Can someone help me read how this differential relay is set? 2

[bdn2004]

The actual trip time? It is a GE PVD Differential Relay.

See the attached (2) pic file of the relay and a cut sheet.

It is connected to a 3000/5 CT, on a 12.47kV bus. The card on the front says it is set at 135V, and 2A.
The cut sheet says its typical operating speed is 20ms (1.2 cycles) at 4X pickup.

Our data from a test report shows that this is set at 0.083 seconds (5 cycles).

 

[bdn2004]

I thought I had this but I guess not...

I am trying to establish the longest pickup time for any fault. That definite time is entered into the 12kv bus in the software to establish the arc flash time for that bus. There’s only one time that can be entered there. Worst case for this purpose is the longest time.

In this case we were told 0.083 is the time - that’s an arc flash Level 1 in this system. I bump it to .107, as per the discussion above it’s an arc flash Level 2. The time overcurrent relays acting as backup give it a Level 3.

 

[jghrist]

To calculate the actual time, you would need to have the excitation curves of the CTs, the fault current (worse case could be minimum fault or maximum fault), and the relay MOV resistance. You would have to calculate the point on the excitation curve where the excitation current and the voltage balance. Normally, this calculation is not done. Usually, you calculate the minimum fault current to trip the relay.

Actually, for any reasonable fault and an adequate CT, you would have at least twice the pickup voltage and the time would be no more that 25 ms. Max pickup time for an SEL-587Z microprocessor high impedance differential relay is 1.25 cycles (21 ms at 60 Hz). You'd have to add lockout relay time and breaker clearing time.