Research On Power Flow Algorithm Of Flexible DC Systems Based On Seventh-order NR Method And Broyden Method

Compared with traditional high voltage DC transmission(HVDC),the flexible DC transmission technology based on voltage source converter(VSC-HVDC)can flexibly and independently adjust the active power and reactive power of each terminal.In addition,flexible DC power transmission is developing toward multi-terminal(VSC-MTDC)and multi-infeed(VSC-MIDC)systems for higher reliability.Therefore,it is of great significance to carry out relevant research on flexible DC transmission technology.Power flow calculation is the premise of transient and steady state analysis of power system.This paper conducts related research on power flow calculation of flexible DC transmission systems.The main contents are as follows:(1)First,the current research overview of flexible DC transmission systems power flow calculation is summarized;then,the transmission principle,steady-state characteristics and control method of flexible DC transmission systems are analyzed,which lays a foundation for subsequent research.The mathematical model of the AC-DC system is established and its power flow equation is deduced,and the solution steps of the unified iterative form suitable for the power flow calculation of the flexible DC transmission systems are given.(2)Two improved power flow algorithms for flexible DC transmission systems are proposed.The first algorithm uses the seventh-order Newton-Raphson(NR)method modified Jacobian matrix,which can effectively reduce the number of iterations of power flow calculation and improve the efficiency of the algorithm.The second algorithm uses the Broyden method in the power flow calculation of flexible DC transmission systems,and uses an approximate matrix that is easier to calculate to replace the complex Jacobian matrix,which avoids the repeated formation of the Jacobian matrix and can effectively reduce the calculation of single-step iteration.quantity.(3)The higher-order Newton method has the problem that the higher the convergence order is,the larger the single-step iteration calculation amount is.However,the Broyden method may increase the number of iterations and reduce the computational efficiency due to the lower convergence order.In order to better balance the relationship between the order of convergence and the amount of computation,this paper proposes a hybrid algorithm based on the seventh-order NR method and the Broyden method.Specifically,the seventh-order NR method is performed at the beginning of the iteration,with a high degree of convergence but longer computation time.After reaching the preset number of iterations,the Broyden method is used to solve the problem.Broyden’s method converges quickly when the initial conditions are close enough to the true solution.The proposed hybrid algorithm can effectively solve the respective problems of the seventhorder NR method and the Broyden method,and at the same time maximize the advantages of the two methods to further improve the efficiency of the algorithm.(4)Finally,based on the modified IEEE standard example,comparative analysis is made in terms of AC and DC power flow results,control methods,algorithm efficiency,various initial states and algorithm performance under heavy load,etc,to verify the effectiveness of the method proposed in this paper to solve the power flow of flexible DC transmission systems.There are 29 pictures,29 tables,and 64 references in the article.