4160 volt 2200kva ac generator problem
4160 volt 2200kva ac generator problem
(OP)
We have had a EM 2200kva 6 pole turbine driven generator in our shop for repair with which we are having a problem.
The unit came in for repair with a low ground reading on the wound rotor (.3 meg ohms). We cleaned and varnish treated the rotor and got it to come up to 4200 MOhms. We assembled the generator and applied 4160v to the stator so we could run it up to speed to test for vibration (rotor leads open). After this testing was completed the rotor was found to be near 0 meg again. We cut open the connections on the rotor to isolate which coil or coils were grounded and found them to be the 2 lead poles.(the 6 poles are connected 1 circuit series).
We rewound and reassembled the rotor. Final testing indicated 200 Gigohms @ 1000v. Again we applied 4160v to the stator so we could run it up to speed to test for vibration (rotor leads open). After a brief run time we started to smell something burning and powered the unit down. Again the rotor was 0 Megohms. We cut open the coil interconnections to isolate the grounded pole or poles and found that, again, the 2 lead coils were grounded.
I guess my question is, could we be damaging the rotor winding by trying to run this generator as if it were a motor and if so why?
The unit came in for repair with a low ground reading on the wound rotor (.3 meg ohms). We cleaned and varnish treated the rotor and got it to come up to 4200 MOhms. We assembled the generator and applied 4160v to the stator so we could run it up to speed to test for vibration (rotor leads open). After this testing was completed the rotor was found to be near 0 meg again. We cut open the connections on the rotor to isolate which coil or coils were grounded and found them to be the 2 lead poles.(the 6 poles are connected 1 circuit series).
We rewound and reassembled the rotor. Final testing indicated 200 Gigohms @ 1000v. Again we applied 4160v to the stator so we could run it up to speed to test for vibration (rotor leads open). After a brief run time we started to smell something burning and powered the unit down. Again the rotor was 0 Megohms. We cut open the coil interconnections to isolate the grounded pole or poles and found that, again, the 2 lead coils were grounded.
I guess my question is, could we be damaging the rotor winding by trying to run this generator as if it were a motor and if so why?





RE: 4160 volt 2200kva ac generator problem
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 4160 volt 2200kva ac generator problem
At its rated load and rated current output of the generator, what is the corresponding rated volts and rated current flowing through the ROTOR due to the rotor excitation?
RE: 4160 volt 2200kva ac generator problem
Muthu
www.edison.co.in
RE: 4160 volt 2200kva ac generator problem
We probably repeated this run up about a 1/2 dozen times during the vibration testing. 2-3 times @ 550v and 2-3 times @ 4160. Speed recorded was 1195 RPM. It took ~ 1 minute to reach this speed.
After the 4160 volt tests we notice that the amortisseur winding became distorted in a couple of spots on the end rings.
How is it possible to induce a voltage into the rotor winding with the leads open circuited?
I've read in some other threads that there should be discharge resistors applied to the rotor leads until it gets up to full speed...could this be the issue?
RE: 4160 volt 2200kva ac generator problem
Think TRANSFORMER.
Consider wound rotor motors.
The "Polarized Field Frequency Relay" used to apply the field of synchronous motors monitored the AC voltage and both the frequency and the instantaneous polarity of the voltage induced in the rotor during the starting of a synchronous motor.
Excess current causes excess heat.
Excess voltage causes insulation breakdown.
Damage to the amortisseur winding may be from excess current.
Damage to the field winding may be due to insulation breakdown (Excess voltage).
When the insulation fails in two places the subsequent current may cause additional heat damage.
The induced voltage difference will be greater across the ends of the windings. Your damage to the end coils supports this suggestion of over voltage on the field winding.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: 4160 volt 2200kva ac generator problem
Muthu
www.edison.co.in
RE: 4160 volt 2200kva ac generator problem
RE: 4160 volt 2200kva ac generator problem
I have seen 500 KVA generators with damper windings. They are invariably present in 1 MW and above.
Muthu
www.edison.co.in
RE: 4160 volt 2200kva ac generator problem
RE: 4160 volt 2200kva ac generator problem
A picture of a German make 1 MVA generator rotor with damper winding before rewinding by us.
Muthu
www.edison.co.in
RE: 4160 volt 2200kva ac generator problem
As Bill said, induced voltage exeded insulation breakdown level.
Why use such motoring test for a generator? Vibration may be tested using a dedicated motor with a "soft" mechanical coupling.
RE: 4160 volt 2200kva ac generator problem
I interpret that you did a direct on line start. In that case I'd think the induced voltage would be highest at initial standstill since slip is highest then. I'll bet the failure occurred immediately and you just didn't notice it for awhile. If variable frequency is not available then perhaps a ramped magnitude voltage would give the rotor winding a better chance although it is not necessarily any better for the armortisseur heating (it may be less current but would last for longer time).
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(2B)+(2B)' ?
RE: 4160 volt 2200kva ac generator problem
Pete adds additional clarity with, "... midpoint of the winding (between 3rd and 4th pole) will be near ground and each end would be the highest voltage..."
(I was initially confused by the original post wording of, "...the 2 lead poles...the 2 lead coils were grounded.")
Is anyone hazarding a guess... as to how much difference-of-potential was actually taking place
that would have "tracked" or penetrated a hole [through], or around the insulation to ground?
From the original post: "...could we be damaging the rotor winding by trying to run this generator as if it were a motor and if so why?"
The answer appears to be a YES.
As iop95 asked: "Why use such motoring test for a generator?"
The opinionated John in me says, "Testing a medium voltage device using the full 4000 PLUS volts to measure
and adjust the 'balance' of a rotating member seems reckless, and the 'tester' appears to have authenticated the action".
You know what though? If I had nothing more than a stone to drive a nail, I'd work with it.
A lifetime craft to learn, and understand for sure.
John
RE: 4160 volt 2200kva ac generator problem
The excess current would have taken you directly to "Smoke".
Bill
--------------------
"Why not the best?"
Jimmy Carter