Effect of Moisture on Space Charge Characteristics of Oil-paper Insulation under AC-DC Electric Field

Space charge characteristics of oil-paper insulation with 0.34%-2.72% moisture contents were investigated under AC superposed DC electric field. The phase resolution of space charge detection was 1.79°. Combined with working condition, the superposition ratio of AC rms to DC was set as 1:3. The results show that homocharge accumulates obviously near both electrodes at 3 s polarization, and the density of the homocharge is related to the alternating field, showing a sinusoidal fluctuation. As the moisture content of the sample is within 0.34%-2.72%, the total charge density inside the sample, as well as the conductivity of the sample, increases with the increase of moisture. When moisture content is between 0.34% to 0.98%, homocharge accumulation of the semi-conductive (SC) electrode is dominant in the bulk of the sample. While moisture content is 2.72%, heterocharge accumulation occurs near both electrodes. This may be due to a lower injection barrier and higher charge mobility after the increase of moisture. Furthermore, the electric field distortion rate increases significantly as the moisture increases, and it even reaches 122% for the 2.72%-moisture content sample, which is triple that of the 0.34%-moisture content sample. As the moisture raises, the enhanced space charge accumulation and electric field distortion pose a great threat to the insulating properties of oil-paper insulation and deserve more attention.