Transient moisture equilibrium in oil-immersed transformer

Abstract

The final goal of the paper is to establish a transient moisture equilibrium model for estimating the moisture in paper of the running oil-immersed transformer. To this end, the presented model for oil-paper insulation is improved according to the insulation structure of oil-immersed transformer. The moisture diffusion process is divided into three steps, i.e., between paper and oil, between support insulation and oil, and in oil. In order to improve the accuracy of the model, the added moisture due to ageing of paper insulation is also taken into account.By (20) in [1], the relation between the moisture in paper and the oil duct near the paper follows:where, C Paper (t) is the transient moisture concentration in paper (%); τ Paper (t) is the equilibrium time constant (h), which is obtained by (25) in [1]; C ss (T(t),C OilDuct (t)) is the equilibrium moisture concentration in paper (%), when the moisture concentration of duct oil (μL/L), C OilDuct (t), and the temperature ( o C), T(t), both remain, and it can be obtained by (9) in [5], as a continuation of authors' previous work, according to which, moisture is distributed between oil and paper stably at arbitrary working temperature as Figure.1 and Figure.2. The coil of transformer are covered by several levels paper, which can be looked upon only one single-sided diffusion paper, with n×l Paper thickness, where l Paper is the thickness of one paper, and n is the number of paper levels, so the equilibrium time constant is expressed:where, A 0 , A 1 , A 2 and A 3 are constants, respectively equal to -7.331×10 -2 , -33.545, 7.9575 and -14.1, according to [1].If the added moisture due to ageing of paper insulation is taken into account, (1) should be changed to:where, v WInPaper (t) is the moisture acreasing rate by ageing of paper insulation. Figure.1 Moisture in oil (ppm ) versus moisture in paper (%) at the arbitrary temperature from 0 o C to 60 o C Figure.2 Moisture in oil (ppm ) versus moisture in paper (%) at the the arbitary temperatrue from 60 o C to 100 o C By substituting equation (2) for the equilibrium time constant, τ Paper (t), into equation (3), the following