1. Influence of Copper on the By-products and Dielectric Properties of Different Oil-paper Insulations Jian Hao 1, 2, Ruijin Liao 1, George Chen 2 and Chao Ma 3 1 University of Chongqing, Chongqing, China 2 University of Southampton, Southampton, UK 3 Gansu Electic Power Reaserch Institute, China Introduction Conclusions Transformer failure caused by the corrosion of copper material in transformer attracts great attention of researchers and engineers. In this paper, 25# naphthenic mineral oil, 25# paraffinic mineral oil, Kraft paper and copper were used to compose four combinations of oil- paper insulation samples. The ageing by-products and dielectric properties of the four combinations of oil-paper insulation samples were compared after being thermally aged at 130°C for 77 days. The influence of copper on the by-products and dielectric properties of different oil-paper insulations was obtained. cquhaojian@126.com University of Southampton, Highfield, Southampton, SO17 1BJ, UK Contact details :  Copper can accelerate the ageing rate of insulating oils and reduce their AC breakdown voltage. The content of copper substance dissolved in insulating oil increases with ageing time at first and then decreases.  Copper has great effect on the moisture content of oil impregnated paper in the ageing process. SEM results show that there is copper product deposited on the insulating paper surface, which will deteriorate the dielectric properties of oil-paper insulation. Figure 7. Color of innermost paper in NK, NKC, PK and PKC (from left to right) (c) tanδ of oil impregnated paper (d)  v of oil impregnated paper Experiments Figure 1. Pretreatment of samples The pretreatment of samples and the flow chart of ageing test are shown in Figure 1. After being pretreated, the moisture content of paper or paper-wrapped copper strip is 0.70%. The oil samples have a moisture content of 9 mg/kg. 400 ml oil and the paper (7.5 g) or paper- wrapped copper strip (paper: 7.5 g, copper strip: 215 cm 2 ) were placed in the same glass vial (500 ml), respectively, as presented in Figure 2. SampleSample composition NK naphthenic oil ＋ Kraft paper NKC naphthenic oil ＋ Kraft paper ＋ copper PK paraffinic oil ＋ Kraft paper PKC paraffinic oil ＋ Kraft paper ＋ copper Table 1. Samples used for thermal ageing test Experimental Results Figure 3. Changes of oil acidity during oil-paper insulation samples ageing process Oil Property: Paper Property: Figure 6. Water content in paper varying with ageing time Figure 4. Breakdown voltage of oils versus ageing time  Figure 3 shows that NKC samples with copper have higher oil acidity than NK samples without copper all the time. However, PKC samples with copper have lower oil acidity than PK samples without copper initially, after aged 29 days the oil acidity of PKC samples becomes higher than that in PK samples.  Figure 4 indicates that the existence of copper substance in oil-paper insulation during the ageing process weakens oil AC breakdown property. Naphthenic oil performs better than paraffinic oil under the same ageing condition. (a) tanδ of oil (b)  v of oil Figure 9. Dielectric loss and volume conductivity of insulating oil and oil-immersed paper of NK and NKC sample aged for 77 days Figure 2. Photographs of samples （ a ） NK Figure 8. SEM results of innermost paper in NK and NKC (a) NK (b) NKC Figure 5. Content of copper substance dissolved in oil  The increased oil acidity will increase the corrosion rate of copper. Therefore, the copper corrosion in PKC sample is more serious than that in NKC sample. The content of copper substance dissolved in paraffinic oil is larger than that in naphthenic oil up to ageing 35 days as shown in Figure 5.  The increase formation of insoluble sludge will cause more heavy copper-organic compounds sediment in the sludge. Consequently, a decreased content of copper substance dissolved in oil is observed after ageing 35 days, and the content of copper substance dissolved in paraffinic oil is less than that in naphthenic oil.  It’s noteworthy that the moisture content of paper aged in PKC is higher than that aged in PK, and the moisture content of paper aged in NKC is higher than that aged in NK.  The innermost paper next to the copper plate of NKC samples aged for 77 days were measured by SEM. Figure 8 shows that there is copper element deposited on the surface of paper.  Figure 9 shows the dissipation factor (tanδ) and volume conductivity (  v ) plots of oil and oil impregnated paper in NK and NKC sample aged for 77 days, respectively. It can be seen that the oil in NKC sample has higher tanδ and  v values than the oil in NK sample. In addition, the oil impregnated paper in NKC sample also has higher tanδ and  v values than the oil impregnated paper in NK sample.