Calculation of Relay Operation Time
For calculating actual relay operating time, we need to know these following operation.
1. Current setting.
2. Fault current level.
3. Ratio of current transformer.
4. Time / PSM curve.
5. Time setting.
Step – 1
From CT ratio, we first see the rated secondary current of CT. Say the CT ratio is 100 / 1 A, i.e. secondary current of CT is 1 A.
Step – 2
From current setting we calculate the trick current of the relay. Say current setting of the relay is 150% therefore pick up current of the relay is 1 × 150% = 1.5 A.
Step – 3
Now we have to calculate PSM for the specified faulty current level. For that, we have to first divide primary faulty current by CT ratio to get relay faulty current. Say the faulty current level is 1500 A, in the CT primary, hence secondary equivalent of faulty current is 1500/(100/1) = 15 A
Step – 4
Now, after calculating PSM, we have to find out the total time of operation of the relay from Time / PSM curve. From the curve, say we found the time of operation of relay is 3 second for PSM = 10.
Step – 5
Finally that operating time of relay would be multiplied with time setting multiplier, in order to get actual time of operation of relay. Hence say time setting of the relay is 0.1.
Therefore actual time of operation of the relay for PSM 10, is 3 × 0.1 = 0.3 sec or 300 ms.