Research On Optimal Control Of Cascaded H-bridge STATCOM Without DC Voltage Sensor

With the continuous expansion of the grid-connected scale of new energy,many problems such as insufficient reactive power of the system and voltage fluctuations at the grid-connected point have begun to emerge.It is necessary to use a fast reactive power compensation device to ensure the reactive power balance of the new energy regional system and maintain the voltage stability of the grid-connected point.This thesis takes the cascaded H-bridge STATCOM as the research object,improves and optimizes the DC side capacitor type and capacitor voltage detection method,and proposes a cluster voltage control scheme suitable for low-capacity STATCOM.The working principle and control strategy of the cascaded H-bridge STATCOM are introduced.This thesis introduces and compares three commonly used modulation strategies,gives several schemes to realize DC side capacitor voltage balance,points out the coupling relationship between the cluster voltage controller,current controller and single capacitor voltage controller,and gives the decoupling condition.A decoupling control system is built to provide a theoretical basis for the next measurement and control optimization of the DC side capacitor voltage.The DC side capacitor voltage detection method is improved.Aiming at realizing the intra-chain voltage equalization control,it is necessary to detect the capacitor voltage of each H-bridge module,which will lead to the problem of increased detection cost.By analyzing the carrier phase-shifted sinusoidal pulse width modulation technology,the mapping relationship between the output voltage of the converter and the capacitor voltage is summarized,and the special switching mode in the modulation process is discovered for the first time.Based on the above findings,an improved sensorless capacitive voltage detection method is proposed to solve the problems of large detection error,heavy computational burden and application limitations in existing sensorless detection algorithms.The correctness of the theoretical analysis and the effectiveness of the sensorless detection method are verified by using the simulation and experiment platform.The DC side capacitor type is improved and a cluster voltage control method suitable for low-capacity STATCOM is proposed.The reliability of film capacitors is much higher than that of electrolytic capacitors,but the achievable capacitance value of film capacitors is relatively low.The application of low-capacity film capacitors in STATCOM will put forward higher requirements for cluster voltage control.To this end,according to the capacitor voltage expression,the variable voltage reference of the cluster voltage controller is obtained to realize the precise cluster voltage control of the single-phase low-capacity STATCOM.Then the reasons for the zero-sequence voltage in the three-phase star connection system and its influence on the low-capacity STATCOM cluster voltage are analyzed,and two cluster voltage control modes are proposed accordingly to improve the control effect of the capacitor voltage.The simulation and experimental results show that the proposed measurement and control method can effectively control the three-phase low capacitance STATCOM.