Input capacitors used for filtering purpose cause pure apparent power to flow between the supply and the UPS. Such power is referred to as leading or capacitive power. Gensets can manage limited amount of pure leading power, as it interferes with the excitation circuit in the AC generator, leading to voltage oscillations and genset’s shutdown. Safe operating area charts show the allowed gensets’ load power factor range for stable operation and pure leading power limit is typically 20% of gensets’ nominal rating.

Diesel generator’s safe operating area. Normal operation is shown in green. In the yellow area operation is still possible, on temporary basis. Operation in the red area is not allowed. During UPS start-up, generators must supply pure leading power and the working point is right on the left horizontal axis. Maximum pure leading power limit is typically 0.2 of genset rated power.

A UPS system restarting after a mains failure exhibits pure input capacitive power. Although it’s small contribute when compared to the nominal UPS rating, at this time the input filter is not balanced by any real power. It is necessary to wait until the rectifier’s start-up to be completed. This issue is magnified when strings of paralleled UPS are to be supplied by strings of paralleled gensets. Due to the inherent sequential start-up of the generators, the very first one kicking in feeds the entire capacitive power of the UPS string. This might be exceeding the genset’s capability, thus precluding a successful changeover.

Several techniques can be put in place to mitigate the issue, such as installing resistors’ banks to compensate, among others. State-of-the-art UPS have decreased their input capacitive power amount, by implementing rectifiers’ electronic filtering technologies. They can also offer adaptive input capacitive power management, to help designing more resilient electrical infrastructures.

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.