Understanding Electrical Conductivity Deterioration of Buffer Layer in High-Voltage Cables Under Electrohumid Stress

In high-voltage cables, the buffer layer (BFL), commonly composed of polyethylene terephthalate (PET) and sodium polyacrylate (NaPA) covered by carbon black, can undergo severe deterioration of electrical conductivity under electrohumid stress, which can easily induce serious ablation faults. However, the deterioration process of BFL conductivity is still lack of comprehension, hindering the condition assessment of cables and boosting the risk of faults. This article investigates the deterioration of BFL conductivity under electrohumid stress. Accelerated deteriorating experiments are carried out on dry and humid BFL specimens for 6 h under the ac voltage of 30 V. The current density of BFL first upsurges and then decreases exponentially with time under the electrohumid stress. Simultaneously, gas products of hydrocarbons, carbon oxide, and hydrogen (H2) are generated, the concentrations of which follow exponential evolution trends as well. Under electrohumid stress, the alkoxy groups in PET fibers of BFL are heavily damaged, and phenolic hydroxyl groups are generated as verified by X-ray photoelectron spectroscopy. Furthermore, electrochemical reactions occur in the BFL-Al system under electrohumid stress as verified by Tafel curves. It is proposed that the conductivity deterioration of BFL originates from the damage to conductive networks, which is mainly induced by the thermal decomposition of PET fibers in BFL through the current heating effect and the electrical barrier by high-resistive solids from electrochemical reactions.