Research On Power Flow Algorithm And DC Voltage Control For AC/DC System With VSC-MTDC

With the development of fully controlled power electronic devices and the growing demand of grid connection for renewable energy,voltage source converter based multi-terminal direct current(VSC-MTDC)has attracted more and more attention.For the AC/DC system with VSC-MTDC,power flow calculation is an important basis for studying the steady-state and transient operation characteristics of the system,while stabilizing the DC voltage is an important precondition to maintain the stable operation of the whole system.In this thesis,both the power flow algorithm and DC voltage control for AC/DC system with VSC-MTDC are studied.The main works are as follows.Firstly,four typical control modes of VSC-MTDC system are determined,combined with the steady state model and control mode of the converter stations.In addition,the transient mathematical model of VSC-MTDC system is established,including the system model and control model,which provides the model foundation for the power coordinated control among the converter stations in later chapters.Secondly,the power flow control algorithm for AC/DC system with VSC-MTDC is studied,in which the node current-injection method and the improved sequential power flow algorithm are proposed.In the node current-injection method,the current-injection at the point of common coupling(PCC)of each converter station is chosen as the interface variable.Therefore,the repeated modification of node type of PCC can be avoided when the control modes of the converter stations are changed.However,multiple interface variables lead to the poor convergence performance of alternative solution.In order to improve the convergence performance,the sequential power flow algorithm is improved,in which the active power transmitted by the PCC of main converter station is chosen as the only interface variable.Compared with the sequential power flow algorithm,the proposed algorithm can reduce the computational burden while ensuring good convergence.Case studies are respectively conducted on the New England system and the Nordic 32 system,and the validity and feasibility of the two proposed power flow control algorithms are verified.The performances of the two algorithms are compared and analyzed,which provides reference for the selection of power flow algorithm of AC/DC system with VSC-MTDC in practical engineering.Finally,the DC voltage control of AC/DC system with VSC-MTDC is studied,in which a novel fast calculation methodology for droop coefficients is proposed.In the case of outage or power step disturbance on any converter station,the linear analytic relationship between the power step amount and the DC voltage variation is derived.Based on the proposed concept of the generalized power sharing coefficient,the fast calculation of droop coefficients can be realized through solving efficiently the quadratic programming model.The calculation of droop coefficients is independent of the power step amount on the converter station suffering from disturbance,without the limitation of preassigning the maximum allowable DC voltage deviation.The DC voltage of the post-disturbance system can be quickly stabilized by implementing the power coordinated control among the converter stations.Case studies are respectively conducted on the New England system and the Nordic 32 system,and the validity and feasibility of the proposed calculation methodology for droop coefficients are verified.The proposed calculation methodology can provide reference for the setting or calculation of droop coefficients of the droop control stations in practical engineering.