Effects of moisture on space charge behavior in epoxy/paper double layer

There are more and more converter transformers, bushings, smoothing reactors use epoxy resin or epoxy/insulating paper composite materials as the main insulating medium, in the UHVDC transmission projects. In practice, there are many interfaces between epoxy and insulating paper exist in paper/paper insulation system. Compared with epoxy resin, there are a lot of traps in paper. The structure of the insulating paper is unconsolidated. So, the insulating paper is easy to moisture absorbing. And because of the imperfect of casting process, the epoxy/paper composite easy to contain water. Therefore, it is necessary to study the space charge characteristics of paper/epoxy composite materials under the influence of moisture. The epoxy/insulating paper composite samples with thickness about 360 μm were obtained by carrying out the vacuum curing process. Sample 1# vacuum drying treatment (100 °C, 24 h) was utilized to remove the moisture. Sample 2# was placed in open air for 5 days. Both voltage-on and voltage-off space charge tests were performed for 1h by Pulsed Electro-Acoustic (PEA) method. Compare the space charge contour of 2 samples, the measurement results are obvious different. On voltage-off space charge tests, the injected charges have been captured by the interface traps. In the effect of applied electric field, water molecules ionization is easy to occur. At the same time, the presence of moisture promoted the dissociation of cellulose molecules -OH, and large amounts of negative charges. The negative charges have been blocked by interface. At 100°C, the cathode inject a large number of electron. As the electrical conductivity of impregnated paper increase interfacial polarization effect is stronger than the charge injection effect.