Influence of insoluble substance hygroscopicity on insulator leakage current

External insulation pollution flashover causes serious damage to the security and stability of power grid. Pollution flashover prediction is prominent in the study on external insulation. Insulator leakage current is most widely used in the pollution flashover prediction. Wet contaminant is an important precondition for pollution insulation discharge. Wet process is affected by the atmospheric environment and the hygroscopicity together. This paper analyzed the moisture absorption of insoluble substances, and chose aluminium phosphate as the strongly hygroscopic material to join the composition of artificial contaminants. The discharge test of polluted insulation was performed in constant humidity condition. The test was performed respectively in the humidity of 55%, 60%, 65%, 70%, 75% and 80%. Some leakage current characteristics were investigated to point out the influence of hygroscopicity of insoluble substances on insulator leakage current, such as current amplitude, fundamental current amplitude, 3rd harmonic current amplitude and phase. Test results show that insulator leakage current is affected by insoluble substance hygroscopicity, and extent depends on atmospheric humidity and the amount of strongly hygroscopic material. As the amount of aluminium phosphate exceeds 20% and the humidity exceeds 70%, the leakage current characteristics of aluminium phosphate sample is much different from that of normal sample. The leakage current amplitude, fundamental current amplitude and 3rd harmonic current amplitude of aluminium phosphate sample is much larger than that of normal sample and the phase is smaller. The leakage current amplitude, fundamental current amplitude, 3rd harmonic current amplitude and phase can reflect the difference of the hygroscopicity of pollution layer. Among them, the phase between fundamental current and voltage is the most sensitive to the hygroscopicity of pollution layer. The strongly hygroscopic material will absorb more moisture under a certain humidity, the dissolution of electrolyte of pollution layer will be promoted, leakage current will increase and the atmospheric humidity of discharge will be reduced.