Experimental investigation of the impacts of mechanical defects and pollution patterns on leakage current harmonics in composite insulators

Composite insulators are widely used in power transmission and distribution systems due to their high pollution resistance and superior insulating performance. However, the combined effects of surface contamination, high humidity, and mechanical defects can lead to an increase in leakage current (LC) and a degradation of insulation performance. Although previous studies have mainly focused on surface pollution, the combined influence of mechanical defects and non-uniform contamination patterns has received relatively limited attention. This study investigates the effects of pollution type and severity (uniform, ring non-uniform (RNU), and ring-longitudinal non-uniform (RLNU)), ambient humidity, and the location and extent of mechanical defects on the sheds and core of 20 kV silicone rubber insulators, with emphasis on the harmonic components of LC. Results indicate that increasing relative humidity and salt deposit density (SDD) generally lead to higher harmonic amplitudes. In contrast, under RNU conditions, increasing longitudinal non-uniformity leads to a reduction in these harmonic amplitudes compared with uniform pollution. This behavior demonstrates the significant influence of contamination geometry on the harmonic response, which is governed by changes in surface electric field distribution and LC paths. Mechanical defects, particularly those located in the central section, significantly increase harmonic levels, disturb LC paths, and increase the likelihood of surface discharges. In some cases, harmonic amplitude was observed to increase by up to 187.03% compared with healthy samples. Based on these results, a predictive model incorporating SDD, relative humidity (H), and pollution non-uniformity index (P) was developed to enhance condition monitoring and improve the reliability of power system infrastructure.