An impedance matrix model of DFIG for harmonic power flow analysis considering DC-link dynamics

Abstract

The doubly-fed induction generator (DFIG) has been increasingly used in modern power systems for interfacing wind energies.Because of the nonlinear characteristics of the DFIG, it is important to properly model the DFIG when conducting harmonic analysis in power systems. This is especially important for low-order harmonics that are routinely encountered by present power systems. Therefore, this paper presents a model of DFIG for harmonic power flow analysis. The harmonic model of the back-to-back converters, i.e., the combined rotor-side converter (RSC) and grid-side converter (GSC) unit, is first established through mathematical derivations, and the result shows that it can be represented by a coupled impedance matrix, which includes the harmonic coupling between the RSC and the GSC and coupling between different harmonics. The studies also show that the coupling between different harmonics is negligible; thus, a simplified 2 × 2 impedance matrix can be practically used with confidence. After integrating with the induction generator model, a full harmonic model of DFIG can be obtained through circuit analyses. The correctness of the developed model has been verified using EMT simulations and lab experiments. The new model fills in a gap in the modeling of renewable energies for harmonic power flow analysis.