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# Short-circuit calculation on derrated generator

## Short-circuit calculation on derrated generator

(OP)
Hello,

I am calculating the subtransient short circuit currents given a set of 8 generators using ETAP and I would like to know if I should use the rating (kW) under ISO conditions or the rating under SITE conditions (derrating power taking into account the winds and ambient temperature)

P(ISO) = 1305 kW
P(SITE) = 1200 kW (given by vendor)

What do you think?

Regards,
Juan

### RE: Short-circuit calculation on derrated generator

The generator reactances are referenced to rated MVA of generator. It has nothing to do with the gas turbine output as such.
Gas turbine outputs are relevant when load flow in a given season or ambient temperature conditions is being performed.

Rompicherla Raghunath

### RE: Short-circuit calculation on derrated generator

(OP)

So, for example

Given the following data:

Generator continuous rating = 200 kVA
Generator Stand By rating = 220 kVA
Prime motor rating (gas motor) = 250 kVA
Derrated motor rating at site = 210 kVA

What number do you think I should enter on ETAP's generator rating for Shortcircuit calculations?

Thanks,

### RE: Short-circuit calculation on derrated generator

You have to know what base kVA the reactances are calculated on. It isn't a choice you make. There is only one correct value. If not clear from the data, contact the manufacturer.

### RE: Short-circuit calculation on derrated generator

If you don't get the information from the manufacturers, you can perhaps use 200 kVA as the base value, this being the rated continuous value. The possibility of getting wrong is low in this case.

As such by seeing your generator rating, the expected SC level will be so low, the equipment ratings would not be a problem.
Probably for the relay coordination it may be important to have the correct value.

If you have facility to test, you can perform the test and measure the actual value as well!

### RE: Short-circuit calculation on derrated generator

If the reactance values are in Ohms the values will not change with a change in the rating.
If the reactance values are in P.U. you must know the KVA base that the P.U. values are based on.
What you give as a continuous rating may be the prime power rating. The prime power rating is most often given and continuous rating and prime rating are not always the same.

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

### RE: Short-circuit calculation on derrated generator

(OP)
Thank you everyone!

From what I have seen in several generator's data sheets, manufacturers never use the word "kVA BASE"

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