As the demand for electricity grows and the certainty of grid-supplied power diminishes, power quality is more critical than ever. Facilities turn to generators for one source of backup power. But generators that protect essential loads may introduce new problems, especially with sensitive electronic equipment. That's why adding an uninterruptible power supply (UPS) to the mix is a good idea. Still, specific problems will arise when you operate UPSs with generators — problems that don't occur when you have stand-alone equipment. In this article, we'll deal with those specific problems and learn how one or both can provide the necessary solutions.
Since the majority of power outages last no longer than 5 sec, a UPS with a 10-min or 15-min battery can provide enough power to ride through the outage or give ample time for an orderly shutdown. Using a stand-by generator is necessary because a UPS is not a power-generating device. It serves only as a bridge for an alternate power source during extended outages. The seemingly straightforward task of sizing generators for use with UPSs is actually quite complex. Some UPS manufacturers work to solve the issues of UPS/generator interface, while others leave the task to the generator suppliers.
While UPSs range in size from less than 300W to more than 3MW, we will 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. Let's look at some of the most common problems that can arise when a generator and UPS work together.
Line Notches and Harmonic Current. Most UPSs use a method of charger control (rectifier) that causes notches on the power feed (generator or utility). These notches can play havoc with some types of generator controls. In addition, chargers typically do not draw sine wave current from the line. The extent that the current differs from a sine wave is often 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 eleventh and thirteenth harmonics dominant. The THD of a 6-pulse rectifier is typically 30% with the fifth and seventh 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.
The UPS manufacturer can solve the problem of line notches and harmonic currents by using a properly designed passive filter (see Fig. 1, on page 24). Most generator manufacturers have derating information to address harmonic heating problems. An input filter (on the UPS) that reduces the harmonics for less than 10% at full load eliminates the need for derating 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 usually can avoid this situation if the UPS has a walk-in feature. This means the UPS rectifier has some means of controlling power flow (usually thyristors), so the power draw of the UPS can gradually be applied to the generator over a 10-sec to 20-sec period. This prevents the protected load from seeing any variation.
Voltage Rise. This problem occurs when a generator is too closely sized to 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 its 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, so the voltage wanders up without control to approximately 120%, limited 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 Fluctuations. Generators have inherent limitations on 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 exasperated. The most noticeable effect of this fluctuation is a chronic alarm on the UPS announcing that it cannot synchronize to bypass.
Good control design from generator and UPS suppliers are needed 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 should not be more responsive than the governor; otherwise, an unstable condition will occur with the UPS battery charger. The UPS should have a control 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 per second.
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 (see Fig. 2).
Synchronizing to Bypass. Some applications require 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 exacerbate the system integration problem. Generally, good control design nullifies this problem. The UPS supplier should be able to increase the acceptance window for bypass frequency deviation and slew rate when it's acceptable to the load.
Automatic Transfer Switch. 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, causing an abrupt phase change in the voltage, the results can be devastating for both the motors and the UPS.
Compatibility issues are inherent when interfacing UPSs and generators. To mitigate downtime risks, it's imperative that you address the issues discussed in this article. By understanding the operational characteristics, load interaction, and control design of these devices, you can ensure your system provides reliable power-supply protection.
John G. Tracy is director of product engineering for Powerware. He has more than 30 years experience in power conversion electronics, including 17 years in UPS product development. Tracy is presently working on algorithms for improving the performance of single-module UPS and multi-module parallel systems.