Analysis and comparison of two control strategies for three-phase series-parallel UPS

In this paper, two control strategies for three-phase series-parallel uninterruptible power supplies are compared. A series-parallel UPS consists of a parallel and a series converter. In this topology, most power required by load is supplied directly from AC line during standby mode. Therefore the overall efficiency is higher than that of a conventional UPS. Traditionally, the control strategy of these structures was based on voltage control of series converter and current control of parallel converter. However, recently an opposite strategy has been proposed, i.e., voltage control of parallel converter and current control of series converter. This paper compares these two control strategies. The comparison is made based on different performance criteria like seamless transition between backup and standby modes, dynamic performance, complexity of reference generation and size of filters in two strategies.