Research On The Control Method Of Converter In VSC Connected To Weak AC System

Based on the fully controlled devices,voltage source converter high voltage direct current(VSC-HVDC)has been widely used in the field of DC transmission because of its technical advantages,with the gradual access of new energy and the increasing transportation capacity of VSC-HVDC,the scenes of VSC-HVDC connected to weak AC grids have appeared,thus the analysis on characteristics of system is becoming a research hotspot.At present,research on VSC-HVDC system connected to weak AC grids focuses on the influence of different kinds of controller in system and their optimation,the selection of control methods in the scene,and the research on effect of network parameters on the stability of the system considering frequency characteristics are rarely involved.The paper analyses the influence of VSC-HVDC controller,s characteristics on the stability of system.Considering voltage source as the AC side,the small signal model of VSC-HVDC connected to the weak AC grids is established in MATLAB,the influence of different controllers on small signal stability of system are analyzed,as well as the applicability of the control methods.Besides,considering the frequency characteristics of AC system,the synchronous generator(SG)is used to simulate the AC system with inertia,and the small signal model of the VSC-HVDC system in this scene is established,then the influence of parameters in generator and VSC controller on small signal stability of system is studied,all the conclusions above are verified by corresponding electromagnetic transient models in PSCAD/EMTDC.Firstly,the research status on characteristics of VSC-HVDC system connected to weak AC grids is investigated.A review is carried out from the aspect of controller characteristic and controller optimization,etc.Secondly,the electromagnetic transient models and small signal models of VSC-HVDC system connected to weak AC grids are established.The VSC-HVDC system models of rectifier side and inverter side based on different control methods are established in PSCAD/EMTDC,and the appropriate state variables are selected to establish the small signal model,active power small step is used to simulate small disturbance to observe the changes of variables,thus to verify the correctness of the small signal model by comparing with simulation results.Thirdly,the applicability of the reactive power controller of VSC-HVDC system is analyzed.Based on the different small signal models,the corresponding characteristic matrices are obtained.Under the same level of reactive power,the dominant poles of the system with different control methods are analyzed,a s well as the critical active power transfer limits of the system.Th e main state variables which influence the dominant poles are analyzed by the participation factor method.The conclusions are: the small signal stability of system is mainly influenced by the state variable—AC voltage of point of common coupling.The constant AC voltage control is more suitable for the VSC-HVDC connected to weak AC grids than the constant reactive power control,so the system is able to reach a higher transmission limit of active power.Finally,considering the frequency characteristics of AC system,SG is used to simulate inertia,the establishment of the small signal model and the eigenvalue analysis of VSC-HVDC system connected to AC grids with inertia are carried out.The electromagnetic transient model of VSC-HVDC system is established in PSCAD,as well as the corresponding small signal model of 26 order in MATLAB,with the methods of eigenvalue analysis,the influence of parameters on the stability of system are analyzed respectively,and the electromagnetic transient model is used to simulate and verify the conclusions,the conclusions are: the small signal stability of the system decreased with the decrease of short circuit ratio and the decrease of impedance angle of line,when the inertia constant is greater than 1.2s,the small sig nal stability of system will enhance followed by decrease but would not lose stability,when damping coefficient of SG ranges from 0-0.5pu,the small signal stability of system will enhance followed by decrease,the controller parameters also affect the sm all signal stability of system,so stability will be optimized by adjusting the parameters of VSC controller within limits.