Research On Control Strategy Based On Multi-Agent System For VSC-MTDC System

The research on the control method of voltage source converter is the key part of the research of the flexible DC transmission technology,and is also the key of the flexible DC transmission technology which is different from the traditional DC transmission technology,voltage source converter based multi-terminal direct current(VSC-MTDC)system has great advantages in the new energy grid connection.The existing DC transmission systems are connected to the AC grid through the various converter stations,resulting in complex system structure and difficult control method,which is easily affected by the AC power grid.Under distributed control,although the traditional droop control method have high reliability,the frequency support on the AC side and the reasonable allocation of power in converter station is a difficult problem,in this paper,a distributed control strategy based on multi-agent system is proposed for VSC-MTDC system.Firstly,The basic principle and structure of VSC-MTDC system based on voltage source converter are analyzed,and the mathematical model of VSC is discussed,and the basic control method of local converter is put forward.The classical constant AC voltage vector control method for the wind-farm side voltage source converter(WFVSC)and the V2-P droop control method for the ground side converter station(GSVSC)for the VSC-MTDC system are discussed and analyzed,the traditional control strategy of VSC-MTDC system is given.Then,under the multi-terminal system control framework based on the upper communication equipment,the optimal communication topology is given by using the network graph theory and using the consistency algorithm.The design method of optimal communication topology for Multi-Agent Syetem(MAS)is proposed,through nonlinear simulation experiments,it is proved that the proposed communication network topology is optimal,which reflects the effectiveness of the conformance algorithm.Then,aiming at the shortcomings of traditional droop control in VSC-MTDC system,some improvements are made,and a complete MAS control strategy is given,the frequency support Agent(FSA)module has been designed,which takes the AC bus voltage and frequency of converter station as input signals,and the power distribution Agent(PAA)module has been designed with the load rate of converter station as input signals,and the coordinated control problem of FSA and PAA have been discussed.Among them,FSA provides fast frequency support for the AC power grid by controlling the power output of the converter station,PAA equalizes the load rate of each converter station in VSC-MTDC system through the output power redistribution between the converter stations.Finally,the 6 terminal VSC-MTDC system has been built based on the power system simulation software DIgSILENT/PowerFactory,through the analysis of the nonlinear simulation experiments,in terms of system frequency stability,when the FSA controller is opened in MAS,the results show that the frequency support effect of the system is better than that of the unopened FSA controller.In terms of system power allocation,when MAS turns on the PAA controller,the power allocation of the system is more reasonable than that without opening the PAA controller.It is verified that the control strategy adopted in this paper can quickly stabilize the system frequency and prevent the overload of converter station effectively.