This paper presents a breakthrough threshold voltage testing methodology for longitudinal insulation design in dry-type transformers, focusing on high-frequency voltage oscillation characteristics between winding segments under lightning impulse. A high-frequency high-voltage oscillation circuit operating at 35 kV output with 20–40 kHz frequency range was developed based on zero-voltage switching (ZVS) circuit principles. The design process incorporated Multisim simulations followed by prototype implementation. Experimental validation involved voltage withstand tests on equivalent axial air duct gaps, with detailed analysis of voltage/current waveform characteristics pre- and post-breakdown. The study demonstrates three key post-breakdown phenomena: frequency elevation, current surge, and transient voltage collapse. By establishing distinct waveform boundaries between insulation integrity and breakdown states, we propose an innovative threshold capture method utilizing pre-breakdown voltage amplitude as the critical insulation parameter. The experimental methodology features a self-oscillating voltage escalation mechanism coupled with synchronized high-speed waveform acquisition (≥100 MS/s sampling rate), enabling precise breakdown threshold identification. This research provides an effective testing framework for optimizing longitudinal insulation coordination in dry-type transformers, particularly addressing high-frequency dielectric stress under transient overvoltage conditions.