Two-port-network-based decoupled impedance modeling method of DFIG system and DC-link coupling analysis

The doubly-fed induction generator (DFIG) has been widely used in the field of wind generation. However, due to the couplings on both dc-side and ac-side, the full-order small-signal impedance model of the DFIG system is hard to build. A conventional modeling method is assuming a constant dc-link voltage to ignore the dc-side coupling. However, the reasonability of this assumption has not been demonstrated sufficiently. Differently, this paper proposes a two-port-network-based impedance modeling method by decoupling the ac-side coupling. With this method, the total admittance of the DFIG system is decomposed into four admittance components listed in a two-by-two matrix, and each of them can be calculated independently. Then, a full-order small-signal admittance model can be obtained by adding these four admittance components. The main advantage of the proposed method is that the complexity of the modeling process can be degraded and the impact of the dc-link coupling on the whole DFIG system can be analyzed quantitatively. It is found that only when the magnitudes of the non-diagonal elements in the two-by-two admittance matrix are at least 10 dB lower than the magnitudes of the total admittance of the DFIG system, the dc-link coupling can be ignored. Otherwise, ignoring the dc-link coupling will make the model inaccurate. Admittance measurements in simulations validate the correctness of the proposed modeling method.