A Comparative Study of the Impact of Moisture on the Dielectric Capability of Esters for Large Power Transformers

There is debate as to whether esters will be suitable replacements for mineral oil in large power transformers, such as those found on the transmission system of the UK. Research has reported that ester oils operate advantageously when compared to transformer mineral oil. However, moisture is known to affect the dielectric capabilities of oils. This paper presents research investigating the impact of moisture on the dielectric properties of esters compared to mineral oil. It was found that AC dielectric strengths of esters are higher than mineral oil over the range of moisture levels selected. When considering the lightning impulse breakdown voltage of esters, although dry mineral oil has higher breakdown voltages than esters, it is quickly reduced to a level comparable to esters with increasing moisture.