Space Charge and DC Breakdown Behavior of Natural Ester Impregnated Insulation Paper with Different Ageing Conditon

Mineral oil is widely used in the transformers as insulation oil because of its preferable low price, good insulation quality and a relatively low condensation point. However, with the consideration of the fact that it almost does not degrade which can cause the contamination of soil, water and our living environment; While on the other hand that fossil fuel would be a scarce resource for the 21st century, it becomes urgent for the researchers to find out its alternative. Natural ester drew the researchers' attention in the 1990s because of the fact it can degrade thoroughly, renewable, and has a better fire safety quality. In order to ensure the long term safety of the equipment using natural ester as insulating fluid under DC operation condition, it is necessary to understand the space charge characteristics of the insulation paper impregnated in natural ester. A thermal accelerated ageing experiment was performed on paper impregnated in natural ester at 130 degrees C. Samples were collected respectively in different days. The space charge distribution analysis was applied on these samples with different electric field respectively using the PEA method. With further degree of thermal ageing, the space charge injection became easier and the amount of its injection increased. The space charge injection threshold voltage was found using ramp test, and the thermal ageing would reduce the space charge injection threshold voltage. The space charge effect on the DC breakdown voltage of natural ester impregnated insulation paper is weaker than the mineral oil impregnated insulation paper after being aged for same time.