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Single phase voltage drop on a generator

Single phase voltage drop on a generator

Single phase voltage drop on a generator

(OP)
We have a small hydro with an 11kV/400V 500kVA Dy transformer. On the LV bus we have 30kW motor load (compressors). A 400V 300kVA (synchronous gen) hydro scheme synchs at 400V to this bus via motorised CB. The plant has been in operation for about three years. Just lately we have developed a fault where we get a neg sequence voltage trip after synch. We see three currents (about equal) and then one voltage on the generator drops from 230V to about 160V. Before synch, no problems, bus voltages also fine as measured when compressor loads are running, so no loss of HV fuse which would be an obvious solution. Generator windings and cables all tested low ohms so no large resistance in windings or cables. Unit synchs ok, and no neg seq at run up. Never seen this before and it has us stumped. Any suggestions out there. High resistance in CB contacts a possibility.... we have to strip the board to measure?
 

RE: Single phase voltage drop on a generator

Before sync are you running on mains or generator?
Is your generator neutral grounded?
Have you meggered your generator windings for an intermittent fault? Do it immediately after a trip. Some faults are temperature related and only show up when the unit is at or near normal operating temperature.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Single phase voltage drop on a generator

Another thing to check is the protection circuit wiring. If there is a bad connection somewhere in the wiring, the protection package may think that there is a problem when there is none.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Single phase voltage drop on a generator

Try to trap this event with a high speed recorder or a multi-channel scope. A voltage collapse of that magnitude will almost certainly leave a signature on the line currents - look for any peculiar phase shift in the affected line or anything else unusual like sudden jump in current magnitude.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Single phase voltage drop on a generator

(OP)
Just to advise on the outcome. The transformer has fuses that supply the switchboard. One fuse had a high resistance joint. At no load there was no voltage distortion on the switchboard, with load one phase voltage dropped due to current flow through the resistance. A new one for me.

RE: Single phase voltage drop on a generator

Thanks for the update.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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