Investigation of the Breakdown Voltage of Insulation Oils at Medium Frequency after Thermal Ageing

Insulating oils and medium-frequency high voltages come together primarily where power electronic components are used, such as in power electronic transformers. For reliable long-term use, the insulation behaviour and, in particular, the breakdown voltage of the insulating oils at medium-frequency voltages is relevant, especially under consideration of ageing. Therefore, this paper investigates the influence of thermal ageing on the dielectric strength of insulating oil at medium frequency with an accelerated ageing test. Three insulating oils are each stored together with copper plates and insulating paper at 120 °C with controlled addition of air for an accelerated ageing. An oil sample is taken at fixed intervals and the breakdown voltage is determined for 2.2 kHz, 5.3 kHz and 8.6 kHz. In addition, the dielectric properties, acid number, water content and viscosity of each sample are determined. The investigations show a clear ageing dependency only for the acid number and the loss factor, all other parameters examined do not show such trend.