Study On The H-bridge Cascaded STATCOM DC Voltage Balance Strateg

The STATCOM based on cascaded H-bridge has widely application prospect in middle and high voltage reactive compensation filed for its modularization and easy extension. However, because of each H-bridge module capacitor is independent, capacitor voltage keeps balancing hardly. The control effect of DC voltage influences STATCOM output ability directly, so it is the key to guarantee normal operation.Based on basic structure and compensation principle of H-bridge cascaded STATCOM, the modulation principles and problems exist in single polarity double frequency CPS-SPWM and PD-PWM are analyzed. According to single polarity double frequency CPS-SPWM, the effect of individual module DC voltage is analyzed. According to PD-PWM, the output voltage harmonic characteristics are analyzed. On this basis, the thesis simplifies PD-PWM strategy.According to the problem exists in pre-charge stage of H-bridge cascaded STATCOM starting process, the pre-charge process using uncontrolled rectifier is analyzed. On this basis, a improved pre-charge control strategy is proposed. The simulation and experiment verify effectiveness and feasibility of improved pre-charge control strategy.The effect factors of individual DC voltage balancing are analyzed through establishing loss module of STATCOM and formula derivation. According to single polarity double frequency CPS-SPWM and PD-PWM respectively, the individual DC voltage balancing strategy which is suit for is selected. Control principles of the two balancing strategies are described and limitation of each balancing strategies are analyzed. Based on PD-PWM, a improved pulse rotation DC voltage control strategy which is suit for 5 level STATCOM based on cascaded H-bridge is proposed. The simulation and experiment verify effectiveness and feasibility of improved pre-charge control strategy.According to problem of phase to phase DC voltage unbalancing exists in star structure H-bridge cascaded STATCOM, power module under non-ideal power grid condition is established and active power which is absorbed by each phase is analyzed. On this basis, the calculation formulas of strategies by injecting zero-sequence voltage and negative-sequence current to balance phase to phase DC voltage are derived according to power relationship. The two balancing strategies are combined based on advantages of each balancing strategy and switching conditions are derived. The simulation and experiment verify effectiveness and feasibility of improved pre-charge control strategy.