Dynamic digital control schemes for three-phase UPS inverters
The purpose of a voltage controller is to produce stable three-phase output voltages with low distortion under all loading conditions. This paper presents the design of such a three-phase, closed-loop voltage controller, implementing a deadbeat controller on the output voltage and the inductor current. A digital signal processor is used to implement the digital closed-loop control algorithms. This, together with delays introduced by sampling the analog quantities, introduces a delay in the control law that causes system instability. The system model is augmented to compensate for these dynamics, hereby including a third deadbeat controlled variable. System stability is then ensured. Load current feed-forward is used to compensate for changes in the load current before the output voltages get distorted, in order to improve the output voltage waveform quality. Practical results show that the designed voltage controller can produce output voltage waveforms that can meet the specifications for balanced and unbalanced loads. This is verified by using a three-phase, four-wire, 250 kW inverter, which shows that the constructed pulse-width modulated control scheme can achieve fast dynamic response as well as low total harmonic distortion. Non-linear loads, however, produce undesirable harmonics that cause large distortion in output voltage.