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/
If the selected transformer has a Wye configured secondary, then a secondary ground can be gained with the installation of the isolation transformer. This ground is isolated from the primary input and provides a couple of distinct benefits:
Grounding prevents the transfer of common-mode noise and transients, both from the primary source to the motor drive, and from the drive to the power system. This can reduce "bearing currents" that often cause fluting.
Introducing a grounded, drive isolation transformer localizes the high-frequency induced ground currents and prevents them from extending upstream of the transformer, minimizing "noise" and related problems often associated with drives.
Consider an isolation transformer when power quality and fault currents are the primary consideration.
Another optional benefit of isolation transformers is that of electrostatic shielding. This provides a shield between the primary and secondary winding, which can provide in the range of 40-60 dB of common mode noise reduction.
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.
The portable generator is designed primerally to provide a 110V center-tapped earth supply as is normally used on UK construction sites. The 230V output is something of an afterthought which is why you end up with it referenced to earth in a weird way.
In the standards for modern appliances in Europe there is no expectation that the "neutral" pin is at earth potential. So powering modern appliances off the generator should be safe.
OTOH in UK house wiring it is normal to assume that neutral is at earth potential. We don't normally put any overcurrent protection in the neutral and we frequently work on circuits with only the live isolated.
As I see it you have a few options, each with it's pros and cons.
- Make sure all circuits are RCD protected with a double pole RCD (note that most RCBOs are only single pole isolating) to mitigate the lack of overcurrent protection in the "neutral" and place warning notices so people don't try to work on circuits that are only single pole isolated.
- Set up the consumer unit with double-pole breakers. This is a good option electrically but can get kind of costly as double pole breakers are not widely used.
- Modify the generator, remove the existing earth reference and add a new one at the neutral end of the winding. Downside here is you will almost certainly be voiding the warranty and the 110V output will no longer be the center-tapped earth supply expected on UK construction sites.
Best Answer
The fundamental logic behind this earthing arrangement is purely of safety. There are 2 scenarios underlying here:
The transformer MEN and consumer board MEN are in physically separate locations, ie pole / house respectively. In this case all earth potentials around the transformer body and dirt are identical.. meaning zero electrocution risks. Similarly, the earth potentials around the house, switchboard and dirt are identical again with zero risks.
The transformer and consumer switchboard are in the same location, adjacent. I've used 2 earth rings, one that connects all equipment metalwork including transformer case to earth and to earth electrode, eliminating earth potential electrocution risks. The 2nd only connects the utility earth direct to the earth electrode .
The big issue is when the Utility supply is under fault conditions, so it's possible to have hundreds (possibly thousands more) of volts presented at the transformer earth, causing step potential electrocution risks. In scenario 1. above, the consumer environment remains largly safe. In scenario 2. above careful engineering is required to avoid a fire, and bonding nearby things like fences, pipes, etc..