Short-circuit Impedance Calculation of 25 MVA Autotransformer with Tertiary Winding using FEM

In this article, the percentage of short-circuit impedance is calculated using the finite element method for a single-phase power autotransformer with tertiary winding. The autotransformer specifications are: 25 MVA, 230 kV, 60 Hz, three-legged core type. A detailed 3D model where mixed formulations in terms of scalar and vector potential and circuit coupling is used. This type of study is essential to design the autotransformer and guarantee the impedance value, and so the safety and operability of electrical power systems. To calculate the percentage of short-circuit impedance the voltage relationship between the short-circuit voltage and the nominal voltage is used. In addition, the magnetic energy storage technique is presented as another alternative to calculate this parameter. The results are validated with the data from the laboratory tests obtaining differences of less than 5%.