Abstract
Insulating oils have been used as dielectric materials in transformers, and also in capacitors and cables. The main advantages of insulating oils are low cost and their superior insulating properties. However, once oils leaks from the equipment, it adversely affects the environment. Therefore, we focused on ultra-pure water (UPW) as one of environmental friendly liquid insulating materials. In this study, we have investigated the fundamental insulation characteristics of UPW using negative lightning impulse (L. Imp.) voltage (1.0/40 μs) and a half-inch (12.7 mm) diameter sphere electrodes at atmospheric pressure (0.1 MPa). The gap length of electrodes, the insulation resistivity, and the temperature of UPW were set in the range of 1.00–2.50 mm, 15.6–18.2 MΩ·cm, and 17–24°C, respectively. In our recent studies, we investigated and compared the L. Imp. breakdown voltages (BDVs) in UPW using Progressive Stress Test (PST) and Up-and-Down Test (UDT) methods. In both PST and UDT methods, the average L. Imp. BDV tends to increase with increasing gap length, and there was no significant difference. In contrast, the average L. Imp. breakdown electric field tends to decrease with increasing gap length in both methods, in common with insulating oils. As a result, there was no significant difference in breakdown characteristics of UPW in two different test methods. On the other hand, it was also found that the average L. Imp. BDV of UPW tends to increase with increasing gap length for both w/wo pressurization, and the BDV of UPW increased approximately 1.3 times by pressurizing UPW in the range of 0.26–0.28 MPa, compared with non-pressurizing (0.1 MPa). Furthermore, the average standard deviation of L. Imp. BDV decreased to approximately 30% by pressurizing UPW, in common with insulating oils. These results indicate that pressurization might prevent air bubble generating in UPW, which decreased the number of weak points in UPW for breakdown.
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