Using UPS systems with generators

Mains power demand for sensitive electronic equipment is growing while confidence in the availability and quality of utility mains is diminishing. Organisations are therefore using generators for a reliable source of backup power; however connecting a generator directly to a sensitive load can cause problems. Inserting a UPS system can mitigate these, but further problems often arise from the interaction between the UPS and generator. These can be solved by sourcing suitable UPS equipment. The UPS battery is essential for protection from short term power outages. However the battery typically has a rectifier charger control circuit which imposes notches on the power feed, interfering severely with some types of generator control. Additionally, some charger circuits draw non sinusoidal input current, creating harmonics, measured as total harmonic distortion (THDi).

These can cause excessive heating in the generators, especially as some UPSs generate up to 30% THDi. Generator output voltage and frequency stability problems can also occur if the full UPS load is instantly applied to the generator output.

Generators have inherent frequency control limitations while UPS inverters have limited ability to synchronise with a fluctuating frequency source. This causes a ‘cannot synchronise to bypass’ UPS alarm. If the UPS must synchronise to bypass for transferring the critical load to the generator, this places even tighter frequency and voltage stability demands on the generator.

KUP’s transformerless technology features solutions to these problems. A THDi of below 7% at full load, a separate battery charger circuit, together with a Generator ‘On’ signal between UPS and generator reduces the UPS battery charging current, in turn reducing load, notching and heating effects on the generator. Step loading on the generator is controlled firstly by soft start of the rectifier current, eliminating high current peaks during mains recovery. KUP. uniquely also limits step loading during mains recovery by sequentially switching on the rectifiers within a parallel configuration. Additionally, KUP’s systems are designed to eliminate excessive excitation and generator voltage rise.

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