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Near-fundamental-frequency oscillation (NFFO) events were observed in a renewable energy system (RES) connected to a four-terminal direct current (MTDC) grid in China, which has the world's largest DC transmission network. The sending terminal of the MTDC grid includes various converter-interfaced generators, such as permanent magnetic synchronous generator (PMSG), doubly-fed induction generator (DFIG), and photovoltaic (PV). The oscillation phenomenon is unique since the NFFO frequencies of 44 Hz and 56 Hz are relatively close to the fundamental frequency. For the first time, field-measured data are employed to investigate the oscillation characteristics. The analysis indicates that a resonance circuit (RLC) exists between the wind farm and the MTDC, and the wind farms display negative resistance at the oscillation frequencies. Further investigations identify the oscillation source based on the distribution of power flow at the oscillation frequencies. The paper also explores potential causes and countermeasures for the observed phenomena.
Xie et al. (Mon,) studied this question.