Determination of a Frequency-Dependent Open Circuit Transformer Model through Grid Impedance Measurements

In this work, a frequency-dependent transformer model is derived with the help of a measurement device that determines the frequency response of the grid impedance at an arbitrary point of connection to the power grid. In this case, the measurement device was connected to a 20 kV medium voltage grid of a wind farm. The disconnection and reconnection of the wind turbine’s transformers revealed the impact the open circuit input impedance of the transformers has on the positive sequence component of the grid impedance. First deriving a positive sequence grid model from the measurement without the transformers, later analyzing the impact the transformers have on the grid impedance, allowed the calculation of the positive sequence component of a frequency-dependent open circuit transformer model, which results in the same impact on the overall grid impedance as observed in the measurements.