IEC 61378-3:2015 provides information to users about specific topics related to industrial and HVDC converter transformers with design, construction, testing and operating conditions differing from conventional transformers used in power systems. In addition, it is the aim of the present part of IEC 61378 to provide manufacturers with the technical background that forms the basis for the principles used within IEC 61378-1 and IEC 61378-2. It is intended that this part of IEC 61378 is used to supplement and not replace or supersede the application guide for power transformers, IEC 60076-8, since many of the general principles contained within it are equally applicable to converter transformers. This second edition cancels and replaces the first edition published in 2006 and constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a) revision of clause on losses to reflect the changes introduced by the second edition of IEC 61378-1; b) addition of a new clause about design review of converter transformer for both industrial and HVDC applications; c) addition of content for transformer connected to voltage source converters; d) reference to the work of CIGRE JWG A2/B4.28 on HVDC transformers.