Soft Transformer Energization: Ramping Time Estimation Method for Inrush Current Mitigation

Uncontrolled transformer energization is known to cause high magnitude inrush currents that can reach up to 10 times transformer rating. Several techniques are proposed in literature to mitigate this issue, including soft transformer energization using a voltage ramp. This technique is regaining more traction with the increased use of converters-based generation. Defining the required voltage ramping time for effective inrush current mitigation is important to avoid fast ramps that can still cause large inrush currents. This paper proposes a model-based ramping time estimation technique that can be used to identify the minimum-adequate voltage ramp time for a given set of constraints, transformer model and network configuration. Key ramping time influencing factors are defined as: residual flux, source to energized core effective impedance and control, in addition to core saturation characteristics. The proposed method considers multiple simulation steps to identify the minimum required ramping time. Simulation case study is presented for a voltage source energization of a 53 MVA three-phase delta-wye transformer, demonstrating the technique capability of determining appropriate ramping times for effective inrush current mitigation. Sensitivity analysis are also carried out to illustrate the impact of varying key parameters such as the transformer core saturation characteristics on the results.