The bridge arms and DC voltage of China’s Four-Terminal Flexible DC Transmission Project exhibit persistent high-frequency harmonics over the medium to long term, causing issues such as overheating losses and electromagnetic interference within the converter stations. To address this issue, this paper first introduces the structure of the Four-Terminal Flexible DC Grid and the high-frequency harmonic characteristics on the DC side, clarifying the impact of control cycles on the harmonic distribution at converter stations. Through analysis of the modulating wave, it is demonstrated that the sideband harmonics originate from the coupling effect between the control cycle and the modulating wave, inducing high-frequency sideband harmonics on the bridge arm. A discrete switching equation for bridge arm voltage was established. Based on double Fourier decomposition, a mathematical model for sideband harmonics was derived, and the flow direction of these harmonics was analyzed. A four-terminal flexible DC system was constructed using PSCAD electromagnetic transient simulation, yielding harmonic distributions in the arm and DC-side sidebands. This validated the accuracy of theoretical analysis and ultimately identified the factors influencing sideband harmonics.

With the rapid deployment of flexible high-voltage direct-current (HVDC) systems, high-power press-pack insulated-gate bipolar transistors (PP-IGBTs) have become critical components. However, field data indicate premature overcurrent failures in PP-IGBTs under modular multilevel converter (MMC) operations. This study systematically investigates the short-term overcurrent failure mechanisms of PP-IGBTs under MMC-specific operational stresses, distinguishing them from conventional short-circuit or avalanche modes. A power cycling test platform emulating MMC submodule conditions was developed to capture the evolution of terminal characteristics. Furthermore, a double-pulse test platform was established to reproduce failure locations under service-relevant stresses. The study reveals a distinctive failure mode characterized by pronounced gate leakage and the eventual loss of normal gate control, leading to functional open-circuit failure—a phenomenon that has been rarely reported in the existing MMC literature. The experimental results indicate a decreasing trend in short-term overcurrent withstand capability with increasing clamping force and reveal a preliminary relationship between peak current and measured emitter-side temperature at failure. These findings provide insights into the failure mechanisms of PP-IGBTs and offer practical guidance for device selection, converter design, and reliability assessment.