Understanding DC-side high-frequency resonance in MMC-HVDC system

Instability on direct-current (DC)-side resonance occurs if the connected converters interact with the DC network. In this study, a single-input-single-output transfer function is built to investigate the DC-side resonance of modular multilevel converter-based high-voltage DC (MMC-HVDC) system. The small signal model is developed for the DC-side resonance of an MMC-HVDC system. Furthermore, the system can be separated into two subsystems, i.e. H(s) and G(s). This study shows that both amplitudes of H(s), i.e. Abs [H(s)] and resonance peak of G(s) affect the encirclement radius of the Nyquist curve, and the encirclement radius reaches the largest at the resonance frequency. More importantly, the power transfer capability is restricted by the DC-side resonance since the more transmitted power leads to larger Abs [H(s)]. The impedance model is verified by both time-domain simulations and frequency responses imposed by impedance frequency scan in power systems computer-aided design/electro-magnetic transient design and control. Besides, an active damping control approach is introduced to suppress the resonance on DC side.