Study Of Some Key Problems Of Matrix Converter

Matrix converter has attracted more and more attention recently due to many advantageous features such as bidirectional power flow, controllable input power factor, sinusoidal input and output currents, and compactness in structure;it is a potential power converter. Due to much effort of researchers, lots of achievements have been made up to now. However, there are still many hard problems which include the complexity of modulation strategy, the high cost, nonlinearity, reactive power and its applications involving matrix converter. In this paper, the issues on modulation strategy, nonlinearity compensation, stability, the modulation and control of four-leg matrix converter and the research for matrix converter used in wind energy conversation system (WECS) will be studied in detail.Based on the in-depth analysis of the basic principles of the double line-to-line syntheses and the space vector modulation (SVM), the consistency of duty ratio calculation of the two modulation strategies have been proved from the mathematical point of view. Then, the consistency is revealed from the issues of switching sequence, the concepts of the'origin switch', power factor and common mode voltage and the ability to suppress the effect to output voltage due to unbalanced grid.Inspired by fictitious rectifier and inverter process of matrix converter, a matrix converter modulation strategy based on mathematical construction is proposed. The core of this method consists in solving the modulation matrix converter. Based on the rectifier and inverter process, it is easy to get a transient modulation matrix, but the physical constraints could not meet here. Then the offsets are added into each row of modulation matrix to satisfy the physical requirements. The offset is a degree of freedom, different offsets lead to different modulations, so the proposed method is a general modulation.Aim to solve the nonlinearity of matrix converter, the nonlinearity sources and features of two-stage matrix converter (TSMC) output voltage are analyzed in details, it is found that the nonlinearity of TSMC has many special features such as narrow pulse, device voltage drops and ripples of filter capacitor which is concerned with modulation strategy, operating mechanism and topologies. To meet the end of reliable operation and compensate the negative affects of nonlinearity, revised modulation strategy and nonlinearity compensation algorithms are proposed. It is found that the classic modulation will lead to narrow pulse which may pose threaten to matrix converter, an improved modulation strategy is proposed by using one time varying distribution factor to overcome the constraints from real practice, and the minimum narrow pulse restriction could be met without special handling. As a result, the waveform quality of TSMC both in output and input is improved greatly. And a dead time compensation method which could keep the local symmetry of pulse is proposed. The experimental results verify the propose method.Stability is a basic requirement of matrix converter, Inspired by the impedance ratio criterion, a general constructive method is proposed, improving the stabilization regions of matrix converter. Based on analysis, it is found that, by using constructive method to modify the output reference voltage and rectifier modulation vector could change the input impedance (or admittance) of matrix converter on D-axis and Q-axis, respectively. As a result, the instability, caused by negative impedance, is eliminated. Plus, stabilization regions are enlarged. Furthermore, the underlying principles of constructive method are stated in detail and several instructive examples which comply with these principles are demonstrated. The essence of some existing methods which are presented to improve the stability of matrix converter is unveiled, and difference and similarity between existing methods and the proposed ones are discussed.In order to reject the zero sequence and negative sequence disturbance due to unbalanced load, a solution using two-stage four-leg matrix converter based on carrier modulation strategy was proposed. In order to improve the comprehensive performance of power converter system, a randomized carried modulation strategy based on Markov chain for four-leg matrix converter was proposed. This modulation is an optimal randomized carried modulation strategy taking the random transient matrix as variables, and taking the output voltage waveform quality and switching times as optimized objectives. And it could reduce the EMI problem of matrix converter.The unbalanced and distorted load currents pose more challenges to four-leg matrix converter's operation. From the perspective of control, this is a servo control problem with external disturbance. To solve the problem, the most commonly used method is to transform AC system in stationary frame coordinate into DC system in synchronous frame coordinate. Then, the classic proportional integral (PI) controller based on cascaded control will work well; however, this scheme is too complicated A new method using robust adaptive back-stepping control is presented for indirect four-leg matrix converter. It is robust against uncertain loads and control input disturbance due to the interaction between input filter and matrix converter.About the application of matrix converter in WECS (wind energy conversation system), two kinds of WECS based on supper sparse matrix converter (SSMC) and permanent magnetic synchronous generator (PMSG) are proposed. For the first scheme, the key feature is that the energy is transferred from rectifier to inverter of super sparse matrix converter. To design a global stable and robust controller, Feedback Linearization theory and H-infinity control are used. The other WECS is composed of PMSG and a very sparse matrix converter (VSMC). In this application, its distinct feature is that the energy generated by PMSG is transferred from inverter to rectifier of VSMC. The PMSG could be controlled directly, so it is easy to control. Because VSMC has only 12 active power switches, this WECS is low cost and reliable.