Interturn insulation characteristics for transformer windings using three ester types under lightning impulse voltage

The capacity of a transformer will increase when adopting high-temperature insulation systems, because the flash point of esters is higher than that of mineral oils. If a lightning surge is applied to a transformer, the electrical insulation inside the winding may be damaged by potential oscillation of the winding. This paper reviews the negative lightning impulse insulation characteristics using the interturn insulation models with three types of esters, which make up one of the fundamental composite insulation systems in transformer windings. In the interturn insulation model, two Kraft-coated wires are pressed together tightly, with a coating thickness of 0.33, 0.55, or 0.77 mm. Partial discharges start in the wedge-shaped gaps formed between the Kraft papers and esters under lightning impulse, and puncture destruction occurs with the increase in applied voltage. Therefore, investigating the behavior of partial impulse discharges is critical. It is concluded that the partial discharge inception voltage of the three ester types is similar to that of mineral oil in the interturn insulation models.