Research On DC-capacitor Voltage Equalization Control Method For Modular Inverter

With the rapid improvement of China's economic strength,the construction of high-voltage and large-capacity power network systems,the development of large-scale,long-distance transmission,increase power transmission capacity,enhance power supply reliability,improve power quality,it is necessary to improve the voltage level of power electronic devices.Modular inverters are modular,redundant,and have excellent output waveform quality,making them ideal for high-voltage,high-capacity applications.As the voltage level continues to increase,the number of inverter sub-modules will increase greatly,which not only adds a large amount of hardware to the system,but also consumes a lot of computing resources,which makes the voltage-balance control of the converter capacitor more complicated and variable;the voltage equalization based on the sorting algorithm Although the strategy can achieve capacitor voltage equalization,there are problems such as high switching frequency,large loss,and large calculation.How to reduce the amount of controller calculation and reduce the number of switching has been the focus of scholars at home and abroad.Firstly,based on the current development status of modular converters at home and abroad,the paper gives a detailed overview of the corrverter main circuit topology,sub-module topology,capacitor voltage equalization strategy and application;then analyzes the working mechanism of the modular inverter,The inverter equivalent circuit and the frequency domain mathematical model in the dq two-phase rotating coordinate system are established.The relationship between the output and the input and the steady-state operating characteristics of the converter are derived,which lays the foundation for the control strategy.Bridge circuit loss,bridge energy pulsation,capacitance dispersion,and pulse delay are used to analyze the causes of capacitor voltage imbalance.Then,the problems of recent:level modulation strategy,traditional sorting algorithm and traditional sorting method are studied,and the improved optimal sorting equalization is proposed.The strategy adds the keyword redundancy processing and reordering criteria to optimize the switching times and the calculation amount of the controller based on the bucket sorting.The overall design of the modular converter is laid,which lays a foundation for the development of the next experimental platform.Finally,based on the MATLAB software platform,a 21-level half?H modular inverter high-voltage direct current transmission system simulation model was built.Compared with the traditional sorting method such as bubble method,the optimized bucket sorting method has the dual function of keyword optimization processing and reordering criterion,and the number of switching times,comparison times,simulation time and sorting times of the converter are significantly reduced,which can be effective.The switching frequency and computational complexity are reduced,and the equalizing effect is the same as the traditional method,which verifies the effectiveness of the optimized sorting algorithm.