UPS Failure Investigation - Harmonics vs Generator Compatibility 1

[NDfan27]

I have been investigating a potential power quality issue at a facility that has been losing multiple UPS devices in one of their server rooms. They were unable to tell me if these failures coincided with any particular event but they believed the issue was on the utility side due to "high harmonic distortion". Due to safety concerns and because the issue was isolated in one part of their facility, we decided to place a power quality recorder in the breaker panel for the server room. After reviewing a couple weeks of recordings, I determined that Voltage THD is just below 5% and the high levels they were referring to was the Current THD. I've placed recorders at other loads around them and I'm pretty sure that the heavily non-linear load that this panel is serving is the source of any elevated harmonics.

However, this didn't explain why they were losing a UPS every few weeks. The IT Director told me that he would discover an UPS with a failure message and there were be a flash and loud pop when he pressed the power button on the device and that this happened several times. From what I’ve been reading about the type of UPS system that they are using, the capacitors are used to correction harmonic distortion among other things. This might help explain some higher THD if the capacitors were damaged in some way. It also sounds like capacitors are more susceptible to voltage and temperature issues than any harmonic issues we’ve been observing. I have not seen any issues with the voltage that we’ve been supplying them and the server room doesn’t seem to be getting too hot when I’ve visited the facility.

It does sound like generators coordinated with UPS systems can have some issues if they are not set up properly. Most of these issues sound like they’ll simply result in the UPS switching back to battery if it doesn't like the generator output, but I found an article mentioning that the automatic transfer switch could cause some issues if there is also motor load, such as HVAC. If the transfer setting is too fast, then the paper says that it can result in damage to both the HVAC motors and UPS. The first time I visited the site for this issue, they mentioned that they just had work done on the HVAC in that room after it failed.

Does an automatic transfer switch issue sound like a reasonable path to investigate for these issues? I've attached the article that I found online regarding UPS and generator issues. I've been working on this problem for several weeks now without any good answers.

Thanks,

 

[FreddyNurk]

The main issue that I'm aware with running generators against UPS' is that the deviations in voltage and frequency that are inherent in smaller size sets will generally be enough for the UPS to flag power quality issues and that it will keep going to battery as a result.

UPS devices are not generally nice to batteries, a lot of then are designed for short run times and to shut off, if allowed to run on batteries for too long they will generally kill the batteries in short order. Its likely that the server load is the reason for the THD, some generator AVRs don't do so well feeding high harmonic loads, but that's speculation at this point in terms of why the UPS units have failed.

I can't comment on ATS arrangements and potential issues, its possible, but a diagram would go a long way in understanding where the ATS might be and whether it would affect the installation.

EDMS Australia

 

[NDfan27]

Thanks for the reply.

I wish that we could get a good power quality recording when they do a generator test but unfortunately the sudden change in frequency seems to mess up the data. The manufacturer of our recorder confirmed that this could be an issue so I really don't have any good data on what the UPS units see when it transfers to the generator.

I agree that the THD source pretty much has to be the server load. There is an oil well with a variable frequency drive nearby but it hasn't run in years. The Voltage THD outside the facility doesn't get above 3% so I think it's isolated to that room. They have UPS units in other server rooms that don't seem to be affected.

I don't have permission to share a layout of their facility but I have a picture that looks like they have a dedicated ATS for this room (along with several others - maybe 5 ATS total). The 750 kW generator produces 480/277V which is the same as the secondary of our padmount transformer. It looks like the ATS can switch from the utility source to the generator and from there is goes through a 75 kVA dry type transformer to step it down to 208/120V. They can pick which loads they want to move to generator power. Not sure if any of that helps.

I apologize if any of the information doesn't make sense. I don't have much experience in troubleshooting power quality issues and it's always from the utility perspective. I'm unfamiliar with most of this equipment. I'm just trying to help our member solve this issue.

 

[ScottyUK]

Some of the UPS designs marketed under the APC badge struggled with heavy voltage distortion, particularly with a multiple zero crossing. I think the problem was the active front end struggling to track the input voltage, but whatever it was the UPS didn't like it at all. One blew its front end, another smoked a bit but remained operable. I don't suppose the problem is limited to this specific brand, I'm just commenting on what I have personally observed.

If you have similar voltage distortion - which is more likely on a generator than on a utility supply - then you may be seeing a similar problem.