PWM Switching Technique for Three-Phase Bidirectional Grid-Tie DC-AC-AC Converter With High-Frequency Isolation

Galvanic isolation based high-frequency transformers have become very attractive in bidirectional dc/ac converters. This paper presents a new pulse width modulation (PWM) switching technique for controlling a bidirectional isolated dc-acac converter employing a high-frequency link transformer. The proposed PWM technique has the ability to control the input dc current and to inject a sinusoidal three-phase current to the grid at unity power factor. The primary-side is an H-bridge converter used to convert the dc voltage to a high-frequency square-wave singlephase voltage. The secondary side is a matrix converter used to convert the grid three-phase voltage to a high-frequency singlephase waveform. The dc-ac-ac converter utilizes a high-frequency link transformer for the galvanic isolation between the H-bridge and matrix converters as alternative for the bulky line-frequency transformers (50/60 Hz). The bidirectional power flow is controlled by the phase shift angle between the primary and secondary voltages of the high-frequency transformer. The mathematical model and the circuit operational modes are presented along with the voltage controllable limit. The feasibility of the proposed PWM switching technique and the accuracy of the mathematical model are demonstrated experimentally by using a 200 V/1 kW laboratory prototype system.