Worst Electrical Fault 3

[tsuro]

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?

 

[davidbeach]

peebee, find a set of sequence diagrams for various transformer connections. The zero-sequence network for a delta - grounded-wye transformer is open on the delta side and shorted on the wye side. So for any line to ground current (fault or load) on the wye side of the transformer, the source zero-sequence impedance is lower than the source positive- and negative-sequence impedance. Generally not an issue for load but can be a significant issue under the right conditions for faults.

 

[stevenal]

When adding + sequence impedances, work from the point in question back to the source. Same when adding - sequence impedances. When adding zero sequence impedances, though, work from the point in question back to the first Dy transformer. Any impedance upstream is essentially shorted out to zero.

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.

 

[peebee]

Thanks.