Study On Suppression Strategy Of Capacitor Voltage Fluctuation Base On Cascade H-bridge STATCOM

With the advantages of strong scalability,high voltage resistance,high degree of modularity,and real-time compensation of reactive power which cascaded H-bridge STATCOM possesses,it has promising prospects and application value in the field of reactive power compensation,and has been a hot topic in the research on power systems in both domestic and foreign academia.One of the core problems that STATCOM needs to solve urgently is the voltage imbalance of the capacitors between the modules due to the independence of H bridges of STATCOM.Besides,the giant traditional STATCOM DC side electrolytic capacitors and the high failure rate limit the improvement of device power density and reliability.This paper focuses on two key issues,namely,STATCOM capacitor voltage balance and suppression of DC side double frequency voltage fluctuations,trying to find a way to ensure the reliable operation of the cascaded Hbridge STATCOM system.In order to compensate reactive power in real time and effectively,it is necessary to establish an equivalent mathematical model for STATCOM on the basis of the coupling of active power and reactive power in a synchronous rotating coordinate system by the method of voltage feed forward decoupling to independently control active and reactive power of the system.At the same time,we take the sampling delay and the inertial of PWM into consideration and correct the PI controller to achieve precise control of the compensation current.The voltage layered control solution is adopted to deal with the problems that the the output current harmonics increases due to the imbalance of the capacitor voltage and reduction of the service life of the capacitor and the switch tube.The influence of the H-bridge modulation ratio on the capacitor voltage is analyzed by the method of establishing a mathematical model between the capacitor voltage and the modulation ratio increment on the basis of the principle that the overall modulation ratio of the cascaded H-bridge is unchanged,and then the individual phase voltage balance strategy for modifying the modulation ratio is explored.By establishing the model of STATCOM three-phase instantaneous output power and forming a mechanism for mutual exchange of active power between three-phase systems in the method of the injection of fundamental zero-sequence voltage,voltage balance between phases could be achieved.To solve the problem of DC frequency double frequency fluctuation,the main factors affecting the H-bridge capacitor voltage fluctuation are analyzed,and the relationship between ripple current and fluctuating power is explored.Derive the threephase STATCOM ripple power model by reducing the voltage fluctuation by adopting third harmonic voltage injection method.A quasi-resonant controller is designed to track the harmonic current on the basis of the 5th harmonic occurrence of the compensation current caused by harmonic injection,and the effectiveness of the voltage fluctuation suppression strategy is verified by simulation.Originating from the double-frequency ripple suppression of single-phase rectifiers,active power decoupling hardly be applied to the research on cascaded Hbridge STATCOM.In this paper,bidirectional Buck / Boost is selected as the basic power decoupling unit to transfer the power fluctuation of the H-bridge to the energy storage element to suppress ripple voltage fluctuations.Two different ways of extracting secondary ripple current command on the DC side of H-bridge is discussed,and the working principle of Buck / Boost decoupling module in current continuous mode and discontinuous mode Parameter design is analyzed.To verify the effectiveness of the above control method through simulation,the overall power decoupling control strategy and voltage balancing strategy of cascaded STATCOM are researched.The control board,sampling circuit and driving board based on DSP+FPGA are designed by establishing the cascade h-bridge STATCOM experimental platform.Through a series of experiments designed to verify the modulation strategy,voltage balance strategy and double frequency fluctuation suppression strategy,the conclusion that the platform can compensate reactive power in real time with high reliability can be drawn.