Investigation of bidirectional switch based three phase AC voltage controller topologies

Abstract

A family of three phase switched mode AC voltage controller topologies, namely- Buck, Boost, Buck-Boost, ĈUK, and SEPIC, has been proposed in this research. The topologies are functionally equivalent to switched mode DC-DC converters. Two three phase AC bidirectional switches have been used to get the SPDT switching configuration in three phase AC-AC circuits similar to those of DC chopper circuits. The three phase true AC bidirectional switch consists of a unidirectional controlled switch, such as, an IGBT, enclosed by two three phase diode-bridges on either side. This is equivalent to three single phase AC bidirectional switches and it provides separate three phase input/output terminals for the switch such that three phase currents can be carried by the same switching device in both directions. The new bidirectional switch gives design advantages like reduction of number of controlled switching devices and associated snubbers, isolated gate driver supplies and control circuits for PWM signals. It also provides the benefits of having simple control circuit, less switching loss, reliable operation and reduction in cost and complexity. Performance of the new AC voltage controllers have been investigated by varying the duty cycle of the gate pulses of high frequency and adjusting the circuit parameters. Simulation results show the ability of the proposed converters to do step-up and/or step-down AC-AC conversion with low input current THD and high input power factor at enhanced efficiency. A study on the closed loop feedback control system on the proposed converter topologies by simulation shows the ability of simple PI-controller to regulate the output voltage of the converters with sufficient accuracy. In response to sudden disturbance of load or input voltage the controller adjusts the duty cycle of the gate pulses to recover the desired output voltage quickly. With the feedback control the performance of the converters has been found satisfactory at desired operating conditions. To prove the workability of the proposed converter concept, the three phase Boost AC voltage controller has been implemented practically in a low voltage, low power prototype setup. Experiments have been conducted with resistive and inductive loads in open and closed loop controls. Some adaptations have been made to the three phase AC-AC voltage converters to further reduce the semiconductor devices and to include possible neutral connection with source and/or with load. The modified converter circuits have been made by two modified three phase bidirectional switches with neutral. The research outcome is expected to result in a family of simple, low cost, high performance three phase AC-AC voltage controller topologies suitable for ACAC applications.