Investigation on the high frequency, high voltage insulation properties of mineral transformer-oil

Based upon the increasing utilization of power electronics in electrical energy systems, high frequency combined with high voltage (HF-HV) phenomena are emerging. Fast switching operations, e.g. by static inverters with their large slew rates, are the origin of the stresses above 50/60 Hz. The impact of HF-HV noise on insulation systems like in power transformers has yet to be investigated and is temporarily unknown. In contrast to drives, where static inverters are in use for some time, their application and interaction with oil-cellulose insulation is rather new. Investigations on that matter are often limited by the ability to generate an appropriate test signal. Although earlier researchers found a significant drop in dielectric strength, impregnated board as well as the mineral oil itself will still need more than 10 kV/mm for breakdown. Further, it is nigh impossible to gather useful data with cellulose material at a gap distance less than 1 mm. A power source with a resonance principle was build to generate sinusoidal voltages with 100 kV maximum amplitude at frequencies up to 200 kHz. As the basic insulation material in the combination with cellulose, mineral oil (Shell Diala D) will be investigated first. Basic tests on the dielectric strength and the comparison with the values at power frequency will be observed. Examinations on the breakdown voltage at 160 kHz show a drop, exceeding a bisection of the 50 Hz values. These investigations will be extended here by testing different frequencies and their dependency upon the flashover strength as well as the detailed comparison with standard test methods for insulating liquids.