Frequency-Coupling Impedance Model Based Analysis of a High-Frequency Resonance Incident in an Actual MMC-HVDC System

Recently, a high-frequency resonance (HFR) incident has been captured in an actual HVDC project of China Southern Power Grid, which is regarded as a new stability issue caused by the interaction between the modular multilevel converter (MMC) and the weak ac grid. To investigate this event, this paper proposes a stability analysis method based on the frequency-coupling impedance model (FCIM). First, the FCIMs of the MMC and the ac grid are established in abc-frame individually with the frequency coupling effect of the MMC taken into account. Next, they are aggregated into a two-by-two impedance matrix by numerical calculation. Then, based on the aggregated FCIM, a criterion is developed to locate the HFR modes and to evaluate their stability. Finally, the results obtained from FCIM-based analysis are verified by electromagnetic transient (EMT) simulations as well as the field measurements.