go to the UPS product page Daniela Zahnd asked on Dec 5th, 2014 in UPS

Can two UPS systems in an (A) and (B) feed synchronized system have different power ratings (different kVA)?

Carlo Kufahl , Product marketing

ABB's answer

Even if very untypical, in case of synchronized UPS (see description below) it is possible to have different power ratings between UPS (A) and UPS (B), as long as the smaller UPS rating is still right sized for the load.

Synchronized UPS

Synchronized UPS are usually used when a plant (load) is supplied by two power supply sources, one is the main and the other is the emergency one (redundant sources). Only one source  is supplying the load; in normal conditions the main source is supplying the load; in case of a power outage on the main source the load is transferred to the emergency source. Since the two sources, which are typically called (A) feed and (B) feed, can have a possible phase shift among them - (A) and (B) are in this case unsynchronized - there is the need to synchronize them upstream the load.

There are basically two ways to do that:

  1. Place a static transfer switch (STS) which when in is able to transfer the load from the main source to the emergency source even if the two sources are unsynchronized. Important note: in asynchronous conditions the STS introduces an interruption which in the worst case can last 20 ms. Such interruptions are not accepted when the load is critical.
  2. Place two synchronized double-conversion UPS systems, one on the main source and the other on the emergency source. Double-conversion UPS systems don’t necessarily maintain the phase shift from input (bypass input to be exact) to output, but have the capability of synchronizing the output either to the output of another UPS (in this case the two UPSs have to be from the same manufacturer) or to any foreign source. In our case the UPS (A) - on the main source - is synchronizing its output to its bypass input (as typical) and the UPS (B) - on the emergency source - is synchronizing its output to the output of the (A) UPS. As a result the two UPS are giving a synchronized outputs. Synchronized outputs (A) and (B) can be transferred trough an STS and the interruption is max. 2-5 ms, which is usually acceptable by the majority of the critical loads, including server loads.

The second solution is by far the best solution because there is no interruption during the transfer. Moreover, applications which require a redundant source typically require also UPS systems plus batteries (sometimes also diesel generators) which guarantee electrical power to the load also in case of longer power outage of both sources.

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  • Carlo Kufahl

    Carlo Kufahl

    Product marketing

    Carlo Kufahl is the Product Manager for 3-phase modular UPS at the center of excellence for UPS, Power Protection Product Group of ABB, Quartino, Switzerland. He joined in 2010 and as a Sales Engineer for two years and then he took over as Product Manager. Today he coordinates and manages the complete life cycle of the 3-phase modular UPS which includes pre-development, development, launch, sales and phase out of products. Carlo holds a Bachelor’s degree in Electrical Engineering from the University of Applied Sciences and Arts of Southern Switzerland. Carlo also holds a Master in Business Administration obtained at Zürich University of Applied Sciences.

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