Sync Window: Induction Motors

[DM61850]

How do you determine the sync window for closing in on induction motors? I came across a motor standard stating that you don't want V/Hz to exceed 1.33 (IEEE C50.41-2000) but I don't really have a way of determining how the system voltage and frequency decays to know how small I need to make the window so that 1.33 V/Hz is not exceeded went the breaker finally closes.

Do people actually dig into this determine how large to make the window or is a general rule of thumb used?

 

[RRaghunath]

The induction motors are disconnected from the bus for slow transfer.
Whereas, fast transfer limits the interruption time to ~20ms by which the slowing of motor (drop of motor frequency) and the phase angle shift is limited.
These are the two modes I am aware in power stations. I have not come across estimation of V/f limits at the moment of transfer.

 

[DM61850]

The angle shift is going to relate to how much inertia the system has. I read that most fast transfer schemes don't supervise sync so maybe this is usually a moot point but in Beckwith's materials and elsewhere, it is mentioned that fast transfer might not be possible depending on the system. One paper I came across had actual data of a bus slipping at 10 degrees a cycle during a fault condition. It wouldn't be hard for anything slipping that fast to exceed the 1.33 V/Hz window.

I believe the slip time is longer than 20ms even if you are doing simultaneous trip close. The relaying has to identify the condition and then send the trip and close and you only get health voltage back when the tie breaker closes. I want to say that I read this is usually around 8-10 cycles which seems reasonable with us having 5 cycle breakers.