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A new method for calculating the parameters of electric arc models for power circuit breaker is developed. The methodology consists in the optimization of the theoretical arc voltage curve, as a function of a set of parameters, with respect to experimental data. Since the solution of the equation for the electric arc model cannot be obtained in general form, then, the theoretical arc voltage is an asymptotic solution obtained for one or several time-ranges. A case of the irregular behavior of the electric arc is analyzed. The developed methods are applied to obtain an improved arc model for a SF 6 power circuit breaker previously published in the literature. The new model has a compact form and exhibits a good correlation between the measured and calculated voltage curves.

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This paper presents an analysis of the thermal behavior (convection, conduction and radiation) in steady state of a dry-type distribution power transformer. Dry-type transformers are mainly used in applications where the risk of fire and environmental policy does not allow the use of oilfilled transformers. The model takes into account the fluid (air) and solves the heat transfer equation for non-isothermal stationary laminar flow; together with the radiation between the components in the transformer and with the external ambient temperature. The finite element method is used to solve an axisymmetric model of the transformer. It is also modeled the effect of forced air in the thermal model. It is concluded that the finite element model can predict the thermal process in the stationary state, and that this tool can be used by manufacturers to design better electromagnetic devices.Index Terms-Transformer, finite element method, thermal model, hot spot.

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