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Offline Overcurrent Protection
What is Offline Overcurrent Protection for Synchronous Generators?

What is Offline Overcurrent Protection for Synchronous Generators? How does it work?

Responding to Speedtronic's 07-Jan-09 (23:50) query..

Are you referring to the inadvertent closing of a generator's connection to the grid or another generator without having first synchronized?

By speedtronic on 10 January, 2009 - 3:15 am

<Are you referring to the inadvertent closing of a generator's connection to the grid or another generator without having first synchronized?>

Actually I have no idea that's why I am asking. Can you please explain both cases.


If I'm wrong, please correct me, but, I have interpreted your question to mean, what backup protection guards against indvertent closure (whether caused by human or equipment failure) of the generator's main breaker while:

a) it's off-line, and not running.

b) it's off-line, but spinning at nominal speed and voltage.

Where, or how, and in what context, did you hear the term "Off-line Overcurrent Protection?"

Regards, Phil

By speedtronic on 11 January, 2009 - 11:54 pm

Well, my understanding about offline is that GTG/STG is spinning at turning gear speed & both its synchronization (main) breaker & excitation breaker are open.

But I think the point you are making about inadvertent energization of main breaker of GTG makes sense. I think this offline over current protection is at back up of reverse power protection for such cases.

Speedtronic... unfortunately there is no "one size fits all" answer. For example, your understanding that it's akin to backup of reverse-power protection won't work if inadvertent energization results from an undesirable closing of the main breaker but then it cannot re-open.

Just think through the different scenarios to consider for two of the most popular system configurations, i.e. GT -> main breaker -> grid transformer vs GT -> unit-transformer w/o the main breaker.

However, all is not bleak. Interlocks can be provided using existing hardware and protective relays, which can ameliorate damage caused by "Inadvertent Energization."

BTW, there are some system configurations where inadvertent energization can occur without involving the GT's main breaker!

By speedtronic on 17 January, 2009 - 11:47 am

Sorry, I was wrong that offline over current is back up for reverse power. Once breaker is closed then Gen. will be no more offline.
I have concluded that offline overcurrent protection is for inadvertent energization of field breaker while main breaker is open.

By Pulpmill power generation on 12 April, 2010 - 4:28 pm

Off-line over current protection is designed to help troubleshooting between generator side faults and power system faults. While off-line, very little current should flow (charging current) to maintain terminal voltage. If you have high current, the problem is within the zone between the generator neutral and the synchronizing breaker. It can be set very sensitive (5% FLA) compared to on-line over current (115% FLA). There is a good explanation in the GE Multilin 489 protective Relay manual.

Inadvertent energization is to protect against motoring the generator if the breaker is closed with the generator at standstill.