Research On Control Of VSC-MTDC System Based On Virtual Synchronous Generator Strategy

With the gradual depletion of fossil energy and environmental pollution,the development and utilization of renewable energy has become the consensus of all countries.Offshore wind power is a clean energy with great potential,so its development and utilization has attracted the attention of many countries including China.However,offshore wind farms tend to be located far from the load center,and the use of conventional AC transmission will cause serious problems of line loss.Voltage source converter based multi-terminal DC(VSC-MTDC)transmission technology is considered to be a most suitable method for connecting offshore wind farms and land-side power grids because it can transmit large-capacity power over long distances while reducing line losses.The traditional control strategies of VSC-MTDC transmission systems contain master-slave control and voltage droop control.However,under these traditional control strategies,when the VSC-MTDC transmission system is used as a DC power source to access the AC grid on the land,it cannot provide the moment of inertia and damping characteristics of the conventional synchronous generator for the AC system,so it cannot provide reliable frequency support for the grid.Therefore,as the scale of the VSC-MTDC transmission system continues to expand,the inertia of the system is relatively reduced,which can decrease the stability of the system.Based on the conventional dc voltage droop control,a new virtual synchronous generator control strategy is proposed in this paper.The proposed strategy can adjust the power output of the converter station spontaneously according to the operating state of the AC grid,and it can simulate the output characteristics of the traditional synchronous generator.The research contents of this paper mainly include the following aspects:(1)This paper introduces the research status,the application field and the topology of VSC-MTDC system.Then,three basic control strategies of VSC-MTDC system,including master-slave control,voltage droop control and voltage droop control with margin are investigated,moreover,the advantages and disadvantages among the three control strategies are compared.Furthermore,the basic principle of the virtual synchronous generator control strategy and several existing control schemes are introduced.By comparing several different schemes and considering the characteristics of the multi-end system model built in this paper,a more suitable scheme of controlling the frequency via virtual inertia is used to simulate the characteristics of the traditional synchronous generator.Finally,the basic strategies of active-frequency control and reactive-voltage control of virtual synchronous generators are derived by means of studying the rotor motion equation and excitation system characteristics of synchronous generators.(2)In this paper,based on the droop control principle of VSC-MTDC system,the V~2-P droop control strategy of VSC-MTDC system is derived.Based on the V~2-P droop control strategy,the active-frequency control strategy and reactive-voltage control strategy of the virtual synchronous generator are combined to obtain the virtual synchronous generator control strategy of the VSC-MTDC system,and the two control strategies are compared.Finally,the most important two parameters(inertial time constant and damping coefficient)of the virtual synchronous generator control strategy are analyzed in detail.(3)Firstly,the five-terminal system model of VSC-MTDC system is built on the DIgSILENT/PowerFactory power system simulation platform,and then the virtual synchronous generator control strategy proposed is verified to be able to improve the frequency stability of the power system through three study cases.