Over current protection

We need relay coordination to minimize the disruption due to fault and operate the nearest relay first. We can not coordinate fuses because the melting time is fixed.

For Fuse:
total clearance time = pre arching time + arching time
The time current characteristic of a fuse has two curves - minimum melt curve and total clearing time

Sectionalizers  can not interrupt a fault. It counts the number of time it seen the fault and operate after preset number. Reclosers have limited fault interrupting capability.


Types of over current protection:

1.  Instantaneous relays - current only ( can not discriminate the fault currents If1 = If2)
2.  Definite time relays - time only ( Faults near to the source has higher currents and it also show higher time)
3. IDMT inverse definite minimum current relays - both current and time

Consideration of  coordination:

• Maximum and Minimum momentarily Short circuit current
• Maximum and minimum ground fault current
• Total time interval

Coordination parameters

• TAP value (pick up current in secondary CT) - Pick up current
• Time Dial (TD) - Time multiplier setting (MTS) or time dial setting (TDS)
• Instantaneous TAP (IT)
• Extremely inverse characteristic

We use extremely inverse relay characteristic in the industry. There should not be any crossing between fuse characteristics and relay characteristics ( we can adjust this using the correct TDS in the relay.when TD is increased it will become more inverse ). It means fuse characteristic always has to be lower than relay (0.20 s gap). If we coordinate two over current relays their characteristic should have 0.4 s gap.

With CT ratio 500:5  and TAP value 5 means.
Pick up current in CT secondary = 5A
Pick up current in CT primary = 500 A