A Sinuoidal Current Limiting Strategy for PR Controller of Single-Phase UPS Inverter

Abstract

Single-phase inverters are categorized into grid-connected inverters and stand-alone inverters. The stand-alone inverter is used in an uninterruptible power supply (UPS) system to provide emergency power to AC loads. When a single-phase induction motor (IM) is used as a load in a UPS inverter, the direct on-line (DOL) starting method, which injects the rated voltage directly, is widely used. This method has the advantages of simplicity and low cost, but it has the problem that the starting current can be as high as 6 to 7 times the rated current of IM, which can damage the semiconductor devices of the inverter and IM. Also, this can cause a reduction in the efficiency of IM. In this paper, sinusoidal current limiting strategy of output current is proposed when a proportional-resonant (PR) controller is used for voltage control of a UPS inverter. The proposed current limiting method calculates a sinusoidal limiting signal from the voltage resonant controller and compares it with the current command, which is the output of the voltage PR controller. In addition, compensation of phase difference between the limiting signal and the current command is included to improve the current limiting performance. The comparison of the proposed method with conventional method for clamping the current is verified through Matlab/Simulink simulations and experiments.