UPS and generators should be best friends, smoothly cooperating to improve site’s resiliency. Often this might not be the case, due to mutual misunderstandings and lack of information. This article is a comprehensive guide and essential knowledge base about UPS input performance and its impact on generators’ operation.

The UPS point of view：

In real operation, whenever a mains failure occurs, the supply is switched between incoming mains and a diesel generator set, or genset. The changeover is break-before-make, and it is usually accomplished within a few seconds. The downstream UPS system switches to battery power during the whole exchange, while its input converter, or rectifier, is off due to lack of input power. When the supply is established by the diesel generator, the rectifier performs a start-up, and the UPS operation is restored to normal double conversion. During this crucial phase it is of utmost importance that UPS and genset run like clockwork, to prevent the changeover from being jeopardized by poor interaction.

From the genset’s point of view：

A UPS system is a load, drawing both active and reactive power, depending on the UPS mode of operation. Up to date uninterruptible power supply running in VFI double conversion draw 99% “clean” real power, that is, a modern UPS is pretty much seen as a linear resistive load by the upstream supply. This simplifies a lot the design and performance of the electrical system.

Exceptions to the above are very light load operation and bypass operation.

• In bypass operation the load is fed through the UPS bypass line by the upstream supply, which cannot thus benefit from the UPS filtering action. In this condition a genset would directly interface with the load power factor, harmonic distortion and load steps.

• At light load, typically below 10% of the rated nominal power, or at no load, a UPS exhibits a predominantly capacitive input power factor and a slightly higher harmonic distortion rejection in percentage of the total absorbed power. Furthermore, the rectifier start-up transient is a load step for the upstream supply.

Although the above conditions do not impact at all the “infinite” mains power, as far as a generator set is concerned, such cases must be carefully evaluated.

If you have any requirements or any kind of query regarding the UPS solutions for your applications, feel free to communicate with our dedicated team at any time at marketing@everexceed.com.