Impedance Modeling And Stability Analysis Of MC-MTDC System

As an essential part of the future energy Internet,the multi-terminal flexible DC transmission system based on Modular Multilevel Converter(MMC)can realize renewable energy consumption and large-scale,long-distance power transmission,which is essential to solve the problem of reverse distribution between energy center and load center in China.However,system oscillations often occur in the operation of current flexible DC transmission projects.In this thesis,we address the oscillation problem of a multi-terminal flexible DC transmission system based on impedance stability analysis theory and conduct research from DC-side impedance modeling,impedance characteristics analysis,oscillation mechanism research,and stability optimization.The main work of this thesis is as follows.First,the DC-side impedance modeling method is studied considering the multiharmonic interaction inside the converter.The MMC DC-side impedance model is established,and the correctness of the constructed model is verified by the sweep method.The modeling method integrates the complex multi-harmonic coupling and dynamic characteristics inside the converter.It establishes different types of DC-side impedance models quickly and efficiently while accurately considering any order of harmonics in the converter,considering accuracy,and providing a theoretical basis for subsequent stability analysis.Second,the stability criterion is applied to study the stability of flexible DC transmission systems.By changing the electrical parameters,control parameters,circulating current control,and the considered harmonic order,the trend of the MMC DC-side impedance characteristic curve is analyzed,the influence of each parameter on the system stability is judged,the root cause of the system stability problem is investigated,and a specific parameter optimization scheme is given.Finally,the simulation verifies the correctness of the analysis method and the parameter optimization scheme’s feasibility.Third,a hybrid damping control strategy is proposed and applied to the MMCHVDC system.Firstly,the closed-loop transfer function of the active controller is determined,the DC-side impedance model is established after adopting damping control,and the characteristic impedance curve proves that the proposed strategy can effectively improve the stability margin of the system.Finally,the simulation demonstrates that the proposed strategy has better dynamic response characteristics than the traditional active control method based on the effective suppression of oscillations.Fourth,the source-load system equivalence is performed for the MMC-MTDC system;the stability analysis is carried out based on the equivalent small-signal model.The hybrid damping control strategy is applied to the MMC-MTDC system to determine the parameter optimization scheme of the hybrid damping control for the MMC-MTDC system.The effectiveness of the adopted control strategy is verified for the MMC-MTDC system through transient simulations.The transient response test shows that the proposed method has a solid dynamic response speed while suppressing the oscillation phenomenon.