Research Of Modulation Strategy And DC Capacitor Voltage Balance For Cascaded H-Bridge Converters

Cascaded H-bridge multilevel converter is widely used,due to its high voltage,large capacity,high output-voltage performance,modularity and ease of controling the output power of each cell independently.The main control objectives of cascaded H-bridge converter in grid-connected applications include keeping dc voltage of each cell stable and ac current being sinusoidal.Hence,when the loads of dc side in each cell are seriously unbalanced,balancing the voltage of the dc side and keeping the system working properly have become interesting fields to research for scholars all over the world.This thesis takes cascaded H-bridge converter as the research object,focusing on the dc side capacitors voltage balance strategy and the power routing algorithms for the overmodulation problem.Based on the fact that ac currents in all cells are equal,the cascaded H-bridge converter is modeled as a single-stage PWM rectifier to analyze its working principle.The double closed-loop control method of proportional integral(PI)controller to control voltage and proportion resonant(PR)controller to control current is studied.The mathematical model of cascaded H-bridge converter is established.The conventional proportional pulse compensation balance strategy is introduced.The effect of the balance strategy on the performance of carrier phase-shifted PWM is studied.Based on the common operation conditions of cascaded H bridge converter in some applications,the principle of dc-side unbalance is analyzed.In allusion to the working condition that the loads of dc side in each cell are seriously unbalanced,the existing modulation algorithms with extended working range are studied.According to the frequency of the compensated components in the power routing modulation strategies,it can be divided into third harmonic compensation strategy and discontinuous modulation strategy.An over modulation compensation strategy is proposed,which can extend the balance operation domain.The overmodulation component of each cell is calculated and compensated to other cells.The theoretical limit of modulation index of each cell is increased to about 1.27,while the operation domain of cascaded H-bridge converter is extending and its power routing capability is improving.Different power routing modulation strategies are compared and extended to multicell condition.An 8 cells cascaded H-bridge rectifier simulation model is designed and established for verification of above theories.Finally,an 8 cells cascaded H-bridge rectifier experimental platform is established to verify the existing modulation methods and the proposed method.The experimental results show that the proposed method effectively extends the operation domain of the cascaded H-bridge converter and improves its power routing capability.