High-Frequency Transient Studies: Modelling Transformers and Reactors from Field Measurements and Design Data

This paper presents a methodology where high-frequency models are developed and applied when detailed design data and Design Review documentation are not available. In this case, the model is built from field or factory information, including measurement of dimensional characteristics during direct inspection of the active part to enable the reconstruction of the equipment design based on reverse engineering. A detailed white-box model is then created with R, L, and C elements in such a way that internal nodes are available to assess internal voltages and dielectric stresses. For system simulations, an equivalent black-box model is created and derived from the white-box model, preserving the terminal frequency response within the frequency range of interest. The voltage waveforms at the terminals of the black-box model obtained in the electrical system study are subsequently applied to the white-box model to calculate the internal response of the windings.

The methodology is presented in a case study for a 230 kV air-core shunt reactor that experienced a dielectric failure and was available for root cause analysis. Dimensional measurements of the failed reactor winding were used to reconstruct the design and geometry in COMSOL software and then calculate the finite element-based inductance and capacitance matrices to implement a 129-node white-box model for use in ATP. The frequency response results of the created model identified resonances at approximately 9.5 kHz, 15.8 kHz, 20.4 kHz, and 23.3 kHz. The system simulation confirmed the existence of voltage signals at these frequencies, which can excite internal resonances and produce a voltage amplification relative to ground of up to approximately 14 times and inter-turn voltages up to approximately 400 times greater than the reference values at 60 Hz, with the greatest inter-turn amplification position coinciding with the region of the winding where the failure occurred.