The Effect Of Linear Birefringence On Sensing Characteristics Of Fiber Optic Current Transformer

An intelligent substation is an important constituent part of a smart grid. An electronic transformer is a key piece of equipment for an intelligent substation, and a fiber optic current transformer(FOCT) is one main type of electronic transformer. With a magneto-glass optic current transformer(MOCT), an FOCT is based on the Faraday Effect and has the advantages of good linearity, a wide dynamic range, and good insulation properties. It represents an important future development direction for electronic transformers.As a result of scientific and technological progress and the efforts of electrical researchers, much practical progress has been made on FOCTs. However, some issues still exist. The sensing fiber possesses intrinsic linear birefringence, which could lead to problems with the responsivity, measurement accuracy drift with temperature, and an inhomogeneous magnetic field. This dissertation combined theories with experiments to investigate the effect of linear birefringence on the measurement accuracy of an FOCT, and improvement methods were proposed. The main contents are as follows.Firstly, a model system for an FOCT was constructed. The micro-element cascading model was constructed, which could be equivalent to one-element model, two-elements uniform model and two-elements nonuniform model on the different limits. Based on analysis method for combining the theory and practice, the scop application for two-elements nonuniform model was obtained. The two-elements uniform model and two-elements nonuniform model were the basic mathematical models used for this dissertation.Secondly, the responsivity characteristic of the FOCT was investigated based on the two-elements uniform model. The proposed responsivity characteristic model indicated that the linear birefringence was the primary factor affecting the responsivity. Because of the linear birefringence, the turns were not linearly related to the responsivity, and the radius also affected the responsivity. The effects of these three factors(linear birefringence, radius, and turns) on the responsivity were analyzed, and the concepts and corresponding calculation methods for the optimal radius and optimal turns were proposed. The experimental results verified the validity of the model and conclusions.Thirdly, the temperature characteristic of the FOCT was investigated based on the responsivity model. The temperature characteristic model for the FOCT was proposed by combining the Verdet constant and linear birefringence. The analysis results indicated that the ratio error was linearly related to the temperature, the coefficient of thermal expansion difference, length of the sensing fiber would affect the temperature characteristic through linear birefringence. The experimental results verified the validity of the proposed model. The simulation and experimental results indicated that the FOCT was an open-loop system in respect to the temperature, which meant it couldnot possess immunity to temperature drift. A temperature compensation algorithm based on temperature sensors could effectively improve the immunity to temperature drift. The results of the compensation experiment verified the validity of this compensation method.Finally, the inhomogeneous magnetic field characteristic was investigated based on the two-elements nonuniform model. An inhomogeneous magnetic characteristic model was constructed to analyze the magnetic crosstalk problem and eccentricity problem. The simulation results indicated that the ratio errors produced by the magnetic crosstalk and eccentricity problems appeared to have waveforms similar to a sinusoid, but they had opposite tendencies. Based on analyses of the effects, we discovered the existence of two optimal azimuth angles at which the magnetic crosstalk did not degrade the accuracy of the FOCT. Based on the simulation results, an improvement method based on an optimal azimuth angle was proposed. The experimental results verified the validity of b oth the inhomogeneous magnetic characteristic model and proposed method. These will provide significant guidance for the design and installation of the FOCT.