With the certainty of grid-supplied power diminishing, as powerfully shown by the August 14 blackout, power quality is more critical than ever. You can turn to generators as a source of backup power, but they may introduce new problems, especially in facilities that use sensitive electronic equipment. That’s why uninterruptible power supply (UPS) systems are popular options for that seamless transition. Still, you may experience specific problems when you operate UPS systems with generators—problems that don’t occur when you have stand-alone equipment.
Let’s look at some of the most common problems that can arise when a generator and UPS must work together.
Interaction problems. UPS systems range in size from less than 300W to more than 3MW, but we’ll focus on the range of 50kW to 500kW. In this size range, most UPS manufacturers use a thyristor rectifier for battery charging, with a passive filter to improve power factor and reduce harmonic feedback. While many engineers would prefer a charger section capable of synthesizing a linear load, the increased complexity of such a converter, as well as its decrease in efficiency and reliability, make this solution undesirable.
Line notches and harmonic currents. Rectifiers, which most UPS systems use for charger control, cause notching on the power feed at the generator or utility. These notches can wreak havoc on some types of generator controls. In addition, chargers typically don’t draw sine wave current from the line. The extent to which the current differs from a sine wave is referred to as total harmonic distortion (THD). These harmonic currents may cause excessive heating in the generators.
The THD of a 12-pulse rectifier is typically 12%, with the 11th and 13th harmonics dominant. The THD of a 6-pulse rectifier is typically 30%, with the 5th and 7th harmonics dominant. The THD of a 12-pulse rectifier is usually low enough to avoid generator-heating problems. But 12-pulse rectifiers are becoming increasingly rare below 500kVA, since they require an input transformer, which increases the unit’s size, cost, and weight.
Some UPS manufacturers solve the problem of line notches and harmonic currents by using a properly designed passive filter (Fig. 1). Most generator manufacturers also have derating information to address harmonic heating problems. By using an input filter on the UPS that reduces the harmonics to less than 10% at full load, you can eliminate the need to derate the generator. This filter should have an input series inductor of about 5% to detune it for other disturbances on the line.
Step loading. When a generator turns on and the switch connecting it to the UPS closes, the immediate application of the load to the generator can cause sudden swings in frequency and voltage. You can usually avoid this situation if the UPS has a walk-in feature. This means the UPS rectifier has some means of controlling power flow, such as thyristors, so the power draw of the UPS can be applied gradually to the generator over a 10-second to 20-second period. This prevents the protected load from varying.
Voltage rise. This problem occurs when you size a generator too closely to that of the UPS and there is little or no other load on the generator. When a UPS is connected to the generator with a transfer switch, the UPS’s charger has turned off so that it may begin the power walk-in routine. If the input filter is the only load on the generator, it may provide excess excitation energy. Most exciter controls have no way to deal with this excess energy, so the voltage wanders up without control to about 120%, limited only by some fundamental generator design constraint—usually magnetic saturation of the generator iron. Most generator suppliers have preloads you can attach to counter this effect. A UPS that disconnects its filter when its charger is off avoids this problem altogether.
Frequency fluctuation. Generators have inherent limitations as to how closely they can control frequency and their response to changing loads. The function is complex and involves such features as rotational inertia, speed of governor response, and the load’s reaction to frequency changes.
The UPS charger also has inherent limitations on how closely it can regulate its power needs from a source with fluctuations in voltage and frequency. Since the generator controls and the UPS charger controls are affected by and respond to the frequency, an otherwise small frequency fluctuation may be problematic and troublesome. The most noticeable effect of this fluctuation is a chronic alarm on the UPS that announces that it can’t synchronize to bypass.
Generator and UPS suppliers need good designs that incorporate the following elements to minimize or eliminate frequency fluctuations:
The generator should have a responsive governor that is properly sized and adjusted for the system.
The generator’s voltage regulator shouldn’t be more responsive than the governor; otherwise, an unstable condition will occur with the UPS battery charger.
The UPS should have a control that’s responsive to fast frequency fluctuations.
The UPS charger should be able to function properly with a slew rate on input frequency fluctuations greater than 5 Hz/sec.
It’s important to note that not all UPS topologies can compensate for frequency variations without accessing the battery. Both standby and line-interactive UPSs rely on battery power to prevent frequency variations from affecting the protected load. Because it recreates the sine wave, a double-conversion online UPS filters frequency variations as part of its normal operation, preserving battery life (Fig. 2).
Synchronizing to bypass. You may have an application that requires the UPS to synchronize to bypass so the critical load transfers to the generator. This usually places tighter demands on the generator for frequency and voltage stability and may worsen the system integration problem. Good control design generally nullifies this problem. Make sure your UPS supplier is able to increase the acceptance window for bypass frequency deviation and slew rate when it’s acceptable to the load.
Automatic transfer switching. Most generator/UPS installations include automatic transfer switches that switch the UPS back to utility power once it becomes available again. The speed of operation can be a problem and may result in a failed transfer. If the transfer switch also has motor loads, such as HVAC systems, the UPS input filter will supply excitation energy during the transfer. This excitation turns these motors into generators, using their inertia as an energy source. If the transfer occurs too fast and causes an abrupt phase change in the voltage, the results can be devastating for both the motors and the UPS. A UPS that automatically senses the transfer switch action and quickly disconnects its input filter can eliminate these problems.
Compatibility issues are inherent when interfacing UPSs and generators. To mitigate downtime risks, you must address these issues. By understanding the operational characteristics, load interaction, and control design of these devices, you can ensure your system provides reliable power-supply protection.
Tracy is director of 3-phase technology for Powerware, Raleigh, N.C.