Ground Fault 2

[HeinzM]

Is anyone able to give me a clear explanation of fault current flow on the attached 1-line with fault locations in case 1 and 2?
The incoming utility service is a 13.2 kV delta. The 480V generator is connected with a 13.2 kV wye-gnd to 480 V delta transformer.
On a recent utility fault on the incoming service, breaker 52F1 (gen-bus) tripped on ground fault and locked out the gen from islanding.
We want to change protective relays, to selectively trip 52F1 or 52U, depending on the fault location.

http://files.engineering.com/getfil...-cca9-4b0a-b149-cda847904d03&file=GND_FLT.pdf

 

[HamburgerHelper]

Ground current comes from anything that supplies grounding. The ground current will come from your wye-grounded-delta transformer to the fault. Deltas block ground current so the fault at the top of the page cannot be supplied ground current by anything below your delta.

Single line to ground faults draw an equal amount of positive, negative, and zero sequence (ground current/3) currents. The delta will block the contribution of ground currents to a fault from those paths but positive and negative sequence currents will still be supplied by anything that can provide them. If your relaying is operating on ground currents, it won't be able to see negative or positive sequence quantities, only ground currents. This helps with coordinating systems because the deltas on transformers effectively segregate systems for ground overcurrent coordination.

 

[HeinzM]

Thanks for your input.
If adding a new relay - SEL487 - to look at the 52U and 52F1 breakers, can such protection scheme determine ground fault current direction? Depending on where the fault happens ( 1 or 2 ), we have to trip the correct breaker to separate the fault and keep the loads energized - either by islanding with the gen (trip 52U for an outside utility fault) or keeping the utility on (trip 52F1 for an internal fault between 52F1 and Xfmr).

 

[mgtrp]

See thread238-437143 for a similar situation (delta source supplying a wye-delta downstream transformer).

If the fault is at location 1, then CTs at both 52F1 and 52U will see positive, negative and zero sequence currents (i.e., they will see ground fault current). If the fault is at location 2, then they will only see positive and negative sequence currents (no zero sequence). This might help you in determining directionality. Otherwise, you need a suitable VT and relay.

Cheers,
mgtrp