Digital control scheme of UPS inverter to improve the dynamic response

This paper presents a digital control scheme for a UPS inverter system. The inverter provides a near sinusoidal output voltage waveform with low harmonic distortion factor and must be controlled with very fast dynamic response and robustness to load changes or parameter variations. The proposed scheme is based on the current regulated voltage controller. The filter inductor current is forced to follow the current command calculated from the desired capacitor voltage and the sensed output load current. Clearly, the sensed output current signal acts as a disturbance feedforward signal which leads to a fast dynamic response of the controller to sudden changes of output load current. In order to guarantee the robustness of the system, a PI capacitor voltage controller is added as in the conventional double feedback loop controller. The natural problems of this controller, i.e. the phase difference between the output voltage and its reference caused by the AC quantity of the feedback signal, are compensated by using a synchronization controller for the output voltage phase and frequency. The algorithm of the proposed scheme is described and digital simulation and experimental results from a 5 kVA prototype are provided for verification.