Editing Uninterruptible power supply (section)

== Power factor ==
{{see also|Power factor}}

A problem in the combination of a double-conversion UPS and a generator is the voltage distortion created by the UPS. The input of a double-conversion UPS is essentially a big rectifier. The current drawn by the UPS is non-sinusoidal. This can cause the voltage from the AC mains or a generator to also become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to that power source, including the UPS itself. It will also cause more power to be lost in the wiring supplying power to the UPS due to the spikes in current flow. This level of "noise" is measured as a percentage of "[[Total harmonic distortion|total harmonic distortion of the current]]" (THD<sub>''I''</sub>). Classic UPS rectifiers have a THD<sub>''I''</sub> level of around 25%–30%. To reduce voltage distortion, this requires heavier mains wiring or generators more than twice as large as the UPS.

There are several solutions to reduce the THD<sub>''I''</sub> in a double-conversion UPS:

Classic solutions such as passive filters reduce THD<sub>''I''</sub> to 5%–10% at full load. They are reliable, but big and only work at full load, and present their own problems when used in tandem with generators.

An alternative solution is an active filter. Through the use of such a device, THD<sub>''I''</sub> can drop to 5% over the full power range. The newest technology in double-conversion UPS units is a rectifier that does not use classic rectifier components (thyristors and diodes) but uses high-frequency components instead. A double-conversion UPS with an [[insulated-gate bipolar transistor]] rectifier and inductor can have a THD<sub>''I''</sub> as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.