Dissolved Gas Analysis (DGA) of Thermally Aged Blended Transformer Oil

Vegetable based ester oils (VOs) are potential substitute of mineral insulating oils (MOs) in transformers due to their enhanced dielectric properties and better biodegradability. Retro filling of operating transformers originally filled with MO by VO is a real possibility, resulting to a mixture of VO and MO, known as the blended oils (BOs). The nature of gas formation (dissolved gases) in blended insulating oils (i.e. MO/VO mixture) needs investigation to understand the fault conditions in such transformers. Fresh MO and VO samples are blended in different proportions, namely, BO1 (25% VO + 75% MO), BO2 (50% VO + 50% MO) and BO3 (75% VO + 25% MO) and are thermally aged in open beaker for 168 hours at 150°C. Dissolved Gas Analysis (DGA) is carried out as per IEC 60567 before and after ageing along with the individual MO and VO. The different DGA fault diagnosis techniques are compared. The DGA data shows that the gas generation varies with the change in MO/VO proportion. Though, most of the gases show decreasing trend in concentration with increase in VO proportion, generation of gases like C
₂
H
₆
, CO and CO
₂
is more in the blended oils (BO) than compared to the individual oils (MO & VO). The nature of gas generation in thermally aged BOs is different than that of thermally aged individual MO and VO. The molecular interaction between VO and MO molecules may be responsible for higher gas generation in BOs. The mechanism of gas formation in BOs needs more investigation.