Electromechanical relays - asymmetrical fault current

[chuckd83]

I have an electromechanical relay (Westinghouse CO-11) feeding a transformer. The instantaneous element is set above a symmetrical through fault, but below an asymmetrical through fault. Will the instantaneous operate for a secondary fault?

 

[cranky108]

Maybe. Most electromechanical relays have some inerta that maybe just enough that they may operate correctly.

 

[chuckd83]

I don't know what that means. Does the instantaneous element on electromechanical relays operate based on asymmetrical fault current (with the DC component), or symmetrical only?

 

[cranky108]

Electromechanical instantaneous elements operate based on the total magnetic force applied to the plunger.

 

[MKFPE]

In industrial settings I have seen recommendations for the high side instantaneous unit to be set at 1.25 times the symmetrical let-through for a 3-Ph bolted secondary fault.

Industrial installations usually don't experience a lot of faults compared to a utility substation feeding overhead distribution circuits. Also the X/R ratio tends to be relatively lower at these installations although this is not always true. Thus this setting rule may offer apparent setting security against undesired tripping based more on the law of averages than physical reasons.

A major utility in my area sets the instantaneous unit at 1.65 x Isc (symm) for a bolted 3-Ph fault on the transformer low side terminals to insure the instantaneous unit will not trip for this fault. The TOC unit is set to coordinate with the transformer damage curve. The desired TOC unit trip time at maximum low side bus fault is around 45 cycles to allow coordination time for downstream relays. Note this can be difficult to achieve if you have a low side main breaker, feeder breakers, and reclosers, etc downstream.

I have followed the above recommendations for many years with no issues.

 

[cranky108]

For microprocessor relay 50 units I set for 125% of the xfmr low side fault current, with a 3 cycle delay. It takes an extra effort to set the delay for an EM relay.

Transformers at utilities typically don't have many faults, but a few. The distribution system has a few. Lightning, animals, bad drivers, backhoe operators, etc.