Tests on rewound generator

[sinhchoo]

We are going to carry out the following tests on a 1800kVA 440V Generator: (stator windings were newly rewound)

1. Insulation Resistance and Polarization Index test
2. DC Resistance test of windings
3. No load output voltage with separate excitation and record excitation voltage and current.
4. Check vibration value and bearing temperature.

What are the acceptable values for all the above tests?

 

[edison123]

I would add an ac hipot at 2 KV for 1 min and a surge (if possible) test at 2 KV.

IR > 100 megohms & PI>2

Vibration < 2.5 mm/sec peak

Bearing Temp < 80 deg C

Muthu

http://www.edison.co.in

 

[TurbineGen]

I agree with edison on the AC hipot. You'll need one rated for motors. One for switchgear and interrupters will likely not work. If not a DC hipot could be a substitute.

Did you perform a loop test on the core before it was rewound to check for lamination shorts in the stator? I don't believe you can do this with the unit rewound, but I could be wrong.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[sinhchoo]

Muthu and TurbineGen, thanks for the reply.

TurbineGen, could you please explain how a loop test is carried out. The stator was found badly burnt at end windings.

 

[ScottyUK]

Was the core itself thermally damaged or just the windings? Core damage in the endwinding region is usually a symptom of under-excitation and caused by the flux distribution which occurs as a result of the weak magnetic field on the rotor. Winding damage could be poor ventilation or possibly by severe discharge activity due to inadequate conductor spacing.


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If we learn from our mistakes I'm getting a great education!

 

[TurbineGen]

Whenever I had a generator rewound I'd run the loop test to check for stator lamination shorts. It was always done before we rewound the unit with the old windings removed. The test is run by wrapping a wire around the stator several times to form a large toroid. It is then fed with AC power to provide excitation to the iron. How much power depends on the size of the machine. For our 200 MW units we were running 480V at 200 amps.

You then let the stator heat up from the magnetization (10 minutes is plenty) and take pictures of the thermal image. The more precise the thermal camera, the better. Hot spots indicate there is likely a short between stator laminations. You can then repair the shorted laminations by inserting an insulating medium between the laminations, or occasionally using phosphoric acid and a DC current works too. I've done both with excellent success.



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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[sinhchoo]

ScottyUK, I was just assigned to handle the testing part of the generator. But this is the second rewind job. The unit fail two months after the first rewinding job. This is why we are going to witness the tests before it is being transported back for installation. I have no record of the first damage but the second damage was found on the end windings.

TurbineGen, thanks for explaining what loop test is. After it is rewound, is there any other tests we can carry out to determine there is any short between stator lamination?

 

[TurbineGen]

I guess you could energize it at reduced voltage. Do you have an auto transformer available? Place a voltage on the empty stator so that you draw about 80% of your full load amperage would be my only suggestion. Keep a close watch on the winding temperature while you do this. Again you'll need a thermal imaging camera. It will need to be a very precise unit to distinguish the difference between the windings and stator iron.

This is just an engineer's educated guess. I've never tried it to see if it would actually work. I've always done the loop test with the stator windings removed.

If the damage was found at the end windings I'd suspect an insulation issue. Stator shorts cause excessive heating to the windings inside the stator slots. It doesn't usually affect the end turns.



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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[sinhchoo]

I have heard about energizing the stator at reduced voltage to cause the winding to draw almost full rated current. My concern is whether the first failure of the stator windings already created some shorting between laminations and therefore lead to the second failure which occurred after two months the generator was put back into service.

I need to check back the scopes involved for the first rewinding job.

 

[edison123]

Rewinding 101 - You never wind or rewind a stator without testing the core first for hot spots since any amount of rolls-royce quality will not save the winding from failing due to hot spots.

Since it has been rewound now, you could do ELCID test to on the wound stator to check the core health. In my opinion, ELCID is inferior compared to full flux loop test described above but it has been gaining a credibility in the past decade.

Muthu

http://www.edison.co.in