Why would we need isolation transformers before and/or after for a UPS system? What are we trying to protect, and from what? I have read that it has to do with grounding and neutral but I didn't understand.
This is 100KVA UPS. The inputs are 3 phase, neutral, and ground, coming from the service panel which has the neutral connected to the ground inside the panel. The output 3 phase, neutral and a ground while only the 3phase and ground are connected to the load (no neutral is connected to the load). When the UPS-company-man saw the installation he said that we must use a isolation transformer before the UPS.
Here is a simple schematic of the this configuration :
simulate this circuit – Schematic created using CircuitLab
Best Answer
Most high-power installations require an isolation transformer. Every variable frequency drive manual I've ever seen requires the same thing. So it's not a UPS-specific requirement.
In general, the isolation transformer limits your available fault current, and keeps electrical disturbances generated by your load from propagating quite so easily through your entire facility. Limiting the fault current is particularly important, because fuses and breakers are only rated for some maximum current they are able to interrupt. And that, in turn, gets listed in the product's UL file. (I'm assuming this UPS is UL listed.) So it's not necessarily that the UPS won't work without the transformer. It's more that if something goes horribly wrong, they can't guarantee it won't catch fire.
I found this resource that explains more about drive isolation transformers, which is pretty consistent with the above: http://www.eecoonline.com/drive-isolation-transformer/
What you're talking about may not be a VFD, but many of the same concerns apply to a UPS or any other switching power supply.