Proper Generator Sizing for Large UPS Systems
In general, proper sizing of the generator is crucial to the success of any installation and requires a good working knowledge of electricity and its characteristics, as well as the varying requirements of the electrical equipment comprising the load. When analyzing any electrical load, consult the manufacturer’s nameplate on each major appliance or piece of equipment to determine its starting and running requirements in terms of watts, amps and voltage.
When choosing the generator output for commercial or industrial applications, in general, select a rating that is approximately 20 to 25% higher than the peak load (for example, if the load is about 40 kilowatts, select a 50 kW gen-set). A higher rated generator will operate comfortably at approximately 80% of its full capacity and will provide a margin of flexibility if the load increases in the future.
UPS have three major types of compatibility categories for generators. The first two are treated the same, Passive and Line-Interactive. These technologies are most common for personal workstations and point of sale applications. They are typically single phase equipment with size ranges of 350 VA – 2000 VA for passive and 500 VA to 5000 VA for line-interactive. These types will not be addressed for this entry.
Double-conversion technology is most common for critical load applications. These UPS systems constantly rectify AC to DC and then invert the DC back to AC. This configuration results in an output that corrects for voltage and frequency deviations. Double-conversion technology can be either single or three phase, large applications (larger than 5000 VA) are almost always three phase. Fortunately double-conversion is the most advantageous type of systems for generator integration’s.
The major sizing recommendation for generator to a three phase double-conversion UPS depends on the presence of a filter (a filter helps manage harmonic distortion). A model without a filter would have an assumed distortion of greater than 30%. Non filtered UPS systems should be sized with a large enough generator for start-up with a consideration of the UPS have no more than 35% of the generators rated capacity. UPS systems with a filter should have a distortion of no more than 10%. For these systems the generator can be sized to 80% of its rated capacity.
The general rules of sizing a generator apply to most applications and can be straight forward. The considerations for large UPS systems will be primarily focused on if the system is single or three phase and if the UPS has a filter to reduce harmonic distortion. If these are known, general sizing can be accomplished and an estimate can quickly be obtained.
It is essential to comply with all regulations established by the Occupational Safety & Health Administration (OSHA) and strict adherence to all local, state and national codes is mandatory. Before selecting a generator, check for municipal ordinances that may dictate requirements regarding placement of the unit (setback from building and/or lot line), electrical wiring, gas piping, fuel storage (for liquid propane or diesel tanks), sound and exhaust emissions.
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