Nonlinear Load Problems with Stand by Generators

If a facility’s electrical loads, such as computer power supplies, variable speed drives, electronic lighting ballasts, or other similar nonlinear electrical equipment, is furnished with switch-mode power supplies, it is imperative that you advise the generator supplier of this situation so that proper steps can be taken to avoid equipment overheating or other problems due to harmonics. Some generator manu- facturers recommend low-impedance generators and have developed winding design techniques to reduce the effects of the harmonic currents generated. The key issue is maintaining the ability to produce a stable voltage waveform and a stable frequency.

This is mostly an issue of voltage regulation system design, but can also be impacted by the governing (fuel control) system design. The winding arrangement is not as critical as the impedance of the machine relative to the utility service. The closer it is to what the utility provides, the more like the utility it will operate and the lower probability of problems. In some instances, the generator may have to be de-rated and the neutral size increased to safely supply power to nonlinear loads. When a standby generator and UPS are integrated in a system together, problems occur that typically do not exist with a UPS system or generator when they are operating alone in a system. Problems arise only when the UPS and standby generator are required to function together. Neither the UPS nor the standby generator manufacturer is at fault, and both manufacturers would probably need to work together to solve the problem. The following are common problems and solutions when applying a design that incorporates a standby generator and UPS.

1. Line Notches and Harmonic Current. The UPS manufacturer using a properly designed passive filter can address the problem of both line notches and harmonic currents. Most generator manufacturers have de-rating information to solve harmonic heating problems. However, an input filter on the UPS that reduces the harmonics to less than 10% at full load eliminates the need for de-rating the generator.

2. Step Loading. When a generator turns on and the ATS switch connecting it to the UPS closes, the instantaneous application of the load to the generator will cause sudden swings in both voltage and frequency. This condition can generally be evaded by verifying that the UPS has a walk-in feature. This requires that the UPS rectifier have some means of controlling power flow so that the power draw of the UPS can slowly be applied to the generator between a 10- and 20-second timeframe.

3. Voltage Rise. This is an application problem that transpires when a generator is closely designed and sized to the UPS and there is little or no other electrical load  on the generator. When the UPS is first connected to the generator by the ATS, 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 increased excitation energy for the generator. The issue is the capability of the alternator to absorb the reactive power generated by the filters. The amount that can be absorbed varies considerably between different machines from the same manufacturer. System designers should evaluate the capability in the initial design of the system to avoid problems. The outcome is that the voltage roams up without control to approximately 120% by some fundamental generator design constraint, typically magnetic saturation of the generator iron. If the value does hit 120% of the nominal, it will be damaging or disruptive to the system operation. However, a UPS that disconnects its filter when its charger is off avoids this predicament altogether.

4. Frequency Fluctuation. Generators possess inherent limitations on how closely they can manage frequency regarding their response to changing electrical loads. The function is complicated and not only involves generator features, such as rotational inertia and governor speed response, but also involves the electrical load’s reaction to frequency changes. The UPS charger, conversely, also has inherent limitations on how closely it can control its power needs from a source with fluctuations in voltage and frequency. 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 exasperated. The most noticeable effect of this fluctuation is a recurring alarm that is found on the bypass of the UPS, announcing that the generator frequency is changing faster than a UPS inverter can follow. In order to minimize or eliminate frequency fluctuation problems, good control design from both the engine generator and UPS manufacturer are required. The engine must have a responsive governor, appropriately sized and adjusted for the system. The UPS manufacturer should have a control responsive to fast frequency fluctuations.

5. Synchronizing to Bypass. Some applications require the UPS to synchronize to bypass so that the critical load may be transferred to the generator. This generally places tighter demands on the generator for frequency and voltage stability. When this is the case, the system integration problem may be intensified. As described above, good control design can usually reverse this problem.

6. Automatic Transfer Switch. Most generator/UPS projects incorporate automatic transfer switches that switch the UPS back to utility power once it becomes available again. The speed of transfer can be an obstacle and may result in a failed transfer. This in turn will lead to nuisance tripping of circuit breakers or damage to loads. If the ATS switch also has motor loads, such as HVAC systems, the UPS input filter will supply excitation energy during the transfer. This excitation source turns these motors into generators using their inertia as an energy source and also their alternator field strength. If the transfer occurs too fast, causing an unexpected phase change in the voltage, the consequences can be devastating for both the motors and the UPS. One of the best solutions is to simply slow the transfer switch operation speed so that the damaging condition does not exist. Rather than switching from source to source in one-tenth of a second, slowing to one-half of a second will resolve the problem. UPS manufacturers can resolve this problem by providing a fast means of detecting the transfer and disconnecting the filter.