| In recent years,a lot of research on acoustic sensing technology based on optical fiber interferometer has been carried out.But most of the research focused on the underwater acoustic detection with low frequency.Compared with underwater acoustic detection,acoustic caused by discharge in gas insulated switch(GIS),power transformers and other power equipment has the characteristics of high frequency,short wavelength,and small amplitude.Moreover,its installation environment is quite complicated.Therefore,higher requirements of acoustic sensors for partial discharge detection are put forward.At present,the research on the optical fiber ultrasonic sensing system based on interferometer for partial discharge(PD)detection is still insufficient.Specifically:1)Disturbance caused by environmental noise is often much greater than the ultrasonic intensity by PD.Thus it is difficult to guarantee the stability of the sensing system under the large noise.For underwater acoustic detection,which is of low frequency,the phase is directly demodulated to avoid interference problems.Unfortunately,the commonly used phase demodulation method is difficult to demodulate high-frequency acoustic.There is a lack of research on the solution of the anti-interference problem of interferometric sensing systems to detect the high-frequency ultrasonic generated by partial discharge of power equipment.2)The intensity of the ultrasonic generated by the PD is weak,which requires the optical fiber ultrasonic sensor of high sensitivity.At present,in the design of the GIS optical fiber acoustic sensor,only the static sensitivity is considered,while the high frequency response performance of the sensor is often ignored.Optimization of the acoustic sensors for high sensitivity GIS PD detection needs further study.3)The large size of power equipment often requires multi-point detection of partial discharge to locate the PD defects.The commonly used time-division multiplexing method in optical hydrophones is costly,and cannot detect PD signal simultaneously.Therefore,this method is not suitable for PD detection in power equipment.At present,multiplexing optical fiber ultrasonic sensing network is still a challenge.In order to solve the above problems,the following researches are carried out in this paper:1)Aiming at the anti-interference problem of the interferometric sensing system,an anti-interference method based on the phase tracking feedback control technology was proposed.This method extracts the error signal to form feedback information,and compensates the phase of the interference signal.This method can ensure that the system response keeps at the highest value.Therefore,the sensing system does not need to demodulate the phase of the interference signal accurately.The output light intensity can directly reflect the ultrasonic signal.Different from the traditional methods,the limitation on the frequency of measured signal can be avoided.Through the optimized control loop,the Michelson interference optical fiber ultrasonic sensing system with phase tracking feedback could suppress the interference caused by low frequency noise below 1.8 kHz.The test results showed that this method can significantly improve the stability of the sensing system.2)In view of the design of high-sensitivity GIS optical fiber ultrasonic sensor,the sensitivity model in frequency domain of the mandrel-type optical fiber ultrasonic sensor was established.Based on this model,the sensor can be optimized to reach peak sensitivity in the frequency band(near 40 kHz)where the ultrasonic concentrated by GIS discharge.The test results showed that the center frequency of the developed mandrel-type optical fiber ultrasonic sensor was 45 kHz.In the frequency range of 20 kHz?80 kHz,the average sensitivity of the optical fiber ultrasonic sensor was 85.8 dB,and the peak sensitivity was 93.0 dB,which was 31.8 dB(38.9 times)and 33.0 dB(44.6 times)higher than the traditional PZT sensor,respectively.In addition,the actual GIS PD detection results showed that the developed optical fiber ultrasonic sensor could detect weak PD signals more effectively than traditional PZT sensor.3)In order to achieve high-sensitivity PD detection in transformer,this paper has designed a fiber coil without mandrel as the transformer optical fiber ultrasonic sensor.The sensitivity model in frequency domain of the optical fiber ultrasonic sensor was established.Based on the model,the effect of the fiber coil size and the angle of the incident acoustic on the sensor sensitivity was analyzed.Moreover,an experimental setup for acoustic sensor sensitivity test was built up.the sensitivity model was verified by experimental testing.Through theoretical analysis and experimental testing,the fiber coil was optimized.The test results showed that the average detection sensitivity of the optimized ultrasonic sensor is 59.2 times of that of PZT sensor.For the same discharge defect,the applied voltage for inceptive partial discharge detection of the optical fiber ultrasonic sensor can be 22.0%lower than that of PZT sensor.4)For the multiplexed Michelson interference optical fiber ultrasonic sensing system,a multiplexing method based on the frequency division multiplexing was proposed in this paper.This method used optical swept interference technology to modulate the ultrasonic information sensed by each sensor into carrier signals with different frequencies.An appropriate sensing fiber length can be set based on the Carson bandwidth rule to ensure that each carrier signal does not alias.Then,through the band-pass filtering technology,each carrier signal can be separated and demodulated to obtain ultrasonic information.Experimental results showed that the multiplexed optical fiber ultrasonic sensor system can perform multi-point detection simultaneously for ultrasonic generated by partial discharge in GIS and transformers.And the discharge defect can be located with high-precision.The positioning accuracy can reach the order of centimeters. |
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