Simulation of Transformer Inrush Currents and Their Impact on the Grid

Sammanfattning: This MSc Thesis’s main objective is to determine the impact of transformer inrush currents in the grid and develop a tool that helps E.ON Energidistribution AB evaluate transformer sizing. The motivation of E.ON is to have a more precise way to estimate the energization consequences of increasing the transformer size in the grid, since currently is being used a rule of thumb that relates the transformer rated power to the short-circuit capacity of the bus at which the transformer is connected. This rule of thumb is established because they have the concern that large power transformers can cause serious inrush currents. An adequate study of the causes and consequences of the inrush current phenomenon is conducted to determine the duration and magnitude of undesired disturbances. Python is used to design simplified transformer models based on banks of three single-phase units due to substantial limitations of more advanced alternatives. PSCAD and Matlab Simscape Electrical, two superior transient analysis software, are used to verify the results of test simulations. The non-linearity of the differential system of equations required numerical methods and the utilization of the Trapezoidal Rule of Integration and the Newton Raphson Method. Then, different strategies are considered as potential alternatives to estimate the voltage dip at the surrounding grid, which is the primary concern for E.ON. For the grid analysis part, PSS/E Xplorer 35.3 (which is a free limited version) is utilized because PSS/E is the preferred tool at E.ON for steady-state and short-circuit calculations. All the strategies are compared and evaluated for a hypothetical energization of a distribution transformer in a reduced network correspondent to the Oland island. Some results coincide with historical data from a similar distribution network, but on-site measurements or dynamic grid studies must validate the results. The in-built Python API of PSS/E allows automating the process and incorporating it with the rest of the program. As a result of this MSc Thesis, E.ON engineers will be able to use a friendly interactive tool that simulates with high accuracy the energization of a simplified three-phase transformer model and estimates the possible effect on the surrounding grid. However, they must be aware of the limitations and assumptions to make suitable sizing decisions.