All-fiber intrinsic sensor of partial discharge acoustic emission with electronic resonance at 150 kHz

An acoustic emission sensor was designed for partial discharges (PD), constructed, calibrated, and tested. The device is based on an interferometric fiber-optic probe and is able to detect PD ultrasound emission at 150 kHz. It may be placed next to the discharge source, thus overcoming the difficulties of acoustic attenuation. The device works in the appropriate bandwidth for narrowband acoustic detection of PD activity, like the PZT transducers mounted on the exterior of the transformer tank. It represents a simple and cheap alternative for detecting acoustic emission, susceptible of being used in a multi-channel optical configuration and able to provide information for locating the source. The sensor is an optical fiber coil exposed to the ultrasonic waves that is interrogated with an all-fiber Mach-Zehnder interferometer. We report first the calibration at the natural frequency of the coil (20 kHz) and at the main frequency of the application (150 kHz), and compared with the response of PZT transducers. The sensitivity decays with the frequency, but it is comparable with the PZT sensitivity by placing the sensor next to the source, which is possible with the immersed approach (or embedded). A certain range of compensation is obtained at low frequencies with a feed-back loop. A second feed-back loop with electronic resonance around 150 kHz is used in order to improve the sensitivity. Thus, the conditioning circuit provides directly the amplified optical phase signal. Results of acoustic emission with both frequencies simultaneously are presented.