Investigation of internal resonances and winding insulation stresses in grounding transformer failures

In Shanghai's 220 kV transformer substations, most 35 kV buses are connected with grounding transformers. In the past two years, several grounding transformers have failed, in which the internal insulation was damaged. In order to cope with the problem, this paper calculates and analyzes the switching overvoltages and electromotive force of the grounding transformer. Also the internal resonance feasibility of the winding was investigated, and electric potential distribution of disk-to-disk and turn-to-turn along the winding during the lightning impulse test was measured. A critically stressed position in the winding insulation was found and studied. Subsequently this paper shows that the main reason for failure of the grounding transformer is turn-to-turn insulating breakdown or damage near the HV line terminals. So effective steps are put forward; one is adding insulating thickness of turn to-turn near the incoming line, the other is decreasing the gradient of intrusive lightning surge.