Effect of Transient Voltages on Power Transformer Design-IV Transition of Lightning Waves from One Circuit to Another Through Transformers

Three preceding papers of the same general title have dealt with transient voltage stresses developed within high-voltage transformer windings. The most important conclusions arrived at by the author and presented in these papers and the discussions are: 1. Transformers of all conventional constructions undergo oscillations when subjected to lightning or switching surges. 2. The amplitudes of these oscillations may be dangerously high, depending on the amplitude and the shape of the applied voltage. Their frequencies range from a few thousand to a few hundred thousand cycles per second.
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3. A lightning wave of a given shape produces very different stresses in different transformers.
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4. In practical design, neither amplitude nor frequency of these oscillations can be controlled by arrangement or proportions of windings.
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5. A lightning wave chopped by flashover of line insulation can produce stresses in transformer windings equal to or even in excess of those produced by a long wave of the same amplitude and front.
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6. Unless means are taken to obtain uniform distribution under all lightning conditions it is entirely possible to design transformers that will pass A.I.E.E. test which are inadequate for service conditions. This follows because test voltages in neither magnitude nor distribution of stresses correspond to that produced by transient voltages in transformers of ordinary construction. This difference is particularly great in transformers with solidly grounded neutrals, where the potential test allows the insulation from high-voltage winding to low-voltage winding and ground to be reduced as the neutral is approached.
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7.