A high performance controller for parallel operation of three-phase UPSs powering unbalanced and nonlinear loads

In this paper, a high performance controller for parallel operation of uninterruptible power supplies (UPS) is proposed. The proposed controller is based on synchronous reference frame that is developed for parallel UPSs. Applying master-slave algorithm in this controller, power sharing and voltage regulation are controlled centrally by master UPS and commands are send through CAN bus to slave UPS. But each UPS performs voltage harmonic compensation locally and independently. Virtual impedance technique is used to share harmonic currents perfectly. Controller also is modified for unbalanced nonlinear loads. The performance of the proposed controller has been verified with simulation results for 20 KVA, 380 V three-phase parallel UPS.