Motor Start Problem

[Toby]

We are having a really interesting problem!

 

We are commissioning two brand new 4160V Siemens motors 900HP and 1600HP. The plant has a 15kV/4160V transformer for just these two motors. The motors are soft started.

 

The motors are ramping up but then trip right away on voltage imbalance and ground fault. The interesting thing is that both motors are tripping on the same fault, voltage imbalance on the exact same phase. B is about 30% higher than A and C.

 

We just had the transformer tested, which is brand new too, and the transformer is OK! We also bypassed the starter by ramping up in 2s and we are getting the same fault.

 

Does anybody have an idea what the solution to this problem could be?

 

Thanks

 

 

[jOmega]

We are having a really interesting problem!

 

We are commissioning two brand new 4160V Siemens motors 900HP and 1600HP. The plant has a 15kV/4160V transformer for just these two motors. The motors are soft started.

 

The motors are ramping up but then trip right away on voltage imbalance and ground fault. The interesting thing is that both motors are tripping on the same fault, voltage imbalance on the exact same phase. B is about 30% higher than A and C.

 

We just had the transformer tested, which is brand new too, and the transformer is OK! We also bypassed the starter by ramping up in 2s and we are getting the same fault.

 

Does anybody have an idea what the solution to this problem could be?

 

Thanks

 

 

Hardly enough information to hazard a guess.

 

For instance ...

 

Transformer kVA is ....... ?

 

Primary is connected ..... Star ? Delta ?

 

Secondary is connected ..... Star ? Delta ?

 

Phase-Phase voltage readings during start-up ?

 

Phase Current readings during start-up ?

 

Phase to ground Voltage readings during start-up ?

 

Phase to ground current readings during start-up ?

 

Have you meggar'd the motor to ground ? and the readings are ?

 

 

The more information you can provide, the more likely the chance that

 

someone will be able to offer suggestions as to the cause of your problem.

 

Kind regards,