QUESTION OF MOTOR STARTUP

Dear All,

I’m trying to start-up a 10 kV-2 MW motor, with no load. But the protection relay tripped 3 times with "Negative Sequence Overcurrent Protection, I2>". I’ve checked the disturbance records, the voltages are perfect (there is no V2 or Vr), but the I2 is very high (startup current = ~760 A, I2 = ~60 A).

The motor is a new one (manufactured in 2004) and we have ran it several times before. I’ve disabled the "negative sequance protection" and the motor started up normally. I again took the disturbance records, but this time the I2 was too small (~ 20A).

Can anybody tell me why we "sometimes" face the unbalance currents, even there is no load and the supply voltages are perfect? and why sometimes we don’t face this high unbalance current during start up?

(PS: We have done the electrical tests again, both on the motor and the cable feeding the motor; but could not find any abnormal value).

Thanks in advance!