Location of Cable Joint with Moisture Based on Short-Circuit Response

Cable joint is the most vulnerable part of power cable. Since cables are often used underground or underwater, the moisture can easily enter into cable joints and make it damp, which is a common defect of XLPE power cables. If they are not repaired in time, they will lead to premature failure of the cable. The broadband impedance spectroscopy (BIS) is a technique being developed in recent years which may become an effective method for diagnosing cables with local degradation. However, the BIS equipment is expensive and it takes a long time to test. Therefore, base on the frequency domain reflection theory and the cable impedance discontinuity point positioning principle, this paper proposes a short-circuit response method for locating the damp joint as impedance discontinuity in cables, which is an inexpensive and speedy method. Because of the energy storage characteristics of cable as a large capacitance, firstly the cable can be charged with DC and then discharge to the ground through a quite small resistance, which will generate a particularly steep wave. The steep wave contains many high-frequency signals. By calculating the spectrum of voltage and current, the cable's broadband impedance spectrum also can be obtained. Further, after calculating the imaginary part of impedance spectrum by fast fourier transform (FFT), it can be clearly seen that the location of damp joint is detected. Finally, it is analyzed that the influence of the background noise of the system on the positioning accuracy and sensitivity. Simulation results show that the short-circuit response method is effective for locating defects and the wavelet domain denoising or increasing the voltage can effectively improve the location results.