Electrical Protection

The difference in currents between phases is indicative of single phase loads being connected downstream. Provided the circuit protection devices are set correctly, and everything in the thread so far suggests that they are, then this is a normal occurrence and no harm would occur to the generator. If a fault were to occur, then the circuit protective devices would operate, and still no harm would occur to the generator or the wiring; that is why they are there.

"Don't worry; be happy."

Permissible Negative sequence capability of 3 phase machines is governed by following relation

I₂² x t = 20

Based on the CT ratio and secondary current, calculate I₂, check that Negative Sequence relay is adjusted correctly. For setting protection relay we use value of 15 instead of 20 (leaving some margin for CT error and safety factor)

40A of unbalance in one phase for 10MW rating appears within limit. But must investigate whether it is actual unbalance load, or Turn Ratio error in that phase CT or wrong instrument.

I am sure protection CT and measuring CT must be different - so as a first check take a Clip on Am meter (Tong Tester) and check secondary currents in all 3 CTs and current wiring to relay -

If the currents in all 3 phases of Protection Relay are same whereas there is a difference in currents of ammeter - then it is a CT error.

If Currents in secondary circuit are balance then it is measuring instrument error.

If unbalance is confirmed by both protection and metering CT then check individual feeders for the cause of unbalance.

If particular phase (B) in all feeders has lower current then it can be:

1. Higher voltage drop in cable connections of B phase -check for any heating joint or connection.

2. Or may be that you are using single phase cables and there are not laid in Trefoil shape or if the length is long, cores are not inter-transposed.

3. Worst can be if inter-turn fault in stator coil of phase B (I hope that is not the case).