Driven by the national “Dual Carbon” strategy, offshore wind power has become pivotal in the energy transition. During the initial commissioning phase of the offshore converter platform for a specific Flexible Direct Current transmission project, disconnector switching operations within the Gas-Insulated Switchgear (GIS) generated Very Fast Transient Overvoltages (VFTO). These transients propagated into the converter valves, causing damage to the submodule bypass thyristors. This incident revealed insufficient understanding of VFTO impacts under the compact design philosophy. Addressing this specific engineering challenge, this paper analyzes the VFTO waveform characteristics at the submodule terminals of the converter valves, utilizing VFTO field measurement data. An innovative submodule-level equivalent test topology is proposed. This topology enables the reproduction of VFTO waveforms across the terminals of power electronic devices, establishing a foundation for laboratory investigation of their effects. An experimental platform based on this topology was constructed, and tests were conducted on IGBTs in the blocking state and their anti-parallel diodes. The results demonstrate that this topology effectively replicates the characteristic VFTO waveforms. Crucially, the tests revealed that bond wire pins of the devices are susceptible to ablation damage under positive-polarity VFTO. A series of experiments validated the effectiveness of the equivalent test topology and elucidated the failure mechanism and typical damage locations induced by VFTO.