Worst Electrical Fault
Worst Electrical Fault
(OP)
Which is the worst kind of fault among, Single Line to ground (SLG) , B-C fault and A-B-C-E(Earth) fault from a protection equipment point of view?
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RE: Worst Electrical Fault
On the other hand, it depends what you mean by "worst". As far as I'm concerned ANY fault is a BAD fault.
When sizing fuses you should consider the fault type which will result in the lowest fault current, and size your fuse to clear that fault.
RE: Worst Electrical Fault
A spring tension connection on a fuse clip overheated and melted at 2400V. Once the copper path was gone, an arc bridged the gap to keep current flowing in the burned open phase. There was only fuses and overcurrent relays on the upstream feeders. This arcing fault persisted for several minutes until the feeder was manually shut down because of flickering lights downstream of the affected phase. Several pound of copper were pooled up in the bottom of the gear and the entire cubicle had to be rebuilt from the resultant damage. As a testament to the switchgear manufacturer's design, no phase-phase or phase-ground fault occured even though ionized gas filled the cubicle during the event.
These are also the worst kind of faults for residences as these are the largest cause of electrically initiated fires.
Hard faults or low impedence faults are easy for protective relaying to "see" and thus clear.
RE: Worst Electrical Fault
RE: Worst Electrical Fault
Comments?
RE: Worst Electrical Fault
RE: Worst Electrical Fault
However, the single phase-to-ground fault may be greater than the three-phase-fault in special cases such as near generator under transient conditions or at the bushings of the Wye-Zigzag transformer.
This is because the zero sequence of the source is less than the system impedance upstream the fault.
The figure in the enclose link may be used to determine the maximum SC.
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RE: Worst Electrical Fault
RE: Worst Electrical Fault
RE: Worst Electrical Fault
I had the opportunity of talking to the late, great JL "Blackie" Blackburn way back in the early 70s when I was working at Westinghouse. I asked him what the worst kind of short circuit that could occur. He said that it was the three phase bolted fault. While a phase to ground fault can generate more SC amps, the three phase bolted fault injected more power (hence more enery)into the fault than any other type
RE: Worst Electrical Fault
RE: Worst Electrical Fault
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Worst Electrical Fault
Either single or three phase faults can have the highest fault current, depending on configuration and distance from source.
As others have said, a 3 phase fault for similar current levels will deliver more energy, and a low current fault may delay clearing times.
RE: Worst Electrical Fault
However, playing with SKM on a recent project, we were seeing high SLG at the y side of a Dy transformer, as was mentioned by davidbeach and others. I don't understand why you would see such high SLG at a Dy -- the transformer zero sequence Z should be equal to the pos & neg sequence Z (right?). So what causes the high SLG fault current at at Dy?
RE: Worst Electrical Fault
The zero-sequence impedance of the system on the source side of the Dy transformer is shorted out.
RE: Worst Electrical Fault
It turned out that the DC had saturated the transformer core and caused a very high primary current. So high that the protective relay more or less caught fire and then, after a few minutes, just died. Had to replace the protection.
That is one of the worst faults I have experienced. And it is interesting because no one had anticipated such a high primary current. A bolted short on the secondary side would have been limited by Xk of the transformer. DC saturation took a lot of that Xk away.
Gunnar Englund
www.gke.org
RE: Worst Electrical Fault
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Worst Electrical Fault
However when there is a bang and the lights go out, your generators stop and the utility company calls in to ask what happened as their power went of, now that's bad.
In fact its almost time to put on ones coat and go home....
Rugged
RE: Worst Electrical Fault
This was half past two in the morning. So I went to my hotel and got some sleep while the HV guys sorted out their problem. The utility company didn't have to call - I was working with one of their fans for the boiler. On their premises.
Gunnar Englund
www.gke.org
RE: Worst Electrical Fault
RE: Worst Electrical Fault
RE: Worst Electrical Fault
During unbalanced loading or fault conditions, the unidirectional zero sequence current component is confined to circulate within the delta. At this point it only sees the winding impedance. Since it doesn't return to the source, no impedance further upstream is relevant.
SLG faults see an impedance that is the average of the sequence impedances, while three phase faults see the positive impedance. Close in faults with low zero sequence impedance will therefore exceed the three phase fault current.
RE: Worst Electrical Fault