Transient Voltage Analysis of a Large 250 MVA Three-Winding Transformer with a Frequency-Dependent Modeling Approach

This study focuses on the transient modeling of a large 250 MVA, 525/154/66 kV, YNynd, transformer with single-phase units. The voltage responses are calculated using a detailed model with frequency-dependent parameters. To validate the simulation results, extensive RSO measurements are conducted on the transformer’s active part in two distinct test arrangements, enabling separate investigation of the HV and MV windings. The results demonstrate that the frequency-dependent model enables accurate estimation of internal transient voltages, including peak overvoltages and subsequent oscillatory behavior. In contrast, simulations based on a constant inductance model exhibit significant inaccuracies in the time-domain voltage waveforms, primarily due to incorrect representation of winding internal resonances in the frequency domain. This work is particularly significant as it addresses transient modeling of a full-scale, three-winding transformer with an iron core, offering substantially greater complexity and practical relevance than studies limited to laboratory-scale air-core windings.