Single phase tripping 1

[matwong]

In a delta-wye 33/11kV system, what will be the imbalance current detected by the 11kV substation relay in the event one of the downstream protective device operated on single-pole tripping ? Say the load current is 60A, is the relay seeing 40A (i.e.{60+60}/3) when one phase is out, and 20A (i.e. 60/3) when two phases is out as imbalance current ? Will the imbalance current treated as "earth fault" current by the relay ? Any simple formulae to calculate ? Thanks.

 

[alehman]

It depends on how the relay calculates "imbalance current". It might indicate it as earth fault current if that's how it's configured. If the distribution is 4-wire and there is a neutral CT, then it should be able to differentiate between imbalance and earth fault. Do you have access to the instruction manual?

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[jghrist]

It is very difficult to say what the remaining phase currents and neutral current would be with a loss of one phase. There could be multiple three-phase loads that trip on phase unbalance. Even if the loads are all single-phase, the neutral current would depend on the relative power factors of the remaining phases. If the load was all single phase with equal current and phase angle, the neutral current would equal the phase current if one phase is lost.

If the earth fault relaying operates on residual current, it will not distinguish unbalance from an earth fault. If the relaying uses a core-balance CT connection around all circuit conductors, then it will not be affected by the unbalance.

 

[JMatt]

Use negative-sequence overcurrent tripping.

May I suggest reading "Negative-Sequence Overcurrent Element Application and Coordination in Distribution Protection" by Ed Schweitzer. This is freely downloadable at selinc.com/literature

 

[stevenal]

http://www.pacw.org/fileadmin/doc/SpringIssue09/single_phase_spring09.pdf