We have two Frame -5 GE gas turbines of 18 MW each. Last week both turbines were on normal operation. Total load was 26 MW and each turbine was on 13 MW.

TG_1 (Mark-II Control) was on ISO mode while the TG_2 (Mark-V control) was on droop mode.
A heavy flash (short circuit) took place in one of the sub stations. Most probably under voltage relay 27 was actuated for a short time, which causes the load shed by the load shedding system.

Then it was observed that the breaker of TG_1 open and load on TG_2 was 5 MW. As the TG_2 was on droop and its frequency was raised to 53.5 Hz.
Now the question is:

Why did the TG-2 go to 53.5 Hz (5446 RPM) while the droop is 4%? And in CSP it is written set point high speed stop 107% and set point low speed stop 95% TNKR3 & TNKR4 respectively? While turbine was in normal operation its signal was on 13 MW but on 5 MW how its signal was raised up to about 107%.

 

It's really difficult to say for certain without being able to look at the CSP of your unit and we don't know how the unit was being operated (Part Load speed control or Pre-selected Load Control or some method of External Load Control), but something drove TNR to 107%.

If the unit was operating on Part Load (not on Pre-selected Load Control), TNR would have remained constant. If the unit was being operated on Pre-selected Load Control, that could have caused TNR to increase since if the load was only 5MW and the setpoint was 13 MW, the sequencing might have increased TNR to try to increase the "load" but the extra fuel resulted in extra speed.

You need to work "backwards" through the CSP from L70R to see what might have caused TNR to increase to the HSS (High Speed Stop). If nothing had increased TNR, then droop would have limited the speed increase, but something drove TNR to 107%.

 

Could you please guide me, how can I check the droop setting of TG-2 (Mark-V Control)? Which BBL and control constant will show the droop setting?

 

There's enough information in this thread to help you:

http://www.control.com/thread/1026248312#1026248517

We don't know if the Mark V uses straight droop speed control or Constant Settable droop speed control, but you should have enough in that thread to help with either.

You haven't responded to whether or not the unit was on Pre-selected Load Control or Part Load when the event occurred.

 

Sorry, I could not mention in the last reply. When this incident occurred, the unit was on part load control. Load was 13 MW and gas FSR value was 30.80 %. Unit is being operated on gas only.

Best Regards,