Convective Heat Transfer Coefficient of Insulation Paper–Oil Contact Surface of Transformer Vertical Oil Channel

Abstract

Insulation paper is the primary inter-turn insulation material for transformer windings. However, insulation paper is a poor conductor of heat and seriously impacts the natural convective heat dissipation in the winding oil channels. In order to study the convective heat transfer performance of the transformer vertical oil channel, the heat transfer characteristics of the two-dimensional boundary layer of the insulation paper–oil contact surface were analysed, and a characteristic number equation with a roughness correction factor was established. Based on the similarity principle and modelling theory, an experimental apparatus was designed to determine the convective heat transfer coefficient of the insulation paper–oil contact surface. Coefficients were obtained for different qualitative temperatures and characteristic lengths. Parametric fitting of the experimental data was carried out to obtain the correction factors. The results show that the natural convective heat transfer coefficient h of the insulation paper–oil contact surface is positively and linearly related to qualitative temperature and inversely related to characteristic length. For temperatures of 312.9 K to 328.1 K and characteristic lengths of 3 cm, h ranges from 116.15 to 144.38 W/(m2 · K). For temperatures of 319.0 K to 337.9 K and lengths of 6 cm, h ranges from 103.79 to 131.14 W/(m2 · K). The correction factor of the characteristic equation coefficient is 1.067. The maximum deviation of h calculated with a coefficient of 0.63 and an exponent of 0.25 is 5.93%. The results are essential for modelling the transformer space thermal circuit and solving the hot-spot temperature problem.