| Matrix Converter (MC), as a direct ac-ac converter, has a number of advantages over conventional converter-inverter systems, such as bi-directional power flow, sinusoidal input current and output voltage, adjustable power factor and without buckly storage components. MC has attracted much attention for its desirable features and been a focus of research in recent years. The modulation strategy is the key of the control for MC and also is the primary facet researched by scholars. So, the modulation strategies for 3×3MC with three-phase balance load and 3×4MC with three-phase unbalanced load are researched in this paper.The application of MC is restricted by its complicated control strategy. For 3×3MC, a novel indirect modulation strategy based on analysis and comparison of existing modulation strategy is proposed. The modulation strategy is based on the equivalent 3×3MC AC-DC-AC topology. For the convenience of inverter modulation, the rectifier-stage modulation output is a constant average DC voltage with the maximum amplitude, while the input current is sinusoidal and power factor is 1, to meet the requirements of non-polluting in power grid. The equations that make duty cycle as variable are written by the above objectives, and the duty cycle expressions are deduced. The inverter-stage is modulated by traditional voltage space vector method, and then the modulation of rectifier-stage and inverter-stage are integrated. The computational complexity of new modulation strategy is lower than space vector modulation strategy's, which has important practical significance.The abnormal voltage of practical power grid should be taken into account before MC is applied into industry. When input voltage is asymmetric, the effect on output voltage and input current is analyzed as modulation strategy under symmetric voltage. The modulation strategy is improved by adding an appropriate quantity of negative sequence voltage component to duty cycle of rectifier-stage, and then rectifier-stage output DC average voltage remains a constant value, while the desired output reference voltage could be realized and the modulation of inverter-stage has no change. The improved duty cycle may be negative, and the realization method of negative duty cycle is given. The improved modulation strategy can eliminate the harmonic components simultaneously in input current, but still contain negative sequence components.The power converter should have ability to supply unbalanced load because the actual three-phase power load may be unbalanced. This requires 3×4MC. In order to apply research results of 3×3MC to 3×4MC, a 3×3MC to 3×4MC modulation strategy based on "supplement" thinking is presented. The 3×3MC modulation strategy in chapter 2 is applied to the 3×3 part of 3×4MC, and N-phase leg is controlled alone. According to analysis of the inverter-stage output voltage, the modulation method of N-phase leg is determined to control the load neutral point potential, and can also realize three-phase symmetrical load voltage in the case of unbalanced load. The method has the advantages that N-phase leg can be masked under balanced load, so 3×4MC can work at 3×3MC state, which can reduce the switching losses and expand the adaptability of 3×4MC. Spectrum of input current is analysed under unbalanced load current, analysis indicates that input current contains low order harmonic currents which frequency are relative to input and output frequency. byConsidering the high power quality requirements of unbalanced loads, an output filter is needed for 3×4MC. A mathematic model of inverter-stage of 3×4MC with output filter is founded. Analysis shows that: the output reference voltages of three-phase leg to N-phase leg are three separate components under unbalanced load. The former two-dimensional voltage space vector modulation of inverter-stage can not meet the requirements, and then a three-dimensional space vector modulation strategy is presented. Principle of three-dimensional space vector modulation is analysed detailedly. Then three-dimensional voltage space vector modulation is applied to inverter-stage and three phase symmetric voltage is obtained under unbalanced load. Analysis for input current also indicates that input current contains low order harmonic currents which frequency are relative to input and output frequency.The experimental prototype of 3×3MC is discussed in detail. The design and realization of main circuit and digital controller is mostly concerned. Finally, the results obtained from experimental system prove the validity of theoretic analysis and feasibility of proposed methods, which gives important theoretical and practical foundation for further research.
|
PDF Full Text Request