Sulfur Hexafluoride-Filled Extrinsic Fabry–PÉrot Interferometric Fiber-Optic Sensors for Partial Discharge Detection in Transformers

This letter presents a side-hole fiber-based extrinsic Fabry–PÉrot interferometric (EFPI) sensor whose cavity was filled with sulfur hexafluoride (SF$_{6}$) for in situ power transformer partial discharge detection. The Fabry–PÉrot cavity was formed by a polished thin diaphragm and a cleaved side-hole fiber that were thermally fused onto a fused silica ferrule. The cavity was first vacuumed and then inflated with 300-kPa SF$_{6}$ through the fiber side holes to increase its resistance to electric stress. Experimental results show that the sensor filled with SF$_{6}$ was able to withstand electric stress of at least 10 kV/mm (ac root-mean-square value) in transformer oil, three times larger than that of a traditional air-filled EFPI sensor.