DC Capacitor Voltage Balancing Control Strategy Of Cascaded U-Cell STATCOM

Compared with the traditional cascaded H bridge static synchronous compensator(STATCOM),the cascaded U-Cell STATCOM uses less switching devices when it outputs the same level,its size and loss is low,so it has better economic value.But there is also the problem of DC side capacitor voltage imbalance,which affects the reliable operation of the system.The Thesis focuses on the modulation strategy and control strategy of cascaded U-Cell STATCOM,enabling the device to run safely and steadily.Firstly,introduce the main circuit structure and working principle of cascaded UCell STATCOM in detail,and establish the mathematical model.A modified PD SPWM rotation modulation strategy is proposed.Which realizes the automatic rotation of the redundant switch state Analysis the switching frequency of the device.Secondly,the DC capacitance voltage balance control strategy is studied from three aspects,using the double closed loop control to solve the overall capacitance voltage imbalance.The goal is to make the sum of the capacitance voltage of all U-Cell modules up to the set value;the two factors that cause the unbalance of the capacitance and voltage in the phase are summarized,and the rotation adjustment is adopted respectively.The control strategy is based on switching state and active voltage vector.Using zero sequence voltage injection to balance capacitor voltage between phases.Thirdly,the five-level cascaded U-Cell STATCOM system is extended,the modulation strategy and control strategy of the 2n+1 level system are analyzed,and deduce the number of switch states of the 2n+1 level system In the modulation strategy,the concept of control wave is proposed to realize the rotation of redundant switches.Combining the control strategy based on the redundant switch state and the active voltage vector,which realizes the 2n+1 level capacitor voltage balance control.Finally,in the MATLAB/Simulink,the modulation strategy of cascaded U-Cell STATCOM and the control strategy based on redundant switch state superposition active voltage vector are simulated,and design the low-voltage experimental platform experimental.The experimental verification was carried out.