Highly Sensitive Optic Fabry-Perot Ultrasonic Sensor for Power Transformer Partial Discharge Detection

Optical sensing has been widely applied in the condition monitoring of power equipment due to its advantages, such as high insulation, anti-interference capability, and high sensitivity. However, for partial discharge detection, the sensitivity and applicability of optical sensors still need to be further improved to ensure their practical application. This paper introduces a silicon-grooved diaphragm-based fiber-optic Fabry-Perot (F-P) ultrasonic sensor that was designed and fabricated for partial discharge detection in power equipment such as power transformers. The groove parameters of the sensing diaphragm were optimized using finite element software. Compared to traditional circular diaphragms, the static sensitivity of the silicon-grooved diaphragm was improved by 4.09 times, while the resonant frequency remained essentially unchanged. The influence of the F-P cavity length on the contrast of the sensor’s interference spectrum was investigated by coupling efficiency to modify the traditional dual-beam interference model, thereby enhancing the sensor’s acoustic pressure sensitivity. The silicon grooved diaphragm was fabricated using micro-electro-mechanical system (MEMS) technology, with a groove diameter of $829.44~\mu $ m, a thickness of $2.09~\mu $ m, and an F-P cavity length of $163.600~\mu $ m. At the resonant frequency of 61.5 kHz, the sensor achieved an acoustic pressure sensitivity of 357.78 mV/Pa. The performance of the sensor was validated by testing in a power transformer with three types of partial discharge defect models. Experimental results demonstrate that the fabricated fiber-optic F-P ultrasonic sensor offers high acoustic pressure sensitivity, good real-time performance, and capabilities in detecting ultrasonic signals. In addition, the developed sensor maintains structural integrity and can function after long-term usage in a transformer environment.