Research On Control Strategy Of MMC-HVDC

The modular multilevel converter(MMC)which has been proposed in recent years attracts more and more attention for its less harmonic content and simple modulation method.It has gradually becoming the main topology of the VSC-HVDC technology when used in engineering applications.Being a branch of VSC-HVDC technology,there are similarities between the MMC and traditional topology especially in fields like the mathematical model,the control strategy,and the fault characteristics.But there are also some special points which need to be studied deeply.Based on a twenty three-level simulation model of MMC established in PSCAD platform,the research of this thesis mostly concentrate on the following four facts.(1)The topology of the main circuit and the sub-module together with the operation principle of the MMC has been analyzed in detail.The mathematical model and advantages of the MMC has been studied in this thesis.(2)In this thesis,the system-level,converter-level and device-level control strategies are introduced,among them,the device-level control using the nearest voltage level modulation.Based on the straightforward strategy,an improved capacitor voltage balancing strategy for the MMC is proposed.It only focuses on the sub-modules whose capacitor voltage exceeds the limits,therefore,the switching frequency and loss of power electronic device are reduced.(3)The characteristic of the circulating current is investigated based on an analytic model.With the double line-frequency,negative-sequence rotational flame,the three-phase alternative circulating currents are decomposed into two dc components and are minimized by a dedicated circuiting current suppressing controller.The controller can also reduce the capacitor voltage fluctuations.(4)Based on the science and technology project of Jiangsu Province,the 380V thirteen-level MMC experimental prototype is developed.Finally,put the MMC experimental prototype on RTDS simulation platform,to verify the performance of the prototype and the correctness of the control strategy in this master's thesis.