Analysis of Leakage Current Characteristics and Defect Fusion Diagnosis of Cable Water-Blocking Buffer Layers Under Multi-Frequency Excitation

The water-blocking buffer layer of high-voltage cross-linked polyethylene (XLPE) power cables is prone to degradation during long-term operation due to factors such as moisture and ablation. The degradation may cause an increase in leakage current, which is affected by frequency. However, the correlation between buffer layer status and leakage current under various frequencies needs further investigation. Accordingly, this paper investigates leakage current characteristics of cable water-blocking buffer layers with different statuses under multi-frequency excitation. Firstly, a multi-frequency leakage current test platform was established. Afterwards, composite specimens consisting of XLPE insulation and a water-blocking buffer layer were prepared in four different states: normal, moisture-affected, slightly ablated, and severely ablated. The leakage current responses of different specimens under various voltage frequencies were systematically examined. The results show that, under moisture-affected conditions, the leakage current increases by 15–30% compared with the normal state. The voltage–current phase difference decreases significantly in the low-frequency range by 9–64°. The total harmonic content increases markedly as the frequency decreases, with the 3rd, 5th, and 7th harmonics exhibiting the highest sensitivity. Ablated samples have limited influence on the leakage current. But localized spikes appear in the waveform. Under normal conditions, the leakage current waveform follows the applied sinusoidal voltage, forming a natural symmetry in both frequency and time domains. However, moisture ingress and ablation defects break this symmetry by introducing harmonic distortion in the frequency domain and pulse-shaped spikes in the time domain, respectively. The method utilizes low-frequency leakage-current harmonics and harmonic–residual spikes to identify buffer layer moisture and ablation defects. The proposed method enables the effective identification of different buffer layer states and provides a technical approach for the defect diagnosis of water-blocking buffer layers in high-voltage cables.