Broadband Fiber Optic Current Transformer for Eliminating Birefringence Effect

With the increase of power system voltage level, monitoring lightning current is of great significance to the lightning protection of transmission lines. Fiber optic current transformer (FOCT) is suitable for current monitoring in ultra-high voltage (UHV) power grid. However, the external environment such as temperature is likely to produce birefringence effect in the sensor, resulting in the decline of measurement accuracy. To suppress the birefringence effect in FOCT, this paper studied a sensing structure coupled a Faraday rotator (FR). Its mathematical model is established by using Jones matrix theory, and the effect of birefringence and FR is analyzed. The temperature characteristic and lightning current response of the sensor are tested. The researches show that the output error is less than 2% in the temperature range of
$\boldsymbol{-20{ }\ ^{\circ} \mathrm{C} \sim 80 \quad{ }^{\circ} \mathrm{C}}$
. Meanwhile, the frequency band can cover 8/20
$\boldsymbol{\mu \mathrm{s}}$
lightning current. Therefore, FOCT studied in this paper has wide frequency band and good robustness, which can be applied to broadband current measurement in complex environment.