Re. High Impedance Source
"Not harm the UPS? I suppose not, because as I have experienced it, the PM250 will just turn itself off when it detects the high impedance source, so the drive will not be operating. I found out the hard way that this is what they do. It turned out that Siemens knew all along and didn't tell me when their salesman was convincing me to use it, then pointed out where it was buried in the manual in cryptic terms."
A small clarification here. It can harm any UPS. Its all dependent on the UPS control system and hardware design. The regenerative VSD/VFD Drive when braking becomes an energy source on the UPS output bus. If the other loads on the UPS output bus are not capable of absorbing the energy, the only other path is backwards through the UPS Inverter IGBT flyback diodes to the DC bus. This reverse energy then pushes the UPS DC bus voltage up to potentially damaging levels.
Generally speaking, the ratio of regenerative loads to steady state loads on the UPS output bus needs to be ~40%:60% respectively. If not, there will be issues, including shutdown or potential damage to the UPS. This is true of all static UPS topologies.
The other option is to put passive braking on enough drives to get the reverse energy down to a level that the other steady state loads can absorb.
Its very similar to a engine-generator control system response to leading power factor loads, e.g., the engine-generator shuts down to protect itself from damage. In fact engine-generators respond to reverse energy in almost an identical fashion.
I would recommend a 7x24 Power Quality monitoring of the SYPX UPS output bus voltage and current with the unit in static bypass. If power fails while its in static bypass it will transfer to battery mode to protect the loads and then go back to normal mode. That way you have some hard data to work with in making decisions.
The SYPX250/500K UPS's also have waveform event capture capability, e.g., to capture events like this, so you should already have output bus voltage and current waveforms for any shutdown. Your FSR can download the logs with the waveform files and forward them to me for analysis.
If you choose to do a PQ study. Whatever PQ Monitor you use has to be able to capture and zoom in on any transient event, e.g., 10ms or less. I recommend RMS Monitoring Services if you don't have a preferred vendor. Keep in mind PQ histograms are averaged values based on the sampling time setting. Therefore, the PQ monitor must be capable of transient event capture too. I would set the current setting @ the UPS output ratings. The RMS Cx Monitors capture all events once they are properly configured for the power system voltage & frequency. No, I don't work for RMS Monitoring Services, I work for Schneider Electric. You can find me on Linkedin.
