generator current inbalance
generator current inbalance
(OP)
We have a large westinghouse generator [530 megawatts] that is tripping on current unbalance when we ramp the megawatt load to quickly the relay is set at 1000 amps we have checked the CT'S they seem ok. The generator would not high pot during an outage but ran after the outage until this problem started to occur any ideas?






RE: generator current inbalance
RE: generator current inbalance
When you say that the generator "would not hi pot" do you mean that a hi pot test was performed and the generator failed?
If so you may have damaged the generator with the attempted hi pot.
yours
RE: generator current inbalance
Jeff
RE: generator current inbalance
It seems that when you say current unbalance you really mean differential. I say this because it seems that you have swapped CT leads from one set of CT's to an other. I am assuming this was a troubleshooting procedure and, according to your post, the fault remained on B phase. If this is the case, it means that, most likely the problem is the relay or the wiring to the relay. If the problem was the generator or the CT the fault would have followed the swap.
The relay should be tested by a qualified technician. Have him apply equal currents at the correct phase angle and ramp them quickly, see what happens.
One word of caution, the generator failed a HIPOT and you are tripping on differential. In the event that you really do have a fault, continuously applying excitation can cause severe damage. Siemens should have made a recommendation on why the generator failed the HIPOT and what might be causing it and what the corrective action should be.
RE: generator current inbalance
If "B" phase on the relay shows the fault, try changing the relay.
If "B" phase on the generator is causing the problem, you may have a fault in the generator. This would also be indicated by failing the hipot test.
RE: generator current inbalance
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RE: generator current inbalance
Seems as if the electricians did an excitation/magnetizing test on the CTs.
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RE: generator current inbalance
RE: generator current inbalance
Assuming Phase B relay still tripped. Test the wiring by using the same procedure as for the relay test but do it from the CT terminals. No need to disconnect or short the CT's. Of course the Unit must be down. You might have to run some temporary wire to one set of CTs. The definitive differential test is primary injection but for a Unit this size it is not practical.
You say the relay was tested but did they test it by simulating the behavior of the Unit when the trip occurs? In other words inject current and ramp at the same rate.