Review of unit protection schemes for auto-transformers

An auto-transformer is a power transformer in which at least two windings have a common part [1]. For standard auto-transformer design this common part is the common winding. Typically auto-transformers are used to interconnect two electrical networks with similar voltage levels (e.g. system intertie transformer). In practice auto-transformer tertiary delta winding is normally included. It serves to limit generation of third harmonic voltages caused by magnetizing currents and to lower zero sequence impedance for five-limb core construction. Standard practice is to size the tertiary delta winding for at least one third of the rated total through power of the auto-transformer. This is done in order to achieve adequate short-circuit withstand strength of the delta winding during earth fault in the HV systems. The application of the unit protection schemes for auto-transformers is somewhat special because such scheme can be arranged in a number of different ways: (1) Based on autotransformer ampere-turn balance (2) Based on First Kirchhoff Law between galvanic interconnected parts (3) Based on zero-sequence currents (restricted earth-fault protection) (4) Dedicated unit protection schemes for tertiary delta winding. Most commonly used auto-transformer unit protection schemes will be described in this document. Advantages and disadvantages of every unit protection scheme will also be discussed.