Instantaneous device setting protecting a generator

[tonyjony]

Wich is the usual setting in % of FLA of an instantaneous relay (50), protecting a generator against external faults?

 

[rbulsara]

Depends on the application and coordination study. You have provided no relevant info. 50 is not used in all gen applications.

Rafiq Bulsara

http://www.srengineersct.com

 

[tonyjony]

It's a 2MW cogeneration system added to an existing plant. I don't have info about the rest of the plant protections yet.I'm trying to save some time starting with the generator protection earlier.
I thought I could use the 50 to cut the deliver of current to the bus it's connected in case of a 3ph short circuit there.

 

[davidbeach]

You could, but you have to do the coordination study. There are no rule of thumb answers.

 

[tonyjony]

Please educate me:

The system is something like this:

network---xfmr---bus---generator
|
other loads

If there is a 3ph short circuit in the bus, should I need to cordinate de generator device 50? Isn't the best thing to trip the generator as soon as possible, no matter what happens with upsteram relays?
That's why I asked for an used value, regardless of coordination.

thanks everybody!

 

[davidbeach]

You need to find someone locally that you can hire to assist you. This web site, nor any other web site, can not provide the training necessary to learn how to do generation protection. There may be regulatory "must stay connected" rules that would not let you trip for outside faults, or there may be requirements that require you to go away for any hint of trouble on the outside system. There are no simple answers.

 

[rbulsara]

Bus is not the only place a fault can occur. You need to allow downstream devices to open in case of a fault on branch circuits and not take the entire facility down. Is this gen runs in parallel with utility? It get more complex then.

Generators also behave much differently under close-in faults than a utility source. It may not deliver the current you may think it will. There are other techniques to address those.

So as davidbeach said, get an experienced professional to do the coordination study and settings. This is not something you want to learn via free advices with answers to hypothetical scenarios.



Rafiq Bulsara

http://www.srengineersct.com

 

[ScottyUK]

Another question would be where are your breakers located? Fault clearance strategy depends on where you can clear the fault.


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

 

[jghrist]

With a 2 MW system, I think bus differential relaying would be justified.

 

[tonyjony]

Thanks everybody for the advices.

This what I did after all:

I deactivate the instantaneous protection and set the Long time overcurrent protection 1.15 FLA (IEEE Buff book). Time delay was coordinated to trip the CB with the 51V (voltage restrain)as backup of the feeders protection.