Negative sequence current from delta source? 1

[kmh1]

I have an application where I need to apply ground fault protection on the HV side of a customer's delta-wye transformer in the event of loss of the HV utility source (which is supplied from a solidly grounded wye connected transormer). There is a generator connected to the LV side of the transformer. The utility has suggested that either zero sequence voltage or directional negative sequence current tripping of the cutomer's generator source be applied in the event of an HV ground fault. I have heard from others that negative sequence current cannot be produced by the delta connected source. Who is correct?

 

[stevenal]

The delta connection can produce the negative sequence current if the fault can. A phase to phase fault for example will produce plenty of negative sequence current. A line to ground fault though has its sequence networks in series. Positive, negative, and zero sequence all see the open circuit at the transformer in the zero sequence network. Once the utility breaker clears, there will be no path for fault current. The voltage solution sounds better.
See IEEE C37.95.

 

[jbartos]

Suggestion to kmh1 (Electrical) Sep 12, 2003 marked ///\\I have an application where I need to apply ground fault protection on the HV side of a customer's delta-wye transformer in the event of loss of the HV utility source (which is supplied from a solidly grounded wye connected transormer). There is a generator connected to the LV side of the transformer. The utility has suggested that either zero sequence voltage or directional negative sequence current tripping of the cutomer's generator source be applied in the event of an HV ground fault.
///The Utility suggestion applies to the high voltage transformer side that solidly grounded wye transformer secondary. The Utility suggestion is viable.\\ I have heard from others that negative sequence current cannot be produced by the delta connected source.
///To the contrary, the negative sequence current may be produced by the delta connected source by any asymmetrical fault.\\ Who is correct?
///In what? In which sense is question posed?\\

 

[jghrist]

Negative sequence current will flow during a high-side ground fault only as long as the utility source breaker is closed. Once the utility source breaker opens, the system is ungrounded and the only current will be capacitive current from the line capacitance to ground. The only way to detect this is from high-side voltage measurements and using over-/under-voltage or zero-sequence voltage relays.

See Protection for Unexpected Delta Sources, by Ken Behrendt, at

http://www.selinc.com/techpprs/6143.pdf

 

[kmh1]

Thanks for the help stevenal and jghrist. I originally took the utility protection engineer's recommendations at face value but questioned them after sitting in on an SEL seminar. The client has implemented the zero sequence voltage tripping scheme but I wanted to confirm the validity of the suggested alternatives for my own sake.

 

[busbar]

There could be a misunderstanding that negative sequence current cannot be produced by the delta... Zero-sequence current cannot be produced in a delta winding.

Offhand, if wye-wye PTs are available, a 59G/zero-sequence overvoltage function can sensitively and reliably sense faults. Negative-sequence directional current can too, but is slightly more complex to incorporate with single-function relay components. If there are minimal upstream contingencies, I'd go with the simpler 59G.

 

[kmh1]

The protective function is applied using an SEL 311C distance relay to supervise ground faults for a separate HCB pilot wire relay (the distance element in the 311C is used to supervise phase to phase faults). The relay has the capability to apply either the 59N zero sequence voltage or 51Q negative sequence current function.