Coupling the capabilities of a UPS and generator might seem the obvious thing to do for large companies, affording them the protection that their sensitive electrical equipment require. However, as Graham Beyleveld, sales director at Meissner Power Systems, points out, the ability of a UPS to work well with the unique operating characteristics of a generator requires special consideration.
Since over 90% of power outages last up to only five seconds, a UPS alone with a 10 or 15-minute battery, can provide enough quality power to allow time for an orderly shut down.
Companies looking for an extended power backup usually turn to generators. However, the seemingly straightforward task of sizing an engine-generator for use with a UPS is complicated by the complex nature of the resulting system. Some UPS manufacturers solve the issues that crop up while others leave the task to the generator supplier. "When engine generators and UPSs are put together in a system, problems arise that do not normally exist with a UPS or generator alone. Neither of the technologies are at fault, however, both sets of suppliers should endeavour to solve the challenges associated with their symbiotic relationship," says Beyeleveld.
When a generator turns on and the switch connecting it to the UPS closes, the instantaneous application of the load to the generator can cause sudden swings in both frequency and voltage. This situation can generally be avoided by assuring that the UPS has a walk-in feature. This requires that the UPS rectifier has some means of controlling power flow so that the power draw of the UPS can gradually be applied to the generator over a 10 to 20 second period.
Voltage spikes occur when a generator is closely sized to the UPS and there is little or no other load on the generator. When the UPS is first connected to the generator by a transfer switch, its charger has turned off so that it may begin its power 'walk-in' routine. If the input filter is the only load on the generator, it may provide excess excitation energy for the generator. Most generators' exciter controls have no way to deal with this excess. The result is that the voltage wanders up without control to approximately 120%.
"Most generator suppliers have preloads that can be attached to the generator to counter this effect. However, a UPS that disconnects its filter when its charger is off avoids this problem altogether," says Beyleveld.
Generators have inherent limitations on how closely they can control frequency and on their response due to changing loads. The function is complex and not only involves generator features, such as rotational inertia and speed of governor response, but also involves the load's reaction to frequency changes. Since both the generator controls and the UPS charger controls are affected by, and respond to the frequency, an otherwise small frequency fluctuation may be exacerbated. The most noticeable effect of this fluctuation is a chronic alarm on the UPS announcing that it cannot synchronise to bypass.
Good control design from both the engine-generator supplier and the UPS supplier are needed to minimise or eliminate frequency fluctuation problems. The engine should have a responsive governor, properly sized and adjusted for the system. The generator's voltage regulator should not be more responsive than the governor; otherwise an unstable condition will occur with the UPS battery charger.
"It is important to note that depending on the specific application, all of the mentioned features have to be considered prior to coupling a UPS and generator system successfully," concludes Beyleveld.
For more information contact Graham Beyleveld, Meissner Power Systems, 011 824 0202, graham@meissner.co.za, www.meissner.co.za
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