분산형 발전을 위한 Quasi-Resonant Step-Down Converter와 병렬 Inverter의 Modified Droop 제어에 대한 연구

Abstract

This thesis proposes a quasi-resonant passive snubber for improving power conversion efficiency of a dc-dc step-down converter and a modified droop-control method to reduce circulating current between single-phase voltage-source inverters operating in parallel via AC bus with resistive line impedance. The proposed dc-dc converter circuit topology and dc-ac inverter control method are described. A quasi-resonant passive snubber for the conventional dc-dc step-down converter is proposed. The snubber uses six passive components to achieve a zero-current turn-on and zero-voltage turn-off of the switch, and to suppress the reverse recovery current of diode. At input voltage of 200 V, output voltage of 100 V, output power of 300 W, and switching frequency of 190 kHz, the snubber increased the power conversion efficiency ηe by 2.8% and stabilized the temperature of MOSFET switch at ~68 °C. The snubber worked well for both MOSFET and insulated gate bipolar transistor (IGBT) switches without increasing the voltage stress. These experimental results show that the proposed snubber is very helpful for improving ηe of a dc-dc step-down converter that operates at a high frequency. The conventional droop-control method, which assumes that the AC bus is purely inductive, is modified to reduce the circulating current between single-phase voltage-source inverters that operate in parallel via an AC bus in a low-voltage microgrid. By analyzing the output power of the inverter and modifying the line impedance compensation method for the AC bus, the modified algorithm is derived for the proposed droop control by considering both the inductive and resistive components of the line impedance of the AC bus. The proposed droop-control method suppresses the circulating current and improves the power-sharing accuracy of the parallel inverters connected by the resistive AC bus even if the resistive component of the line impedance is large. The proposed control was tested on two single-phase voltage-source inverters operating in parallel. The experimental results show that the output currents of the two inverters were almost the same under various test conditions of installation interval, output power and cross-sectional areas of cables. These results mean that the proposed droop-control method is suitable for reducing circulating current between single-phase inverters operating in parallel via an AC bus that has resistive line impedance.