How to Use the Velocity of Electromagnetic Wave to Diagnose the XLPE Cable Insulation

Abstract: The research on the diagnosis of XLPE power cable insulation is of great importance for learning the cable insulation aging status and ensuring the power transmission system in safe and reliable operation. A new method was put forward in the paper, that is, using the transmission velocity of electromagnetic wave in an XLPE cable to determine whether the aging happens in the cable. The principle and theoretical basis are illustrated followed by the introduction of test platform. Two cables with different levels of water treeing aging are measured by the proposed method. During the process of handling the data, how to get the round-trip time effectively is also determined. Results indicate that using electromagnetic wave velocity to diagnose the XLPE cable insulation can reflect the insulation status of the two cables. The comparison between the real value and theoretical value shows that the method is of high accuracy and reliability, which can provide new ideas for the diagnosis of XLPE cable insulation. 

Key words:  XLPE cable, insulation diagnosis, signal detection, dielectric constant, water treeing, refraction and reflection      

Introduction

The XLPE power cables have been widely applied in the power transmission system due to many advantages including high reliability, small floor area, simple operation and less maintenance work. The reliability and operation condition of power cable is directly related to the safety and stability of the whole power system. In order to learn the aging of cable insulation and avoid the accidents, it becomes increasingly important to use the diagnosis test to conduct the evaluation of cable conditions. Research indicates that the water treeing is a major reason for decrease in XLPE cable insulation, and that is inevitable. Under certain conditions, low AC electric field can make some areas develop and form water treeing. The amount of water treeing serves as an important parameter of reflecting the insulation aging of cable. Common methods include DC component method, DC superposition method, low frequency superposition method, tanδ online detection method, PD online detection method, earthing line current method and harmonic component method. However, not many methods of XLPE cable insulation diagnosis are massively applied in real projects.