A Study on Highly Efficient Single Power-conversion Bidirectional Inverters for Low Voltage Battery System

Abstract

This dissertation proposes highly efficient single power-conversion bidirectional inverters and their respective control algorithms. Since the bidirectional inverters directly convert AC grid voltage to DC low battery voltage using only one power conversion stage, these inverters have simple structure, high power density, and low manufacturing cost. Firstly, a highly efficient grid-connected single power-conversion bidirectional inverter with low input battery voltage and a control method for it are introduced. The proposed bidirectional inverter consists of a bidirectional DC-DC converter and an unfolding bridge, and the power conversion stage only corresponds to a bidirectional DC-DC converter. The bidirectional DC-DC converter can perform bidirectional power conversion between the low input battery voltage and a rectified sine wave due to its step-up/down voltage regulation functions. The unfolding bridge unfolds the rectified sine wave into the grid voltage and provides a current path to the grid. The study also proposes a control algorithm to regulate the grid current through a single power-processing stage. The control algorithm is comprised of a feed-forward nominal voltage compensator and a repetitive control scheme. The feed-forward nominal voltage compensator presets the operating point to lighten the burden of the grid current control, and the repetitive controller provides precise control of the grid current. Secondly, a highly efficient direct single power-conversion bidirectional inverter without commutation problem for an off-line uninterruptible power supply (UPS) system and a control method for it are introduced. The proposed bidirectional inverter provides direct single power-conversion between a low voltage battery and a grid. For grid-connected inverters using bidirectional switches, the commutation problem is a crucial issue that must be solved to achieve higher power capability. Because of the novel circuit structure, the proposed inverter can provide a commutation path in any switching case and has high reliability. In the proposed modulation, which combines frequency variable modulation with phase-shift modulation, the power transfer function for the phase-shift angle is linearized and the closed-loop controller design is simplified. In addition, the proposed modulation strategy minimizes the reactive power flow during the switching period, while guaranteeing the zero-voltage-switching (ZVS) turn-on of all switches. With minimized reactive power, current stress and power losses are significantly reduced. Proposed phase-shift angles and switching frequency are obtained in analytic form, thereby that they are computationally simple and easy to implement. All proposed inverters are analyzed theoretically and implemented practically to evaluate their performance. Finally, the experimental results show that the proposed converters improve overall performance such as high power conversion efficiency, power density, power quality, and manufacturing cost.