Abstract-Inadvertent failure of power transformers has serious consequences on the system reliability, economics, and the revenue accrual. An accurate estimation of transformer life can, to a very large extent, mitigate the problems, besides satisfying the conflicting requirements of optimum utilization of the equipment and safeguarding the reliability.In this endeavour, the authors have planned long duration aging experiments on scaled-down (prorated) models [1] of a transformer, incorporating all of the essential features of actual equipment under normal operating electric stress and accelerated thermal stress. In continuation of the authors' earlier experimental investigations, an elapsed life assessment study has been instituted by acquiring insulation-aging data under accelerated thermal stresses.
The paper presents certain aspects of electrical / thermal failure of dc power cables. Closed form theoretical formulations for computing the critical stress and temperature due to an external heat source in the form of a steady current through the conductor is presented. The criticality here implies an unstable state of the dielectric and is shown, more often than not, to be different, from that corresponding to thermal decomposition limits. Formulation and solution of continuity equations under first and second kind boundary conditions taking account of electric stress and temperature-dependent dc conductivity is covered. Using the suggested model, stress and temperature distribution in the body of the insulation can be obtained to a reasonable degree of accuracy.Index Termsdc cable, thermal breakdown, critical stress, critical temperature, stress distribution, temperature distribution, thermal instability.
The degradation of electrical insulation in transformers is more often due to thermal stress. In this paper, authors have attempted to work out a method of assessing the magnitude of temperature and the hottest region in the body of a large transformer with a reasonable degree of accuracy. A theoretical model has been developed based on boundary value problem of heat conduction in transformer winding using finite integral transform techniques. The model requires, in addition to electrical parameters of the transformer, information on the actual design data. The authors believe that this model predicts HST with precision and also indicates a possibility of online data acquisition.
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