Improved Power Flow Algorithm For AC/DC Systems With VSC-HVDC

At present,HVDC transmission technology is in a high-speed development stage in the world.China’s Southern Power Grid and East China Power Grid have formed a large-scale AC/DC power system.With the energy shortage,environmental problems and the continuous development of renewable resources,which will further promote the development of China the world’s most complex AC/DC grid.Therefore,it is of great significance to carry out related research on the operation problems of AC/DC systems.HVDC has affected its further development and application due to its complex structure and operation,and the reactive power consumption of a large number of converter stations.In this regard,with its own unique technical advantages,based on the voltage source converter（VSC）and pulse width modulation technology,the voltage source converter based HVDC（VSC-HVDC）technology received universal attention.The power flow calculation of AC/DC systems with VSC-HVDC is the prerequisite for the analysis of the system’s steady-state and transient characteristics,and it is also an indispensable link for studying its corresponding control operation mode and protection.To this end,this paper conducts related research on power flow calculation of AC/DC systems with VSC-HVDC.The main contents are as follows:（1）Firstly,the HVDC and VSC-HVDC transmission systems are summarized,and the current research status of VSC-HVDC power flow calculation is reviewed.Then,the principles and characteristics of VSC-HVDC transmission are analyzed,including the basic concept and working principle of VSC-HVDC,as well as the operating characteristics and control methods of VSC-HVDC under steady-state conditions,which laid the necessary theoretical foundation for subsequent research.（2）The mathematical model and power flow equation of the AC/DC system with VSC-HVDC are discussed and established in detail.Based on the standardization analysis of DC system physical quantities,the necessary prerequisites for VSC-HVDC steady-state modeling are given first.Then,the power flow equations applicable to the calculation of AC/DC systems with VSC-HVDC are derived in detail,and based on the analysis of the steady-state characteristics and power flow equations of VSC-HVDC,a solution step of a unified iterative form suitable for power flow calculations of VSC-HVDC is given.Finally,aiming at the problem of increasing the dimension of Jacobian matrix of AC/DC system caused by VSC-HVDC feed in.According to different control methods of VSC,some rows and columns corresponding to the required variable and the Jacobian matrix are eliminated to reduce the matrix dimension and calculation cost.（3）Based on the existing VSC-HVDC power flow algorithm,two improved unified iterative methods are proposed.In the first algorithm,the ₁₊₂ order Newton-Raphson（NR）method is applied to the AC/DC systems with VSC-HVDC to modify the Jacobian matrix.The second algorithm is a new hybrid algorithm that combines the ₁₊₂ order NR method with the simplified Newton method（SN）,which can effectively combines the advantages of the two.（4）Finally,based on the modified IEEE standard examples,the validity of the proposed method for solving AC/DC power flow with VSC-HVDC are verified by comparing the results of AC/DC power flow,control mode and algorithm efficiency,etc.