Abstract: Recently， capacity of grid-connected wind power has increased， and power electronics technology has been widely used， causing new sub-synchronous oscillation problems. In order to cope with the problem， this paper proposes a method for analyzing the resonance characteristics of wind farms on the basis of impedance measurement. The effectiveness of the method is validated through the first IEEE benchmark model. A time-domain simulation model including direct-driven PMSGs and power electronic converters is established and it is analyzed on the basis of harmonic impedance measurement， which avoids complex formula derivations and calculations. The results show that the converter behaves as capacitive with negative impedance. The capacitive impedance of the converter forms a resonant loop with the inductance of the AC grid and causes unstable sub-synchronous oscillation.

Key words: direct-drive PMSG, sub-synchronous oscillation, harmonic impedance, frequency characteristicse