In order to satisfy the requirements of measurement accuracy and relay protection setting simultaneously，the transformation ratio selection of current transformer in high voltage side of auxiliary transformer is a common problem faced in projects． Based on the research about the principle of relay protective configuration，the setting calculation of protection relaying and the precise work current range of protection devices，the upper and lower limits for the transformation ratio of current transformer are derived． And then，by analyzing the relationship between measuring winding and protection winding，the recommended transformation ratios of measuring winding and protection winding used in high voltage side of 10 kV and 35 kV auxiliary transformer are given for practical using．