A practical impedance modeling method of MMC-HVDC transmission system for medium- and high-frequency resonance analysis

Abstract

The modular multilevel converter-based high-voltage direct current (MMC-HVDC) transmission system can arouse resonance that threatens the safety and stability of the power grid. In this paper, the impedance model and resonance mechanism of an MMC-HVDC transmission system are investigated at medium and high frequencies. First, an MMC-HVDC transmission system equivalent circuit model is proposed based on the partitioning impedance modeling method. Second, the concept of normalized sensitivity index is proposed to quantify the influential effects of crucial factors on the MMC equivalent impedance, including the control gains, control link delay, and circuit parameters; subsequently, the influencing factors are graded, and reduced-order impedance models are proposed for different situations. Then, the resonances in the MMC AC- and DC-side systems are analyzed considering both active and passive resonance, i.e., with or without a harmonic source, to systematically reveal the resonance mechanism of the MMC-HVDC system. Finally, a two-terminal MMC-HVDC simulation model is built to verify the proposed model and resonance analysis.