February 2012
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The Generator and the UPS

An issue I had to deal with recently; an unstable generator/UPS relationship.  When the generator was running under load, it surged repeatedly causing the UPS to drop out and not recharge.  Eventually, the UPS ran out of juice and shut down, killing the power to the Sine Systems remote control and telephone system.  Of the two, the remote control was the biggest pain to fix, as it lost it’s timed commands and would not reduce power at sun set for the associated class D AM station.

What went wrong?  This is a chart of typical problems with generators operating UPS loads:

Symptom Potential Problem
Fail to “lock on” to generator power Improper generator frequency or voltage
Poor generator regulation
Unrealistic performance requirements
Instability of generator Voltage regulator sensitivity
Control loop compatibility
Filter/control interaction
Governor or AVR problem
Fail to sync bypass Frequency or voltage out of range
Poor frequency stability
Unrealistic performance requirements
Changes to total load on the system
Generator output voltage distortion
Instability at specific load levels Control loop compatibility
Instability at load changes Control loop compatibility
Metring errors Generator output voltage distortion
Loss of voltage control Excess capacitance in filters vs. load

Table courtesy of Cummins Power Generation.

Generator excitation methods can be the culprit in many of these situations.  Generators often use one of three types of excitation for their field coils:

  • Shunt excited SCR (silicon controlled recifier)
  • Shunt excited PWM (pulse width modulation)
  • PGM (permanent magnet generator)

Of the three, permanent magnet generator is the most stable since the AVR (automatic voltage regulator) is powered by a separate small generator which is unaffected by the load on the main generator output.  SCR and PWM both use the generator output windings, which makes them susceptible to load inducted voltage distortion brought on by non-linear loads.  Therefore, in locations where large UPSs are known to be part of the load, PGM excited generators are the best choice.

PMG generator diagram

PMG generator diagram

Sometimes, the generator is already in use before the UPS is installed.  In that case, there are some remedial steps that can be taken.  The speed which the voltage regulator reacts to changes in the load is often the culprit in many of these situations.  It may seem counter intuitive, however, the faster the AVR reacts, the more fluctuations there will be in the voltage and frequency.  A UPS can operate under a wide range of voltages and frequency, provided they do not rapidly change.

Depending on other loads, it may be necessary to dampen the gain on the AVR to slow it’s reactions down.  This will work if there are not large intermittent starting loads on the generator such as air conditioning compressors.

Another method would be to delay the UPS transfer to generator power until after all the other loads have been satisfied.  This will ensure that the generator voltage and current fluctuations are damped by the existing load.

The generator’s size needs to account for the equipment attached to the UPS and the battery charging load. With a larger UPS, the battery charging load can be significant. Generators that are improperly sized will not be made to work under any circumstances, hence the “unrealistic performance requirements” noted in the chart above.

You can read the entire Cummins Power white paper on generators powering UPS loads here.

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2 comments to The Generator and the UPS

  • Bob M.

    I had a similar experience. Old (1980s) Onan 7kw propane genny, mechanical governor, with a 1kw AM for the load. A good TripLite UPS seemed very picky about input voltage and frequency. It didn’t like the generator power, so it ran on the batteries until they discharged and it had no other choice but to (reluctantly) switch back to external power, until the batteries charged enough for it to switch off the gen, and so on.

    Some UPSes are better than others. For example I tried a Liebert GXT2 at the same site (this is a true on-line UPS) and it had absolutely nothing to complain about, using the generator power to charge the batteries while the inverter powered the load. Not all UPSes will do this, even though it seems like such a basic thing to do. Some of the APC SmartUPS units will switch between AC and battery if the frequency is off by more than 0.25 Hz, although you can de-sensitize some of them.

    Triplite had no such de-sensitization procedure. Their recommendation: replace the generator. (Yeah, like we’re gonna spend $6000 because their $1000 UPS can’t deal with imperfect power; that’s why there’s a UPS there in the first place.)

    Bob M.

  • Kris Kirby, KE4AHR

    The Liebert GXTs are also somewhat annoying because it will beep at you if you have it in bypass. Not a bad function, but annoying when you know it’s in bypass.

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