A UPS (uninterruptible power supply) and a standby generator can work together to provide continuous, reliable power during an electrical outage. However, the systems must be correctly sized, configured and tested to ensure the generator can support the UPS and its connected equipment.
There are several important factors to consider when designing a UPS and generator combination.
Correctly Sizing The Generator
The generator must have sufficient capacity to supply the UPS, power the connected critical equipment, and recharge the batteries following an outage. Allowance also needs to be made for UPS conversion losses and other site loads, for example, air conditioning, emergency lighting, alarms, and communications systems.
The generator must be able to accept changes in electrical load without excessive voltage or frequency fluctuations. The actual generator capacity will depend on the characteristics of the UPS, the connected load, and the entire installation.
Voltage And Frequency Compatibility
A UPS and generator must be compatible in terms of voltage, frequency, and their permitted operating tolerances. Generator voltage and frequency can fluctuate when loads change, particularly during start-up or when larger loads are connected.
If these fluctuations fall outside the UPS's acceptable input range, the UPS may remain on battery power rather than accepting the generator supply. This can reduce available battery autonomy and prevent the batteries from recharging.
Load Acceptance And Power Quality
The generator must be able to respond to sudden changes in demand, including the UPS's battery-charging requirements. UPS input characteristics, including harmonic distortion and power factor, can also influence generator selection and performance.
The generator's ability to handle step loads depends on its design, rating and operating conditions, so it is important to assess the actual installation rather than rely on a general sizing rule.
System Design, Commissioning And Testing
Involving the UPS and generator suppliers early in the design process can identify compatibility issues before installation. It also ensures control systems, automatic transfer arrangements and operating sequences work together correctly.
The completed installation should be commissioned and tested under actual operating conditions, including any changes in load or transition from mains supply to the generator.