Insulation stresses in transformers, with special reference to surges and electrostatic shielding

The paper discusses various types of abnormal voltagestresses in transformers, and in particular those due to surge phenomena. A brief resume of the theory of the effects of surge voltages on transformer windings is given, and a method of determining initial impulse-voltage distribution by means of a calculating board is explained. The principles of electrostatic shielding are considered and the application of a simple, economical and effective type of shield to commercial transformers is discussed. Illustrations of actual transformers fitted with such shields are given, and the efficacy of the shielding is demonstrated by means of cathode-ray oscillograms. An account is given of researches carried out by means of the recurrent-surge oscillograph, and results are compared with those obtained at high voltage, using a continuously evacuated cathode-ray oscillograph. In conclusion, reference is made to the problem of co-ordination of insulation, and the relative merits of various protective measures are briefly discussed. It is shown that a shielded-winding transformer, when properly co-ordinated with line protective devices, possesses a high degree of immunity from breakdown due to lightning or other transient voltages.