Our 300 mw generator was tripped yesterday due to a fault in an out-going feeder. Protection acted was under frequency trip in relay panel. Please explain a bit what it is and how it is acted? Why the necessity of under or over frequency protection?

please Sir do this favor for me...

 

Sanantu...

typically off-normal frequency tripping protects auxiliary plant apparatus during run-up and run-down operation.

However, when an alternator is tied to an infinite-grid off-normal frequency tripping most likely provides back-up relaying. That is, for the case when primary protection fails to detect sudden grid-frequency excursions.

Regards,
Phil Corso

 

Hi Santanu,

Under/ over frequency protection is set to protect the alternator and turbine at unsafe level.

Under frequency is likely to be set to active somewhere between 90% and 96% of your normal frequency and can have a time delay.

You mentioned that a feeder fault caused the under frequency event. I can think of four things which may have caused this.

1. During the feeder fault larger amount of current would likely have been supplied by the generating unit. This extra current would have come from the rotation energy stored in the spinning rotor. As this rotation energy was turned into electrical energy the speed would have reduced and the under frequency would have occurred.

2. After the feeder trip your generator was islanded and was unable to supply the load required and slowed down until the under frequency occurred.

3. The feeder fault caused a voltage drop/ disturbance to the extent that the under frequency protection could not measure correctly.

4 . False trip: Incorrect installation or setting, crossed or damaged wire and so on.