Research On Coordinated Control Of Multi-terminal HVDC Transmission Based On Differential Game Theory

At present,under the situation of massive renewable energy grid-connected development,the grid must transmit its power in a timely manner.Commonly this requires the grid have higher reliability and better flexibility.Therefore,the multi-terminal DC transmission technology based on voltage source converters(VSC-MTDC)become more and more concerned.This technology not only has the advantages of traditional largecapacity DC transmission technology for large-capacity power transmission and good control performance,but also has the ability to improve power quality and cater well to the needs of an environment-friendly economy and society.VSC-MTDC constitutes an important part of the modern power grid and has adverse impact of grid-connected new energy generation such as wind power on the normal operation of the power grid and power quality.At the same time,VSC-MTDC can meet the needs of multi power supply and multi drop power receiving.So the prospect of multi-terminal DC transmission is very broad.However,there are still many problems to be solved in the VSC-MTDC transmission technology.For example,to determine and optimize the coordination and control strategy of the VSC-MTDC.First of all,it is necessary to ensure the DC voltage of the multi-terminal DC grid system can maintain stable and active power balance under different working conditions.The controller of the VSC must be well designed.And its control principle and strategy selection method must be mastered.Conduct optimization research based on upperlevel controllers so that the stability of power grid operation can be improved.Based on the detailed investigation of the current status and trend of the traditional two terminal DC transmission and multi terminal DC transmission technology,this paper analyzes the current topology of the voltage source converter and current source converter,and compares the advantages and disadvantages of the two.And then choose voltage source converter which is more reliable as the research focus.The mathematical model of the voltage source converter in the three-phase stationary coordinate system is established,and it is subjected to the PARK transformation to obtain the mathematical model under the dq synchronous rotating coordinate system.Based on the mathematical model of VSC,the active and reactive power control is decoupled so that the active and reactive power can be individually adjusted,and the VSC outer and inner loop controllers are designed.The theory of differential game was born in the middle period of the last century.It developed on the basis of optimal control theory and game theory.Now it has become an extremely rich subject and has become an effective method to explore the dynamic conflicts of interest among individual participants.Naturally,the differential game has a great advantage in researching and solving multi-agent dynamic coordination decision problems.The control strategy of each converter has an impact on the control strategies of other converter stations,and the relationship between the game is generated.Based on the characteristics and application scope of the differential game theory,it is determined that it can be used in the field of coordinated control of multi-terminal DC systems.And from two aspects of differential game theory: cooperative differential game and non-cooperative differential game,the game models of multi-terminal DC transmission systems are established respectively.Then,the genetic algorithm is used to solve their equilibrium strategies respectively,and the optimized PI control is obtained.Finally,by building a multi-terminal DC transmission system model on the Matlab/Simulink platform,the feasibility of the control parameters of each converter under the differential game condition for the coordinated control of the DC system is verified,and the cooperative game and the noncooperative game are compared in multi-terminal DC transmission.The control effect of the system under transient conditions of load disturbance or three-phase short-circuit is better than the control effect of unoptimized parameters.But considering the actual development trend,the interests of the converters belonging to different subjects have certain differences.Therefore,the multi-terminal DC transmission coordination strategy based on noncooperative differential game theory is more suitable for objective and practical power operation and control.