Interface Polarization Behaviors of Water Tree Aged XLPE Cable During PDC Measurement

Understanding the dielectric response of water tree aged cross-linked polyethylene (XLPE) has great significance in diagnosing the insulation failure of power cables. In this work, the polarization and depolarization current (PDC) characteristics of XLPE cables with different degrees of water tree aging are investigated. The conduction current, calculated as the difference between the PDCs, is analyzed. A novel phenomenon is that the conduction currents of aged samples increase first and then gradually decrease to a stable level over time. A model based on the nonlinear dielectric response theory of water trees is established to explain this behavior, attributing it to asymmetric charge accumulation and dissipation at the water tree-XLPE interface during the charging and discharging stages. Further analysis reveals the characteristics of the interface polarization current component during PDC measurement. During polarization, the interface polarization current first increases and then decreases, while the relaxation time constant first decreases and then increases. During depolarization, the interface polarization current gradually decreases with the increased relaxation time constant. To evaluate the degree of water tree aging in XLPE cables, the interfacial polarization coefficient K is defined. A K value greater than 1 indicates the presence of water trees in the cable. Moreover, the K value increases with the degree of water tree aging, and increasing the test voltage can enhance detection.