Multimodal Optical–UHF Diagnostics for Early Partial Discharge Detection in Fatigue-stressed GFRP Insulating Rods

Surface micro-defects on glass-fiber-reinforced polymer (GFRP) insulating rods in gas-insulated switchgear (GIS) can initiate incipient partial discharges (PDs) long before visible degradation. A multimodal framework integrating ultra-high-frequency (UHF) PD sensing and optical single-photon counting is developed to detect incipient PD under mechanical fatigue. Hollow GFRP rods containing representative micro-defects (scratch, subsurface bubble, micro-crack) are mechanically cycled for 10,000 operation-equivalent cycles to emulate breaker actuation. After each 1,000-cycle block, specimens are energized with AC high voltage in SF6while phase-resolved UHF and photon-count PD signals are synchronously recorded. Photon counting identifies PD inception one fatigue block earlier (ΔN ≈ 1,000 operation-equivalent cycles) than UHF for all defected specimens, while both channels remain at baseline for the pristine rod until ≈7,000 cycles. Under typical GIS duty rates, this lead corresponds to several to tens of service years. Phase-resolved PD patterns reveal distinct defect-specific behaviors: scratches show the earliest PD onset and later exhibit discharges on both voltage polarities; bubbles develop dominant negative halfcycle activity under advanced fatigue; and micro-cracks confine PD activity to a narrow phase band near 270°. The combined UHF–optical method provides early warning of insulation degradation and enables quantitative defect severity ranking, supporting practical online condition monitoring of GIS components. An IEC 60270–compatible phase-resolved pulse-current reference is also derived from the coupling capacitor to quantify apparent charge (pC) in the weak-discharge regime.