Surge Transfer in 3-Winding Transformers [includes discussion]

When an impulse wave hits a transformer winding, transient voltages are produced in all the windings. This paper discusses the voltages transferred to the terminals of the nonimpulsed windings of 3-winding transformers and autotransformers. Only two components of the transferred surge have practical significance: the electrostatic component and the electromagnetic component. Analytical expressions of the terminal voltages have been obtained for both these components for various terminal conditions of the nonimpulsed windings. Comparisons are made with tests made on an electromagnetic model of a 33,333-kva 3-winding transformer. Recommendations are made for the protection of the tertiary windings of transformers and of the apparatus connected to these windings.