VFD Troubles

We are using a pump system that incorporates a 100HP gear pump charged with a 15 HP impeller pump. The 15 HP pump is started with a Schneider Electric soft start and the 100 HP is driven with an Eaton VFD. The 15 HP starts first and after 30 seconds the VFD starts and ramps up to speed.

We have nearly 30 of these units currently running and they have been so over an 6 month period. This last one installed gave us an overvoltage trip on the soft start as soon as the VFD was started. We had set the percentage parmeters and the time delay settings on the Soft Start to the maximum allowed. Either pump would operate correctly and stay running by themselves, but is soon as the VFD came on the soft start would trip.

Voltages phase to phase were 478 and phase to ground 276. STarting voltage flucuations were 2 volts or less. The 15hp was drawing 8 amps. The soft start was replaced with the same results.

We replaced the VFD and it started working correctly, for a few hours. We returned in the morning to find it was down again. When it was restarted the soft start again tripped. We replaced the Soft Start with a size #2 direct starter. It worked fine for about 15 minutes then a lot of smoke came out, they think it was the VFD. Not certain what happened next, but I had them check for faults or alarms on the VFD. They turned it back on and all was clear and normal (bus voltages, frequencies, etc). They restarted the pumps and the VFD popped and cracked and came up with an Over Voltage alarm.

All these VFD's have a line reactor filter and after 10 sec another reactor with capacitors kicks in on the incoming line. The second reactor never came in. The other puzzling thing is it appears to be a utility line problem but a mile down the line we have an identical setup that has been running problem free for 4 months.

They want to install another VFD but I think it will do the same thing after a day or two. What can be going on here and how can we check this?