Influence of water content in natural ester liquids partial discharge inception voltage

The use of natural ester liquids in power transformers, as an alternative to mineral oils, is increasing in the last decades due to the higher interest in reducing the environmental and fire risks. This is possible thanks to the fact that natural ester liquids present much higher flash fire points and biodegradability, also satisfying the general principles of a circular economy. Furthermore, their diathermic properties are completely comparable with those of mineral oils and this, if the active part is properly designed, may allow more efficient cooling with a consequent increase in specific power, especially in temporary overload conditions. Also, the electrical properties of mineral oils and natural ester liquids may be considered comparable, especially in terms of AC breakdown voltage and partial discharge behavior when testing these insulating liquids according to standardized methods. The breakdown voltage (IEC 60156) and partial discharges determination (IEC 61294TR) at power frequency are test procedures mainly intended to indicate the presence of contaminants such as water and suspended solid particles. Nowadays, being in progress the preparation of an ASTM Project WK65707 on PD detection in dielectric liquids and being envisaged the revision of the existing Technical Report IEC 61294 TR, in this paper are reported some considerations concerning the influence of different water contents on Partial Discharge Inception Voltage (PDIV) of one commercial natural ester liquid and a traditional mineral oil.