Application of 2D linear modeling for computing zero-sequence short-circuit impedances of 3-phase core-type YNynd transformers

This paper shows the application of 2D linear modeling for computing zero-sequence short-circuit impedances (Z0SC) of 3-phase core-type YNynd transformers. Firstly, a basic description of these impedances is shown. The proposed 2D model is based on two steps: (a) the search of the currents to satisfy the condition of induced voltage equal to zero in short-circuited windings; (b) the use of constants (KX and KP) to approximate 2D results to results of real 3D geometries. KX is related to the computed reactances, and KP is related to the computed values for the power losses during Z0SC tests. Induced currents in short-circuited windings are found by an iterative process, in order to apply constant current densities in the model of each winding. Results of the model are accurate in comparison with measured values in five transformers. Tertiary is the innermost winding in four units, and the outermost winding in one unit. Open-delta cases and closed-delta cases are considered, as well as cases with and without magnetic shunts on the tank walls. Equivalent permeability of tank steel has an influence on results but a reasonable accuracy is obtained with an intermediate value of this parameter; thus, non-linearity is not a concern in this case. A good accuracy is also obtained for a wide range of KX and KP; in fact, reactances can be estimated without considering that 2D geometry is not an exact representation of real 3D geometry (unlike power losses during the tests, where this fact must be taken into account).