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6. Parallel Operation
6.1.
Theory
When two or more UPS units are deployed in parallel, they are connected according to the
decentralized Static Switch method (see below). Special technical solutions and control algorithms
are implemented to synchronize all units in the system to prevent circulation of energy between
them and to isolate a faulty unit from the common power bus. In addition, an active current
sharing circuit regulates the output voltage of each UPS to provide optimal load sharing between
the units.
One of the parallel UPS systems assumes the role of "Master". This unit is synchronized to the
mains power. The other units operate as "Slaves" and are synchronized to the Master.
The function of Master is assumed by the first system to be switched on during
the parallel start-up process. This is transparent to the system operator.
Decentralized static switch method
6.1.1.
Every UPS is equipped with a static switch to ensure that the UPS will continue to supply power to
the load (from the mains or from a secondary ac input source) in the event of a problem with the
inverter or an overload of the UPS. Thus, the static switch performs a very important function.
UPS for 208 Vac Grid, Installation Guide, doc. rel. 3.0
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