A preordainment approach for design of auxiliary damping controller and SSSC tuning to enhance SSR mode stability in DFIG based windfarm

Sub-synchronous resonance (SSR) is a growing concern in wind turbine generators-based series capacitive compensated networks at low wind speed and peak compensation levels. It is feasible to increase the stability of SSR mode caused by a transmission line's passive series compensation by providing active compensation using a flexible ac transmission system controller cooperating with an auxiliary damping controller. This paper presents preordainment and enhancement of SSR mode in DFIG-based series capacitive compensated line by incorporating an additional damping controller into the static synchronous series compensator (SSSC). The auxiliary damping controller modulates the reactive voltage injected by SSSC into the transmission line by considering the appropriate input control signal, reflecting the SSR instability, and giving the maximum damping to the SSR mode. The optimal input control signal is determined via residue analysis, and the design procedure of the auxiliary damping controller is presented using root locus plots. The root locus plots offer a visual representation of the loci for each mode and preordain eigenvalues for new stability, which aids in the design of the auxiliary damping controller. This feature allows to predetermine the SSR mode eigenvalue while before connecting it with SSSC. Accordingly, it provides a controlled dampening to SSR mode such that the desired damping ratio for SSR mode can be achieved without influencing the stability of other modes. The impact of the designed auxiliary damping controller is examined at high compensation levels and low wind speed. The eigenvalue analysis results show that the preordained eigenvalues found using root locus plots are correct and the proposed approach predominantly enhances the stability of SSR mode. The fast Fourier transform (FFT) analysis and transient simulations are used to show the detuning and dampening of SSR mode. The proposed work is executed using MATLAB/Simulink.