Parallel control of the UPS inverters

This paper deals with the design of good quality output voltage waveform and excellent power sharing of uninterruptible power system (UPS) inverters with parallel-connection capability. In this sense, the controller is designed by using two nested loops: 1) the inner loop is performed by using the output voltage and inductor current double loop controller, providing a good static and dynamic response of a single UPS. 2) the outer loop calculates the output active and reactive powers and adjusts the output voltage amplitude and phase during the load transients. Integrated with the power control and the output filter current control, the proposed load-sharing control can improve the current-sharing quality and stability of the paralleled UPS system. In order to eliminate the high frequency circulation current around carrier wave frequency, the triangle carrier wave synchronization is used. Experimental studies using two parallel 30 KVA UPS´s are presented to show the effectiveness of the proposed control.