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.
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2023, Vol. 38 
