Ground current on start-up

[gronk2003]

When starting a large motor (14000 HP Synchronous), with a 600/5 zero sequence ground CT around all 3 phases on the feeders leaving the starter, will you see any ground current on start-up?

 

[dpc]

You may see some current in the CT secondary due to slightly uneven flux distribution. The zero sequence flux summation CT assumes the resultant flux from each phase conductor will sum to zero. But in reality, the flux distribution is not perfectly uniform so there will be some residual flux and a small amount of CT secondary current. This can be especially true during starting due to high currents. False tripping of the ground relay is not too unusual. It helps to keep the phase conductors centered within the CT.