Wideband modeling of a 45-MVA generator step-up transformer for network interaction studies

A stable and passive six-terminal model is developed for a 45-MVA generator step-up transformer based on frequency sweep measurements in the range 5 Hz-10 MHz and curve fitting with rational functions. The modeling employs separate treatment of the zero sequence system in combination with a new variant of the fast residue perturbation method for passivity enforcement. For use in system level simulations, the model can be employed in PSCAD, EMTP-RV and ATP. With ATP, the representation with an equivalent electrical circuit requires some care to prevent the occurrence of near zero circuit elements. Comparison with measured time domain responses at reduced voltage shows that the model can accurately reproduce transient voltage transfer between the windings while taking into account the loading effect of the connected system. The model also reproduces the transient recovery voltage (TRV) when interrupting short-circuit currents. As an application example, the model is demonstrated for simulation of resonant voltage transfer from the high-voltage side to the low-voltage side. In this setting, it is shown that the transformer input impedance can at high frequencies greatly influence the shape of the impinging overvoltages, thereby giving a self-protective effect against incoming overvoltages.