Summary
Wind‐farms are usually placed in remote areas, and thus high‐voltage AC‐based transmission systems with series capacitive compensation may be cost‐effective approaches for connection of wind‐farms to the grid over long transmission lines. However, the risk of subsynchronous resonance (SSR) is the main issue raised in series compensated transmission systems. This article first analytically investigates the effect of SSR phenomenon on the doubly fed induction generator (DFIG) dynamic behavior in a series compensated grid, and then proposes a proficient damping approach for complete suppression of SSR oscillations. The proposed damping approach is created by partial/full compensation of rotor back‐emf voltages by modifying the rotor‐side converter control. It is shown that under full compensation of rotor back‐emf voltages: (a) the rotor, stator, and grid currents become independent of SSR and (b) the DFIG and rotor circuit do not contribute to the induction generator effect, and subsynchronous oscillations have vanished quickly. In this article by the modal analysis and simulation results, it is shown that the proposed approach has a satisfactory response under different values of short circuit ratio and different series compensation levels. Next, it is shown that the proposed approach has superior performance compared to the rotor virtual resistance emulation approach.
Integration of doubly‐fed induction generator (DFIG) based wind turbines (WTs), in series‐compensated power grids, may result in a phenomenon known as sub‐synchronous control interaction (SSCI). Indeed, SSCI, created due to interaction between WTs controllers and series‐compensated power grid, may worsen damping of sub‐synchronous resonance (SSR) oscillations. This paper deals with the analytical assessment and mitigation of SSCI phenomenon in series compensated grids comprising of DFIG‐based WTs. In this way, first, theoretical and mathematical expressions and equivalent circuits regarding the SSCI phenomenon and the impacts of the RSC and GSC controls on the SSR oscillations damping are given and then a comprehensive impedance model for the DFIG‐WT at the sub‐synchronous frequency is extracted. Next, the rotor current control is modified, and an efficient approach based on the feedback linearizing technique (FLT), different from approaches presented in the related literature, is presented to mitigate the negative role of the DFIG in SSR frequencies. By mathematical analyses and developing related formula, it is shown that the proposed approach changes the impedance of the rotor circuit to infinity at the SSR frequency. This in turn results in suppression of the SSR oscillations initiated as a result of induction generator effect and SSCI phenomenon.
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