| The modular multilevel converter has high modular structure,high output level,can adapt to different voltage levels and power levels,and does not require special filter devices.It is very suitable for high voltage and high-power flexible DC transmission.The HVDC-Flexible based on MMC has broad application prospects in the aspects of passive network power supply,isolated load power supply and city center power supply.Therefore,it is of great significance to study related technologies concerning MMC,such as modulation technology and control strategy.In this paper,the topological structure and working principle of the MMC were analyzed and studied,and established a simplified equivalent model of MMC-HVDC to facilitate the back analysis.Then expounded the selection principles of MMC main elements on the basis of the operation principle and mathematical modeling,and provided theoretical basis for the subsequent study of the MMC modulation strategies and the control strategies.Several modulation strategies suitable for MMC topology are compared and analyzed.According to the characteristics of MMC,this paper choose Nearest Level Modulation strategy.In order to solve the problem of sub module capacitor voltage imbalance,proceed from sub module capacitor voltage stability control and balance control to achieve the control of the sub module capacitor voltage.In the stability control,established the corresponding mathematical model.In the balance control,an improved method for MMC balance control was proposed to avoid time ordering of capacitance voltage.The improved method can reduced the switching frequency and improved the operation speed of the converter.Then through the perspective of energy fluctuation to analyzed the mechanism of the modular multilevel converter circulation,and the corresponding circulation control controller is used for the three-phase circulation between the MMC converter bridge arm.Finally,studied the hierarchical control principle of the control system of the flexible MMC-HVDC transmission system.On this basis,a double closed loop control system which can control the active power and reactive power independently was designed.After that,this paper build a 21 level MMC model in PSCAD/EMTDC software to simulate the strategy in before.Through analyzing the simulation results,it is verified that the proposed sub module capacitor voltage balancing control strategy enables the sub module to fluctuate within the allowable range.The circulating current control controller can better suppress the circulation of the bridge arm,and it shows that the double closed loop controlsystem has good steady state operation ability and dynamic response ability. |
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