Inverter duty wire for generators

[gentech1975]

Does anyone have experience with generators operating loads with high harmonics, VFD, SCR etc?
I've been researching a problem with a generator that has blown holes in the windings twice now. The first rewind was with Invertor duty wire and class H slot liners. It worked for approx 6 months and blew the windings again.
4 gen sets operate a particular location and so far it is just the one unit that is having the problem. All units are the same size and manufacturer. I've given instructions to inspect the wiring, grounds etc and to do a power quality survey.
Any help is appreciated.

 

[davidbeach]

Look at the AVR. Does this generator have a different AVR than the others?

 

[waross]

I agree with David as the first step.
Another important check is the proper functioning of the Single/Parallel switch.
Check also that the settings are the same on all the AVRs. Also check that the quadrature CTs are properly sized. Improper sizes or settings in the quadrature circuit may allow excess reactive current in the generator. Some of the older AVRs had a wire wound resistor with a sliding tap. This should be set so that the quadrature action is the same on all machines.
Also, you may wish to check for blocked cooling passages and inoperative anti-condensation heaters.
respectfully

 

[gentech1975]

The AVR programs are all the same and the droop CTs are factory installed. The control panels indicate equal KW, droop, KVAR etc.
All units are set in the parallel mode Isochronous.

 

[waross]

You may have a problem with the rotor. An intermittent fault in the rotor may cause high voltage pulses in the windings. An intermittent fault in the AVR may also cause problems.
respectfully

 

[waross]

PS;
Have you checked the rotating surge suppressors?
respectfully

 

[catserveng]

Have you compared all operating parameters on each of the units while under similar loads?

Are the AVR field outputs levels close for a given load? As mentioned before, are the units propotionally sharing the VAR load? You should record both the field volts and field current. We find using a scope looking at the field output to be very helpful. Paralleled units are sometimes not properly adjusted for stability since the system tends to set the voltage level, but we do find some regulators are trashing around pretty hard while others in the system are floating along. We have found this to lead to generator problems if allowed to continue.

Does the unit have stator temperture detectors? What are the readings?

In some applications we have installed surge capacitors depending on the type of harmonic problems the system has.

Is the neutral connected? Any circulating currents?

Is the generator operating within it's reactive capability curve? Where is the power factor? What is the THD and what harmonics are you seeing?

Does this unit show any mechanical indications of problems, such as a higher vibration level?

What is the winding pitch of the machine, is it appropriate for the high harmonic loads?

This is the type of information we would start to gather in troubleshooting this type of problem, hope that helps.