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Stability analysis of VSCs connected to an AC grid

Li, Xiangyu 2021. Stability analysis of VSCs connected to an AC grid. PhD Thesis, Cardiff University.
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As a typical renewable energy resource, wind power has been extensively exploited in the past years. With the continuous growth of wind power installed capacity, power electronic devices are widely used due to their good control performances. However, the interaction between power electronic devices and the power grid will lead to power system stability problems. Subsynchronous interaction (SSI) between wind farms and AC grids, as one of the most severe power system stability problems, has aroused great concerns. Different from the SSI between wind turbine generator (WTG) controllers and fixed series compensation or between generators and high voltage direct current (HVDC) controllers, recently, a new type of SSI is detected which is caused by the interactions between power electronic devices of WTGs and weak AC grid. The work in this thesis focuses on the mechanism and characteristics of this new type of SSI. A simplified system model with permanent magnet synchronous motors (PMSGs) connected to weak AC grids is established to investigate the new SSI. The linear impedance model of the studied system is conducted. The correctness of the proposed impedance model is validated by the comparison between the analysis in MATLAB and time-domain simulations in PSCAD/EMTDC. In the model of a single VSC connected to an AC grid, the mechanism of the SSI is investigated. The characteristics of the studied system under various conditions are analysed. The effects on system stability of different factors have been studied including the number of connected WTGs line reactance, Phase-locked loop (PLL), feed-forward voltage low-pass filter, current loop and outer loop of the VSCs. Time-domain simulation results verify the correctness of the analysis. An equivalent model of multiple VSCs connected to an AC grid is presented to investigate the characteristics when VSCs with different control systems and control parameters are connected to an AC grid. A new approach based on the Generalised Nyquist Criterion (GNC) is proposed to analyse the system stability. Compared to the existing traditional criteria, the new criterion has the advantages of better accuracy and simplicity. The new approach is validated in time-domain simulation. The study of this research work contributes to the stability analysis of power systems in the subsynchronous frequency

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: stability analysis, renewable energy, wind power, vsc, weak AC grid, impedance model
Last Modified: 26 Nov 2021 13:46

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