Impedance Modeling And Stability Analysis At AC/DC Side Of Back-to-back HVDC Transmission System

Back-to-back DC transmission system based on modula multilevel converter has good application prospects in the fields of new energy generation interconnection,high voltage and large capacity transmission and asynchronous grid interconnection.However,the complex control of flexible DC systems,including large-scale power electronics,has led to Broadband oscillation problems.The impedance analysis method is a common method to analyze this problem,but the existing modeling process of this method often ignores the influence of the internal harmonic coupling of the MMC and some controllers,which has errors with the actual model and cannot accurately reflect the broadband impedance characteristics,and the research scenarios mostly focus on the MMC access to wind farms or AC grids,with less stability analysis on the DC bus.In addition,the enhancement measures for the stability margin of the AC and DC sides need further study.The article analyzes the stability issues of the MMC back-to-back HVDC transmission system and modeling the AC/DC side resistance.The main research content is as follows:Firstly,based on the basic structure and operating characteristics of MMC,the average model of bridge arm is derived,and the causes of internal harmonic components and their distribution law during steady-state operation of MMC are analyzed.On this basis,the harmonic state space method is used to establish an AC-DC side impedance model with the electrical part considering the multi-harmonic coupling characteristics;the controller part considering the main controller,circulating controller,voltage feedforward and the influence of delay links,to ensure the accuracy of the model and to build a unified modeling system for AC and DC sides,which provides the basis for stability analysis.Secondly,for the phenomenon of high-frequency oscillation on the AC side of Hubei of Yu-E project,combined with the AC side impedance model,the influence of system parameters on the stability of the AC side of MMC was analyzed using the impedance analysis method.The oscillating mechanism for the high-frequency oscillation event on the AC side is clarified from the resistance viewpoint.The stability enhancement strategy of control link optimization is proposed to maximize the attenuation of the negative damping characteristics of the oscillation-prone frequency band of the MMC and eliminate the risk of oscillation in specific multi.The stability enhancement strategy is proposed to maximize the attenuation of the negative damping characteristics in the oscillation-prone frequency band of the MMC,eliminate the oscillation risk in specific multi-band frequencies,and improve system stability.Finally,to analyze the DC-side stability problem of the two-end flexible-straight system more conveniently,an improved modeling method introducing the transformation matrix instead of the control system role is proposed to establish the impedance models of the two sides of the converter stations,and the fast switching of the control of different converter station models is achieved by constructing the control loop model separately.Based on this model,the influence of each parameter of the converter stations on both sides is clarified.The risk of resonance on the DC bus is found,which leads to design guidelines for the system electrical parameters and controller parameters to suppress the oscillation to improve phase margin.The PSCAD/EMTDC simulations tool is used to verify the models created in this study and the stability assessment findings that go with them.These results are close to the actual engineering field conditions.The research can ensure the grid’s secure and reliable operation while fully using back-to-back DC transmission technologies.Furthermore,it also provides inspiration for the stability problem analysis and impedance modeling of other power electronics.