"Semi" Paralleled Diesel Generators 1

[ghostbuster7]

3-1200 KW. diesel generators are started up and paralleled together thru a Woodward electronic package.

The entire stator winding assembly in 1 geny all of a suddenly becomes loose and mechanically rotates by approx. 10 degrees.

Question:
Before any breakers have a chance to trip,would you expect to find these extra torsional forces to break welds on the other 2 geny stator assemblies?

 

[dpc]

Possibly. It depends on what cause the first generator to fail. If it was closed in out of phase, that could create a pretty good transient torque on the other generators as well.


"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[ccjersey]

Why would it?

I would bet on two against one winning out. Also would expect the stator rotation to happen slowly enough (relative term here!)for the phase angle to never get too far out.

 

[dpc]

Way too many unknowns to say anything definitive. None of the generators is going to "win" when subjected to fault current.

We don't know what happened to cause the first generator to fail, so how can we say it is not possible that some damage could have been done to the other two generators?

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[waross]

I have seen a pair of 600 kW and three 350 kW generators gensets paralleled several times a day as the load on the system changed. They ran for an unknown (to me) number of years with a 30 degree error wired into the synchroscope.
They would usually stay online if the operators closed the breaker at exactly 12 Oclock on the 'scope.
If they were a little early a breaker would trip.
If they were a little earlier they would shear a one inch key in a coupling.
Is this a new installation?
Are there operators on-site and do they become involved with paralleling?
Were you there when the incident occurred?
Did the incident occur during paralleling or when already on line and sharing the load?
If two sets were already happily paralleled and the third came in out of step, I would expect the third set to take the hardest hit. It would take 100% of the current but the two sets in sync would each take 50% of the surge current.
It's a good thing that it only rotated 10 degrees. Much more and it would have chopped its own leads off.
Could be faulty welds and/or a block load hitting the system.
Could be an out of step closure.
Could be operator error.
Could be a wiring issue.
Bottom line. Over the years I have seen a lot of equipment badly abused and only once saw a stator spin. The motor stator that spun had been in service for a long time when it let go. It did chop its leads off. (Then it coasted to a stop.)
I strongly suspect that the generator stator to case welding was faulty if the stator spun, no matter what caused the stress.



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

 

[ghostbuster7]

Clarification Points:

1.All standby rated genys at this site are over 15 years old
2.They have run essentially problem free(minor problems only)
3.There usual operation in test mode is runup all genys in a parallel scheme(Woodward controller) all unloaded
4.Next is to transfer some building load or in house resistive load banks on them
5.At time of failure(stator shifting)approx.100kw. of building mechanical load only was transferred onto the 3 paralleled Genys using Asco switch with in-phase monitoring
6.Failure occurred at the instant the transfer occurred
7.Stator rotated up to the point where the leads(were being guillotined) grounded out to the geny frame
7 Short time trip setting on geny breaker was actuated
8.It is suspected that outside rainwater was sucked in and got into the very dryed out and broken down lead insulation
9.This insulation failure(possibly phase to ground) caused severe short circuit forces in the geny and snapped the stator welds before the breaker was able to trip

ANSWERS TO QUESTIONS
1.Yes the operators were present and started this process(NOTE: these operators have routinely done this at least twice per month for the last 5 years)
2.I was not at site when this occurred
3.All welds on the other 2 surviving genys were surveyed at site and found to be all cracked (all genys sent out for repair)
4.Transfer switches were personally checked by me(after the failure) with scopes and chart recorders and found to be transferring essentially in-phase

 

[waross]

How is the condition of the Asco switch.? The Asco's (smaller sizes) that I have seen have very close spacings. It may be that the "in phase" monitor of the Asco failed or the frequency was too far off and the Asco flashed over internally. That could break the weakest, cracked weld.

3.All welds on the other 2 surviving genys were surveyed at site and found to be all cracked (all genys sent out for repair)

100 kW shouldn't be a problem with three 1200 KW sets in parallel. Bad insulation would probably fail as the sets were run up. BUT a transfer switch flashover between utility and genset may cause the torque surge that broke the weak welds.
Remember, if the event happened at the instant of transfer, the odds heavily favor a transfer problem over an insulation problem that could happen any time.

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

 

[ghostbuster7]

When I checked,the transfer switch involved in the failure was transferring in phase as earlier stated.
However,we did also measure voltage spikes generated by this transfer switch greater then 1000 volts each time when transferring to geny power.

These spikes were only generated under no load conditions.With load, the spike level dropped down below 300 volts.

Other Asco transfer switches were checked and all were found to do the same thing as above.

 

[edison123]

Were these welds in the back iron (i.e. OD) of the stator core to the stator shell ? Can you post some pics here ?

 

[ghostbuster7]

I have pictures of the cracked welds(real close-up pictures using a flexible 3' long camera device made by Rigid).This allowed for deep visual inspection within the stator to examine for potential damage.

I do not have pictures of the actual stator assembly,but will attempt to describe.

The stator winding is hydraulically compressed.Sandwiching the stator winding together at each end are large metal rings approx.1" thick,4" wide and outside dia.approx.same as I.D.of steel alternator enclosure.With compression being held on this stator assemly,these steel rings are welded to the enclosure at approx.10 points around the circumference.

NOTE:The stator winding is not mechanically attached to this enclosure.The only thing preventing it from rotating is the frictional pressure between these steel rings and the stator.

 

[edison123]

The reaction forces on the stator core against the stator frame are quite large. There must be an axial lock for the entire length of the core with the frame against rotation, like an axial key or axial welds along the stator core length.

Welding of the two end core pressure plates alone to the frame would not suffice.

 

[ghostbuster7]

Edison

On this brand of stator there is NO NO NO mechanical locking system between the stator core and frame.On the repair of these units,there will be 1/2 " dia. holes and steel pins installed around the circumference.This will mechanically pin the core to the enclosure.

 

[edison123]

yeah ? no wonder the welds broke and the core assembly turned. lousy design.

 

[ghostbuster7]

UPDATE:
It was found all the generators were set up on a scramble start to get to the dead bus.On retest with the new rewound generators,it was found 2 generators tried to close in (out of phase)on the dead bus at the same instant in time .There was a huge earth shattering vibration sent thru the floor when this happened.Timing circuits have been now installed to prevent this from re-occurring.

 

[ScottyUK]

Wheeee, that would be exciting. Thanks for the update.


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

 

[itsmoked]

WOW! I would like to have seen that. (From a distance!)

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alehman]

I saw the result of a similar incident a number of years ago. Upon arrival I was shown several large drums full of miscellaneous engine parts that had been gathered up. I seriously doubt all the King's horses and all the King's men would have been sufficient.

They do make relays designed to prevent such things.

Alan
----
"It’s always fun to do the impossible." - Walt Disney