In order to satisfy the demands for the wide-frequency current measurement of the DC current measuring device for HVDC transmission systems

by referring to the requirements of the national standard

the frequency response characteristics and step response characteristics and performance improvement method of the DC fiber-optic current transformer(FOCT)are investigated. Meanwhile

considering the effect of feedback delay

two-order frequency response characteristic of the phase modulation circuit and moving average output filter

the high-order mathematical model in the discrete domain of the DC FOCT is developed. It is ascertained by the simulation analysis of the frequency response characteristic and step response characteristics of the DC FOCT that the forward gain of the closed-loop signal-detecting system and the order of output filter are both crucial factors affecting the wide-frequency current measurement accuracy. The overshoot can be suppressed

and the wide-frequency current measurement accuracy and bandwidth of the DC FOCT can be improved by the optimization of the forward gain and order of the output filter. The test device for the frequency response of the DC FOCT is built

and the equivalent test method of the step response is also presented. It is shown by experimental results that the wide-frequency measurement performance of DC FOCT can meet the requirements specified in GB/T 26216. 1—2010.