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When a large power transformer is energized, a high magnitude inrush current may result in harmonic resonance overvoltage (HRO), if the system has resonance phenomenon. Such overvoltages may cause large stresses on the system. This paper investigates the energization of two winding 220/22 kV main power transformer in a weak system of a grid-connected wind farm. To achieve this aim, the produced HRO during the energization of the main power transformers and the affecting factors are investigated. In addition, the effect of the sympathetic inrush phenomena is studied. Finally, the sequential energization of the three-phase transformer and the pre-insertion resistor method are discussed as mitigation methods. The investigated scenarios and results will contribute in developing the power system restoration procedures for existent wind farm, its extension, and future-planned wind farms. Keywords: Grid-connected wind farms, Harmonic resonance overvoltage, Pre-insertion resistor, Sequential energization of transformers