Recommendations on balancing transformer and line insulations on basis of impulse voltage strength

The danger of over-insulating transmission lines without a corresponding increase in the insulation of connected apparatus is pointed out. To avoid trouble from lightning reaching the apparatus, the Transformer Subcommittee of the Electrical Machinery Committee has prepared a set of proposed rules covering: (1) A practical method of defining the impulse strength of transformers, and (2) recommendation on coordinating the transformer and adjacent line insulation on the basis of impulse voltage strength. It is pointed out that commercial methods for determining the impulse strength of transformer windings by acceptance test in the factory are not feasible. However, investigations in the laboratory show that the impulse strengths of transformer insulation and of standard line insulators or of air-gaps vary similarly with different forms of waves; also that the impulse ratio of strings of line insulators of different lengths is approximately constant for the same impulse waves. The forms of impulse waves actually occurring on transmission lines have not been fully determined. For these reasons the impulse strength of transformers is expressed in terms of the 60-cycle dry flashover voltage of non-shielded suspension type insulators. If the transformer and line insulation are properly balanced with a given wave, it eliminates the necessity of having to consider the different shapes of waves encountered in practise. Three methods are recommended for coordinating the transformer and line insulation, namely, (1) the use of an effective lightning arrester; (2) the use of a horn-gap; and (3) the maintenance of average line insulation for a limited distance from the station.