For Stating a Gas turbine, Generator is made to run as a motor & SFC is used for Stating. I would like to know why are we using SFC? & when will the excitation come into picture.?

It all depends on what type of excitation system you have installed.
One possibility is, that the Static Frequency Converter (SFC) can start the turbine, relying on the generator damper winding in similar manner as the rotor windings of an asynchronous motor, and a completely different possibility could be that the excitation is on from the start, and the generator start as a synchronous motor.
- And a lot of options in between.

BTW: Please refrain from using TLAs in your initial post; that would improve your chances for a varied reply.

Thank you Sir, for your reply and your suggestion has been noted. The two situations pointed out are possible. Actually in the plant the generator , compressor , Turbines are all on a single shaft. I have a doubt that if we are using SFC then we can start with giving low frequency to the stator... and if the frequency is low then can magnetic locking between stator and rotor can take place directly ( without excitation) and after that we can increase the frequency to stator & after reaching say 75% speed the turbine will be self sustaining and the SFC supply can be cut off with the rotor being driven by the turbines then at this point if we give the excitation it can run as a Synchronous Generator. Please tell me whatever I have written above is it right?

Yes, you are correct.
If it is possible for you to apply excitation from standstill (through slip rings, or "rotating transformer"), you should be able to treat is as a synchronous motor all the way. Your main problem would be to adjust excitation and SFC voltage to match.

I practice I have seen a separate induction motor used as a starter, with a SFC (VFD), with the nominal output voltage of the SFC much larger than the nominal voltage for the motor, and driving the motor well above its nominal speed, like if you were running a 6-pole motor at 150Hz. The motor would become well and truly overloaded during the short start period, but would have plenty of time to cool down between each start attempt.

The second case regarding rotating transformer is not being implemented. Can u please elaborate your point on giving excitation from standstill. My doubt is if SFC is serving the purpose of magnetic locking by giving low frequency then why is the excitation required at the start. we can give excitation once the turbine become self sustaining and is driving our rotor then to make it synchronous we can give excitation at near synchronous speed.

HI engineers
the synchronous motor did not start from zero speed .it requires the synchronous speed.
but for starting the gas turbine is some thing different it use the generator as motor but not as a synchronous motor.it used a principle of a cyclo converter ie on and off the signal to the winding in sequence to make the generator rotate.
but when the excitation get on the answer is :if you work in this field you should know !!!

Mr. Kndsen as you have stated we have to apply excitation to motor from standstill along with SFC. So later stage when SFC is cut off & now then turbines are rotating the rotor, we would need the excitation to rotor at that time even? So is the excitation always required from start of operation till the end?