Static exciter - varistors 4

[Skogsgurra]

I had this question a couple of weeks ago:

"In a 2000 kW hydro-electric generator with a static exciter, there are MOV varistors across the rectifier bridge that feeds the pole wheel. Are they there to protect against inductive overvoltage in case someone opens the excitation winding?"

- No. I said. -The rectifier bridge takes care of that. It acts as a free-wheeling diode. I think that they may be there to take care of out-of-sync situations where lots of voltage is induced in the excitation winding.

Now, this guy gets back: "No, my customer didn't buy that. They say it is for inductive kick-back".

I know there are lots of ignorants out there. But these guys work for one of Scandinavia's largest power companies and do nothing but power generation things. So I ask myself if they know something that I don't know. And that my fellows in Eng-Tips know.

What could that be?

Gunnar Englund

http://www.gke.org

 

[GTstartup]

It's common for rotating exciters to have capacitors installed across the diodes to suppress commutation voltage transients that can occur even under normal conditions. Since a varistor is really just a capacitor this is probably just a design variant of what is commonly seen on every rotating exciter.

 

[GTstartup]

Correction. Varistors are not capacitors above their threshold voltage. However the remainder of my post applies, I think

 

[waross]

Thanks again skogsgurra
I have a gift for mixing up energy and power in conversation as well. I meant what you said.
Maybe I should join that support group. I forget their name, but their motto is "Dislectics Untie".
respectfully

 

[davidbeach]

Ok, the definitive answer:

I believe that the MOV is used for exactly what you said, pole slipping, along with negative currents (currents flowing out of the F+ terminal, back toward the rotating rectifiers) associated with out of phase paralleling and non-symmetrical loading / short circuits on the main armature windings. Having said that, I checked with a generator manufacturer and they commented that MOVs are typically not robust enough to live through a pole slip, and are mainly used to protect against out of phase paralleling.

As far as inductive "kick-back", as you described, the two diodes stacked on top of each other for each phase of the bridge act like a fly-back diode, providing a path for current to flow. I am aware that for high di/dt, diodes can have very short lived, but significant forward voltages across them. Having said that, these voltages are small (10's of volts) compared to the ratings of the devices, thus an MOV should not be necessary.

 

[waross]

Thanks for your time and trouble to get this answer davidbeach.
respectfully

 

[Skogsgurra]

Thanks davidbeach!

This is very much in line with my thinking. It is also what happened - the MOVs exploded when the generator got out of sync due to a mishap in the excitation circuit (a wire was loose and the excitation just dies so the generator got completely out of sync)

To sum up: There cannot be any inductive kick back in a circuit like this. So the MOVs (Varistors) shouldn't be needed. In spite of that, they are installed, probably to take care of transients and (perhaps) pole slips. Pole slips are too energetic (MW during seconds = Megajoules) for normal MOVs, that typically are 100 - 1000 joule devices. So the MOVs have one mission: take care of transients that get in from outside (line). I think that it is better not to install them where they are dificult to reach. Much better to install overvoltage protection in the switchyard so that transients don't get to the generator. Most switchyards have such diverters. (MOVs with an arc gap and magnetic "blowing").

I think that I have got the answers I needed. Thanks to all involved.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

Some excellent information in this thread - thanks to David especially. I have learned something today.


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I don't suffer from insanity. I enjoy it...

 

[jtkirb]

I will chime in here- is this a custom built exciter, or one off the shelf fitted to this application?

The Basler exciter I just worked on yesterday had a blown MOV. This was applied to a brushed rotor. The MOV is in there to protect the electronic components from the outside world- whatever that may be.

You can't always say "there is no way to open up a brushless exciter" since you have to look beyond typiclal failures. An open circuit caused by some other failure could be a possible result.

The designers were covering their bases. Cudos to them.

JTK