Current on grounding conductor at motor startup.

[11bgrunt]

A water supply company has two pumps moving water to fill the reservoir.
The complaint reports that each motor has a problem and they are different problems. The 150HP trips on unbalanced current. The 600HP is going out on ground trip. A PQA was connected on the 150HP and a recording was taken.
No motor testing has been done since the plant was built two years ago.
The water company is measuring small continuous currents on the ground bonds when one of the motors was running but not when both were running, which does not happen often.
The question here is; there was a recording Fluke 345 meter on the grounding conductor at the 150HP switchgear when it started. The grounding conductor had a 25A+ during startup. The recordings are being reviewed and the water company will be doing more test.
Both motors are 480Y/277VAC with soft starts.
What can cause the ground currents?

 

[synchro]

The waveform on the grounding conductor for the 150HP suggests that there's a ground fault at an intermediate point in a winding of the motor. The current drawn by winding(s) located between the ground fault and the motor supply terminals would show a characteristic higher starting current and lower running current as shown in your attachment. If there was a ground fault upstream of the motor, then the ground fault current might actually go down slightly when the motor starts because of the higher voltage drop on the conductors to the motor during starting.

 

[welltodocase]

An intermediate point in one of the windings of the 150 HP motor seems to be ground faulted, as shown by the waveform on the grounding wire. As shown in your attachment, the current taken by the winding(s) situated between the ground fault and the motor supply terminals would increase during startup and decrease during operation. In the event of a ground fault upstream of the motor, the increased voltage drop on the wires to the motor during beginning might actually result in a decrease in the ground fault current.