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| Research on Cable Local Defect Detection Method Based on Phase-Sensitive Characteristics of Frequency Modulated Continuous Wave |
| Zhao Shujing1, Zhan Bobo1, Gong Liangtao1, Wang Wei1, Li Chengrong1, Meng Xiaokai2 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. Electric Power Research Institute of State Grid Shanxi Electric Power Company Taiyuan 030001 China |
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Abstract Cables play a very important role in the operation of society and are closely related to industrial production and people's lives. Minor defects develop into serious defects that eventually lead to cable failure. In this regard, it is important to be able to detect and locate defects in cables before they occur. Frequency domain reflectometry (FDR) is developing rapidly because it has higher sensitivity than time domain reflectometry (TDR) in terms of cable defects and fault location. In recent years, the broadband impedance spectroscopy (BIS) method based on FDR has made great progress in the field of cable defect location. The BIS method measures the impedance spectrum of the cable through instruments, and analyzes the impedance spectrum to obtain the location spectrum of the cable. However, the BIS method is easily interfered by noise. In order to improve the anti-noise ability of the FDR method, a frequency modulated continuous wave (FMCW) method based on FDR was developed. With the same background noise, FMCW measurements are better and can locate defects further away.
FMCW has obtained higher sensitivity than BIS method by measuring the frequency difference, but it is still not sensitive enough for the detection of minor defects in cables that are further developed into serious defects due to the influence of operating environment and other factors. In response to this difficulty, inspired by the use of phase characteristics to improve the distance resolution of the FMCW method in the process of high-precision liquid level measurement, we propose a phase-sensitive-FMCW method to improve the sensitivity of the FMCW method to the measurement of cable defect state changes.
First, based on the principle of FMCW cable defect location, the basis for using FMCW phase characteristics to detect defects is proposed: the phase of the mixing signal is affected by the chirp signal, and the phase signal is double modulated by both amplitude and phase. When the reflected signal changes, the phase also changes. Even if the defect position remains unchanged, the strength of the reflected signal will cause the final output signal phase to be different, which is the basic principle that the FMCW signal is sensitive to the defect phase.
Then, the cable defect detection simulation experiment is carried out in combination with the cable distribution parameter model. The simulation test results show that the cable location spectrum based on FMCW can effectively locate the location of cable defects; the cable phase spectrum based on FMCW can effectively reflect the changes of cable defects, and the phase-sensitive characteristics of the phase spectrum can be used to detect the changes of local cable defects. After that, the RF coaxial cable in the laboratory is simulated and tested with different degrees of defects, and the phase change at the defect is observed. There are four types of defects, namely capacitive defects caused by heating, defects caused by copper screen breakage, and resistive defects on the copper shield and conductive defects caused by parallel resistance.
The simulation and experimental results show that: (1) This paper proposes a local defect detection method for cables based on the phase-sensitive characteristic of FMCW, which can improve the sensitivity of the FMCW method to the detection of cable defects. (2) Through simulation, experiment and calculation, the relationship between phase change and defect severity is clarified, the principle of phase sensitive-FMCW method is explained, and the maximum range of phase change at defect is 180°. (3) Taking the ratio of the phase and amplitude changes caused by the defect to the value interval as the sensitivity, the sensitivity of the capacitive defect caused by the local abnormal temperature rise of the cable can be increased from 36% to 71% when the local temperature difference is 37℃. The sensitivity of phase sensitive FMCW method is better than that of FMCW method for resistance, conductivity and copper shield damage defects, which can increase the detection sensitivity by 7%~62%.
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Received: 21 September 2022
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2023, Vol. 38 
