High-voltage direct current (HVDC) transmission based on modular multilevel converters (MMCs) has become the backbone of modern power grids, enabling efficient long-distance power transfer and grid interconnection. However, MMC-based converter valves face an underexplored vulnerability: external transient overvoltages from lightning strikes or switching events can couple into converter stations through parasitic capacitances and distributed network paths, imposing unexpected electromagnetic stresses on semiconductor devices and insulation systems.
Researchers have addressed this gap by developing a comprehensive multi-scale wideband model that captures transient voltage behavior across three hierarchical levels: individual sub-modules, complete converter valves, and entire converter stations. The model incorporates realistic high-frequency characteristics of a 4.5 kV/3 kA IGBT-based sub-module and accounts for parasitic elements throughout the valve tower structure.
Simulation results reveal critical findings. When external disturbances enter the converter valve network, they propagate differentially across sub-module terminals, with sub-modules closest to the disturbance source experiencing the most severe transient voltage peaks and fastest voltage rates of change (dv/dt). The analysis identified multiple resonance bands exceeding 10 MHz within the valve tower structure, indicating that high-frequency oscillations can become amplified rather than attenuated by the distributed parasitic network. Stray inductance in sub-modules proved particularly influential, significantly affecting both peak voltage magnitude and oscillatory characteristics.
These insights have immediate practical implications for HVDC system designers. Insulation coordination procedures must account for high-frequency transient coupling that conventional surge arrestors may not adequately suppress. Converter valve structural improvements—such as optimized grounding strategies and parasitic parameter management—can mitigate transient overvoltage exposure. As HVDC technology scales to higher voltages and faster switching rates, understanding and controlling high-frequency transient propagation becomes essential for ensuring long-term reliability and minimizing unplanned maintenance shutdowns across global HVDC transmission networks.
