Design of power transformers to withstand surges due to lightning, with special reference to a new type of winding

The general principles on which the voltage distribution in a transformer winding is dependent are outlined. The effects of different impulse voltage waveshapes on the voltage distribution in core-type transformers are described and discussed. Breakdown/time-lag curves for insulation across coils and between turns are given, and from these and the distribution measurements it is shown that the impulse voltage which would give rise to an inter-turn breakdown is very much higher, in general, than that required to give a breakdown across coils. The problem of obtaining adequate insulation across coils to meet the impulse-voltage level required for transformers for installation on the highest-voltage systems is emphasized. A new type of winding is described of relatively simple construction which has been developed to solve this problem. The voltage distribution in a typical example is compared with the conventional core type of winding already discussed. The oscillograms of the voltages in this winding due to a chopped wave show that, unlike the conventional winding, the chopped wave does not give rise to more severe conditions than full-wave tests. It is concluded that by better utilization of insulation it is possible to obtain a very much higher impulse level with the new type of winding than can be obtained with a conventional winding using the same amount of axial insulation.