A high-performance DSP-controller for parallel operation of online UPS systems

In this paper, a novel controller for parallel-connected online uninterruptible power supplies (UPS) without control interconnections based on the droop method is presented. The control approach consists in drop the frequency of every module when its output power increases, resulting in an unavoidable nominal frequency deviation. Consequently, this method in its original form is only applicable to off-line or line-interactive UPS systems. As opposed to the conventional droop method, the proposed control scheme endows proper transient response, strictly frequency and phase synchronization with the AC mains, and excellent power sharing even for nonlinear loads. Hence, this controller is suitable for paralleled online UPS systems. Experimental results are obtained from two parallel-connected 1-kVA online UPS by using TMS320LF2407A DSP.