If my GT load is running at 20MW (max base load 30MW) and the machine is connected to grid and the plant load is around 40 MW, this means I am importing 20MW load. If the grid is disconnected due to some reason, how long will my machine sustain by loading before I take plant load shedding action. This is for me to set set load shedding timing.

 

In as few words as possible, you have precious little time (seconds or even a fraction of a second) to shed load to prevent tripping the turbine and generator.

If the plant load (lights and motors) is 40 MW when the tie breakers opens and the GT is rated at 30 MW, the GT will not maintain frequency, whether or not is in Isochronous- or Droop Speed Control mode. The unit will likely trip very fast on either exhaust overtemperature or underfrequency (presuming typical configurations of Control Constants and protective relays), but it will likely not run for long once separated from the grid.

If the unit is in Droop mode, the frequency will drop fairly quickly and the output will increase to near 30 MW, before some underfrequency relay will likely open the generator breaker and/or trip the turbine if it doesn't trip on exhaust overtemperature. (When the frequency decreases the airflow through the compressor will decrease which will cause the exhaust temperature to increase even if the fuel were not increased; but because of the speed error the fuel will be increasing at the same time, so it will be a "race" between exhaust overtemperature detection and underfrequency detection for which will trip the turbine-generator first.)

In such cases it is customary to have some kind of automatic "load-shedding" which is activated by the opening of the tie breaker to disconnect sufficient load (in your example, more than 10 MW of lights and motors, so as not exceed the GT's rating of 30 MW) so as not to cause the GT to trip on exhaust overtemperature or underfrequency. One *cannot* reduce the load on the GT by clicking on the Lower Spd/Load button. The load is the sum of all the lights and motors the GT is trying to power, so the only way to reduce load is to disconnect some of the light and motors, and to disconnect them quickly, to prevent an exhaust overtemperature or an underfrequency trip.

It is also customary for units which are to be operated while separated from the grid to switch them to Isoch mode at the same time the tie breaker opens. This will tell the GT control system to adjust the fuel to try to maintain frequency, presuming the load (lights and motors) doesn't exceed the GT's power output capability. So, automatic load shedding (disconnecting of lights and motors) *and* switching to Isoch are required to maintain frequency when a unit is disconnected from the grid is supplying load independent of any other generators and their prime movers.

However, if the load were 40 MW and the GT is only capable of 30 MW, even if swtiched to Isoch mode the unit would likely trip on either exhaust overtemperature or underfrequency; there's simply too much load for the GT to run at rated speed and frequency.

There are some special forms of Droop speed control that will allow the unit to remain in Droop speed control mode and attempt to control power, but they are the exception rather than the rule. (In other words, they ain't common.)