Inverse time overcurrent protection for traction transformer

Traction transformer is often on short-time overload state to meet the needs of traction load. In order to prevent the damage to the traction transformer caused by overthermal which is produced by load current, inverse time overcurrent protection is often used in China. Firstly, this paper introduces inverse time overcurrent protection and three standard characteristic equations of inverse time overcurrent protection prescribed in IEC60255-3. Secondly, combined with the China's railway standard, this paper discusses flexibility that whether the IEC standard inverse time protection curves are suitable to Chinese high-speed railway traction transformer or not, the results show that the three characteristic equations can not meet the requirements of China's traction transformer completely. Finally, in order to fit the overload capacity of traction transformer, inverse time overcurrent protection curve for China's high-speed railway traction transformer is introduced, and the corresponding characteristic equation of inverse time protection is also deduced. (4 pages)