I am currently working on using a high voltage breaker to synch a generator to the grid. The breaker manufacturer states that they cannot withstand a possible 2PU voltage situation across the breaker terminals for more than 1 minute. This condition could exist if the generator is at full voltage and 180 degrees out of phase with the grid.

Being unfamiliar with generator operation, my question is when can I expect to see voltage at the generator terminals? Does the voltage gradually increase as the generator is brought up to speed using the LCI system or is it not present until the excitation transformer is energized?

Thanks in advance for any help/advice.

 

Hmmmm.....

When a gas turbine employs an LCI (static starter) as a starting means, the generator is not connected directly to the grid during starting. It is excited (because every motor needs two magnetic fields to operate) and the LCI provides a variable frequency at a lower than rated voltage to the stator windings to rotate and accelerate the turbine shaft during starting and acceleration.

Once the unit reaches near rated speed, typically somewhere around 95% of rated speed, the LCI is disconnected from the generator terminals and the "normal" generator terminal voltage builds up. And synchronization takes place.

I can't imagine a situation where the generator would be at rated voltage and at exactly 180 degrees out of phase with the grid voltage for more than a few seconds at maximum. Generally, most synchronizers and turbine governors are configured to have the generator speed/frequency higher than grid frequency in order to have positive slip, so most times the voltage differential due to phase difference is continuously varying.

And, any synchronizer that can't close the breaker in less than a minute isn't worth what was paid for it. Unless the grid frequency is extremely unstable and the synchronizer/governor is incapable of matching the frequency oscillations breaker closure should occur very soon after reaching rated speed--provided all the normal permissives are met. Is there something about the operation of the plant that requires prolonged operation (more than a minute or two) at rated speed prior to synchronization (HRSG (Heat Recovery Steam Generator warming, for example)?

A generator which does NOT use an LCI for starting will excite the generator field to build up terminal voltage at different times during starting depending on several different factors. But, again, a generator used as a motor by powering it with an LCI will typically have a lower-than-rated voltage applied to the generator terminals by the LCI during starting, and when near rated speed, the LCI output is disconnected from the generator terminals and normal generator terminal voltage is allowed to build up to rated just prior to synchronization. And synchronization should take place within 10-30 seconds of enabling, and prior to and during synchronization generator speed/frequency should normally be different from grid frequency which would prevent prolonged operation at 180 degrees out of phase with rated generator terminal voltage. For operation 180 degrees out of phase with grid frequency, the generator frequency would have to be exactly equal to grid frequency (the synchroscope would be stopped/stationary at the 6 o'clock position). And, that just usually doesn't happen for very long.

It seems there is something amiss with the understanding of the 2PU voltage differential for 1 minute requirement for the breaker being considered. Hopefully the above explanation helps to understand how a typical LCI might work during a starting sequence. The exact operation of a LCI would be specific to the manufacturer and application/site. And, the particulars of turbine operation at rated speed and during synchronization would also be particular to the manufacturer and application. But, in general, this should help to understand the basics and perhaps allow you to converse with the breaker manufacturer in an effort to understand if their product can be applied to this usage.

Hope this helps!