Research On The Voltage Balance Strategy Of Modular Multilevel Converter HVDC Light

HVDC Light in power transmission has great advantages in solving the non-synchronous AC system inter connection,supplying power to the remote passive loads and satisfying the demanding of protecting environment.HVDC Light based on modular multilevel converter(MMC)is a new topology of HVDC technology,the characteristics of MMC make it suitable for HVDC systems.In this paper,the working principle of the HVDC Light?structure of topological and working principle will be introduced in detail.Meanwhile,a appropriate modulation algorithm will be proposed.It also studied the circulation characteristics of MMC and the related factors influencing circulation,build the circulation model of MMC.The results showed that circulation cannot be completely removed,but it can inhibit circulation control voltage components through improved control strategies.By analyzing the mathematical expression of the capacitor voltage,proposed that suppression of the circulation can reduce DC capacitor voltage ripple control strategy.Then it analyzes the harmonic expression MMC DC capacitor voltage power unit,analyzing the balancing factors which can influence the MMC sub-module capacitor voltage,from the perspective of the capacitor voltage fluctuation,it study the principles and values of capacitance calculation method,analyze the MMC sub-module pre-charge process,gives a general rule of sub-module does not control the charging stage,and establish a three-phase equivalent MMC model does not control the charging,at last the impulse current formula based on this model is proposed,which can effectively be applied to the selection of the current limiting resistor;Additionally,a control strategy for sub-module pre-charging is presented,which can take advantages of AC side to make the MMC start up stably.Based on the analysis of the modulation strategy and the module control strategy,propose a new voltage balance control method based on carrier phase shift,and make improvement in the average voltage controlled current inner part,we design a controller based on proportion resonant and band trap,Using band trap control,circulation is extracted to proportional resonant controller for tracking compensation,thereby fast-tracking of circulation,and join suppression capacitor voltage ripple control section in the traditional control strategy.Finally,through constructing the simulation model,the results demonstrate the effectiveness of new voltage balancing control strategy,indicating that the control method has good steady-state and dynamic characteristics and achieving a stable and balance control of DC capacitor voltage.