IEC 61378-1:2011 deals with the specification, design and testing of power transformers and reactors which are intended for integration within semiconductor converter plants. The scope of this International Standard is limited to application of power converters of any power rating. Typical applications are: - thyristor rectifiers for electrolysis; - diode rectifiers for electrolysis; - thyristor rectifiers for large drives; - thyristor rectifiers for scrap melting furnaces, and - diode rectifiers feeding inverters for variable speed drives. The standard also covers the regulating unit utilized in such application as step down regulating transformers or autotransformers. This edition includes the following significant technical changes with respect to the previous edition: - addition of winding connections with phase displacement (30°); - addition of transformers with more than one active part in the same tank; - change of reference power definition; - addition of regulating transformer feeding converter transformer; - addition of considerations about current sharing and hot spot temperature in high current windings for various winding arrangements; - addition of transductors used for d.c. voltage regulation together with diode rectifiers. The contents of the corrigendum of January 2012 have been included in this copy.