In case of total power failure in the grid, the running gas turbine trips. We have naphtha as running fuel in our facility & we will do a heavy fuel purge. The machine will come on DC barring gear or if that fails, we will do hand barring. Now, a black start diesel generator with pressurized starting air is available at our facility for, which we will use to charge the GTP MCC switchgear, which houses the various LT AC auxiliaries for the gas turbine like AOP, AC JOP, etc & put the machine on AC barring gear. Again, using the diesel generator set, we start the cranking motor of the gas turbine & take it up to FSNL. Next, the field breaker of the excitation circuit of the brushless generator is closed & generator terminal voltage is brought to less than 1 kV [Usual value is 10.5 kV]. The same is done to prevent any inrush current in the generator transformer [supplying to grid at 220 kV]. Next, the isolators of the generator transformer are closed & the generator circuit breaker is closed in dead bus mode. Then, the excitation voltage is raised so as to bring the generator terminal voltage to 10.5 kV, the normal value. Now, the unit auxiliary transformer [which is connected to the LV side (10.5 kV) of the generator transformer], needs to be synchronized to the 6.6 kV bus (which has the gas turbine starting motor & currently being energized by the diesel generator)]. Now, my doubt is what pre-select load set point to be given to the gas turbine? Is a low value like 0.3 MW possible? Or should we go for isochronous mode of operation (islanding mode)? I have seen this option in the Mark 5 HMI operating screens, but I have never used it? We have 2 gas turbines, both frame 9E & 115 MW base load each and Mark 5 control system. Has anyone operated their gas turbine in this islanding mode?

 

KRManish,

That's a pretty ambitious plan. The two most ambitious parts are getting the generator terminal voltage to start so low (1 kV), and getting the output of a Frame 9E to be stable at 0.3 MW, whether in Isochronous- or Droop speed control mode.

I think it's going to take some pretty smart fellows to be able to limit the GT excitation to less than 6 kV at rated speed. It's probably doable, but it's probably going to take some special hardware and switching, and then to be able to "jump" the voltage to rated when necessary.

0.3 MW out of 115 MW is (0.3/115=0.26% of rated), and it's probably going to be difficult to control fuel flow-rate at that low an output. The frequency will likely be a little unstable--whether in Isochronous- or Droop speed control mode.

As for the Governor Mode, well, that's going to be dependent on the mode of the diesel's governor. You don't want two machines both in Isochronous speed control mode synchronized together--that will be REALLY unstable and will probably result in another black-out. So, if the diesel is operating in Isoch (and it will probably be), then you want to operate the GT in Droop mode.

As for Pre-selected Load Setpoint, I would strongly recommend against trying to set a Pre-Selected Load Setpoint of 0.3 MW (if the Speedtronic turbine control will even take it--there's usually a lower limit and an upper limit, and 0.3 MW is probably less than the lower limit). Also, the deadband of Pre-selected Load Control is probably something like 0.5 MW or 1.0 MW, so it may go into reverse power while trying to maintain 0.3 MW. It would be much better to just use the RAISE- and LOWER SPEED/LOAD commands to manually try to control load.

Also, if you automatically synchronize the unit it's probably going to try to go to Spinning Reserve--which is usually 5-10 MW. And, in trying to do so the frequency is going to go high, possibly causing the diesel generator to trip on reverse power.

That's going to be a touchy situation; best of luck. Please write back to let us know how it goes!