Generator Excitation

[Vincente]

Ladies; Gentleman
Hopefully you can help me to understand a problem we are dealing with - I was called in to troubleshoot why our generators at a certain building were running and had not timed out. When I arrived I found that we had a breaker trip that feeds one side of two substations, which tie over automatically, building normal after tie over. I will not get into details of cause. The problem is that on one side of these units the fire pump feeders are tied in ahead of the main which kept the generators on line. We asked our management if we could turn off while we dealt with tripped beaker problem and fire watch building explaining the problem with running these unloaded, this was denied.
About five hours in one of the paralleled generators tripped offline with a excitation fault. Now before this, we noticed that these generators showed a load of over 600 amps with no load connected, this load was on generators them self, on our protective relays (paralleling gear) and feeder breakers that were open.
Question -
Is it normal to see this kind of current on unloaded generators?
Is the excitation fault on protective relay saying that we have a problem that will cause damage to generator if we were to test? We called in generator rep who started this generator and said all is good? Being I'm not a generator guy does this seem correct? I would think it would need more investigation?
Was the current we experienced caused by this problem and we can expect same on non faulted generator? Thanks!

 

[iceworm]

Question -
Is it normal to see this kind of current on unloaded generators?
Nope

Is the excitation fault on protective relay saying that we have a problem that will cause damage to generator if we were to test?
For a paralleled generator - Yes. Loss-of-Excitation can cause the gen to get where it will slip poles.
Islanded - No.

We called in generator rep who started this generator and said all is good?
Interesting. I would want to see the report on the LoE fault resolution. And the report on the 600A load with all the CBs open

Being I'm not a generator guy does this seem correct?
No
I would think it would need more investigation?
Yes. As much as it takes to understand the causes of the faults/abnormal operation.
Or, you believe your generator guy and walk away.

Was the current we experienced caused by this problem and we can expect same on non faulted generator?
With the limited information we have - I don't know.

We asked our management if we could turn off while we dealt with tripped beaker problem and fire watch building explaining the problem with running these unloaded, this was denied.

That is a problem. If the management is not allowing reasonable troubleshooting - engineeringly speaking - yer screwed

 

[petersonra]

could be all kinds of things.

I don't see how a generator could have 600 Amps of current if the downstream breaker is open unless there is some serious instrumentation fault or a short circuit. If you have a short circuit somewhere it might explain the excitation fault.

 

[synchro]

The circulating current between unloaded paralled generators should typically be held to no more than 25% of the generator nameplate rating. The generator's manufacturer should have a spec on this.

 

[ATSman]

As post #2 and #4 stated 600A of current reading on one gen that is paralleled with no load connected tells me that one gen is trying to motor (drive) the other gen. The relay trip on loss of excitation is evidence of this. When a gen looses excitation it can no longer produce output so instead of putting out power it takes in power and becomes a motor driven by the other gen. I am not a gen guy and am not sure if loss of excitation is the same as reverse power but this is from google "When a generator loses its excitation, the rotor current decreases gradually and the field voltage slowly decays as dictated by the field time constant. As a final outcome, the generator starts to consume reactive power from the power system instead of supplying it."
Quora website:
"In case of a synchronous generator, loss of field excitation will lead to its operation as an asynchronous generator only if damper winding has been provided.
The machine would have operated as an induction motor had there been a loss of prime mover.
This happens simply because in case of failure of the field, the synchronous generator does not supply any power. As a result the entire input power from the prime mover goes into increasing the kinetic energy of the rotor. This results in an increase in the rotor speed over the synchronous speed. Direction of current changes in the stator but the direction of rotation of stator magnetic field remains same. Now recall the condition of operation of conventional induction generators-

• They must be grid connected (or must have static capacitors for excitation) to allow then to draw magnetizing current.
• The rotor must be rotated at super-synchronous speed in direction of rotating magnetic field.

Hence the Asynchronous operation
The reverse power relay could still trip if it has been set for very small values. The current drawn by the machine during the above operating condition is mostly reactive in nature. The above operation will result in overheating of the rotor due to flow of continuous induced currents through damper winding."

You should be able to run each gen separately to determine which one is the culprit, then call in a gen vendor to further t-shoot and make the necessary repairs. It could be a bad voltage regulator, loss of residual magnetism, etc, etc?

 

[Vincente]

Thanks for the reply,
I believe I stated some of the above incorrectly in that these were not synced being that the load breakers on paralleling gear were open meaning they were on their own. Maybe problems with both? Excitation from end generator.
I will pass this on to my superiors and appreciate your expertise!

 

[iceworm]

I would not put any faith in anything I said. My post 2 was in response to the OP - not the amended post 6. I'm right square in the middle of, "Insufficient data for a creditable response."

Sorry, I don't have a help

 

[mbrooke]

could be all kinds of things.

I don't see how a generator could have 600 Amps of current if the downstream breaker is open unless there is some serious instrumentation fault or a short circuit. If you have a short circuit somewhere it might explain the excitation fault.

You could if one of the gens is severally over excited producing VARS and the other is "absorbing" them.

 

[petersonra]

You could if one of the gens is severally over excited producing VARS and the other is "absorbing" them.

I got the impression from the OP that the current reading was with the generator isolated by a downstream trip.

 

[mbrooke]

I got the impression from the OP that the current reading was with the generator isolated by a downstream trip.

Could be- I'm just guessing to be honest.

 

[Chillmattic]

I'm a turbine operator. My guess would be you were consuming KVAR's and that is were the amp reading was coming from.