Undervoltage relays: working backwards to find fault current from relay pickup?

[brainsalad]

Hello,

If I had an undervoltage relay in a 120V relay loop receiving input from 4.16kV bus 4200/120 PT's, would it be appropriate to apply symmetrical components to determine, given the relay pickup (say 55V/120V) and the system sequence impedances, a fault current which would depress the bus voltage to the point of relay pickup (i.e, to the point when the PT's produce 55V output)? I guess I am unsure about the application of the source (generically) "V" in the 3-phase, LG, LL, LLG symmetrical component equations.

For example, if the symmetrical component application to 3-phase faults was modified to something like I_f = V / Z_1 where I_f is 3-phase fault amps, Z_1 is positive-sequence seen from the fault and V is the voltage (some transformed amount of the 55V pickup) which causes the relay to pickup. The same would be applied to the other typical faults (LL, LLG, LG).

This is related to an earlier inquiry which wished to determine (3-phase,LG,LL,LLG) fault currents and associated depressed phase voltages, to show that overcurrent relays could operate in fast enough time to prevent undervoltage relay operation (which would initiate source transfer to the faulted bus).

Thanks for your thoughts.

 

[davidbeach]

If you have a very good system model you could get close. But you probably don't as it take a lot of iterative work to get the zero sequence impedances right. Far better to have the currents directly.