4160 volt 2200kva ac generator problem 1

[accordlx]

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?

 

[waross]

You are probably inducing a voltage in the rotor windings that is well in excess of the insulation breakdown level. The induced voltage may be expected to be the highest at the ends of the winding.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racookpe1978]

The 4160 volts is across the stator output connections, right?

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?

 

[edison123]

The synch motor (as you ran it) is not self starting. Probably you got the speed solely from pole face mounted damper winding, which would have got overheated with full stator voltage applied. Did your pole faces appear overheated after you pulled out the rotor? How long did it take to get to full speed?

Muthu

http://www.edison.co.in

 

[accordlx]

The generator is self exciting. We do not know the dc excitation voltage as is is not stated on the nameplate.
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?

 

[waross]

"How is it possible to induce a voltage into the rotor winding with the leads open circuited? "
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

 

[edison123]

Your amortisseur winding acted as an cage rotor helping the start and it is meant for short time duty during normal operation, not for a full minute overcurrent. If you got the full speed at 550 V to check the vibrations, there is no need to go to 4160 V, which will overheat the damper winding. Also, it is advisable to short the main rotor terminals during this low voltage run as a motor.

Muthu

http://www.edison.co.in

 

[ScottyUK]

How comon is it for a 2.2MW machine to have an amortisseur winding? I didn't think they were particularly common on small sets, but I don't deal with them day-to-day so I could be wrong.

 

[edison123]

Scotty
I have seen 500 KVA generators with damper windings. They are invariably present in 1 MW and above.

Muthu

http://www.edison.co.in

 

[ScottyUK]

Thanks edison, I hadn't realised they were used so far down the size range. My little bit of learning for the day.

 

[edison123]

Scotty

A picture of a German make 1 MVA generator rotor with damper winding before rewinding by us.




Muthu
www.edison.co.in

 

[iop95]

Voltage are induced in open coils, no need to be closed. And are much higer in an open coils.
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.

 

[electricpete]

Yeah, that scenario makes some sense. I guess with a string of six series poles open circuited and with no connection to ground, the capacitive coupling will establish the voltage to ground (rotor). Assuming symmetry, the midpoint of the winding (between 3rd and 4th pole) will be near ground and each end would be the highest voltage (E/2 at one end and E/2 opposite polarity at the other). Then once one end fails, the ground immediately attaches at that end and the opposite end goes to full voltage E and would fail shortly after. Once you have those two shorts there is now a path for loop current flow through the faults and probably after awhile that gave the smell.

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).


=====================================
(2B)+(2B)' ?

 

[dArsonval]

The theory of "induced voltage' into the rotor has total merit, via the TRANSFORMER effect.

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

 

[waross]

Now if you had only shorted the coil end together, you would not have had to wait for the the coil to go to ground in two places to produce smoke.
The excess current would have taken you directly to "Smoke".

Bill
--------------------
"Why not the best?"
Jimmy Carter