Research On The Method Of Low Frequency Oscillation Damping Control Strategy In Comlpex Power System With VSC-MTDC

With the construction of the Global Energy Internet,the interconnection of power grids in the world is increasing.Large-scales of new energy is connected to the power grid,especially wind farm.Its volatility,intermittency,and randomness not only increase the pressure of system peak regulation,but also affect the stability of power system.In the worse condition,the direct conflict between the large capacity of the transmission line and the weak network may cause low-frequency oscillations.In this paper the influence of low-frequency oscillations in the interconnected grid is studied based on a complex grid with VSC-MTDC and an effective wide-area damping control strategy is proposed.Firstly,the network structure of VSC-MTDC system is described in this paper.The linear mechanism and nonlinear mechanism of low-frequency oscillation in the interconnected power grid are analyzed.The classification of low-frequency oscillations,and the existing analysis method of low frequency such as eigenvalue analysis,time domain simulation,and Prony algorithm are inducted.The concept and properties of Power Spectral Density(PSD)are introduced,and the application of PSD in power system is briefly described.A low frequency oscillation analysis method based on PSD is proposed.Then,the control strategy of the multi-terminal system is introduced.The mathematical models of the inner-loop current control and the outer-loop voltage control are built.The principle and mathematical theory of the droop control of the multiterminal system are analyzed.The Prony identification method is proposed to find the transfer function of the complex system.A wide-area damping control strategy is proposed based on the phase compensation method,completing the function that monitoring of the interconnected power grid oscillation,identificating the system oscillation modes and designing the damping controller.A case study was conducted in the interconnected system through time domain simulation and power spectral density analysis to verify the validity and correctness of the strategy.Finally,a simulation model for a complex power grid with VSC-MTDC is built in DIgSILENT/PowerFactory.The wide-area damping performance of the interconnected system is studied and analyzed from several aspects,including the selection offeedback signal and the position of the damping controller.The study found that in order to achieve the purpose of suppressing the interarea oscillation,the higher the complexity of the power grid,the need to configure the damping controller in more converter stations.