Stator winding Fault

[jan63]

A couple of days back one of our generator tripped on generator differential protection and smoke started coming from it.The CO2 did not operat on auto so it was manually released.Also the transformer diiferential protection operated resulting in spray on trf.
Inspection revealed 60 to 70 bars effected.The lower sid of bars out of slots was effected.Six or seven had completed melted while other seemed to have suffered from heat.It was a class B insulation.
I want to ask whether the slots in direction of rotation will suffer from heat or the one behind.The rotation is clock wise.I hope I make my self clear.Also why did trf.differential protection operated

 

[rbulsara]

CT saturation due to high fault currents could lead to a false transformer differential trip, even if the fault is outside the protection zone.


I have no comment on the slot damage.

Rafiq Bulsara

http://www.srengineersct.com

 

[slavag]

Many reasons for the unwanted 87T operation.
As Rafiq saied, firstly: CT saturation.
You have send us more information.
SLD
Size of generator and Xfr
CT size
Type of protective relay
Setting of 87T, etc

Best Regards.
Slava

 

[dpc]

I assume you are certain the generator was not in the transfsormer differential zone? For unit transformer, it is fairly common to have a differential relay covering both the transformer and the generator.

 

[edison123]

The direction of rotation has got nothing to do with winding failure. Since yours is a class B insulation, it's probably over 30 years old and has reached the end of useful life and hence the failure.

Muthu

http://www.edison.co.in

 

[jan63]

I agree that direction of rotation has nothing to do with winding failure.My point was that since the co2 did not operate on auto and there was a delay in maunual release.The flash or fire will damage which bars considering the direction of rotation.
Also the transformer differential also covers the generator but since the gen.differential operated why did the trf.differential operated.
Winding was installed in 1977

 

[davidbeach]

All differential zones that include the generator should have operated. The zone that is both transformer and generator would not hold off just because the zone that is generator only operated. Wouldn't want that to happen anyway, that's the whole point to having multiple overlapping differential zones around a generator.

 

[slavag]

Hi.
Isn't Xfr differential-it's block generator-transformer differential.
Operated in the same time as generator differential. Is OK.

 

[wolf39]

jan63:

The melting of some six or seven of stator bars has been caused by arcing. This was the primary cause of destruction. The secondary part of winding destruction has been caused by the heating energy from the place of arcing. This energy has been "transported" from the place of origin to the other stator bars "downstream" of the arcing area because of rotor rotation. Modern insulation is regarded as not burning by itself. Manufacturers even claim self-extinguishing properties. However, as long as arcing is present considerable energy is feeding the environment. In your case CO2 application was delayed, this further deteriorating the situation.

Was there a delay in de-excitation also (protection failure)?

Regards

Wolf

http://WWW.HYDROPOWER-CONSULT.COM

 

[jan63]

No delay in de excitation.But the excitors are rotary exciter so the field breaker has a delay.This winding had a past history of similar faults.Three times the unit was shut down due to burning smell.This time either the fault was sudden or the noses did not work.But your point on down stream is not applicable.Consider the fault at 12 o clock.The burning effect was at 11:50 to 12:00 and not 12:00 to 12:10

 

[PHovnanian]

jan63. It appears that the question of why the stator damage propagated 'backwards' against the rotor's direction has yet to be answered.

Here's a thought to mull about: Which direction would an arc (a conductive plasma) be driven in the magnetic field between the stator and rotor?

 

[esee135]

Likely 7 stator bars in sequence is a line to line failure in the end winding region. I assume your stator pole is less than 7 bars wide.

Did you determine the nature of the failure? It sounds like a failure in the end winding region which spread to the adjacent bars causing your cascading failure.

Multiple failures of the winding from the same machine is typical end of life behaviour. Does your company perform routine HV testing (hvdc ramp/negative charge PI)?

I assume you have a stator bar versus a stator coil. Each winding system has different failure modes.