Re-Examination of DFIG-Based Wind Park Small-Signal Instability

Doubly-fed induction generator-based wind park (DFIG-WP) can interact with both series-capacitor compensated and weak (low short circuit ratio) grids, thus causing instability, and called as series-capacitor sub-synchronous oscillation (SSO) and weak grid SSO, respectively. However, the frequency spectra of phase voltage/current signatures of those two SSOs are significantly different. There is one dominant resonance frequency in series-capacitor SSO, but there are two frequencies (resonance and corresponding mirror frequencies) in weak grid SSO in addition to the fundamental frequency. To address this issue, this paper deeply analyzes the DFIG-WP instability mechanisms through impedance-based stability assessment (IBSA). Both the IBSA and electromagnetic transient (EMT) validation simulations demonstrated that the resonance takes place in the sub-synchronous range in a series-capacitor compensated grid and is mainly affected by the rotor-side converter (RSC) control parameters. However, the weak grid instability is found in the super-synchronous range with its mirror frequency in the sub-synchronous range and is affected by both the RSC and grid-side converter (GSC) control parameters. Therefore, the classification of weak grid SSO needs revision. The utilized DFIG impedance model in IBSA accounts for the DC coupling between RSC and GSC (AC-DC coupling phenomenon). The contribution of this coupling on DFIG impedance and its impact on DFIG-WP instability are demonstrated. In the considered test system, the AC-DC coupling provides a considerable amount of positive resistance in both resonance conditions. Hence, its omission may lead to pessimistic IBSA results. This paper also provides recommendations for instability prevention and/or mitigation through DFIG control parameter modification based on the guidance of the stability contour analysis.