Ground Fault Current Split - two online gens with one grounded

[KenAlmon]

Assume two 4160 V generators online on an islanded system; one is grounded through a transformer/resistor, the other ungrounded (as recommended by various sources including Eng-tips).

My question relates to a fault on the ungrounded gen. It is a straight-forward exercise to show the path the fault current flows when the fault is on the grounded gen, but what about the ungrounded gen? If a fault occurs say 50% down one winding, what "circular" route will the current flow? This is probably best illustrated with a drawing, but I am not sure this forum will allow that...

TIA

Ken Almon

 

[peterb]

Regardless of where the fault is, the current path is through the faulted phase lead of the grounded generator, through the busbar and feeder conductors to the fault point, then through the ground system back to the grounded generator neutral grounding connection, then to the faulted phase lead, etc.

 

[busbar]

Offhand, it mat be worthwhile determining that each stator winding-differential [87M] relays will be equally sensitive for the two units. The cost of repairs and lost generator output normally influences protection philosophy. Some cases might favor standalone grounding unchanged by generator breaker status.

Discussing differential protection, C37.96-88 §2.4.7 states: “These [87] relays detect low-magnitude fault currents during normal loads and do not trip falsely during high-magnitude external faults. Differential relays cannot detect turn-to-turn faults in the same winding.”

For the least amount of long-term trouble, it’s worrh seeong that the type, ratio, and kneepoint of CTs should to be identical—called out and proven in commissioning.

 

[KenAlmon]

Peterb:

Thank you for your timely response.