field discharge resistor - small hydro generator

[BrkfldEE]

I have a hydro generator that is 12.5MVA, 13.8KV. The rated field volts and amps are 250V, 240A; and according to my datasheet the field resistance is 0.904 ohms. The rotating exciter has nameplate info of: 80KW, 250V, 320A, shunt wound. I am removing a field breaker that is in the exciter field and installing a field breaker in the generator field. How do I figure the size of the field discharge resistor? My research so far tells me that it should be 1 to 1.5 times the field resistance, is that right? How do I determine the Watt rating?

 

[Skogsgurra]

If you choose same resistance as the field winding, then the power needed would be the same as the excitation power, in your case 60 kW. That would obviously create a lot of unnecessary power losses if connected permanently and the exciter cannot deliver that extra power, anyroads.

It is (or was) quite common to use a contactor that connects the discharge resistor and then open the field winding connection. Then, the energy that the resistor must absorb is equal to the energy in the excitation winding (or half of it, actually). If (assuming a time constant around 5 seconds) the energy that the discharge resistor needs to absorb is something like 144 000 Ws.

That is a lot of energy and you will probably need a discharge resistor able to dissipate at least a few kW continuously. I would go to a reputable power resistor manufacturer and get advice from him.

It is also possible to disable the rotating exciter by opening its excitation winding. That will not need a big discharge resistor. But may be unsuitable for reasons that I don't know anything about.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[dpc]

The time rating required will be a critical factor in sizing the resistor. Big difference between a resistor with a 10-second rating and a continuous rating. The kW rating just determines the resistance. The time rating determines the physical size.

 

[waross]

Given that solid state rectifiers are much smaller, more efficient and more dependable than the days when the special breakers for discharge resistors where common, how about a free wheeling diode?

Bill
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"Why not the best?"
Jimmy Carter

 

[BrkfldEE]

I am installing a Basler DECS-250 voltage regulator to provide field current to the rotating exciter. This AVR has a free-wheeling diode across the exciter field. Is the generator field breaker that I am installing un-necessary? Is there any advantages to installing this generator field breaker? Maybe I should just rely on the free-wheeling diode in the exciter field.