Gas turbines are not synchronized--regardless of rating. The generators driven by gas turbines are synchronized to a grid or "bus", which can be one other generator, or two other generators, or five hundred twelve other generators. Synchronization of any generator to another generator or other generators is exactly the same--regardless of prime mover. There are several good texts on the subject.
Synchronization of three-phase generators involves the following basic procedures:
1) Ensuring that the phases of the generator to be synchronized are connected to the proper phases of the bus--in other words that Phase A will be connected to Phase A, Phase B to Phase B, and Phase C to Phase C. This is usually done prior to initial synchronization and should be done ANY time the mechanical phase leads of either the generator or the bus are "disturbed" (physically removed/replaced) for maintenance or repair. Also, any time the electrical connections to the PTs (potential transformers) used for the synchronizing circuit have been "disturbed" the phasing should be re-verified (since the PTs are used for sensing phase angle and they can be reconnected such that the phasing would appear to be correct when it was actually not).
2) Ensuring that the frequency of the generator to be synchronized is just slightly greater than the frequency of the bus; this is generally referred to as "speed matching" (since speed and frequency are directly related, changing the speed of the prime mover changes the speed of the generator).
3) Ensuring that the terminal voltage of the generator to be synchronized is equal to, or usually just slightly greater than, the bus voltage; this is generally referred to as "voltage matching."
4) Ensuring that the voltage sine waves of the three phases are exactly equal in magnitude to each other--this is generally referred to as "in phase" though some texts refer to it as "phase opposition." (The idea is that since voltage is a 'difference is potential' that when the two voltages are equal in magnitude there is no potential difference between them (11.9 KV vs. 11.9 KV = 0 KV) so the contacts of the generator breaker can more be easily closed; if the voltage on the generator side of the breaker contacts was 4.7 KV and the voltage on the bus side was -7.9 KV, the difference in potential would be (4.7 KV - -7.9 KV = -3.2 KV) which is a lot of potential/force for the closing springs to overcome.) Synchronizing lights and/or a synchroscope are used to indicate this potential difference; the synchronizing lights are extiguished (not lit) and the synchroscope needle is at the twelve o'clock position when the potential is zero.
When the frequency is nearly equal and the voltage is nearly equal and the difference in potential is nearly zero, the generator breaker can be closed. The energy input to the prime mover can then be increased to cause the generator to accept some of the load from the other generators on the grid/bus (also known as "loading" the generator).
Automatic synchronization schemes and controls are available to perform the 2nd, 3rd, and 4th procedures. The first, again, is usually performed prior to initial synchronization of the unit to the bus or any time maintenance or repair has caused the phase leads to be physically removed/replaced or the PT leads have been disturbed.
Failure to match speed/frequency will cause excessive power to flow out of the generator or into the generator at the instant of synchronization. By having the energy input to the prime mover just slightly greater than that required to maintain synchronous speed there will be a small amount of power flowing out of the generator after the breaker is closed. If the speed/frequency of the prime mover was less than the bus when the generator breaker was closed then power would flow INTO the generator and back into the prime mover through the load coupling.
Failure to match terminal voltage to bus voltage will cause excessive VARs to flow "into" or "out" of the generator when the generator breaker is closed. If the generator terminal voltage were much greater than the bus voltage when the breaker was closed that would result in a large amount of VARs flowing "out" of the generator; if the generator terminal voltage were much less than the bus voltage when the breaker was closed then a large amount of VARS would flow "into" the generator.
The original equipment manufacturer of any generator-prime mover package usually provides detailed instructions about how to synchronize that particular package. So, the exact names of the switches/pushbuttons and meters and indicators may vary, but the principles never change.
Given your contributions to this forum, I guess we all can cut you a little slack. I have learned much about the control of these machines from your posts. Thank you.
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