Fiber Extrinsic FP Sensor For Detection Of PD-induced Acoustic Emission In Liquid Dielectrics

Acoustic emission detection technology is an effective way to monitor sudden failure caused by partial discharge (PD) in liquid-solid composite insulation of electrical equipment. Compared to piezoelectric acoustic sensors, fiber optic sensor buried in the liquid medium can detect PD-induced acoustic directly. The signal transmission and sensing process are not subject to electromagnetic interference from the environment. Extrinsic fiber Fabry-Perot (FP) sensor is made of a diaphragm as coupling components to acoustic and light, end face of optical fiber and a cavity composed. The sensor has compact and flexible structure without polarization component, which has potential prospect for PD detection in liquid. Research in the paper was carried out about structure design of FP sensor and preparation, characteristic description, simulation and experiment to the sensor and its component. The effect of environmental factors and elimination methods were studied.Structure design method of extrinsic fiber FP sensor is set up for monitoring PD in liquid medium. The relationship was obtained by simulation and ultrasonic excitation between the sensitivity and first-order natural frequency of the sensor and structure parameters of the diaphragm. FP sensors were made of pigtail and silicon diaphragm by MEMS or quartz diaphragm respectively. The structural parameters and performance changes of components of the sensor were studied to improve the sensitivity. Reason about the additional modulation in spectral characteristics of FP sensor were analysed and the solutions were put forward.Drift of operating point of the sensor resulted in decreased gain and distorted waveform by acoustic excitation. The laws of change of the operating point on the sensor characteristic curves was obtained by theoretical estimates and experimental tests with the change of cavity length caused by thermal expansion of the sensor structure. Modulation and demodulation methods of FP sensors on the acoustic signal were investigated, which focused on the significance of operating point in the intensity modulation.A distribution source with tunable center wavelength was put forward to launch the sensor in order to track automatically operating point with changes of FP cavity length caused by temperature and hydrostatic pressure. Drift of the operating point of the sensor by changes of ambient temperature and hydrostatic pressure result in degradation of transmission characteristics. According to interference principle FP sensor, characteristic curve of FP sensor can be changed by the source with tunable center wavelength. This technology can be able to ensure the ideal point, and the source and the experimental system were built up to test the effect of adjusting operating point.The structure parameters of the sensor were determined to detect PD-induced acoustic emission. Seven types of sensors with different materials, structural parameters and different end-reflectance were prepared to detect PD-induced acoustic signals in the system set up in the lab. The test was achieved in the operating point-adjusting systems with the source of tunable center wavelength. PD-induced acoustic signals were test under different discharge voltage. The effects on PD-induced acoustic detecting of the structure parameters and frequency characteristics of the sensor, such as thickness and effective area of the diaghragm, face reflectivity, and natural frequencies were analyzed. FP sensors with 90% end reflectivity of the pigtail and 4mm×4mm×60μm silicon diaphragm with gold coating film were able to detect 0.3～130kpC of discharge level at 2cm from discharge source in the laboratory test.