Critical Power Supplies has pleasure in discussing the difficulties faced with generator use and UPS Systems

Frequency variations

The voltage and frequency of the power produced by a generator depends on the engine speed. The speed is controlled by a system called a governor. Some governors are mechanical, and some are electronic. The job of the governor is to keep the voltage and frequency constant, while the load on the generator changes. This may pose a problem where, for example, the startup surge of an elevator can cause short “blips” in the frequency of the generator or the output voltage, thus affecting all other devices powered by the generator. Many radio transmission sites will have backup diesel generators – in the case of amplitude modulation (AM) radio transmitters, the load presented by the transmitters changes in line with the signal level. This leads to the scenario where the generator is constantly trying to correct the output voltage and frequency as the load changes.

It is possible for a UPS unit to be incompatible with a generator or a poor mains supply; in the event that its designers had written the microprocessor code to require exactly a 50.0 Hz (or 60.0 Hz) supply frequency in order to operate; with this condition not met the UPS could remain on battery power, being unable to reconnect the unsuitable supply voltage.

This problem of input frequency requirements should not be an issue through the use of a Double Conversion / online UPS. A UPS of this topology should be able to adapt to any input frequency, using its own internal clock source to generate the required 50 or 60 Hz supply frequency.

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 causes the voltage from the generator also to become non-sinusoidal. The voltage distortion then can cause problems in all electrical equipment connected to the generator, including the UPS itself. This level of “noise” is measured as a percentage of “Total Harmonic Distortion of the current” (THD). Classic UPS rectifiers have a THD level of around 25–30%. To prevent voltage distortion, this requires generators more than twice as big as the UPS subject to the design of the equipment being used.

There are several solutions to reduce the THD in a double conversion UPS :

Passive power factor correction: (Passive PFC) Classic solutions such as passive filters reduce THD 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.

Active power factor correction

An alternative solution is an active filter. Through the use of such a device, THD can drop to 5% over the full power range. The newest technology in UPS units is a rectifier that doesn’t use classic rectifier components (Thyristors and Diodes) but high frequency components (IGBTs). A UPS with an IGBT rectifier can have a THD as small as 2%. This completely eliminates the need to oversize the generator (and transformers), without additional filters, investment cost, losses, or space.