Protecting Critical Loads with Commodity UPSs

SIDEBAR No. 1: Configuration Categories

UPS systems are devices with batteries that provide backup/ride-through power when the utility supply is interrupted. These systems use rectifiers to charge the batteries from the utility supply and inverters to convert the batteries' DC outputs to AC. They also may have constant-voltage or tap-switching transformers, which can provide voltage regulation. In addition, they typically include surge suppressors to protect loads from transient overvoltages.

In general, UPS systems fall into one of three configuration categories:

• Standby (offline) units basically switch from the utility supply to batteries when they sense voltage changes and typically do not offer steady-state voltage control;

• Line-interactive units also switch from the utility supply to batteries when input voltages change, but typically use ferroresonant transformers to provide voltage control; and

• Online, where power is constantly supplied by the battery through an inverter, with the battery continuously recharged through a rectifier and a rectifier/inverter combination providing isolation between the source and load. The inverter can directly handle voltage control, as can a ferroresonant transformer (if included). The online design is typical of larger UPS systems but less common in the commodity UPS market (e.g., less than 1kVA).