"Single-Conversion" refers to the single AC-to-AC conversion normally seen by the load. For example, input to the UPS passes only through an isolation transformer, regulating transformer, or filter. To bring the standby UPS components on-line, an abnormal condition must exist.

There is a variety of single-conversion topologies, including line-interactive and tri-port configurations. In a line-interactive system, the inverter interacts with the line to buck, boost, or replace incoming power on an as-needed basis. The inverter may either operate continuously or switch on by control logic.

A tri-port system is similar, but is always on line. The utility power normally passes through an isolation transformer and filter or a regulating (ferroresonant) transformer.

This type converts DC to AC by using power switching at a 20 kHz to 50 kHz rate. A linear feedback loop is part of the circuitry. The output is a pulse-width modulated positive and negative square wave. A simple output low-pass filter removes the high frequency carrier for a smoothed sine wave.

There are two harmonic distortions associated with UPS systems. One is the UPS' ability to minimize the amount of harmonic distortion caused by the UPS-fed loads. The other is the amount of harmonic distortion the UPS reflects back onto the distribution source that's providing its input. This second characteristic affects the power quality of other loads.

Input filters can mitigate reflected harmonics at the UPS input. Step-wave and PWM systems have greater sensitivity to load-generated harmonic distortion than other types of UPS systems. Sometimes, you must derate the system capacity to compensate for load profiles with a high crest factor (the ratio between the peak of the current waveform and its rms value).