Sampling-Rate Enhancement of φ-OTDR Systems for Acoustic Emission Detection of Partial Discharges in GIS

Acoustic emission (AE) detection methods based on phase-sensitive optical time-domain reflectometry (φ-OTDR) have shown strong potential for partial-discharge (PD) detection in gas-insulated switchgear (GIS). However, an inherent trade-off between the sensing fiber length and the system sampling rate restricts the detection of high-frequency PD acoustic signals. To overcome this limitation, a φ-OTDR system incorporating an auxiliary fiber loop is developed to increase the effective sampling rate without sacrificing the sensing range. In this configuration, each periodically injected probe pulse serves as a temporal seed, undergoing a fixed frequency shift during each loop circulation and forming a probe pulse sequence with periodic and equally spaced frequency offsets and enabling frequency-multiplexed Rayleigh backscattering interrogation. This multiplexing substantially increases the sampling rate of AE signals by raising the equivalent probepulse repetition rate. Experiments on a practical 110 kV GIS demonstrate that the developed system achieves an effective acoustic-signal sampling rate of 800 kHz, enabling distributed sensing at 24 sensing points. Metallic particle PD defects are successfully detected and localized, with a signal-to-noise ratio approximately 4 dB higher than that of a conventional piezoelectric transducer. These results demonstrate the effectiveness of the proposed system for high-frequency distributed AE detection in GIS PD monitoring.