Research On Coordinated Control Strategy Of MMC-MTDC System

In recent years,the multi-terminal flexible HVDC transmission system based on modular multi-level converter has become one of the research hotspots in the field of electric power due to its technical advantages of Two-terminal MMC-HVDC transmission system with high voltage and large capacity,as well as the operation characteristics of MMC-MTDC system with flexibility,reliability and economy.Because of the complex structure of AC-DC hybrid power grid,when the power fluctuations occur in the network,there may be unbalanced power distribution among MMC stations,resulting in the instability of DC voltage in the DC network,thus increasing the difficulty of coordinated control of the MMC-MTDC system.In view of the above problems,this paper focuses on the research of MMC-MTDC transmission system coordination control strategy design and optimization control method.Firstly,analyzed the topology structure,the working principle of MMC and MMC operation features,summarized the MMC-MTDC mathematical model of transmission system,this paper introduces the MMC-MTDC transmission system coordination control strategy and the DC voltage control method of inner or outer loops and the design of controller.In RT-LAB simulation platform,the MMC-MTDC transmission system simulation model is set for the subsequent control method of theory analysis and experiment validation to provide technical support.Then,in order to solve the problem that the MMC-MTDC system coordinated control has the unreasonable power distribution between the converter stations and the DC voltage instability,the MMC-MTDC coordinated control strategy based on the combined mode is proposed to apply the DC voltage margin.Combination of control and droop control,adding additional active power control,indirectly adjusting the slope of the characteristic curve,increasing the power margin of the converter station,making up for the shortcomings of the DC voltage margin and droop control.The system is subject to disturbance.After that,the control mode can be smoothly switched to find a new stable operating point and the DC voltage of the system is prevented from being short-term and the DC voltage is relatively stable.The effectiveness of the proposed control strategy is verified by simulation analysis.Finally,aiming at the traditional droop control with the fixed droop coefficient has the disadvantages of control inflexibility and poor dynamic voltage adjustment,and faced with some complex conditions such as frequent power fluctuation.An adaptive control method with adjustable droop coefficient is proposed.According to the DC voltage,transmission power limit and boundary of safe-operation margin,considering the dynamic voltage and power error of the DC network,the improved droop control method applied into the MMC station is obtained by modifying the droop coefficient.To improve the dynamic response of control system,the lag compensator is introduced to optimize the structure of inner-loop controller of MMC,then the dynamic response and steady-state characteristic are analyzed by using the time-frequency analysis method.The model of five-terminal MMC-HVDC transmission system including wind farms is built based on the MATLAB simulation platform.The different operating conditions of the MMC-MTDC system are designed to verify the effectiveness and feasibility of the proposed method,and simulation results show that the proposed method not only enhances the system response speed,but also improves effects of the power distribution and DC voltage control.