Abstract：Real-time monitoring of transformer status is essential for the stable operation of the power system. Immunity to electromagnetic interference, high sensitivity and high insulation enable optical fiber sensing to be widely used in power transformer monitoring. Among them, heterodyne interferometer method uses optical frequency shifter to modulate signal to high frequency. This allows the effects of environmental disturbances on interferometer to be effectively suppressed and brings the advantages of high sensitivity and lower minimum detectable limit. The frequency of optical heterodyne interferometer sensing signal is hundreds of megahertz. Traditional software demodulation method has the disadvantages of high cost and large data volume. Thus, the optical heterodyne interferometer is restricted in the detection of partial discharge ultrasound. This paper proposes a phase demodulation method based on dual phase detectors, which reduces the sampling rate and data volume, and realizes real-time demodulation. Firstly, the characteristics of the phase detector is analyzed. When the phase difference of two input signal is around 0° and ±180°, the output voltage has a nonlinear relationship with the phase difference. To avoid of the nonlinear region, a phase demodulation method based on dual phase detectors was proposed. Whenever a phase detector works at nonlinear region, the other will provide accurate measurement of phase signal. Secondly, bandpass filters are connected to the phase detectors. The low-frequency carrier with high amplitude is filtered out and the weak high-frequency phase voltage signal is preserved, so that the measuring range can be reduced to improve the sampling accuracy. Thirdly, both output signals of the two band-pass filters have attenuation and distortion because of nonlinear regions, but the nonlinear regions of the output signals are staggered from each other. An algorithm to avoid nonlinear regions is used to obtain a stable signal. The algorithm uses the method of envelope to identify the position of the nonlinear region of one output signal. Then the nonlinear regions are compensated with the linear regions of another output signal. The key performance parameters of the phase demodulation method such as signal-to-noise ratio (SNR), linearity and minimum detectable limit (MDL) were tested. The result shows that the SNR can reach to 36.4 dB (the phase modulation depth of test signal is 0.873 mrad), and the minimum detectable limit of 0.005° is achieved. The phase modulation depth and the amplitude of output voltage have a good linearity. The optical Mach-Zehnder interferometer ultrasonic sensing platform was built, and the comparative testing indicates that the dual phase detectors demodulation method is 15 dB higher in SNR and 58% lower in MDL than traditional differential cross multiplying (DCM) software demodulation method. Then, the experimentations of partial discharge ultrasound detection have been done. When the test voltage was gradually increased, the ultrasonic signal of partial discharge was detected by the dual phase detectors demodulation method at lower test voltage. And it only takes 0.8 s to demodulate sensing signal with a time length of 1 s. The following conclusions can be drawn: (1) The dual phase detectors phase demodulation method has excellent response linearity in the standard ultrasonic signal test, and the SNR is 15 dB higher than DCM software demodulation. (2) The MDL of the dual phase detectors phase demodulation method reaches to 0.005°, which meets the requirements of weak phase signal demodulation in the detection of partial discharge ultrasound. (3) The dual phase detectors phase demodulation method uses the phase detectors to reduce the frequency of the interference signal, and greatly reduce the sampling rate and the amount of data. This makes the dual phase detectors phase demodulation method 1 124 times faster than DCM software demodulation method. Therefore, a real-time phase demodulation method with high signal-to-noise ratio and low minimum detectable limit was proposed for the detection of partial discharge ultrasound.
[1] Ma Guoming, Wang Yuan, Qin Weiqi, et al.Optical sensors for power transformer monitoring: a review[J]. High Voltage, 2021, 6(3): 367-386. [2] 陈伟根, 张知先, 李剑, 等. 电气设备状态参量智能传感技术[J]. 中国电机工程学报, 2020, 40(增刊1): 323-342. Chen Weigen, Zhang Zhixian, Li Jian, et al.Intelligent sensing technology for power equipment state parameters[J]. Proceedings of the CSEE, 2020, 40(S1): 323-342. [3] 江军, 马国明, 宋宏图, 等. 基于侧边抛磨光纤布拉格光栅的变压器油中溶解氢气传感器[J]. 电工技术学报, 2017, 32(13): 264-270. Jiang Jun, Ma Guoming, Song Hongtu, et al.Dissolved hydrogen sensor in power transformer oil based on side polishing fiber Bragg grating[J]. Transactions of China Electrotechnical Society, 2017, 32(13): 264-270. [4] 刘云鹏, 李欢, 田源, 等. 基于分布式光纤传感的绕组变形程度检测[J]. 电工技术学报, 2021, 36(7): 1347-1355. Liu Yunpeng, Li Huan, Tian Yuan, et al.Winding deformation detection based on distributed optical fiber sensing[J]. Transactions of China Electrotechnical Society, 2021, 36(7): 1347-1355. [5] 陈起超, 张伟超, 白仕光, 等. 非本征光纤法-珀传感器局部放电检测研究进展[J]. 电工技术学报, 2022, 37(5): 1305-1320. Chen Qichao, Zhang Weichao, Bai Shiguang, et al.Research progress of extrinsic fiber Fabry-Perot interferometer sensor in partial discharge detection[J]. Transactions of China Electrotechnical Society, 2022, 37(5): 1305-1320. [6] 张楠, 孟洲, 饶伟, 等. 干涉型光纤水听器数字化外差检测方法动态范围上限研究[J]. 光学学报, 2011, 31(8): 92-98. Zhang Nan, Meng Zhou, Rao Wei, et al.Analysis on upper limit of dynamic range of fiber optic interferometric hydrophone using digital heterodyne detection scheme[J]. Acta Optica Sinica, 2011, 31(8): 92-98. [7] 姚雨杭, 潘成, 唐炬, 等. 交直流复合电压下流动变压器油中金属微粒运动规律和局部放电特性研究[J]. 电工技术学报, 2021, 36(15): 3101-3112. Yao Yuhang, Pan Cheng, Tang Ju, et al.Motion behaviors and partial discharge characteristics of metallic particles in moving transformer oil under AC/DC composite voltage[J]. Transactions of China Electrotechnical Society, 2021, 36(15): 3101-3112. [8] 马星河, 张登奎. 基于改进经验小波变换的高压电缆局部放电噪声抑制研究[J]. 电工技术学报, 2021, 36(增刊1): 353-361. Ma Xinghe, Zhang Dengkui.Research on suppression of partial discharge noise of high voltage cable based on improved empirical wavelet transform[J]. Transactions of China Electrotechnical Society, 2021, 36(S1): 353-361. [9] 李泽, 王辉, 钱勇, 等. 基于加速鲁棒特征的含噪局部放电模式识别[J]. 电工技术学报, 2022, 37(3): 775-785. Li Ze, Wang Hui, Qian Yong, et al.Pattern recognition of partial discharge in the presence of noise based on speeded up robust features[J]. Transactions of China Electrotechnical Society, 2022, 37(3): 775-785. [10] Zhou Hongyang, Ma Guoming, Qin Weiqi, et al.A multiplexing optical partial discharge sensing system for power transformer using a single photodetector[J]. IEEE Transactions on Power Delivery, 2021, 36(3): 1911-1913. [11] 史荣斌, 林仲钦, 秦炜淇, 等. 基于有源光纤光栅阵列的局部放电声发射检测与定位技术[J]. 中国电机工程学报, 2023, 43(1): 358-368. Shi Rongbin, Lin Zhongqin, Qin Weiqi, et al.Partial discharge acoustic emission detection and localization based on active fiber grating array[J]. Proceedings of the CSEE, 2023, 43(1): 358-368. [12] 周宏扬, 马国明, 张猛, 等. 基于迈克尔逊光纤干涉的变压器局部放电超声信号检测技术[J]. 中国电机工程学报, 2022, 42(21): 8016-8025. Zhou Hongyang, Ma Guoming, Zhang Meng, et al.Partial discharge ultrasonic signal detection technology in power transformer based on the Michelson optical fiber interferometer[J]. Proceedings of the CSEE, 2022, 42(21): 8016-8025. [13] He Haijun, Yan Lianshan, Qian Heng, et al.Enhanced range of the dynamic strain measurement in phase-sensitive OTDR with tunable sensitivity[J]. Optics Express, 2020, 28(1): 226-237. [14] Jiang Fei, Li Honglang, Zhang Zhenhai, et al.Undersampling for fiber distributed acoustic sensing based on coherent phase-OTDR[J]. Optics Letters, 2019, 44(4): 911. [15] Ren Baokai, Cheng Jin, Zhao Longjiang, et al.Research on the frequency response and dynamic range of the quadrature fiber optic fabry-perot cavity microphone based on the differential cross multiplication demodulation algorithm[J]. Sensors, 2021, 21(18): 6152. [16] Zhang Nan, Rao Wei, Meng Zhou, et al.Investigation on the maximum signal handling capability of fiber optic interferometric sensor based on the digital heterodyne demodulation scheme[J]. Optik, 2014, 125(19): 5771-5775. [17] Posada J E, Garcia-Souto J A, Rubio-Serrano J. Multichannel optical-fibre heterodyne interferometer for ultrasound detection of partial discharges in power transformers[J]. Measurement Science and Technology, 2013, 24(9): 094015. [18] Yao Chenyu, Gao Shoufei, Wang Yingying, et al.Heterodyne interferometric photothermal spectroscopy for gas detection in a hollow-core fiber[J]. Sensors and Actuators B: Chemical, 2021, 346: 130528. [19] Mohamadou Y, Momo F, Theophile L, et al.Accuracy enhancement in low frequency gain and phase detector (AD8302) based bioimpedance spectroscopy system[J]. Measurement, 2018, 123: 304-308. [20] Vieira J M, Fabiani B M, Silveira E S, et al.Calibration procedures for microwave vector comparators[J]. Sensors and Actuators A: Physical, 2021, 331: 112955. [21] Chen Zhao, Zhao Yanping, Chen Gen, et al.Design and implementation of power and phase feedback control system for ICRH on EAST[J]. Nuclear Science and Techniques, 2018, 29(2): 1-8. [22] Grossmann J, Suslov A, Yong G, et al.Highly sensitive simple homodyne phase detector for ultrasonic pulse-echo measurements[J]. Review of Scientific Instruments, 2016, 87(4): 044901. [23] Shrivastava A, Gupta V.Methods for the determination of limit of detection and limit of quantitation of the analytical methods[J]. Chronicles of Young Scientists, 2011, 2(1): 21-25.
2023, Vol. 38 
