Thermal impact analysis of circulating current in high power modular online uninterruptible power supplies application

In a modular uninterruptible power supplies (UPS), each DC/AC module was designed to work in a circular way considering the reliability and power stress issues of the whole system. Thus unsynchronized PWMs will occur if any of the DC/AC modules is plugged in or out of the system at any time. On the other hand, modular difference is another issue that will bring such kind of circulating current. In order to reduce system size and cost, each module shares the same DC and AC bus without any isolation or passive elements in the system. Consequently, potential zero-sequence current is possible to occur and should be paid specific attention. In this paper, a four-module online UPS system is designed. And thermal and loss distribution condition are investigated under different circulating current condition with conventional three phase H-bridge topology.