Although I joined sometime ago and have read afew threads, this is my first post on the forum.

I've just had a call from a client reporting that they're getting sustained surges of 700V in an industrial type plant. This is blowing various VFDs they are using, apparently. Most time the voltage sits at about 400V.

They have a 315KVA mini-sub, council supplied, on the property which then feeds a 450-amp/400V possible load. A typical council supply to commercial properties.

I haven't been to site so the fault they're reporting is second-hand knowledge. They have, however, told me that their neighbours are experiencing the same problem, which leads me to believe it's a supply problem on the HV/MV side. I'd assume this is a council problem and is also outside my skill set. I don't know whether HV fluctuations are handled by the mini-sub or not.

My question is how to fix? They'll contact the council but is there something I can install inside the property to stop the damage?

Is there some sort of large surge arrstor/capacitor I can install on the incoming supply to remove these surges, even if they are for long periods of time?

This is uncharted water for me but I need a solution and am hoping you guys can help.