how to select the right varistor for a syncronous generator 3

[mottt]

I got a syncronous generator, and the varistor blow up, and i don´t know the value of the new varistor.... the generator is 2100 KW and the voltage of the stator is 3000V how to select it? i think dynamic resistence is important,

thanks a lot

 

[dpc]

Are you talking about a surge arrester on the generator terminals?

 

[waross]

Hi dpc;
This is probably a varistor on the rotating diode plate. Load upsets may at times reflect back to the rotating DC field and are snubbed by the varistor. I have replaced them but have always had spares available and did not have to size them.

Bill
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"Why not the best?"
Jimmy Carter

 

[mottt]

yes, it´s on the rotating diode plate, and i don´t know how to size them and as it´s a generator i have no good relations with the manufacturer, cause some faults in the commisioning i can´t ask them.

thanks.

 

[fangas]

Try here

http://www.marathonelectric.com/

I used to work for them & they have a pretty good apps dept.

Give'em a call, explain your problem, they'll probably give you a hand.

Ed

 

[catserveng]

You probably have a couple of options,

Just call the manufactuers parts department or local dealer and buy the part, probably less than $100 USD. If they are like most companies the parts department doesn't talk to sales or engineering unless they have a hard question, pay with a credit card and you'll likely have your part within a day or two. When you get the new one in hand, it will likely have the varister manufacturers number on it, then you can cross reference it for future use.

Try your local motor rewind shop, most have cross references and technical info for repair of generators.

Contact an aftermarket supplier, like Amps Abundent, who sell diodes and surge suppresors for several different brands.

Take it out, lots of generators running out there with them gone, but the other side of that is that the varistor is there to protect the diodes during abnormal conditions and transients. Do you know why the varistor failed?

Hope that helps.

 

[Skogsgurra]

I have had this on a few occasions.

The varistors are there to protect the diodes from switching transients on the grid. With external protection and (in larger units) transformer protection, such transients are less likely to happen.

What happens, though, is pole slip. Bad synch or just loss of excitation. In those cases, the induced voltage is quite high and the energies involved are so, too.

No varistors can take that much energy and, consequently, fail.

With those facts it is easy to do as catserveng says: take them out. First: They are there for unlikely events like transients on the grid being coupled from stator to rotor, which is a low power event because of high leakage inductance and fast transients. Second: They can never be designed to handle pole slip and therefore fail everytime that happens, which can be quite often..

Conclusion: Don't use them.

Some designs use thyristors with a voltage sensitive trigger to short out the excitation when pole slip occurs. It also serves well as a transient protection since the low pass characteristics of the stator/rotor coupling path gives the trigger circuit some time to activate the thyristors.

Gunnar Englund

http://www.gke.org

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