Effect of water on AC breakdown properties of vegetable-oil-based insulating fluid mixed with mineral oil

This paper reports the effect of water on AC breakdown properties of the vegetable-oil-based insulating fluid mixed with mineral oil. When we replace the mineral oil with the vegetable-oil-based insulating fluid in the high voltage electrical equipment, there are the concerns of containing residual mineral oil. It is called “retrofill”. We mixed the mineral oil into the vegetable-oil-based insulating fluid (0 to 20 [wt%]) and added the water (0 to 600 [ppm]), observed AC breakdown voltage (electric strength Fb). These results were shown as the following: (i) Fb decreased with the increase of water. (ii) Fb decreased with the increase of the ratio of the mineral oil to the vegetable-oil-based insulating fluid with the water content of 0 [ppm]. On the other hand, with the water content of 600 [ppm], Fb rose with the increase of the mixing ratio. (iii) At the water content of 150 and 300 [ppm], Fb showed the maximum value in the mixing ratio of 10 [wt%].