Large motor earth fault

[hhhansen]

Hi out there
From at cement plant an earth fault current of > 150 Amps has been reported on a large cement mill slip ring motor. Rated line to line voltage is 11 kV. Megging of both cables (parallel single phase cables) and motor (stator, rotor) have been performed sucessfully. A capacitor bank of 1500 kvar is installed in the station with outgoing power feeder to the motor.

What seems like a mystery to me is how an earth fault can occur. Is there something I haven´t paid attention ?

I am looking forward to recieve some comments on this.

 

[DanDel]

Did you experience a ground fault trip? By slip ring, I assume you mean a wound-rotor motor, though I haven't seen one at that voltage.

 

[asymptote]

At 11kV (6351volts) 150 amps of earth fault is going to leave it's mark if realy is a fault.

How is the earth fault relay connected. Residual or balanced. Is the relay IDMT or instant. How is the source neutral/star point connected and how big is the network connected at the PCC?

 

[slavag]

Hi.
Maybe trip was in the start time?
If it's residual connection of earth fault relay are possible maloperation of EF relay during start of motor, becouse partly saturation of CT.
What is a time delay of EF protection?
Regards.
Slava