Research On Improved Droop Control Strategy Of Voltage Sourced Converter Based Multi-terminal HVDC

As one of the core directions of the new DC transmission technology,the flexible DC transmission technology has been studied and applied more and more in recent years.The multi-terminal flexible DC transmission technology has the advantages of high resource utilization and flexible control.Flexible DC transmission is widely used in new energy grid connection,AC system interconnection,construction of passive network power supply,construction of urban DC transmission and distribution network.In order to ensure the reliable and stable operation of VSC-MTDC system,it is of great theoretical and practical value to study the structure,operation mechanism and control strategy of the system.Firstly,the structure of the HVDC transmission system is described,and the mechanism of the controllers is analyzed by using the simplified structure model of the system.The topological structure of MMC,the core component of the system,is introduced.Taking five-level MMC as an example,the working principle and operation characteristics of MMC are explained in detail.The mathematical model of MMC based on ABC coordinate system is established,the mathematical model based on DQ coordinate system can be obtained by Park transformation,the mathematical model based on DQ coordinate system is used to decouple active and reactive power,and the mechanism of action of the controller of outer ring and inner ring is analyzed.Secondly,three kinds of system-level control strategies are analyzed by operating curves and operating principles,namely master slave control,voltage deviation control and droop control.The master-slave control strategy relies too much on the communication system,and the dynamic response ability of master-slave control and DC voltage deviation control is inferior to that of the droop control.Simulation verification proves that the droop control strategy has excellent dynamic adjustment ability,but the DC voltage deviation in the system operation is relatively large.Finally,an optimal droop control strategy is proposed,which uses the DC voltage deviation of the system to set the adaptive factor in the droop coefficient,so that the system can adjust the droop coefficient in real time according to the operating conditions,and realize the droop coefficient control.The PSCAD/EMTDC simulation platform is used for steady and transient simulation verification.The results show that the proposed strategy can improve the dynamic response speed of the system,reduce the DC voltage deviation,and reduce the peak value of DC voltage during transient regulation.The optimal droop control strategy can improve the stability and reliability of the system.