Tap Changer and C37-13

[Alex68]

Dear all,
this is my first time using ANSI C37-13 to calculate short circuit currents at generator terminals.

I reading the std, the example of the annex A in particular, I noted that the calculation procedure does not consider the possibility that the tap changer of the step up transformer is at a tap different from the central one or a voltage value different from the rated one.
Actually the HV grid contribution to the fault consider only the rated value of the HV grid voltage and the nominal ratio of the transformer.

Should I consider also different conditions (high value of the grid voltage and different tap)?
Or does the standard impose the use of the rated values even though I didn't find any indication?

Thank you
Alex

 

[dpc]

You should use the highest pre-fault voltage that could reasonably be expected to occur to get the maximum fault current.

At power plants, we generally use at least 1.03 per unit.

 

[Alex68]

Thank you, DPC.
I suppose that 1.03 p.u. is your practise but it isn't imposed by the Std.
What about the OLTC?

 

[dpc]

The real issue is what is the pre-fault voltage. The LTC will just try to maintain whatever setpoint voltage is.

If you are trying to calculate the maximum SC current to verify breaker ratings, you need to use the highest voltage that can reasonably be expected.

If you know the voltage can be higher than 1.0 pu, it's your responsibility to use that voltage for calculations even if not explicitly required in ANSI C37.

 

[Alex68]

Thank you