torque, is the winner of the selection, because the other regulators are not active. During a shutdown, the TSP_MO1X signal is forced to 50%, by a ramp if we have a normal shutdown or by a step if we have an emergency trip.

Before the end of the start sequence, the minimum value of regulators is 50%, so the output is activated as a torque request; in this phase the TSP_TNH, speed regulator via motor torque, is the winner of the selection, because the other regulators are not active. During a shutdown, the TSP_MO1X signal is forced to 50%, by a ramp if we have a normal shutdown or by a step if we have an emergency trip.


Note: As far as Low NOX operation is concerned, in case of power decrease in Premix mode, MKVI will provide an APPROACHING PREMIX TO LEAN_LEAN signal for QGII operation to indicate nearness to “Lean-Lean” mode operation. This indication allows the operator to increase the VFD generator absorbed power, since during this mode NOX emission limits are not guaranteed.


Regulators
TSP_P, TSP_FQRG, TSP_SRV regulators perform a decreasing of power available or a  torque request that are generated by the fuel governor in order to avoid:

TSP P : low fuel gas intervalve pressure (96FG-2A/2B/2C);

TSP_FQRG : fuel gas control valve (VGC-1) max opening condition;

TSP_SRV : fuel gas ratio valve (VSR-1) max opening condition;

TSP_LTA : high load tunnel temperatures (TT-IB1/2/3).


TSP_TNH
This is the regulator active during the start up sequence and during the cooldown. Depending on the unit status (Crank, Firing, WarmUp, Acceleration, End of Sequence, Cooldown, Water Wash), the MarkVI will select the appropriate speed set point and the appropriate speed rate of change in order for the GT start up to follow the predefined schedule. During startup, the minimum value of TSP_TNH is forced to 50%, therefore only

the TSP_TNH regulator output is selected by the Max Selector and this signal will develop the torque demand sent to The VFD. After completing the sequence, the minimum value of TSP_TNH drops to zero and the other regulators are enabled.

TSP_FSR
This regulator performs a torque request when the unit is very close to temperature via fuel control, which means that helper motor torque is needed to achieve further load request. Obviously in this condition the THEORETIC POWER AVAILABLE FOR GENERATION shall be zero, so a generation-to-helper commutation command is sent to the VFD, and the regulator output is immediately activated as torque request.

Under normal operation, when power can be generated by the VFD, the TSP_FSR regulator is outputting 0% and the Power Available multiplier is 1. If there is a process load change or the PMS makes an additional power demand on the VFD, the Power Available calculation will be controlling the Power Available Signal to the PMS/VFD, and will prevent the GT from hitting the high temperature limit. If the Power Available calculation is not able to

prevent the GT from hitting the high temperature limit, then the TSP_FSR regulator will reduce the multiplier for the Power available signal from 1 toward 0, if this is not enough to overcome the disturbance, the VFD will enter helper motor mode. It is possible that other override regulators

may be selected by the Max Selector, which will reduce the Power Available signalor enable the helper motor.

In the event that the VFD is in the generation mode and it does not respond to a reduction in the Power Available signal, then the Mark VI will change the Generation Mode Available bit L83GA from 1 to 0, when Power Available becomes 0. The Generation Mode Available bit (false) L83GA is sent directly from Mark VI to the VFD, that will leave the Generation Mode.
TSP_MAN
This signal is a manual value to the maximum selector. As soon as MANUAL_ON command is sent from HMI, the TSP_MO1X value is copied to TSP_MAN variable. From now on, the TSP_MAN value can be changed by the operator from HMI until MANUAL_OFF command is sent. Analog signals are ramp actuated.


Alarms and Trip
The following alarms and trips have been implemented:

SPEED DOESN'T FOLLOW SETPOINT = Excessive (Speed set-Actual speed) difference during start sequence – hold speed set point ramp

TORQUE NOT FOLLOWING TRIP = Excessive (Torque request-Actual torque) difference during start sequence –Trip

TORQUE NOT FOLLOWING ALARM = Excessive (Torque request-Actual torque) difference
after start sequence –Alarm

1 comment: