Investigation of Partial Discharge Features and Inception Voltage in Cable Insulation Defects

This study investigates the partial discharge (PD) characteristics of bubble, protrusion, and scratch defects in 10 kV XLPE DC cables under 70°C DC stress. For bubble defects, discharge counts increased from 10 to 30 pulses per 20ms window, with peak amplitudes rising from 50 to 120 arbitrary units, reflecting cumulative electric field distortion and insulation deterioration. Protrusion defects exhibited initial high activity with 25 pulses and 100 amplitude units, gradually decreasing to 5 pulses and 30 units due to tip erosion and field redistribution. Scratch defects displayed a two-stage evolution: discharge counts rose from 8 to 18 and then stabilized at 10, while amplitudes increased from 60 to 90 and declined to 70, indicating early damage accumulation followed by discharge channel stabilization. Statistical analysis showed that, within a 10ms sampling window, protrusion defects generated 100 pulses with a maximum amplitude of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$800\mu\mathrm{V}$</tex> and discharge energy 30 times higher than bubble defects, while scratch defects produced 80 pulses with up to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$800\mu\mathrm{V}$</tex> amplitude and 20 times the energy of bubble defects. Discharge frequencies reached 10000 pulses/s for protrusions, 8000 pulses/s for scratches, and 3000 pulses/s for bubbles. PD inception and extinction voltages decreased linearly with temperature, and bubble defects consistently exhibited the lowest PDIV, followed by scratch and protrusion defects. These results provide comprehensive insights into the discharge evolution and degradation mechanisms of typical insulation defects, supporting condition assessment and lifetime prediction for HVDC cable systems.