Breakpoint and Moisture Detection Method Based on Distributed Brillouin Optical Fiber Built in Buffer Layer of High-Voltage XLPE Cable

In recent years, the water-blocking tape in high-voltage (HV) crosslinked polyethylene (XLPE) cable has suffered from penetrating moisture 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, with a single-mode fiber of 30 m built in it. Meanwhile, a distributed detection system for built-in optical fiber of 110 kV cable was established and the criterion for Brillouin optical time domain reflectometry (BOTDR) breakpoint detection method was proposed. The comparison test of breakpoint detection methods for built-in optical fiber was carried out using BOTDR and optical time domain reflectometry (OTDR). Besides, detection tests of moisture water blocking tape including its drying process were launched using BOTDR. The relationship of Brillouin frequency shift (BFS) with moisture length, position, and drying process of the tape was studied. The results have demonstrated that BOTDR plays a better role in detecting breakpoint with poor-quality end face than OTDR. By using BOTDR to detect moisture tape in cable, the BFS decreases sharply when moisture length over some thresholds and linearly falls with increasing moisture length. As time passes, the BFS gradually increases, which can characterize drying process.

I. Introduction

Due to safety, reliability, and aesthetical considerations, the electric power industry uses underground power cables in densely populated urban areas for transmission and distribution networks [1]. With advantages such as high dielectric strength and excellent electrical and mechanical performance, it has played a significant role in power systems. 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, a new-type defect of high-voltage (HV) cables has occurred in Australia, Korea, and a large number of cities in China, such as Beijing, Shanghai, and Guangzhou. Defect positions are not in terminations, joints, or insulation, but between the insulation screen and corrugated aluminum sheath, which is usually called buffer layer [see Fig. 1(a)]. It acts as a 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 water migrates into power cable, sodium polyacrylate will swell into gel to prevent its further penetration [3]. 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 the aluminum sheath [see Fig. 1(b)]. Fig. 1.

Diagram of (a) HV XLPE cable and (b) buffer layer defects of cable.