We've got a dual-conversion split-phase(120/208 transformerless) Powerware 9170+ 18 kVA UPS that is feeding an isolation transformer (208 to 120/240)configured as an SDS. The transformer was added in an attempt to eliminate common-mode noise from the UPS (may not be an issue in a server farm, but a VERY BIG issue in our application, with includes 15-20 radio broadcast and production studios). While the common-mode noise is dealt with quite nicely in this arrangement, there's still objectionable high-frequency noise showing up on the secondary of the transformer, noise that is quite evident in our low-level, high-gain audio circuitry.
Ideally, PowerWare should clean up their inverters and filter them better... but of course, we have to get them past the first step (admitting they have a problem) before we can get them to work the program . Based on the discussions to date, I don't see that happening in the near future.
I know the best way to filter this noise is with a good EMI filter between the UPS and transformer. I also know that I've run out of budget, so I'm experimenting with a few alternatives. I've found that a couple of 30-60 mfd capacitors from the secondary legs to ground do a very credible job of shunting the objectionable noise to ground and reducing it to an acceptable level. I've thought this through and can't think of any problems that would be created by these caps, but then again I don't deal with capacitors on power lines all that frequently, so I may be forgetting something.
Any reason why these two capacitors on the secondary could create a problem?