Modeling of Power Transformers for Grid-forming Converters Based Soft Energization Studies

Power transformers modeling for transient studies has been classically approached with the assumption that the transformer is energized from a constant amplitude (1 pu) voltage source. The rising penetration of converter-based resources with limited overcurrent capabilities, and the inherent converters voltage control flexibility is recently increasing the utilization of soft energization through a voltage ramp as a reliable transformer inrush current mitigation technique. To address this trend, this paper proposes a new theoretical model for soft transformer energization studies that can be used in conjunction with voltage-controlled sources such as grid-forming power converters (GFCs). The model considers the voltage ramp-rate impact on the magnetic core flux characteristics to quantify the key influencing factors on the inrush current arising during soft energization and can be used to identify the occurrence of core saturation during the energization process. The derived model performance is successfully validated against the EMT transformer models in PSCAD and MATLAB/Simulink software tools for single and three phase transformers.