Generator pole slip

[Ben87K]

Hi all,

I am hoping to receive some advice and explanation about the reason of pole slip and methods of improving it.
As far as I know, the pole slip happens while rapid load changes in utility, long elimination of short circuit
in the grid (more than 400ms) or power swing. As long as I understand correctly there are
stable power swing, which has to be blocked and not to be tripped by relay protection, an unstable one, which has
to be tripped immediately, as well as pole slip on a generator.

Our grid is not stable and some events happen all the time. However, I believe that not all the generators connected to
the grid experience pole slip because after the event the generators are still in operation,
but generators at our power plant experience pole slip quite often and have to be tripped. There is some explanation
on the internet about the electrical centre that might be the reason, but I am not sure.
Is it possible to improve generator stability and eliminate pole slip by implementing boost of excitation or power system stabilizer?

Thanks for your advice in advance,
Ben.

 

[crshears]

A prime cause of pole slip is generator under-excitation; proper and correct AVR settings can greatly mitigate pole slip.

These are addressed by:

1] careful and appropriate AVR functionality choices and calibration by Design Engineering Services and Protection and Control, and

2] plant operators properly adjusting AVR setpoints on operating plant to match prevailing system conditions so as to both provide sufficient unit stability and preclude excessive VAR absorption.

 

[jwatty]

Pole slip is due to electrical energy (Pe) out of the generator not matching mechanical power (Pm) in. This can happen for a variety of reasons, but the most obvious is a sag in the system voltage. If the generator's terminal voltage sags, Pe < Pm, so the generator will speed up. This means the generator's operating point will move into the under excited region of the power "D" curve. Anything that slows this acceleration is going to improve your stability margin - operating in a leading VARS state, more aggressive AVR (or turning on the AVR if it's just manually set), updating settings on the under excited limiters, for example. Changing the tap of your GSU could help, but you'll need to review all limiters and protection settings.