Insulation Properties and Degradation Mechanism for XLPE Subjected to Different Aging Periods

Due to its excellent insulation and mechanical properties, cross-linked polyethylene (XLPE) is widely used as the insulation candidate of power cables in power transmission and distribution systems. However, its performance can be affected due to various stresses, which endangers safe operation of the power system in service. In this paper, dielectric and insulation properties of XLPE materials of 110 kV cables with the service lives of 0, 15, and 30 years were tested and evaluated. Results show that with increase of service life, dielectric loss caused by the polarization process increases significantly, and dielectric strength decreases as well. Power frequency breakdown field strength and crystallinity decrease significantly, and carbonyl index increases. The degradation process of XLPE can be divided into two stages: the structure of the crystalline region of the original sample is relatively complete, and loss can be almost ignored. After long-term service, the crystallization region is destroyed, and thermal oxygen reaction occurs. Intermolecular force is weakened, which makes breakdown easier to occur. However, there is no direct relation between the carbonyl index and macroscopic characteristics. This paper reveals the deterioration law of the XLPE cable insulation, which provides a theoretical basis for evaluating cable aging condition and is of great significance to improve cable operation safety and maintenance quality.