Ultrasonic Detection For Partial Discharge In Transformer By Using Fiber Optic Sensing

Effective monitoring of partial discharge can find the defects of insulation in power transformers in time and avoid insulation faults. Ultrasonic detection is one of the most important non-electric testing technologies for partial discharge. However, the traditional ultrasonic detection technology has the drawbacks of low sensitivity, low detection efficiency and poor ability of on-line monitoring in the field application. To solve the problems mentioned above, a novel ultrasonic testing system based on optical fiber sensing technology was proposed for partial discharge detection of power transformers.In this paper, the response characteristics of the fiber Bragg grating (FBG) to ultrasonic was analyzed. The correlation between the wavelength shift of FBG and the amplitude and frequency of the ultrasonic was obtained. The ultrasonic testing system based on optical fiber sensor was proposed for partial discharge in transformers, the detection principle of the system was researched. According to the principle, the detection amplitude formula of the system was concluded. And the key factors determining the system sensitivity mainly included the slope of the linear zone of FBG spectral, optical power and so on.The characteristics of different types of fiber grating were studied and sensitivity test was carried out to select the high sensitivity sensor. Results of the experiment showed that he average detection sensitivity of Phase-Shifted Fiber Bragg Grating (PS-FBG) was 13.63 dB higher than the ordinary FBG and 8.46 dB higher than Piezoelectric Transducer (PZT) in the bandwidth of 50 kHz-400 kHz. The sensitivity characteristic of PS-FBG was conducted. The results showed that PS-FBG had a directional sensitivity distribution and axial of its gate area had higher detection sensitivity.The problem of stress and temperature cross-sensitivity of the ultrasonic testing system based on optical fiber sensing technology was researched. The method of directly wavelength scanning was put forward to compensate the temperature changes, which was suitable for PS-FBG ultrasonic detection system. To improve the efficiency of wavelength scanning, this method was optimized based on the principle of cross-correlation. And the experiment shows that the optimized solution meet the demand of temperature compensation in the process of ultrasonic detection.To test the performance of PS-FBG ultrasonic testing system, partial discharge detection test in transformer oil was conducted. Suspension model and needlepoint corona model were put in transformer oil and used as partial discharge source respectively. The detection sensitivities of the PS-FBG ultrasonic testing system and the Piezoelectric Transducer sensor were compared. Experimental results showed that PS-FBG could not only detect the ultrasonic signal produced by partial discharge model, but also its sensitivity was higher than the traditional PZT sensor.