Comparative statistical analysis on AC breakdown voltage of thermally aged nanofluid with mineral oil

Insulating properties of the conventional transformer oil can be enhanced by the substation of appropriate nanofluid (NF). This study report based on the comparative analysis of statistical AC breakdown voltage (ACBDV) in mineral oil (MO) with a well dispersed TiO₂ based NF after accelerated thermal ageing. The studies have been performed measuring 60 ACBDV data as per ASTM D1816 standard using spherical electrode under AC source for a specified ageing period of the liquid. The breakdown probabilities of both the oil are estimated using normal and Weibull distribution functions. Suitable distribution functions are used for this hypothesis. The ACBDV probabilities of the sample population are evaluated by seeking the histogram, frequency of distribution, mean and standard deviation. Thermal ageing of two different liquids namely MO and TiO₂/MO-NF is carried out in two similar ageing vessels as per standard testing condition. Investigations of ACBDV are carried out for the four distinct ageing duration (100, 300, 500 and 1000 hour) of these two liquid. The effect of ageing on ACBDV of the two liquid at similar ageing duration are compared and analyzed. At upper aging duration, ACBDV of MO is drastically dropped compared to TiO₂/MO-NF. Hydrophilic nature of MO may be the key reason of generating oxidative product such as oxide, peroxide and water in contact with cellulosic paper insulation and hence decrement in its BDV. It is observed that the measured ACBDV data follows better normal than Weibull distribution for both the liquid at each ageing durations. ACBDV at the level of risk 0.1%, 5% and 50% probability for U0.1%, U5%, and U50%, the dielectric tolerance of TiO₂/MO-NF is far more superior to MO under homogeneous field of AC. It has been observed that AC withstand voltage of tested TiO₂/MO-NF are well suited for the transformer application.