Inversion Currents and Voltages in Auto-Transformers

Line grounds on the secondary of an auto-transformer, fed from a grounded system, tend to invert the neutral of the auto-transformer. If the auto-transformer is isolated, this may lead to larger voltages, and if grounded, to larger currents than what would ordinarily be expected. The analysis of an important installation (75,000-kv-a., three-phase units at Detroit) is given below, for various possible connections and conditions of operation. Features of the analysis are: (A) A novel theory and method of auto-transformer circuit representation was developed to handle some aspects of this problem that otherwise appeared elusive. The method is applicable to all networks involving auto-transformers. (B) The rather startling fact is brought out and explained that fault currents to ground on the secondary lines of an auto-transformer may be larger when stepping-up than when stepping-down. The prediction has been verified by test.