Asynchronous Motor - Decay of motor fault contribution

[EddyWirbelstrom]

IEC60909-0 uses empirical formula to calculate the decay in asynchronous motor fault current contribution at given times after fault inception.
Can anyone provide actual information on the decay of fault current contribution from asynchronous motors ?
The plant in question has 23MW of 6.6kV motors, and 51MW of 440V motors. The plant has 80MW of local generation at 6.6kV, and a 132kV Utility interconnection.
We wish to use the hi-set pickup of directional relays to provide reactor protection. The directional relay must be delayed by approx. 40 msec to allow it to determine direction.
How much will the asynchronous motor fault current contribution have decremented 40 msec after fault inception ?

 

[ijl]

It is difficult to give any general answer, because the decay of the current depends on several factors, such as the motor type and pre-fault loading, location of the fault, etc. Simulation with ATP, EMTP or similar program would perhaps give the best answer. You might also get some idea of the decay of the current by using a power system analysis program that calculates the currents as a function of time. See

http://pp.kpnet.fi/ijl

 

[dpc]

The larger the motor, the longer the decay time. I do know of a refinery that was blacked out because the directional relays mis-operated due to motor contribution into a fault well out on the utility system - so be conservative.