Generator Instantaneous Overcurrent Trip

[BigJohn1]

I have a re-occurring problem where a simple 1.5MVA 600V hydroelectric generator periodically goes out on "instantaneous" overcurrent for not obvious reason.

The unit has been meggered and there are no faults. It will go online and run fine for days sometimes, and then some days it will trip a couple times.

The only common theme seems to be that when it trips it is usually heavily loaded, at around 800-900 kVA at unity PF, maybe 70-90% gate on the turbine.

There are other machines tied to the bus that are not getting knocked offline, so I'm ruling out an external fault at this point.

In the past I was getting my fault readings upstream and they were uniformly about 4kA per phase at the time of trip. The most recent trip I got readings off the breaker at the generator and got 2kA on phases A and C and 700 on B, which sounds like a problem to me.

I'm tempted to do a TTR on this thing, but I don't honestly think that's the problem: When online this generator runs like a top and the phase currents are perfectly balanced.

I'm almost tempted to say this thing is being pulled out of synchronization, but how? Any ideas on this?

-John

 

[davidbeach]

What do the relay event reports show?

 

[BigJohn1]

I've got no targets on the Basler relay. Since this is a low-voltage setup the overcurrent is built into the breaker and not handled separately, and that's what's causing the trip.

This is not a breaker problem, though. For unrelated reasons I had to change the breaker out and this has re-occurred on the new breaker as well.

-John

 

[DTR2011]

Perhaps you can set the basler relay to trigger an event report, when the LV breaker trips?

 

[davidbeach]

Yeah that, or even better yet crank the settings on the breaker as high as they can go and have the Basler relay do the overcurrent protection. That way you'd get an event report for all trips. (Or at least more of them.)

 

[pwrengrds]

I would not expect to see an instantaneous trip on a generator circuit breaker.

If the Basler unit is providing protection could the instantaneous trip be turned up higher? Where is the instantaneous set at?

Is the breaker set per a coordination study or was it just left as found?

On a 1500 amp generator I wouldn't expect to see an instantaneous trip at less than 7500 amps (5x).

 

[rasevskii]

How is the 600V system grounded? How is the generator neutral grounded? Is it ungrounded or resistance grounded? Or an old machine with a delta conn winding? What about the excitation system: Old DC exciter/newer AVR or static system?

Why would the gen be dropping out of sync? Loss of excitation/bad brushes/slip rings/other fault??

Is the plant unattended/attended? Is anyone around when it trips?

Some background info needed on the plant...an old crock rehabbed or something completely new...

If a ground fault were to occur in the stator, and the system has a solid neutral ground (a bad idea) then you would possibly get a trip on phase overcurrent.

Where in the world are you, approximately...

rasevskii

 

[BigJohn1]

This is a 60 year old ungrounded generator, rehabed with a solid-state exciter and Basler relay.

The breaker definitely isn't set per a coordination study, but at this point I don't have any reason to doubt those instantaneous readings of several kA and regardless of the breaker settings, I don't believe I should ever be seeing that on a properly functioning generator under these load conditions. Agree or disagree?

I don't know the trip settings off hand, I'll have to grab them.

It's not a manned station so nobody has ever seen the trip. The operators come in the morning and the thing is offline.

I guess my next course of action is to see if I can tie into the Basler and get it to tell me some useful info.

-John

 

[BigJohn1]

Regarding my theory about why it might be dropping out of synch, I don't have a reason to believe it is, except that I can't think of any other events that would account for such high currents.

What else might cause those really high impulse currents? Thanks.

-John

 

[pwrengrds]

Its hard to say how much current you might see during normal operations without some idea of what's connected or how stiff the connected line is. If someone starts a large motor or transformer nearby it's going to spike your generator current unless the something else provides the current for the inrush. That is likely what your seeing.

 

[rasevskii]

Have a close look at the excitation system:any loose connections, bad PT fuses/fuseholders/sliding contacts on the HV or LV side of the PTs... It can be that the excitation goes berserk and takes the stator current to a high level momentarily. Even a bad voltage setting pot on the AVR can cause problems. Try reducing the gain or response time of the AVR, static systems are often set for far too fast response times.

The old DC exciter and regulator was likely very slow acting and the new system is likely unnecessarily fast or is set for too high ceiling excitation level (the max DC momentary output of the thyristor bridge).

Is this a new problem that developed long after the unit was put in service after the rehab?

Is there any possibility that rats or vermin are getting into the control panels...not unheard of.

Is there overheating at the main breaker on one or more poles or on the main drawout blades (discoloration of the conductor)...

just some ideas...

rasevskii