| The modular multilevel converter based high voltage direct current(MMC-HVDC)technology has significant advantages in areas such as regional grid interconnection,island power supply,and renewable energy generation grid connection.There are broad prospects for development in the field of DC transmission.As the voltage level and transmission capacity of a flexible HVDC transmission system continue to increase,the number of sub-modules of the inverter bridge arm increases and the control becomes more complicated.Reducing the amount of sub-module switching control calculations,reducing the switching frequency of sub-module power devices,and reducing the internal losses of the converter are still prominent issues that need to be addressed in high-voltage and high-capacity MMCs;realizing renewable energy power generation through flexible HVDC transmission systems delivery is also an important issue that needs to be addressed in flexible HVDC applications.The improvement and optimization of the control system for high-voltage,high-capacity flexible HVDC transmission is an effective means to solve the above problems.Through the theoretical research and experimental analysis of the MMC-HVDC control system,the MMC-HVDC station base control and valve base control and control strategy are optimized,and a suitable high-voltage large-capacity flexible HVDC transmission system and its optimized control scheme for wind power networking are proposed..The system structure of the modular multilevel converter,the internal topology of the sub-module,and the working principle are analyzed.The base station controller of the converter is improved,and a direct current control scheme based on the bridge arm is proposed.The calculation method of the circulation between the bridge arms of the upper and lower bridge separation control modes and the circulation suppression strategy realize the comprehensive control of the DC component,the AC component,and the secondary loop flow of each bridge arm of the MMC converter.A valve-based controller control method based on grouping sequencing modulation method is designed.The sub-module capacitance voltages after grouping are sorted in-group and group-wise,and the sub-module matrix with capacitor voltage order is formed,and the sub-module capacitance voltage deviation is introduced.Value and capacitor voltage deviation allowable value,effectively reducing the sub-module capacitor voltage sorting calculations and power device switching frequency,reduce converter losses.Based on the design of the converter control strategy,a control system for the wind farm through the flexible HVDC transmission system was designed,and a control strategy suitable for the converter station on the wind farm side was proposed.The additional frequency controller was designed to realize the change.The flow station tracks the power generated by the wind farm in real time;the power operation interval of the converter stations at both ends of the flexible HVDC transmission network system is optimized to ensure that the converter station smoothly passes the transient disturbance during the start of the pitch angle controller.Avoid latch-up accidents.The simulation of the proposed control strategy was verified by physical dynamic simulation system and real-time digital simulation system.The results show that the proposed control method has good steady-state and dynamic characteristics.The research in this paper preliminarily realizes the optimal control of high-voltage and large-capacity flexible HVDC transmission systems,and provides technical support for the design of high-voltage large-capacity flexible HVDC transmission systems and large-scale wind power networking through flexible HVDC transmission. |
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