Simulation Study on the Influence Characteristics of Ultra-High Frequency (UHF) Harmonics on Bypass Devices in Flexible DC Converter Systems

This paper focuses on the impact of ultra-high frequency (UHF) harmonics in flexible DC converter systems on the bypass devices of converter valve sub-modules. To explore this, wide-frequency and electromagnetic transient simulation models are established, and the voltage time-frequency characteristics of bypass devices under different working conditions on both the rectifier and inverter sides are analyzed. The results reveal that on the rectifier side, the harmonic frequency of the upper bridge arm is approximately 130 kHz, while that of the lower bridge arm is around 240 kHz, with the voltage peak during conduction significantly higher than in the off state. Additionally, the voltage peak first rises and then falls as the wall bushing capacitance changes, and stabilizes once the transformer-to-ground capacitance reaches 400 pF. These findings provide crucial theoretical and data support for optimizing the design of flexible DC converter systems, particularly in enhancing the reliability of bypass devices under UHF harmonic interference.