Black-box modeling approach for evaluating internal resonances in high-voltage windings

• Transient events can trigger internal resonances in transformer windings. • First-mode resonance can cause severe overvoltages in transformer windings. • Frequency response-based modeling captures winding dynamics without design details. • The method enables post-fault analysis assessing transformer resonance frequencies. This paper presents a black-box modeling approach for evaluating internal resonances in high-voltage windings of power transformers. The proposed method relies on frequency response measurements, specifically admittance and voltage transfer functions, over a frequency range of 20 Hz to 1 MHz. First, the black-box modeling methodology is presented, followed by a discussion of the frequency response measurements performed in a high-voltage laboratory environment using a Sweep Frequency Response Analysis (sFRA) instrument. Based on these measurements, Y-matrix and H-matrix models were developed and validated in both frequency and time domains. The analysis revealed critical resonance frequencies, especially the first-mode resonance, which cause significant overvoltages that pose a risk to the integrity of the winding’s insulation. These results highlight the importance of considering internal resonances during the design phase of power transformers to mitigate the risk of dielectric failure. Furthermore, the technique proves especially valuable for post-failure analysis, as it enables the investigation of resonance frequencies in a failed transformer or similarly designed unit, providing insight into whether these resonances contributed to the failure event in the operational power grid.