Development Characteristics of Winding Interturn Insulation Partial Discharge in a Full-Scale Converter Transformer

To investigate the development characteristics of winding interturn insulation partial discharge in a full-scale converter transformer, an interturn insulation defect specimen was installed inside a ±400 kV full-scale converter transformer. A partial-discharge test was conducted under stepwise alternating-current (AC) voltage, while pulse-current, high-frequency, ultra-high-frequency, ultrasonic, sound-pressure, and vibration signals were acquired synchronously. To reduce the subjectivity of discharge-stage classification, a dimensionless discharge-development index was constructed by integrating the 95th-percentile apparent charge, discharge pulse count, and phase occupancy, and the stage-transition points were identified using piecewise-linear change-point analysis. Two change points at approximately 1.9 and 4.3 min divided the discharge process into the inception, development, and severe stages. The apparent charge increased from approximately 1.5 × 102 pC in the inception stage to the order of 103 pC in the development stage, and then rose sharply to approximately 1.6 × 105 pC in the severe stage. Meanwhile, the discharge activity changed from an intermittent low-intensity state to a continuous high-intensity state. After the test, insulation-paper ablation and carbonization, conductor exposure, and a continuous breakdown channel were observed in the defect region. These post-test observations confirm severe final damage to the interturn insulation, although the time sequence of the individual damage features could not be determined from the final morphology alone. Under the present sensor arrangement, the HF channel provided the earliest identifiable response at approximately 1.9 min, whereas the UHF and internal ultrasonic channels supplied complementary evidence during subsequent discharge development. The external ultrasonic, sound-pressure, and vibration responses were more strongly affected by propagation paths, structural coupling, sensor location, and background disturbance and were therefore treated as supplementary indicators. These findings support a staged multisensor interpretation strategy for interturn partial discharge in a full-scale converter transformer rather than a universal ranking of sensor performance.