Research On Multi-objective Modulation Strategy For Cascaded H-bridge Converter

The increasing global energy demand has put pressure on energy supply and environment protection.Countries have successively formulated the corresponding energy structure transformation strategies in the post-fossil energy era.Environmental governance,energy conservation and emission reduction are important elements for ensuring China's economic and social sustainable development and building a conservation-minded society.The use of clean energy,such as solar energy and wind energy,and energy conservation in the field of high power transmission are important components of China's environmental governance and energy conservation and emission reduction.Cascade topology has a modular structure,easy to expand to high voltage,high power applications,relatively simple control,good output characteristics.It is an effective topology to solve the high voltage and high power transmission field,and it is also the most suitable topological structure for large-scale photovoltaic power generation and energy storage power plants.This paper mainly studies the problems of unbalanced dc-link voltages and unbalanced power among modular units existing in the application of cascaded H-bridge topology.This article takes the cascaded H-bridge converter as the research object.First,a simplified modulation strategy applied to cascaded H-bridge converters is introduced.On this basis,when the dc-link voltages of the cascaded H-bridge converter is unbalanced,a theoretical analysis of the modulation range and the limited degree of modulation is carried out in detail.And a modulation algorithm based on switch state flipping to achieve modulation index expansion is proposed to improve the utilization of the dc-link voltages.Secondly,when the the dc-link voltages of cascaded H-bridge converter is balanced or unbalanced,the calculation principle and influencing factors of its dc-link power output are analyzed in detail.A method for calculating the output power of dc side power supply of cascaded H-bridge converter based on FIFO is proposed.Based on this,the control strategy of dc-side power output of cascaded H-bridge converter when its modulation wave frequency is fixed and changed is described.When the dc-link voltages of the cascaded H-bridge converter is unbalanced and its modulating wave frequency changes,based on the guarantee of the balanced output of three-phase current,the accuracy of its dc-link power output control is verified through experiments.In addition,the mathematical model of permanent magnet synchronous motor in ABC coordinate system and dq coordinate system are introduced,and the vector control scheme of permanent magnet synchronous motor i_d=0 is described.And realize high-performance control of permanent magnet synchronous motor under unbalanced dc-link voltages of cascaded H-bridge converters.Based on this,an effective control of the output power of the dc side power supply of the cascaded H-bridge converter is realized.Finally,a vector control system of permanent magnet synchronous motor based on cascaded H-bridge inverter is built with DSP+FPGA as the core.On this system platform,the multi-objective modulation strategy of cascaded H-bridge converter studied in this paper is verified experimentally.