Sub-Synchronous Oscillation Suppression Strategy for Grid-Forming Direct-Drive Permanent Magnet Synchronous Generator with Uncertainty and Disturbance Estimator Supplementary Damping Branch
Chen Jian1, Ren Yongfeng1, Meng Qingtian1, Xue Yu2, He Jinwei3
1. College of Energy and Power Engineering Inner Mongolia University of Technology Hohhot 010051 China; 2. Beijing Tianrun New Energy Investment Co. Ltd Beijing 100020 China; 3. School of Electrical and Information Engineering Tianjin University Tianjin 300072 China
Abstract：Under the background of continuous growth of wind power installed capacity, grid-forming converters are used to solve the problem of system inertia loss caused by high proportion of power electronic equipment in wind power system. During the interaction between the grid-forming direct-drive permanent magnet synchronous generator (D-PMSG) and the weak grid, its equivalent capacitance and the weak grid inductance produce resonance. Especially in remote areas, the sub-synchronous oscillation occurs with the resonance diverging under condition of negative damping. Compared with the traditional sub-synchronous oscillation dominated by shafting dynamics, the oscillation phenomenon in the new power system often begins with small signal negative damping divergence and ends in nonlinear continuous oscillation, which shows the characteristics of wide oscillation frequency range. In order to solve the problem of sub-synchronous oscillation caused by the lack of damping, an oscillation suppression strategy of supplementary damping branch based on uncertainty and disturbance estimator (UDE) in grid-forming D-PMSG is proposed. The sequence impedance model of grid-side converter is established, and the stability of virtual synchronous generator (VSG) with or without voltage-current double closed-loop is analyzed. The sequence impedance of VSG without voltage-current double closed-loop is inductive as a whole, which is similar to the characteristics of traditional synchronous generator. Under this mode, there is no interactive resonance with the weak grid, and the system stability is good, but it cannot achieve accurate voltage regulation and current limiting. The positive sequence impedance of VSG with voltage-current double closed-loop is capacitive as a whole, and the negative damping characteristics appear locally, which has the risk of sub-synchronous oscillation. In order to improve the damping characteristics of grid-forming D-PMSG, the UDE supplementary damping branch is introduced into current loop. The mathematical model of UDE damping controller is established, and the VSG sequence impedance model after introducing UDE controller is derived. The influence of UDE controller parameters on system stability is analyzed based on sequence impedance model with UDE. The parameters of reference model and error feedback gain are mainly used to improve the performance of the controller itself, which has little influence on the stability of VSG. The filter bandwidth of UDE controller has a great influence on the stability of the system. Increasing the filter bandwidth can improve the damping characteristics, but it is more affected by noise. Therefore, the choice of filter bandwidth needs to be compromised. After introducing the UDE supplementary damping branch, the negative damping interval of the VSG is sharply reduced, and the stability of the system is significantly improved. Conclusions can be drawn that: (1) The sequence impedance model of VSG in the frequency domain is derived. After introducing double closed-loop control, the grid-forming D-PMSG presents negative damping and capacitive characteristics in a specific frequency band, and the sub-synchronous oscillation occurs under interaction with the weak grid. (2) The UDE supplementary damping branch control model is established with the idea of filter bandwidth, which can suppress the sub-synchronous oscillation effectively within a certain range of grid strength. This method solves the problem that the traditional supplementary damping control must change the control parameters frequently, and improves the ability of the system to adapt to complex working conditions. (3) Under the step disturbance of grid voltage and active power command, the system can also suppress sub-synchronous oscillation well, which presents obvious anti-interference ability and good system robustness.
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2024, Vol. 39 
