UPS and generator compatibility is not a new issue. However, with the increased use of UPS systems, often added to existing generator systems, the challenge persists. UPS input filters are used to maintain the input current distortion of UPS modules to a specified low value. Because these filters are sized based on the load, they can cause the upstream electrical system to see excess capacitance and leading power factor, when operating only at partial load. To solve this problem, UPS manufacturers either switch the filters in and out of use at certain pre-defined load limits or use an active filter that adjusts the filtering according to the load. The voltage regulator of the generator system can lose control of the system voltage when presented with a leading power factor, not allowing the generator to pick up the UPS load.
Make sure to obtain the generator's reactive capability curve. This shows its capability to both produce and absorb power. Comparing this with information obtained from the UPS manufacturer on the amount of KVAR units (a measure of reactive power) produced by the UPS module at differing load conditions will help spot any potential compatibility problems.
Compatibility between the UPS and its load is also critical. UPS loads due to low distortion/high efficient power supply designs are moving from lagging to unity power factor (a power factor of 1) and in some cases leading power factor.
Many UPS modules, especially the older legacy type, are not designed to support higher power factor loads and must be de-rated accordingly because more of the KVA units capacity must be used to support the load. Newer UPS designs are more compatible with and are rated to support leading power factor loads with little or no de-rating due to their output filter design and controls. Often this problem is not apparent up front because the initial loads on the UPS are a small percentage of the full load rating so the system is naturally de-rated. However, as the loads increase and become closer to the full load, this problem becomes more evident.
Engine generators continue to be the preferred means of supplying standby power for durations longer than can be supported by the UPS. Many current projects involving trading floors, command centers or office power upgrades occur in existing buildings, making the addition of engine generators challenging. Many of these issues are not directly electrical in nature but more mechanical.
For example, louver location and air supply are crucial to an emergency generatorÕs success. A generator requires a certain amount of air for both combustion and cooling. If the louvers are too small, the generator may have to be de-rated, or worse, may overheat when operating at design day temperatures. (Make sure to always test the generator at least once at full load on a near design temperature day to make sure air requirements are met and the generator will not overheat).
Ideally, inlet louvers should be placed behind the generators and the air drawn across the engine and exhausted out through the radiator. The exhaust louver should not be located adjacent to the inlet louvers or hot exhaust air could be short circuited into the inlet louvers. Some general rules of thumb include: The effective open area for the radiator exhaust should be roughly equal to the radiator area. The effective open area of the inlet to the room should be 1.5 times the radiator area. Another solution is to provide remote radiators for the heat rejection of the engine thus reducing the amount of required ventilation air.
Sound attenuation is also another important design factor. The sound level originating from an engine generator can be in the range of 95 dB. Many cities, states and industrial parks have their own sound emission requirements, often between 55 and 65 dB at the property line. Meeting these sound requirements may demand the use of sound baffles and high-attenuated mufflers.
Whether the project involves a new or existing building, the decision that has to be made is where to place the generator Ñ indoors or outdoors. If located indoors, the issues of inlet air, exhaust combustion, and fuel storage should be addressed. Often these become constraints that make it more feasible for the engine generator to be mounted outdoors. When mounted outdoors, however, the engine generator is enclosed in a sound-attenuated enclosure that may be subject to extreme weather conditions. Ice and snow can inhibit access to this enclosure, making testing and maintenance more difficult.
As the demand for power reliability continues to spread from industry to industry, creating a reliable, efficient, scalable and compatible infrastructure of UPS, generator and switchgear systems will be crucial. Avoiding the many pitfalls that can accompany these designs will be even more so.
Paul Schlattman is vice president and Robert W. Weber, PE, is senior vice president and director of electrical engineering for the mission critical facility group, Environmental Systems Design, Inc. in Chicago. 
Reliable Backup Power Requires Careful Design Attention
Preventing UPS and Generator Compatibilty Problems
