MV Protection Trip
MV Protection Trip
(OP)
A 33kV feeder of ours connects to a remote substation B with a single 33/11kV Dyn11 transformer connected at B, and an 11kV distribution board at B. The relay on the 33kV feede CB at Substation A has Overcurrent & Earth Fault protection. There was a phase to phase fault and the 11kV CB at substation B opened on OC prot. About 2 ms AFTER the correct opening of the 11kV CB at sub B, the 33kV feeder at sub A tripped on 33kV earth fault prot, which we find strange. The 11kV feeder which faulted runs for about 2 miles on conjoint poles carying the incoming 33kV supply with the 11kV underbuilt bu about 2.5 metres. Any ideas as to why this may have happenned?






RE: MV Protection Trip
Also, did it happen on the first trip of the 11 kV relay or was it possibly after some reclosing operations?
Alan
RE: MV Protection Trip
The 2ms difference indicates that both breakers started opening at the same time, just that other took 2ms longer to trip. The trip action has already taken place. ( Also how did you synchronize the clocks in the two relays?, but that is a different question)
It probably has to do with the sensitive E/F setting on 33 kV side and/or also the type of E/F scheme employed. Not enough info. Harmonics during a fault can fool a realy to think that there is unbalance between phase currents, showing up as E/F. Difficult to say without any waveform captures.
Rafiq Bulsara
http://www.srengineersct.com
RE: MV Protection Trip
RE: MV Protection Trip
Please add information.
What are a current amd time setting on the 11kV side OC function?
What is a current and time setting of the 33kV side EF function?
What is a tipe connection to EF input ? dedicated toroid, Holmogen connection.
It's known result of phase to phase fault on the Y ground side.
Best Regards.
Slava
RE: MV Protection Trip
11 kV OC Set to 210A (11 kV) with a 0.15 TMS Standard Inverse. The 33 kV HV CB on the 33kV side of the Dyn11 transformer is set to 20 A EF and it didn't see a thing, so i'm leaning towards some sort of mechanical interaction.
All CTs measure the neutral spill of the CT circuits for EF, no fancy coils etc.
RE: MV Protection Trip
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If we learn from our mistakes I'm getting a great education!
RE: MV Protection Trip
RE: MV Protection Trip
Are you have DR file of event.
11kV OC is IDMT, EF of 33kV is DT with low current setting.
You must check time grading.
RE: MV Protection Trip
On the other hand, I'd never want to set primary protection at 1 second. Both systems could probably be sped up.
RE: MV Protection Trip
RE: MV Protection Trip
I dont know the conclusion what may have happened...
RE: MV Protection Trip
Alan
RE: MV Protection Trip
Exectly. It's not zero sequency, it's some kind of unbalance
and reasidual or Holmogren connection see it as ground-fault.
RE: MV Protection Trip
On the other hand, if the 33kV relay is actually a negative-sequence relay looking at imbalance, that would be a different matter.
The 11kV tripping in 0.45 sec (still way too long for something that close to the 11kV source) then the hot gas from that fault established the ground fault and the ground fault was able to continue without the support of the 11kV fault.
RE: MV Protection Trip
Ia+Ib+Ic=0, if Ia+Ib+Ic not =0 you have ground-fault trip, and not important what is a reason:
really ground fault, unbalance, inrush or disconnected CT wiring. If you have dedicated toroid, of course no trip, but residual connection it is something other.
And two phase fault on the wye side is unbalnce voltage on the delta side and as result of unbalance voltage-unbalance current.
BTW, what is 0.45sek trip from/to, alingstone , are you have event list, DR fail.
Best Regards.
Slava
RE: MV Protection Trip
But any time you have the sum of all three phases flowing into a delta winding, and there is no fault between the CTs and the transformer, the only residual current you can ever see will be due to CT mismatch. Never from the other side of the transformer.
RE: MV Protection Trip
What is the power of 33/11kV transformer and 33kV CT ratio?
RE: MV Protection Trip
could harmonics or CT saturation play a role?
Rafiq Bulsara
http://www.srengineersct.com
RE: MV Protection Trip
RE: MV Protection Trip
I agree with you. I missed the fact that was distribution circuits underbuilt with transmission lines.Could be an evolutive fault to other line.
RE: MV Protection Trip
Short circuit forces from 11 kV fault current in the 11 and 33 kV conductors cause poles and conductors to swing. Swing path of one of the 33 kV conductors intersects a down guy or other grounded object.
I have heard of this scenario causing a line to line fault on the higher voltage circuit when the conductors slap together. Slight alteration for this situation.
RE: MV Protection Trip
RE: MV Protection Trip
Thanks, just wanted to know your opinion. Your suspicion seems more probable of all scenarios.
I am still not clear what the breaker B operating 2ms AFTER A means. Did it pick up 2ms later or completed opening. Were they acting simultaneously or there is enough time difference to allow for hot gases to travel up 2.5m (8+) feet), cause flash over plus the relay time. Simultaneous operation would be more puzzling.
Rafiq Bulsara
http://www.srengineersct.com
RE: MV Protection Trip
RE: MV Protection Trip
Rafiq Bulsara
http://www.srengineersct.com