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.