SZB (DZB) series three- phase dry-type auto-transformers are single-coil type transformer. Its primary and secondary sides share a winding, and the transformation ratios are of fixed and adjustable types, voltage buckling and boosting can be realized. Compared to isolation transformer of the same capacity, auto-transformer has advantages of material-saving, simple structure, small size, light weight and high efficiency. It is particularly economical when the transformation ratio aligns to the occasion. However, due to the sharing of winding between the primary and secondary sides, the initial stage has electrical contact, which shall be considered in some occasions., the main role of dry-type, auto-transformer is to change the voltage class.