Loss of Excitation on 10 MW Turbo Power Generator


Thread Starter


Dear All

We have 3 Power GT PGT10 driving a JEUMONT Generators. Usually 2 generators are running in parallel mode and the 3rd one is on Standby. We lost the PMG (Permanent magnet generator) on one of those GENs and the root cause of the damage was due to the PMG Bearings failure.

A new OEM genuine PMG has been installed but we still have the same trouble. The Gen Main Breaker opens due to excitation failure.
During the PMG replacement we also removed the exciter end cover and we cleaned all the armature/Stator area from dirt and dust than we checked and tested diodes, connections, cables and found fine.

My questions are:

How to solve this problem? And what's going wrong with this Generator and its exciter?

When the PMG has stuck and has burned, what's its effect on the other components of the generator?

Thanks In advance
after closing the generator circuit breaker are you sure there is load?

If there is no load, you will not see the Amps (excition).

it is like you are closing the breaker on dead bus-bar (no load).

best luck

A PMG usually generates an AC voltage that is fed to the "AVR" which rectifies and controls the amount of DC voltage applied to a stationary field which then produces AC on a rotating armature on the shaft which is fed to a rotating diode wheel which produces DC which is applied to the rotating synchronous generator field(s) to produce AC voltage at the generator terminals. Not all PMG exciters are exactly like this, but they are all very similar still.

If the unit can be synchronized then it's likely that there is generator terminal voltage during synchronization, but it seems that something is happening when the generator breaker closes and the unit tries to produce current to power the load. The PMG has to provide sufficient voltage/current to the exciter to be able to produce DC to apply to the stationary field of the brushless exciter.

If the configuration is such that the loss of excitation detector is looking at the DC output of the exciter (AVR) then either the input to the exciter is low, or the output is low.

Some loss of excitation relays look at VAr direction, and if the VArs are exceedingly "negative" (leading) that's usually the result of low excitation and so the generator breaker is opened (tripped) and/or the unit is tripped. So, it's important to understand how the "loss of excitation" condition is being detected--but there's still the issue of why VArs are excessively negative, which is usually the result of low excitation. But, the problem may be a sensor issue--and there may be no loss of excitation. You don't seem to know, yet, if there is truly a low exciter output or if the VArs are going excessively "negative"--or if it's a sensor problem. (Either type sensor could have been damaged when the previous PMG failed.)

And, when you say "... we still have the same trouble....", did the unit originally trip on loss of excitation--which would seem reasonable if the PMG failed, because if there was no "input" voltage to the exciter (AVR) there would be no output voltage from the exciter (AVR). (And, the VAr magnitude/direction would likely go negative (leading) on loss of exciter (AVR) input/output.)

You don't say when the Main Generator Breaker opens--immediately after synchronization, or at some light load, or some higher load?

Does the AVR produce and alarms/alarm messages during synchronization, or when the apparent loss of excitation occurs?

If the PMG was damaged, then it could be that something was damaged in the exciter (AVR). So, simply replacing the PMG may not be enough.

Again, if you're able to produce generator terminal voltage and synchronize the unit before it opens the Main Generator Breaker on loss of excitation, then it's likely there's still some problem with the exciter (AVR), or possibly the rotating diode wheel (if there is one), or something like that.

You need to look at the generator and exciter (AVR) drawings to determine where to take voltage measurements to ensure everything is working properly.

Please write back to let us know what you find!
Dear CSA;
I also posted this topic on eng.tips.com forum and an electrical had recently a similar trouble. Here is below his response:

<i>I recently had a similar problem on a larger diesel engine driven generator, the unit tripped due to a loss of excitation fault and the PMG was damaged, we made repairs similar to what you described, the unit was put back on line and ran about 2 hours, then tripped on loss of field. Unit was checked, no obvious problem, restarted and put back on line, tripped again after a short time. Unit had a Basler DECS regulator, we found the AVR was failing after being on line for a short time, noted that each time we put it online it took less time to trip. Perhaps you have a similar issue? Not sure what type of voltage regulator you have, but if the PMG rotor comes in contact with the PMG stator, it may produce a spike or power issue that could damage the AVR, at least in my experience.

What provides the excitation fault indication for your system, a loss of field (ANSI 40 device), or a signal from the AVR?</i>

See link: http://www.eng-tips.com/viewthread.cfm?qid=395512

About my concern,

A double check had been done and the electrical team had replaced the 12 diodes of the rectifier. Now, the unit is running about a week and no trip happened.

I think the problem is solved.

Thanks lot Sir CSA