Analysis and Compensation of High-Current Error for Fiber-Optic Current Transformer

The nonlinear relationship between the output of the fiber-optic current transformer and the current to be measured is derived by means of the mathematical model of the quarter-wave retarder and the spun elliptical birefringent fiber. The nonlinearity caused by the azimuth angle and retardation errors of the retarder, as well as the linear birefringence in the sensing fiber is determined to be the main factors leading to the basic measurement error, composite error, and maximum peak instantaneous error. The high-current measurement accuracy of the transformer can be improved by reducing the alignment error and phase delay error of the retarder and shortening the pitch of the spun elliptical birefringent fiber. Based on the DC equal ampere-turn method and linear interpolation, a nonlinear error measurement, modeling, and compensation method for fiber-optic current transformers is proposed. The linear response of the transformer to Faraday effect over a large dynamic range is obtained and the power frequency high-current measurement accuracy is improved by the error compensation. It is shown by test results that the scale factor error of the fiber-optic current transformer can satisfy the requirement of 0.2 accuracy class within the rated accuracy limit primary current.