Development of a direct matrix converter for micro-grid applications
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Matrix converter is a direct ac-ac converter topology which does not contain a dc-link passive component, unlike conventional ac-ac frequency converters. Electrolytic capacitors which are used as the energy storage component have a limited lifetime. They are also bulky and unreliable at very high temperatures. Matrix converter is able to generate controllable sinusoidal ac outputs regarding magnitude and frequency directly from an ac power supply. The other significant advantages offered by matrix converters are the capability of regeneration and adjustable input power factor. The main objective of this thesis is design, implementation and the stability analysis of the matrix converter for power flow control applications in the context of the microgrids. In this regard, different applications with bidirectional or unidirectional power flow capabilities are considered as the case study. These include using the matrix converter as an interface link between a microgrid and the utility grid, between a variable frequency source such as wind turbine and the microgrid ac bus, and between a variable frequency load such as induction machine and microgrid ac bus. As bidirectional power flow control between the utility grid and the microgrid is significantly affected by the stability issue, the stability analysis has become an essential part of this research. Details of the input filter design are presented due to the considerable effect of the filter components on the system stability. The effects of the system parameters on the matrix converter stability are investigated using the small-signal model of the converter. Two methods of stability improvement using the damping resistor and the digital filter are studied in detail. In order to increase the efficiency of the converter, an optimum solution based on the combination of the damping resistor and the digital filter is suggested, and the performance of the proposed method is analyzed. The operation of the matrix converter as an interface between the utility and a microgrid for bidirectional active and reactive power flow control is studied in detail. To control the active and reactive bidirectional power flows, a vector-oriented control method is used. Two main modulation strategies, the Venturini and space vector modulation, are analyzed and the simulation and experimental results are compared. Due to the better performance of the space vector modulation, this technique is selected for modulation of the designed matrix converter. Different current commutation methods are studied in detail and the simulation and experimental results of four-step semisoft current commutation are presented as the selected commutation method in this work. Furthermore, a comprehensive simulation study is carried out to investigate the operation of the proposed strategies for modulation, protection, stabilisation and bidirectional power flow control of the matrix converter. To validate the proposed stability analysis and numerical simulations, a prototype direct matrix converter has been developed. The simulation results related to each of the research sections are confirmed through the experimental tests.
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