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A reflection fiber optic current transformer (FOCT) based on equal-ampere-turn method and range self-expanding approach is proposed to measure ultra-high current in this paper. A flexible sensing coil is utilized in FOCT to suit the needs of the measurement environment and to enable installation and removal. At the same time, two significant difficulties have been raised: one is the nonlinear difficulty introduced by the equal-ampere-turn approach and the range self-expanding method; the other is the non-uniform magnetic field errors generated when the flexible sensing coil is put on site, including conductor eccentricity and non-closed angle, which can amplify the nonlinear problem. Therefore, in order to eliminate the non-uniform magnetic field error, a closed structure of λ/4 waveplate and reflector is designed for the reflection structure, and the sensing coil skeleton is optimized. Experimental results show that the FOCT with closed structure and sensing coil skeleton has a good range self-expanding ability. The accuracy at rated current can also fulfill the requirement of 0.2S. Finally, the FOCT was subjected to a temperature cycle test in the range of -40°C to 70°C, with a ratio error of less than ±0.2%. So the method's validity and feasibility are confirmed.
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