Distributed Optical Fiber Detection Based on BOTDR for Buffer Layer Defects of High-Voltage XLPE Cable With Corrugated Aluminum Sheath

In recent years, buffer layer defects have occurred in high-voltage (HV) crosslinked polyethylene (XLPE) cable frequently, which affects its safe and stable operation, while there are no effective detection methods yet. In this article, a 110 kV cable sample was prepared, embedded with single-mode optical fiber. Meanwhile, a distributed optical fiber detection system for buffer layer defects based on Brillouin optical time domain reflectometry was built. The partial discharge (PD) test and Brillouin optical time-domain reflectometer (BOTDR) detection test for defective cable were conducted to study the relationship between Brillouin frequency shift (BFS) variation of built-in fiber and temperature rise resulting from dielectric loss, white powder, and damaged insulation. During applying voltage of 96 kV for 7 h, the BFS variations within white powder and damaged insulation defects are significantly larger than that within non-defect which is only determined by dielectric loss. The BFS variations are closely related to the number and depth of damaged insulation holes, as well as circumferential angle of holes against optical fiber. But the BFS variations within moisture water blocking tape, poor electrical connection, and damaged insulation screen cannot be distinguished from that within non-defect.

I. Introduction

Due to safety, reliability, and aesthetical consideration, the electric power industry uses underground power cables in densely populated urban areas for transmission and distribution networks [1]. With advantages such as low dielectric constant, high dielectric strength, and excellent electrical and mechanical performance, it has played a significant role in power system. According to the statistics of CIGRE TB 815, by the end of 2019, the worldwide length of crosslinked polyethylene (XLPE) land cables above 110 kV is 14 353 km [2], and the demand is still increasing. However, in recent years, the new-type defect of high-voltage (HV) cables have occurred in Australia [3] and a large number of cities in China, such as Beijing, Shanghai, and Guangzhou. The defects are not located in terminations or joints, but between insulation screen and corrugated aluminum sheath, which is usually called buffer layer (see Fig. 1). It acts as longitudinal water blocking layer and has other functions such as reducing stress extrusion and maintaining excellent electrical connection between insulation screen and aluminum sheath. The water blocking tape is a kind of fluffy polyester fiber containing super absorbent resin, whose main component is sodium polyacrylate (C₃H₃NaO₂)_n. When moisture migrates into power cable, (C₃H₃NaO₂)_n will swell into gel to prevent its further penetration [4]. After dissecting the defective cables, it is found that there are ablation holes on the outer surface of insulation screen, white powders on the water blocking tape, and corrosion traces on the inner surface of aluminum sheath (see Fig. 1). It is known that white powders are generated by the reaction between damp water blocking tape and aluminum sheath after moisture penetrating into the cable [5]. Because the tape is fluffy and porous, swelled water blocking powders are prone to contact with aluminum sheath to react after absorbing moisture. As time goes by, the moisture in water blocking powders gradually evaporates, leaving white powders on the tape and corrosion traces on the aluminum sheath. Since white powders are of high resistance [6], their appearance disrupts the excellent electrical connection between insulation screen and aluminum sheath [7], which results in partial discharge (PD) [8], [9] or uneven distribution of capacitive current [10]. The maximum withstand temperature of buffer layer of cable is 200 °C [11], when the temperature exceeds this threshold, the overheating decomposition will occur. Finally, the defect developing in the long term will result in premature failure of HVac XLPE cables within ten years, a duration significantly shorter than their designed lifespan. Therefore, as the features of buffer layer defects, white powders, and damaged insulation are supposed to be paid special attentions. Fig. 1.

HVac XLPE cable with corrugated aluminum sheath and buffer layer defects.