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 insulation in a dc cable is subjected to both thermal and electric stress at the same time. While the electric stress is generic to the cable, the temperature rise in the insulation is, by and large, due to the Ohmic losses in the conductor. The consequence of this synergic effect is to reduce the maximum operating voltage and causes a premature failure of the cable. The authors examine this subject in some detail and propose a comprehensive theoretical formulation relating the maximum thermal voltage (MTV) to the physical and geometrical parameters of the insulation. The heat flow patterns and boundary conditions considered by the authors here and those found in earlier literature are provided. The MTV of a dc cable is shown to be a function of the load current apart from the resistance of the insulation. The results obtained using the expressions, developed by the authors, are compared with relevant results published in the literature and found to be in close conformity.Index Terms -Maximum thermal voltage, thick slab, dc cable, breakdown, insulation, heat flow patterns, external heat injection.

Inadvertent failure of power transformers has serious consequences on the power system reliability, economics and the revenue accrual. Insulation is the weakest link in the power transformer prompting periodic inspection of the status of insulation at different points in time. A close Monitoring of the electrical, chemical and such other properties on insulation as are sensitive to the amount of time-dependent degradation becomes mandatory to judge the status of the equipment. Data-driven Diagnostic Testing and Condition Monitoring (DTCM) specific to power transformer is the aspect in focus. Authors develop a Monte Carlo approach for augmenting the rather scanty experimental data normally acquired using Proto-types of power transformers. Also described is a validation procedure for estimating the accuracy of the Model so developed.

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