Grounded Stator Winding Sychronous Generator 3

[aolalde]

In a cruise ship, 5 identical synchronous generators are connected in parallel sharing the total load.
One generator failed to ground. Preliminary testing, with Megger and feeding AC to the fault shows one grounded spot on phase 2,

Generator Name Plate Data: 14000 kVA, 6600 V, 1224.7 Amps, 60 Hz, 514.3 rpm, ODP, PF 0.80
Stator Winding: 126 slots, 126 coils lap, 42 groups of 3 coil each, 7 circuits Wye
TPC 21, coil span 1-7, six terminal leads.

Since removal of this generator from the ship is very expensive and time consuming, the owner wants to clear the ground and work the generator with the coil or coils grounded cut out of the winding (capped).

Is this possible? Assuming a single coil is grounded what is the best way to clear that ground fault?
Should it develop dangerous internal circulating currents?

Have any one solved a similar problem? Many thanks in advance for your comments.

 

[edison123]

Thx aolalde. I look forward to your feedback.

In the mean time, I will try and see how to connect the 6 ckts for 14 poles.

 

[edison123]

aolalde

See, if this sounds right.

For 6 parallel circuits and with 6 coils in series to produce the rated voltage, the next lowest coil configuration is 108. For each phase, then the coil grouping will be

2 2 2
2 2 2
4 2
4 2
4 2
4 2

This means

1. 18 coils are to be cut out.
2. All series connections and jumpers have to be undone and reconnected as above.

If this is a global VPI winding, I am not sure how easy it will be to do the above.

If it is a resin rich and the ground fault is in the top coil, I would lift the coil and reinsulate it in-situ.

 

[edison123]

The pole band can be improved to

2 2 2
2 2 2
3 3
3 3
3 3
3 3

 

[electricpete]

kumar - The email address that I have for you doesn't work any longer. Send me another email when you get a chance.

=====================================
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[motorspert]

Aoldade,
You obviously know your stuff. The best way to effect this repair is to remove one parallel circuit from each phase. This will give an electrically balanced winding and output will be limited to 6/7 of the normal rating.

The magnetic circuit will be unbalanced, so you are right in removing the other two parallels at 120 and 240 mechanical degrees from the orginal fault.

The good coils do not need to be removed, just cut the lead and cap the leads. The failed coil will need to be removed or cut to ensure no circulating currents are allowed to flow. Remember it will be sharing a slot with another coil which could be at rated volts.

 

[aolalde]

Happy New Year

We did exactly as posted by motorspert.
The results were 100% satisfactory. After testing at 85% of original full load (the machine capacity was de-rated), we did not notice any difference on the modified generator as compared to the others working in parallel. We got almost the same temperature rise, vibration and excitation, all into conservative figures.

 

[edison123]

aolalde

I am sure you must have been working through the holidays but then you have started the new year a happy man with a very happy client.

Congratulations. And, thanks for the feedback.

(I tried to give you another LPS but the site doesn't allow it apparently)