Research On Control Strategy Of Flexible HVDC Transmission Based On MMC

The flexible DC transmission system based on Modular Multilevel Converter(MMC)has the advantages of modular design,low harmonic content,etc.,and has strong advantages in power supply reliability,and can be easily and efficiently connected.Renewable energy realizes multi-source power supply and multi-drop power reception.It has broad application prospects in the future grid connection of new energy,long-distance load power supply,urban DC distribution network and other power transmission fields.This article takes the MMC-HVDC system at both ends as the object,and studies the control strategy of the MMC-HVDC system under steady state and failure.This article takes the MMC-HVDC system at both ends as the object,and studies the control strategy of the MMC-HVDC system under steady state and failure.Firstly,on the basis of analyzing the topological structure and working principle of MMC,the mathematical model of the MMC-HVDC system in the three-phase static coordinate system and the two-phase rotating coordinate system is established,and the parameters of the main equipment components in the system are calculated and selected.Secondly,the converter valve-level control of MMC-HVDC is studied.In order to solve the problem of sub-module voltage imbalance,different sub-module voltage equalization control strategies were studied on the basis of two modulation strategies of recent level approximation and carrier phase shift.At the same time,in order to improve the high switching frequency under the traditional sequencing method,change For the problem of large current loss,an improved sub-module voltage equalization control strategy is proposed.In addition,in view of the problem of circulating current in the bridge arm of MMC-HVDC,the mechanism of circulating current is analyzed,and a control strategy to effectively suppress the double-frequency circulating current is designed.Thirdly,the converter station-level control of MMC-HVDC is studied.According to the mathematical model of the AC side,the current inner loop control and the power outer loop control based on the rotating coordinate system are deduced,and the active and reactive power decoupling control of the system is realized,and the two are designed in combination with the system-level control and the converter valve-level control.The terminal MMC-HVDC control system verifies the correctness of the above control strategy on the simulation platform through power step and power flow flipping.Finally,the mathematical model of the MMC-HVDC system when an asymmetric fault occurs in the AC system is derived.On this basis,a control strategy to suppress negative sequence current is designed through the phase sequence decomposition link,which improves the operating capability of the MMC-HVDC system under asymmetric conditions.The effectiveness of the negative sequence current controller is verified through single-phase grounding and two-phase grounding faults in the PSCAD/EMTDC simulation environment.