Digital control of single-phase UPS inverters with modified PWM technique

The purpose of a voltage controller for UPS inverters is to produce stable output voltage with low distortion under all loading conditions, especially under nonlinear loads and dynamic loads. This paper presents the design of a novel digital robust feedback controller for single-phase UPS inverters based on pole-placement method. With the polynomial design method adopted, all closed-loop poles of the system are placed at origin with feedback of output voltage for achieving fast dynamic response. Furthermore, the feedback of load current is designed so as to reduce the voltage distortion introduced by load current to a small amount so that it can be ignored. To eliminate the effect of the time delay caused by sampling and calculation, a modified PWM technique is introduced. The proposed control strategy is capable of guaranteeing high quality steady-state and dynamic responses. The design of the proposed digital controller is very simple and requires only a reasonably accurate knowledge of the output L-C filter parameters. Also, only the output voltage and load current are sensed. Simulation and experimental verification have been carried out on a 1-kVA PWM inverter. Results of simulations and experiments show that the proposed controller can achieve very low total harmonic distortion (THD) and fast dynamic response under various loads with low switching frequency.