The flexible DC transmission system offers significant advantages in renewable energy integration and long-distance power transmission. However, the continuous ultra-high-frequency harmonics caused by the switching of converter submodules lead to localized overheating of main equipment, resulting in degraded insulation performance and damage to multiple main equipment units, posing a threat to operational reliability. Traditional wide-frequency models neglect the stray parameters of the converter valve, limiting their applicable frequency range and making it difficult to accurately predict the leakage current characteristics of main equipment caused by ultra-high-frequency harmonics. This paper proposes a wide-frequency equivalent circuit model that accounts for the stray parameters of the converter system. By employing a layered modeling approach, it comprehensively characterizes the harmonic propagation properties of the converter valve tower and passive main equipment. Electromagnetic transient simulations in PSCAD are used to compare the results of the traditional model and the proposed model, verifying that considering stray parameters can accurately reproduce the harmonic characteristics of leakage currents in wall bushings, significantly outperforming the traditional model. This provides a theoretical basis for the design optimization of flexible DC systems and the suppression of ultra-high-frequency harmonics.