Sub-synchronous resonance analysis in DFIG-based wind farms: Definitions and problem identification — Part I

This paper is part I of a two-paper series (Two-P-S) to review, analyze, and mitigate sub-synchronous resonance (SSR) in series compensated doubly-fed induction generator (DFIG) based wind farms. The IEEE first benchmark model modified to include a 100 MW DFIG-based wind farm is employed as a case study. In this work, MATLAB/SIMULINK is used for eigenvalue analysis and PSCAD/EMTDC for time-domain simulations, respectively. Part I of the Two-P-S focuses mainly on the identification and definition of the main types of the SSR that occur in DFIG wind farms, namely: (1) induction generator effect (SSIGE), (2) torsional interactions (SSTI), and (3) control interactions (SSCI). Regarding the SSIGE, first a simple definition of the SSIGE is given; then, using eigenvalue analysis and time-domain simulations, it is shown that the DFIG wind farm can be highly unstable due to the SSIGE; finally, the impact of wind speed and compensation level variations on the SSIGE is explained. Regarding the SSTI, first a descriptive definition is given; then, the real world possibility of the SSTI in DFIG wind farm is studied; finally, the impact of the stiffness coefficient and compensation level variations on this type of SSR is investigated. Regarding the SSCI, since it may be confused with the SSIGE, a simple definition of the SSCI and its mechanism in DFIG wind farm are presented. In part II of the Two-P-S, three methods to mitigate the SSR in DFIG wind farms will be presented.