Wind farm resonance characteristics analysis based on harmonic impedance measurement method

Recently, subsynchronous oscillation (SSO) has been captured in direct-drive permanent magnetic synchronous generator (PMSG) based wind farms in many areas without series compensation in nearby power grids. However, there are fewer reports related to this new type of SSO. In addition, an analysis of the new SSO based on existing methods is computationally cumbersome, and the control effect of the converter was generally simplified in previous studies, which lead to serious errors. This paper proposes a method for analyzing the resonance characteristics of wind farms based on impedance measurement to simplify calculations and improve accuracy. The correctness and effectiveness of the proposed method are validated through the IEEE first benchmark model. A time-domain simulation model of a direct-drive PMSG is established and analyzed by harmonic impedance measurement. The results show that SSO exists when direct-drive PMSGs are connected to a weak AC system. The converter of the direct-drive PMSG exhibits a small negative value at the oscillation mode frequency. This means that the converter behaves like a capacitor with negative impedance. The capacitive impedance of the converter forms a resonant loop with the inductance of the AC grid and causes unstable SSO.