Gas Turbines Synchronization with Grid @ 11kV, 50Hz

A reverse power relay should be there as a prime consideration. Frequency deviations in grid is of concern if it a hydro and yours is GTG. Correlate under and over voltage relays with those of the grid.

Look, you first check the capability curve of generator and determine how much active & reactive power you can share.

Set the turbine governor droop to share the active power depending the governor characteristic. Mind, that once the gen is synchronised with the grid you have no control over frequency of your generator. Controlling governor that is fuel in to Turbile simply changes the sharing of active power. Of course, you have to have a Reverse power relay and a Low forward power relay in circuit. Hope ,the AVR control circuit have inbuilt reactive power limiter.

Study the features of AVR be it EX2000, 2001 etc and Turbine Control system Mark-V or Mark-IV if its GE turbine.

Our concern is how gas turbine protects against variation in frequency in fixed power mode, i.e. Frequency up/down from Grid to 48 Hz or some time raise up to 51 HZ, the frequency relay (81) will cutoff the C.B, but we want to know how control system behave for these frequencies band changes and what parameters we have to considered to control this variation .

None at all.

Once synchronized your GT will follow the frequency pattern imposed by the grid or else the protections will act to trip which is highly undesireable.