Differences in response to generator frequency out of spec

[nt4workstation]

What explains why different double conversion online UPSs respond differently to frequency problems on line power?  I realize that load & battery condition are the biggest factors, but is there something in the unit design that can compensate better for frequency deviations?  

Recently had a six-hour power outage.  At one site, the generator ran normally for about 45 minutes and then started varying wildly in frequency, with a range of 45 to 62 Hz as recorded by one of our three-phase UPSs.  

Clearly the generator needs service and I got a ticket started on that.  

Some UPSs held their own until we got commercial power back.  Others were dead after 45 minutes on variable frequency power.  

So this question remained, as we're constantly buying UPSs, and of course I'd like to buy stuff that thinks dirty power is better than nothing.  

[Teken]

At a high level no UPS from any maker will function correctly if the voltage source is providing dirty output power.

Dirty power encompasses voltage, frequency, THD, and Slew.

You should be considering both On Line (Double Conversion) and Line Interactive topologies both provide pure sine wave.

Only an extensive live field test in the environment will confirm if the hardware under consideration will operate as expected. 

99% of the consumers fail to ever bother to perform any tests. Than later, complain the hardware failed to meet their expectations when in reality they did absolutely no validation or testing and just assumed the black box would function during a grid down / lights out event! 🤦‍♂️

I suggest you hold off getting the generator repaired. As you can use this worse case scenario and condition to test out other UPS Systems.

As it relates to operational runtime any location that requires longer (on battery) runtime should be using extended runtime systems such as the SMX, SRT, SUA, SURTA, Smart-Ultra, etc.

You simply can’t expect a single unit to provide any reasonable runtime especially if the load is high.

The only goal for the vast majority of UPS systems is to allow connected devices to be gracefully shutdown or to bridge the time from UPS to generator power.

Depending upon specific model and firmware revisions you’ll find double conversion units from APC are more sensitive and limited in handling dirty power.

More specifically Line Interactive units allow the power quality, high / low transfer voltages to be adjusted.

In Online Double Conversion units APC muddies the waters by using the phrase (Bypass)! 🤦‍♂️

The Power Quality setting defines when the unit will go into (Battery Only) mode. The transfer voltage setting defines when the unit will engage the AVR circuit.

Its the AVR circuit that will allow the system to accept a wider (dirty) input voltage without going into battery only mode.

Which extends the operational runtime and the service life of the RBC (Replacement Battery Cartridges).

Given the above some units will allow the end user to also define the frequency and slew rate. This can also help the system to accept dirty power from the utility / generator.

Neither topology from any UPS maker will have the ability to adjust / define THD! The vast majority of systems use filters, transformers, and optical isolation to isolate and reduce noise.

Why is this important to know?!?

Because you will find out in some conditions the input voltage, frequency, slew is fine. But, if you take a scope to measure and view the waveform it will be filled with 1st, 2nd, 3rd harmonics.

As such the UPS will refuse to go back to line power and remain in battery only mode and thus draining the RBC!

Obviously insuring the generator is providing clean output power makes sense. But, as you see yourself things break and that’s why other measures should be in place to compensate for the same.

Depending upon the size and voltage of the UPS line conditioners can be installed at the POU (Point of Use) or at the panel.

Some units are a giant AVR vs others are just passive filters . . . 

You can adjust the Power Quality, Transfer Voltage, Frequency, Slew, to a wider range. Doing so will force the system to accept common dirty power.

NOTE: Do not adjust these values until you have validated the connected loads are capable of accepting this output power!

The vast majority of electronics can accept dual voltage / frequency. As such they operate fine as the hardware is designed to operate in both extremes.

While other devices which don’t support dual voltage / frequency and may also include highly regulated PSU will begin to draw more power and heat up!

Question's Ask . . . 👍 

[nt4workstation]

Thanks for the rundown.  I wish we could hold out the generator as a test case, but we don't have the luxury.  It's already had some maintenance, and a test showed it stayed consistent for 45 minutes.  Load is heavier during an actual outage though, and we can't simulate that without a lot of people involved.  Anyway, the process has started to get the generator replaced.  

Thanks for the comments on the power settings - we have 208/120 3 phase throughout.  The most finicky UPS on this generator is 208V double conversion and it's loaded to about 10 amps during an outage, 5 amps during a test, so not inconsequential load.  Bypass upper is 255, lower is 160, and auto 50/60 Hz output.  All of the downstream equipment is rated 100-240VAC 50-60 Hz.  Screenshot attached.  

I appreciate the comments about power settings.  I also looked at one of the 5000VA units and I had that set to low sensitivity and as wide a range of voltages as the dropdown menu would allow.  Also attached.  

Outside of sensitivity and the listed voltages, I don't see anything I can change for these.  BUT I am thinking, if I can put the 3000VA unit in bypass next time, that would be worth a try.  I am guessing the downstream equipment can handle the generator power better than the UPS.  

Smart-UPS 5000SURTD3000XLT