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Single Phasing Generator

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KLINGERMAN

Electrical
Jul 19, 2010
2
I have a generator connected to the grid using a delta-wye transformer. The generator is a 12MVA machine, and runs at 4.16KV. Recently, while it was operating, the C phase opened, however it took longer than we would have expected for the negative sequence relay to operate. I am familiar with symmetrical components, but not with "single phasing". I have done a lot of research, and I think I am now just more confused than ever before. I know the FLA for each phase before the C phase of the breaker opened was 1665amps. I also know that since it was connected to the grid, the voltages would have been unaffected by this event, and lastly I know the two remaining currents would have shifted to 180° apart.

I guess most of my confusion is coming from most of the things I have read say the currents would have behaved similarly to a Ph-Ph fault (.8667 at 180° apart) however, I also know that there was still 12 MW worth of water (a hydro unit) flowing through this machine. Some of this power was consumed in the form of heat, but I have a hard time believe the entire 4MW of power that were on the C phase was consumed as extra heat inside of the machine.

I have gone in a few circles with this question, I hope it is clear what I am asking. I can provide a drawing if necessary.

Thank you.
 
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Do a little research on the problems related to negative sequence currents in generators and motors. With loss of one phase, there will be a large amount of negative sequence current that can be very damaging to the rotor which sees this as a "double frequency" current. Also do some research on ANSI relay type "46".

The bottom line is that negative sequence currents much less than full-load can cause serious overheating damage to the generator.



David Castor
 
You are hinting that your main problem is to understand where the 4 MW worth of water went.

If you realize that water flowing through a turbine is not at all like a rack-and-pinion, you may have less problems. Water can pass through a turbine without delivering much energy to it.

If you think about it, what quantity do you expect to see reduced when the generator power drops? Pressure? Flow? Water speed? None of those? Some of them, but not so much?

Gunnar Englund
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
I am aware of the effects of negative sequence, and how to calculate the amount of negative sequence available in the circuit. I am trying to understand the amount of current that would be available to operate a negative sequence relay if a pole were to open on the low side of the transformer.

I know how to calculate the negative sequence currents once I have the phase currents. I am unsure what the phase currents would go to though.

I appreciate the responses!
 
Where did C phase open? Between the generator and the transformer or on the transformer secondary?
Is the transformer secondary neutral floating or connected to the grid neutral?
Under some conditions, and assuming that there is 12 MW of power being input into the generator, you may experience a 50% overload on the windings and a 300% overload on one phase of the transformer.
With C phase open on the transformer primary, you will have A-B phase delivering energy to the transformer.
B-C and C-A will act as an open delta and deliver energy to A-B phase of the transformer.
The current will split 50%-50% between A-B phase and the open delta equivalent, B-C,C-A.
This is the standard connection to convert a three phase alternator to a single phase alternator. Because of the phase angle differences the rating is now 2/3 of the original rating or 8MW. If you input 12 MW of power, the windings will all be carrying 1/3 more current.
Now you are feeding all the power to only A-B phase of the transformer so you will have 12 MW in a 4 MW winding.
I have made an assumption that C phase went open between the generator and the transformer. If that is a false assumption, something else may have happened.

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