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Electrical transients generated in power systems due to switching operations comprise of voltages and currents of complex waveshapes ranging over broad spectrum of frequencies. Such transients when travel through transformers can excite the winding's natural resonate frequencies. The resulting winding resonance leads in severe internal voltage amplification and abnormal stresses on the insulation. In the present paper the authors have conducted studies to ascertain how the winding insulations of a 400 KV EHV power transformer are stressed when one or more of the winding's natural frequencies are triggered by power system transients.