Lightning studies of transformers by the cathode ray oscillograph

Study of transformers has been under way for some time to coordinate the strength of transformers and transmission-line insulation under lightning conditions. This study has taken two forms; first, the transient dielectric strength of the line end of the winding and second, the distribution of transient voltages and therefore the stress caused by them throughout the winding. Theoretical studies and spark-gap tests of transient voltage distribution in transformer windings have been previously published by the Institute. Since then an extensive study of transient voltage phenomena has been made by the cathode ray oscillograph on power transformers connected to a short transmission line and subjected to artificial lightning waves sent along the line. The effects of the transmission line, concentrated inductance, and transformer entrance bushings on traveling waves were studied, as well as the effect of traveling waves of various service conditions in producting internal oscillations in ordinary transformers: The non-resonating transformer was studied under similar conditions. A striking agreement between the oscillographic records and theoretical conclusions previously published was found, sufficient to establish beyond any doubt the following conclusions: 1. Very high-voltage oscillations occur throughout the entire winding of even a grounded neutral transformer. 2. Points of the winding near the grounded neutral may rise to 95 per cent of the crest voltage of a very short traveling wave (three microseconds long). 3. Entrance bushings have a negligible effect on the shape of the incoming traveling wave. 4. In case of sudden voltage changes, concentrated inductance in series with the transformer, unless by-passed by a suitable device, causes rise of voltage across the transformer terminals as well as internally in the windings. 5. Arc-over of line insulators by a traveling wave produces severe oscillations in a transformer the amplitudes of which are roughly proportional to the arc-over voltage of the line insulators. 6. Grading the insulation between high voltage and low voltage and ground in ordinary transformers with grounded neutral is a dangerous practise when the transformers are subject to lightning. 7. All the above conclusions apply to concentric winding core type as well as interleaved and shell type transformers. From theoretical studies the non-resonating type of transformer has been developed and its action checked by tests. This type of transformer eliminates voltage oscillation within the winding and therefore local concentration of transient voltage. An Appendix entitled Present Status of the Cathode Ray Oscillograph on the Measurements of Transients, by H. L. Rorden and J. C. Dowell, both of the General Electric Company, Pittsfield, Mass. is included in the complete paper.