Comparison of Different Coupling Types of Fiber-Optic Fabry–Perot Ultrasonic Sensing for Detecting Partial Discharge Faults in Oil-Paper Insulated Equipment

Power transformer internal partial discharge fault detection is a guarantee for the safe operation of the power system. Fiber-optic Fabry–Perot (F-P) sensors are widely used in the partial discharge detection of power equipment due to the antielectromagnetic interference, high sensitivity, and built-in fault location. Based on two different types of sensing principles, F-P sensors are divided into two categories: F-P cavity length change (indirect-coupled F-P) and medium refractive index change (direct-coupled F-P). At present, most scholars focus on indirect-coupled F-P studies, and direct-coupled F-P sensors based on the change of refractive index of the medium are less studied. This article focuses on the effect of fluid parameter variations on its internal spatial optical waveguide in the liquid-phase environment, revealing the acousto-optic direct-coupled F-P sensing mechanism in oil. Two types of F-P sensors were developed and the band characteristics, directional performance, and sensitivity of F-P sensors were comparatively tested. The results show that the indirect-coupled F-P sensors have a narrow and highly sensitive response region, typically 20–30 kHz, but are less directional and insensitive to lateral signals. Direct-coupled F-P sensors have a wide bandwidth, a wide dynamic range, a flat response in the 1–200 kHz range, and omnidirectional detectability. In this article, a comparison of partial discharge fault detection of oil-paper insulation equipment was carried out using two types of F-P sensors. The results show that indirect-coupled F-P sensors detect partial discharges with partial signal distortion, while direct-coupled F-P sensors can effectively detect different types of partial discharge ultrasonic signals of oil-paper insulated equipment.