GRID Disturbance

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Thread Starter

Sikandar7641

I have two 6FA Machines each machine attached to 132kV 100MVA transformers having breaker 52L. during the grid disturbance, 52L breaker opens due to RoCoF (rate of change of frequency) setting in MARK VI. with opening of 52L breaker, Generator circuit breaker 52 G also gets open. what could be the possible reason as i'm stuck could not find out any possible reason?

there is no tripping on generator protection relay.
No tripping on main power transformer protection relay.
machine island mode is not selected.
 
Basically, you haven't provided enough information to be of too much help. We don't know if both machines trip (52L AND 52G; what the island load when 52L opens; what load the machines were running at when the 52L(s) opened; what the frequency(s) of the two machines were after the 52L(s) opened; etc. We only the most basic of descriptions, and it's not very helpful.

This is a question best answered by GE. They have been under a lot of scrutiny (and pressure) to improve the off-frequency operation of their heavy duty gas turbines, and this may be a response to that initiative. I, personally, have never seen RoCoF logic in a machine before, so I can't make any comment on how it should be set or even what it is intended to do/protect.

I would suspect that if the unit is operating in droop speed control and the 52L breaker opens, and at least one of the units doesn't automatically transfer to Isochronous speed control that the frequency of one or both of the GTs could go high enough to result in reverse power for one or both of the GTs long enough to open 52G--but that's just a SWAG (Scientific Wild-Arsed Guess) because we would have to know a LOT more about the configuration of the plant and the interlocks and protective relays and island load, etc.

You should work with the packager of the turbines to understand the logic, its intent, and if it--and other sequencing--can be tuned to prevent nuisance 52G openings. The logic may also be a response to the plant Architect-Engineer's design requirements, or the grid/utility regulator's requirement--and the settings may never have been properly vetted. It could be quite complicated to sort, but, I feel very confident in saying that it's not a Mark VI (or VIe) problem--it's a programming/configuration/coordination problem. The Speedtronic is doing what it's programmed and configured to do; it just sounds like there's an issue with settings and/or coordination with other functions (such as reverse power--because that's <b>usually</b> the only thing in the Mark VI that opens the generator breaker <i><b>without</i></b> a Process Alarm. You may need to run some trends and capture some data to see what is opening 52G.

And, please, write back to let us know how this progresses!
 
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