Autotransformer

A transformer in which a single continuous winding serves as both the primary and secondary, with the secondary voltage tapped from a portion of the primary winding. Autotransformers are used extensively in EHV substations (e.g., 765/345 kV, 500/230 kV) because they are physically smaller and less expensive than two-winding transformers for the same MVA rating when the voltage ratio is modest. However, because there is a direct electrical connection (not just magnetic coupling) between the two voltage levels, autotransformers transfer fault current across voltage levels and affect zero-sequence network behavior.

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