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Power transformer in service is exposed to various voltage and current stresses. The ability to withstand short circuit is an essential requirement for power transformers. There are different types of short circuit: single - phase to earth, double - phase with or without simultaneous earth fault and three - phase short circuit. These various short circuit conditions result in different stress conditions for the windings. Mesh analysis and symmetrical components are the two methods most commonly used for determining the magnitude of short circuit currents. In this paper, both methods will be presented with results compared on a real-case transformer. Also, a generic scheme using the symmetrical components approach is designed in order to standardize the short-circuit currents calculation for all power transformer types and to reduce the time required for obtaining results.

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Total losses of a power transformer are subdivided in several distinctive parts. The I2R losses are easy to calculate and can be precise to a level of measurement repeatability and tolerances of the guaranteed material properties. The additional losses inside the windings can be calculated almost with the same precision using analytic methods. The third part of the load losses that consists of stray losses is the smallest and the most difficult to estimate. However, stray losses estimation is very important in the design phase of a power transformer. Not only because the guaranteed design parameters have to be satisfied, but also the utmost care has to be taken that local losses density across the power transformer does not exceed levels that are permissible in the long term loading conditions of a transformer. The additional losses estimation process presented in this paper models the additional losses level in a transformer as a unit.

During voltage transients, the windings of a transformer are coupled by electric and magnetic fields. To calculate the transients inside the transformer, a network model is typically used. The accuracy of obtained calculation results depends mainly on this model in which the windings are lumped into R, L and C circuit components. The windings are usually represented by discs, or groups of discs, with the corresponding resistances, inductances (self and mutual) and capacitances (series and to the ground) [1]. In case of impulse voltage, wave’s steep front and consequently high frequency oscillations are the main reason why capacitances modelling is of major importance for the calculation of voltage distribution in winding and between windings.