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Resonant overvoltages caused by transformer energization in relatively weak systems, especially during power system restoration, are investigated in this paper. These transient overvoltages may result in failures, especially for surge arresters. A theoretical analysis is performed to obtain the transformer inrush current harmonics and the transmission-line lengths for which the harmonic resonance is likely to occur. The temperature rise of surge arresters is proposed as an index to diagnose a harmonic resonance case. An appropriate electrothermal model for metal–oxide surge arresters is developed using the thermal model and proposed combined electrical model. Comprehensive transient simulation studies are performed to find out the harmonic resonance cases using the proposed index. Obtained simulation results are in good agreement with the analytical results. To reduce such harmonic resonance cases, controlled closing of the transformer can be utilized. In this paper, the proper instants for transformer energization are proposed in such a way that the overvoltage is reduced to acceptable values. [ABSTRACT FROM PUBLISHER]