Mid- and High-Frequency Resonance Characteristics and Suppression Strategies of VSC-UHVDC for Large-Scale Renewable Energy Transmission

Mid- and high-frequency resonance (MHFR) is highly likely to occur at the sending end of voltage source converter-based ultra-high voltage direct current (VSC-UHVDC) for large-scale renewable energy transmission. It is of great importance to investigate the resonance characteristics and the corresponding suppression strategies. Firstly, this paper introduces the overall control scheme of VSC-UHVDC for large-scale renewable energy transmission. Then, the impedance models of VSC under grid-forming control with AC voltage coordinated control are established. The mid- and high-frequency impedance characteristics of VSC-UHVDC are analyzed. The key factors affecting the impedance characteristics have been revealed, including the AC voltage control, the voltage feedforward, the inner current loop, the positive-sequence and negative-sequence independent control (PSNSIC), and the control delay. The MHFR characteristics at the sending-end system are analyzed in the whole operation process, including the black start and the normal power transmission operation. An integrated control scheme is proposed to address the MHFR problems. Finally, extensive case studies are conducted on a planned VSC-UHVDC project to verify the theoretical analysis.