Medium Voltage Motor Failures

Submersible pump motors thru 450hp, 2300V, fail along a group of pumping stations each supplied by one 12470-2300V transformer [some ungrounded] and each with a power factor correction capacitor switched with the load.

Measured tests [across multiple sites]show normal voltages prior to motor start, yet unbalanced current / voltages [as large as 25% current unbalance] during operation.

Motor protection relays are inadequate.

Q: Can the capacitors be ruled out as a potential cause of the unbalance problem.

Q: If the system is ungrounded, then it should still require a fault to occur to throw the voltages out of balance [and damage the motors]. However, the same phenomenon is occuring on multiple sites. What else could cause this??? [Motor leads were rotated between tests and phase results did not change thereby ruling out motor windings]