Small-signal stability of grid-connected MMC-HVDC systems: impact of PLL bandwidth and SCR

Modular Multilevel Converters in High Voltage Direct Current (MMC-HVDC) systems with Synchronous Reference Frame- Phase-Locked Loops (SRF-PLLs) have been extensively studied for power transmission networks. This paper investigates the impact of the Short Circuit Ratio (SCR) and SRF-PLL bandwidth on the small-signal stability of grid-connected MMC-HVDC systems. Small-signal state-space models of the systems were established, through which the impedance/admittance of the MMC-HVDC and power grid were calculated. An impedance/admittance-based stability analysis with generalized Nyquist stability criterion was then applied. The analysis results were compared and verified through non-linear time-domain simulation in PSCAD. This analysis demonstrates that the SRF-PLL bandwidth range that maintains stability is narrower for lower SCR conditions. This range is further constrained when there is a higher current output from the MMC-HVDC.