UPS and Standby Generators

[Anonymous user]

I have a standby portable generator(a mid level quality unit big enough to run the house at about 80%) and the house is wired with a manual transfer switch. I need a layman's education on the quality of the power that comes from such a generator and whether it's true that UPS "clean up" the power supply for computers and other electronic devices. I currently have two APC UPS units in use in our home connected to two computers. My limited grasp of the topic has led to some research that talks about three different types of UPS;

“stand-by” (which switches over to the battery only when power is lost, otherwise running on line power)

“stand-by interactive” (which has an inverter that somewhat regulates the power, but the main power comes from line power until it switches to battery)

“double conversion” (in which all the power comes from the inverter regardless of whether the line power is on or off)

My understanding is that the double conversion type does the best job of cleaning up the output . I do not even know what types of systems my two UPS have; they are an older Backup UPS-650 and a fairly new XS-1300.

I guess my bottom line is whether it's safe to use my eletronic devices while on generator power, especially the computers plugged into the UPS.

Many thanks!

Jim

[Erasmus_apc]

I would only say that the UPS can be used for the LCD TV if it's a very expensive model and you would like the chance to turn it off manually for some reason, as well as add in some surge protection. That is up to you.

The UPS will certainly give cleaner power to it, which is of course beneficial to all electronic devices, but for LCD TV's I am not aware of any problems they may have with losing power suddenly. I would just be concerned with feeding it noisy or "bad" power.

[Erasmus_apc]

While I cannot vouch for the exact power quality properties of such a generator system (the manufacturer should have more info), the general problems we do see in regards to generators in use with our UPS systems is a combination of harmonic distortion (THD), frequency, and jumpy voltage. Loads that have a high in-rush current and/or loads that have generate harmonics (also non-linear loads) can cause voltage, frequency, and "noise" issues at the generator level, which can then get fed back through the output to be seen at the UPS/load input. We usually see this at APC in the form of a generator being loaded too heavily or loaded with other motor-based equipment, or a generator that is not properly sized against the existing UPS system, etc.

When we do see these issues, it could just be a frequency being out of range, or jumping around to the point where the UPS goes to battery from time to time. It could also be too much noise on the line, or voltage Dv/Dt (delta/change in voltage/time) causing the UPS to stay on battery, since it can't evaluate if the power will be acceptable. APC recommends that with our smaller stand-by and line-interactive units, that you size the generator to 3-5x the size of the total load. For safety's sake, the more over sized the generator, the better.

Your research was correct in finding that "double conversion on-line" UPS' provide the most reliable power output. Due to the fact that these systems take input, and recreate the sine wave to power the output, they can operate in a wider range of input power, and they are guaranteed to output a pure sine wave that is free from distortion and set at your specified output setting. Our line-interactive UPS systems have their inverter connected to the output line of the UPS at all times, and also include a tap-changing transformer that allows for the adjusting of output without going onto battery; the UPS "boosts" or "trims" voltage that is too high, which is called Automatic Voltage Regulation. The stand-by UPS is a simple UPS that basically works as you explained.

Your Back-UPS 650 is a stand-by UPS, and your XS1300 is a line-interactive UPS. With these UPS' and a properly sized generator, you should be safe to run your electronic equipment. If needed, you can set the "sensitivity" of the UPS systems to a lower setting so that they will not be as prone to switch to battery power if they see transient/voltage issues from the generator. The stand-by UPS will only go to battery if it sees a power problem severe enough to necessitate a transfer to batt, which will provide 120v good power to the load, and your line-interactive UPS will prefer to boost/trim the power if a voltage issue arises, and will otherwise transfer to battery.

[Anonymous user · Accepted solution]

Thank you for that useful answer.

As a followup, I would ask if it would also be important to put a UPS on our LCD TV/VCR for the same reasons? In truth, being without power is a rare event, but this winter the power was down several days and that generator paid for itself!. We have the backup because we want to keep our water system, heat and freezer/fridge going. We can live without TV. Since I work at home and use a computer to do so, that's also important if an event will go on for days. Still, it would be nice to know about using other electronic devices while on generator power. Again, many thanks.