This part of IEC 61378 applies to oil-immersed three phase and single phase convertor transformers for use in HVDC power transmission: It applies to transformers having two, three or mutiple windings. This part of IEC 61378 does not apply to: - convertor transformers for industrial applications, see IEC 61378 Part 1; - convertor transformers for traction applications, see IEC 60310.