A master and slave control strategy for parallel operation of three-phase UPS systems with different ratings

Parallel operation of UPS system has been used to increase power capacity of the system or to secure reliable supply of power to critical loads. During parallel operation, load sharing control to maintain the current balance is critical for reliable operation, since load sharing is very sensitive to differences in components of each module such as amplitude /phase difference, line impedance, and output LC filters. To solve these problems various control algorithms have been researched. However, these methods cannot be applied to UPS systems with different ratings. For this case, master and slave control algorithms for parallel operation is adequate. If the ratings of UPS systems are different, the value of passive LC filters will be different, and it will affect current sharing. This paper presents the general problems associated with parallel operation of UPS systems, and control strategy for parallel operation with different ratings. The validity of the proposed control strategy is investigated through simulation and experiment with two UPS systems.