Overvoltage on the Neutral Insulation of Dry-Type Transformers during Impulse Tests

For power transformers intended for operation in networks with an insulated neutral, the relevant regulatory documents currently in force stipulate tests of the impulse insulation strength of star-connected windings without a neutral brought out by applying a voltage impulse to three-line terminals connected together. In this case, a voltage is induced on the neutral, which may exceed the test voltage applied to line terminals. This circumstance must be taken into account at the transformer designing stage in choosing the transformer insulation. The article presents the results obtained from transient voltage measurements on the neutral of 6–35 kV dry-type transformers for a capacity of 40–630 kVA in case of applying a full wave-shape lightning and switching impulse voltage to one- and three-line terminals connected together. Possible measures to reduce dangerous overvoltages on the insulation of the neutral of dry-type transformers' high voltage winding are considered.