Research on EFPI Acoustic Emission Sensors for Partial Discharge Based on Solid Acoustic Medium Coupling

The Extrinsic Fiber Fabry-Perot Interferometer (EFPI) sensor is widely employed to detect partial discharge (PD) ultrasonic signals within oil-immersed transformers. However, under the influence of transformer oil, the sensor faces significant challenges, including a reduction in overall detection performance and poor diaphragm stability, particularly in transformers already in operation. This study examines the impact of gas and liquid media on the detection characteristics of the sensor diaphragm and proposes using an acoustic impedance matching medium together with a vibrational freedom air gap. These enhancements guide and amplify the excitation of the PD ultrasonic signals on the sensor diaphragm externally mounted on the transformer oil tank wall, thus enabling effective partial discharge detection from outside the transformer oil tank. The impedance-matching medium has a thickness of 2 mm and a radius of 10 mm, while the vibrational freedom air gap has a depth of 0.2 mm and matches the constrained radius of the sensor diaphragm at 0.9 mm. The diaphragm itself has a thickness of 30μm, with a primary resonance frequency of 109 kHz, and a static pressure sensitivity of 60.01nm/kPa. In terms of detection sensitivity, the external EFPI sensor is twice that of sensors with the same diaphragm size, 3.99 times that of sensors with equivalent detection frequency, and 1.6 times that of conventional PZT sensors. These results underscore the proposed externally mounted EFPI sensor’s exceptional value for engineering applications.