I. Introduction
Reducing the weight of on-board equipment is the development trend of higher performance electric multiple units (EMUs) because the weight directly affects the energy consumption and the wear between wheel and rail, and this effect is more significant at the higher running speed [1]. The on-board traction transformer (OBTT) is the key equipment of the traction system of EMUs, which is used to provide traction power and provide power supply for auxiliary equipment. The weight of the existing oil-cooled OBTT accounts for about 15% of the weight of a single carriage [2], [3], [4], which is the heaviest electrical equipment in the EMU. This inherent disadvantage limits the development of EMUs toward higher speed, higher efficiency, and better performance. Dry-type OBTT (D-OBTT) is considered as a lightweight substitute for oil-immersed OBTT because it does not contain heavy components, such as cooling oil, tanks, conservators, and oil coolers, and is directly cooled by the airflow (train-induced wind) generated during train operation. At present, Hitachi Energy Ltd. has launched an 800-kVA small capacity fan-cooled D-OBTT that can be used for low-speed level intercity EMUs [5], [6]. However, the large capacity D-OBTT for high-speed EMUs using train-induced wind cooling is still blank.