Motor tripping on overcurrent protection

[KRISHNA746]

One of our substations has a 6.6kV switchboard with two incomer(NC) and bus coupler(NO) scheme, this board further feeds motor loads.

During starting of one of the motor P1 515kW FLC-56() ,it so happened that motor tripped on over current protection(450A 0sec) and corresponding incomer of that section tripped on feeder differential protection.

On further checking it was observed that b phase of incomer failed. However motor was healthy.

I would be grateful if i could get clarifications on following points from community.

1.During failure it was observed from bus PT and line PT that there was voltage dip in Y and B phases, however there was not fault in Y phase of incomer.

2.Motor P1 relay (Schneider P141) recorded current of around 3kA in Y and B phase, however motor was healthy.

 

[waross]

(450A 0sec), that is about the expected starting current of the motor.
If you are seeing a record of 3 kA, check the wiring between the bus and the motor. Something may have flashed to ground.
A hi-pot test of the wiring may detect a wiring fault.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

You have mentioned that feeder DIFF protection had tripped for a motor fault which cannot happen.
Unless how the CTs are connected to the DIFF relay & motor O/C relay is seen it is difficult to give a logical answer.
Therefore, could you please upload the SLD showing how the CTs are connected?

 

[wcaseyharman]

To really answer your question would require a lot more information about the system, the protection settings and the exact sequence of events, but generally it seems there are two possibilities that I see for why the motor would have tripped:
- if the motor had developed enough speed during starting before the incoming line fault, the motor would contribute fault current to the fault. Depending on the relay settings this sudden current could cause a trip.
- an incoming line fault during the starting sequence would create high imbalanced voltages leading to very high negative sequence current to the motor. The relay may have settings for unbalance current or voltage and tripped accordingly.
- I suppose a third but less likely option is that the electrical designer added some communication to the feeder relays from the bus relay to trip the feeder breakers for a bus fault, but of course this all depends on the system design.