Comparative study of mechanical and electrical strength of kraft paper in nanofluid based transformer oil and mineral oil

Abstract

Transformer is a long run device in a power system network. The life of the transformer is affected by the insulating system provided to it. When transformer oil comes in contact with cellulosic paper, it generates oxides, peroxides and several other harmful gases that lead to the formation of sludge and impurities in the insulating oil. This sludge, not only affects the dielectric properties of the oil, at the same time it is also responsible for the formation of hot spots in the core and hence leads to failure in the winding. Adequate insulation in the winding is maintained by using insulating kraft paper, but sludge contamination degrades the electrical and mechanical strength of the cellulosic kraft paper. To understand the mechanism behind the formation of sludge and decay in the properties of the kraft paper, thermal ageing study has been performed followed by electrical and mechanical strength analysis. Keeping the same kraft paper, a comparative study of mechanical and electrical strength has been carried out after thermal ageing in two different insulating oils namely: mineral oil (MO) and exfoliated hexagonal boron nitride (Eh-BN) based nanofluid (NF). Initially the thermophysical and electrical properties of the MO are studied. However, non-biodegradable nature, inferior thermophysical and electrical properties of the MO motivate the researcher to develop a new class of NF (Eh-BN/MO) for transformer application. Eh-BN nanofiller are dispersed into the MO and extensively sonicated to avoid the agglomeration in the NF. Five different concentrations of Eh-BN nanofiller from 0.01-0.1wt.% are dispersed into the MO to prepare the Eh-BN/MO-NF. Due to enhanced stability and superior thermophysical and electrical properties, 0.01wt.% concentration is chosen to prepare Eh-BN/MO-NF for ageing study. It is also observed that thermophysical and electrical properties of 0.01wt.% of Eh-BN/MO-NF are far more superior to the existing MO. Hence to understand the effect of the preferred NF to the paper insulation inside the transformer, ageing study is necessary. Ageing study of both MO/kraft paper and 0.01wt% Eh-BN/MO-NF/kraft paper are performed in two different ageing vessels at same parametric conditions and the electrical and mechanical strength of the aged kraft paper are examined in different ageing duration. The moisture absorbances in both the cases are studied and lower moisture contamination and higher breakdown strength has been observed for kraft paper in NF. INSTRON tensile test measurement setup is used to measure the tensile strength of the virgin kraft paper, MO/kraft paper and 0.01wt.% NF/kraft paper. It is observed from the experimental results that both modulus and tensile strength of the aged kraft paper in the Eh-BN/MO-NF are superior to the aged kraft paper in MO. It is concluded that Eh-BN/MO-NF can reduce the effect of sludge formation and hence better insulation stability can be achieved for enhancing the life of the transformer.