AUTO-TRANSFORMERS

At higher voltages, large auto-transformers are used to interconnect the 400kV-500kV systems with either the 230kV-275kV systems or 120kV-145kV systems (the exact values depend on the country being considered). The HV and MV windings in an auto-transformer are connected together unlike in other transformers. The result is that the MV winding (common winding) is shared between the MV and HV systems. This results in a smaller and less expensive transformer than if the windings were separated as in a conventional fully rated two winding transformer.