Distributed Discharge Detection Based on Improved COTDR Method With Dual Frequency Pulses

Optical fiber distributed acoustic sensing (DAS) is a potential approach to realize on-line monitoring and rapid localization of power cable partial discharges (PDs). Distributed detection of discharges requires a DAS system with a wide frequency band and low noise level. For this purpose, an improved coherent optical time-domain reflectometry (COTDR) method based on dual-frequency probe pulses was developed. The method doubled the length-bandwidth product (LBP) of traditional DAS systems without significantly increasing the complexity of signal demodulation. A discharge signal extraction algorithm was also proposed based on the detection results of the dual-frequency pulses to exclude the effects of fading noises and vibrations. The improved COTDR system demonstrated the frequency band extension by detecting and recovering the 50 kHz sinusoidal vibration. The sensing distance was 1.1 km and the spatial resolution was 7.15 m. Afterward, the multi-point weak spark discharge acoustic signals were correctly measured and localized, and the frequency components up to 71.36 kHz were detected. The proposed COTDR method provides a prospective solution for distributed PD detection for power cables.