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.
However, this is only advantageous if relatively close voltage ratios are used. A separate delta connected tertiary winding may be provided for harmonic suppression or for the supply of local loads at lower voltages.
These auto-transformers are generally more complex than generator step-up transformers and their dielectric design more difficult. Their performance under short-circuit has also to be carefully analysed at the design stage, as they are connected to large systems with high short circuit fault capacities capable of the bulk infeed of fault current during a short circuit on the external system.