12 Lead Generator Connection Question

[catserveng]

We have become involved with a failure analysis and repair of a competitors generator. The unit is a 225 kW, 12 lead machine connected series wye for 480 VAC line to line. The unit is in standby service with 256 hour since install. The failure was in the slots of the stator core near the end turns. There was no report of opperational problems, excessive load, or faults prior to failure.

Our shop removed the unit and sent to a local motor shop for repair. No apparent problems were noted except that it appears on the B phase (T2), the coils are swapped. The lead marked 5 is connected to the neutral star point, lead 2 is connected to lead 11, and lead 8 is on the T2 terminal.

We have asked some of our available generator "experts" to see if this could be a contributor to the failure. While all agree this isn't correct, none have been able to offer if it is a possible cause of failure.

The unit was tested weekly by the customer's operation staff with no apparent problems, the connected load is about 45% of unit rating. There is no data for winding temp or field measurements.

Have seen generators conected in just about every way possible, but this is one I haven't seen before and if anyone has some experience or insight, I would appreciate it.

Thank you,

 

[davidbeach]

Swapped end for end, or in the wrong order? If in the wrong order, there shouldn't be any issues, and if there were no apparent operating problems then that is what I would guess, not having connection diagrams memorized. Both windings have 240V from neutral end to terminal end and connected in series it shouldn't make any difference which is where provided the insulation is all the same. If one winding was insulated for only 140V to ground and the other for 277V to ground there would be a problem if they were in the wrong order.

 

[catserveng]

David,

They were in the wrong order. Normally on the T2 phase in series 11 would be at the star point, 8 would be connected to 5 and 2 would be on T2.

We've been kicking this around a bit, the technical guys here want to try hooking the replacement up the same way and take some readings, for some reason management seems reluctant to let us do that.

Thanks for the reply.

 

[mottt]

is this a Stamford Newage generator?? if yes i could help...

 

[waross]

Hi catserveng;
Just a shot in the dark.
We both know that all the windings should withstand more than 480 volts to ground, but could the coil swap have left two windings physically adjacent with more than the normal voltage difference between them. This would not in itself cause a failure, but in the event of insulation deterioration due to condensation or some other cause it may make one of the swapped windings the first to fail in a failure that was going to happen regardless of winding swaps.
Were the swapped windings involved in the damage?
DB, a parallel example may be a transformer with two 120 volt secondary windings. The normal connection to supply 240 volts is to connect X2 to X3 and take power from X1 and X4. This is similar to connecting X1 to X4 and taking power from X3 and X2.
respectfully

 

[TurbineGen]

Could the coil swapping have been for a main-neutral interchange procedure to extend the insulation life of the coils? This has been considereded by many utilities on larger machines, though I haven't heard it done on a 480V machine. Just a guess.

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

 

[catserveng]

We got the gen end in the shop yesterday and I spent some time taking a hard look at it.

There are actually two coils grounded, one of the coils affected by the question I posted, and another coil on T3. The coil on T3 is the one that blew out and damaged the iron core.

There are signs of corrosion present at several points where the windings leaves the slots. Further investigation found the space heater was open, so obviously not working for some time. Since install about 6 year ago there has been no insulation tests performed. The unit is installed at a shoreside facility about 55 feet from the water's edge.

Based on these inspections, we feel this is likely a general breakdown in the insulation caused by environmental conditions and lack of a functioning generator space heater. While the apparent misconnection of the leads doesn't appear correct, it is hard to call it a factor in this failure based on the info we have.

Thanks to everyone for their responses.