Insulation failure and partial discharge mechanisms of BOPP films under complex electrical stresses

Abstract Parallel capacitors across the interrupter contacts of circuit breakers in ultra-high-voltage (UHV) converter stations can be exposed to power-frequency voltage with superimposed harmonics and elevated operating temperatures. To examine the associated insulation response, this study used biaxially oriented polypropylene (BOPP) films and established an electro-thermal test platform for breakdown and partial discharge (PD) measurements under power-frequency voltage, selected characteristic harmonics (550 and 650 Hz), selected high-frequency harmonics (10 and 20 kHz), harmonic proportions of 1%-5%, and temperatures of 45–85 ℃. Within this test matrix, adding harmonics was associated with lower characteristic breakdown and PD inception field strengths than the power-frequency reference in most comparisons, although the dependence on harmonic proportion was not monotonic for all characteristic-harmonic conditions. Within the high-frequency subset, the measured field strengths were generally lower at 20 kHz than at 10 kHz under matched proportions. Increasing temperature reduced the characteristic breakdown field strength across the tested harmonic conditions; the PD inception field strength was generally lower at elevated temperatures, but some temperature sequences overlapped or were non-monotonic. PRPD and PRPS patterns also varied with waveform. These conclusions provide a solid theoretical reference for insulation condition assessment and fault prevention and control of converter station parallel capacitors.