Abstract: This paper focuses on a typical 35kV main transformer low-voltage busbar breakdown fault in a wind farm. Through on-site investigation, fault reproduction, and theoretical analysis, the study systematically examines the complete mechanism of insulation failure caused by a "floating conductor" due to installation process errors. The fault directly originates from the busbar wall bushing sealing insulation panels, where the illegal use of metallic materials for connection creates a "floating conductor." This structure induces a floating high potential through capacitive coupling, causing severe local electric field distortion, gradually eroding and carbonizing the solid insulation medium, and eventually developing into a through-breakdown fault under operating overvoltage excitation. Based on the analysis results, this paper proposes a prevention and control system for hidden installation risks from three dimensions: "source control - process supervision - long-term operation and maintenance," providing theoretical guidance and practical solutions for preventing similar faults.