This paper proposes to use time domain reflectometry (TDR) to detect water trees in underground residential distribution (URD) cables. Water trees are very dangerous to underground cables. They can grow for years without any change on the performance of the cable, and then can cause the cable to fault unexpectedly. Therefore, a method to detect water trees in URD cables is proposed in this paper. The method implements an optimal pulse generator placement algorithm on a distribution feeder for performing TDR. It can determine the exact location of a water tree in a cable, and it does not rely on the length of the cable or the type of cable used. The method can also detect multiple water trees on the same cable, and can be installed on existing systems, since it does not require data of the healthy system. It is a periodic monitoring system where pulses are sent throughout the year. The proposed method has been verified using multiple case studies on a real distribution feeder. Using TDR, optimal pulse generator placement, and subtraction of cables' measured signal in time domain with similar measured signals a critical distribution feeder's health has been monitored successfully.INDEX TERMS Time domain reflectometry (TDR), underground residential distribution (URD), water tree.
Water Tree is a corrosion phenomenon in cross-link polyethylene (XLPE) insulation. It is commonly found in underground cables. Water tree induced fault is difficult to detect due to its high impedance and difficult to model due to its random nature. In recent years, underground cables have become more popular in the power industry. They are resistant to environmental damage and has reduced space requirement. They are suitable to areas with high environment hazard or heavily populated areas where space is a constraining factor. As a result, studying and modeling the structure and effect of water tree become increasingly important. Since majority of the fault inducing water trees are vented trees which originated from the surface of the cable insulation, the mathematical model focuses on this particular type of water tree. To reduce the complexity of the model, the shape of the water tree afflicted region of the insulation is assumed to be ellipsoidal and the permittivity of the region is assumed to be linearly changing. Finite element analysis is used to analyze the water tree affected region. The resultant capacitance is calculated and compared with a physical model from Comsol. The result obtained using the proposed mathematical model and the result obtained using physical simulation through Comsol package agrees with each other. Hence, this method can be used to analyze the effect of water tree fault in large power systems.
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