APPLYING AC DECREMENT PER ANSI C37.010 1979

[newpower]

ANSI C37.010 1979 says to apply the AC decrement to short circuit current when the current is fed predominately from generators through:

-Not more than one transformation
or
-A per-unit reactance external to the generator which is less than 1.5 times the generator p.u. subtransient reactance on a common system base.

Does anyone have any pointers on how this should be applied from a practical standpoint?

My teamate and I are currently reviewing all of the breakers on our system & tracing the fault current back through system to generation & comparing the reactance of the system to the generator's reactance if there is only one transformation.


Any advice, help, or experience provided would be greatly appreaciated.

 

[bigamp]

From the short circuit (SC) decrement characteristic calculate a generator "equivalent reactance" at various times throughout the characteristic, starting at say 0.1 seconds and ending at the steady state value. This reactance is the reactance that the generator appears to have such that it produces the SC current as shown on the decrement characteristic for a dead short at the generator terminals.

Normally 5 or 6 different times will give a good enough picture. You then use the calculated value (of generator equivalent reactance) as the generator reactance in the SC calculations and you do the calculation for each time interval (easy enough on a spreadsheet). This will give you the SC current at various points in your network that is a function of the generator decrement characteristic. You can then draw the actual SC current as function of time on your relay co-ordination drawings.

You get some interesting results because most decrement characteristics hit steady state current in the 1.5 to 5 second range but some IDMT relays on a system would want to be operating before the curernt has reached a steady state value. The decrement characteristic can aid relay grading.

The above approach is really worthwhile (essential) where the generator impedance is a significant proportion of the total impedance to the faulted point, as under these conditions the fault current will be determined by the generator. For cases where the circuit impedance downstream of the genererator is much greater than that of the generator the decrement characteristic makes very little difference.

Regards

 

[newpower]

Thanks for the information!