Abstract
This thesis presents design and implementation of a three-phase three-wire bi-directional inverter with two operation modes, grid-connection mode and rectification mode. In grid-connection mode, the power is transformed into AC and injected into the utility grid. In rectification mode, power from the utility grid is transformed into DC to supply DC loads. The circuit is divided into two parts, including a power stage with the configuration of a three-phase three-wire full-bridge inverter and a control stage with a single-chip micro-controller Renesas RX-62T implementing the control law of Division-Summation (D-Σ) Digital Control. With the proposed control method, the limitations of abc to d-q frame transformation can be overcome and the core size can be reduced significantly, since inductance variation is taken into account. Additionally, the proposed control method can accommodate the effect of three-phase voltage distortion and imbalance. First, plant derivation based on D-Σ digital control is discussed. Next, the controller is derived with the aim of canceling the variation effects of dc voltage, switching period and inductances. Then, the derivation of duty ratios based on Two-Phase Modulation with the reallocation of T0 and T7 is explained in detail. In addition, the mechanism of DC-bus voltage regulation and selection of the inductor core are also presented. Finally, the inverter has been implemented and tested. Simulated and experimental results have verified the feasibility of the proposed control scheme. There are two major contributions in this research. First, the derivation of duty-ratios is different from that in the literature related to D-Σ digital control. In this research, the derivation follows the procedure of the conventional control system theory in which the plant is derived first, and then the controller is designed. Finally, a system control block diagram is obtained and stability is analyzed. Secondly, the D-Σ digital control is applied to a 60 kW power converter system and the effects caused by nonideal characteristics of the components are considered and improved.