All Fiber Optic Current Sensor

Compared with current traditional sensors, all fiber optic current sensor(AFOCS) has the advantages of simple structure, reliable insulation, light weight, small size, no saturation, good linearity, and its output signal can be directly connected to a computer device. AFOCS can completely satisfy the construction and technical requirements of smart substation and EHV grid. Based on the demand of current measurement and the shortcoming of AFOCS products, such as the high cost and the difficulty in realization of light patch, the main contents can be divided into following parts:1. Taking Sagnac AFOCS as the main object, by analyzing a variety of optical structures, the mathematical model and signal processing model of AFOCS is derived by using Jones matrix, which lays the theoretical foundation for in-depth study of AFOCS. Meanwhile, a new In-line type of AFOCS structure is designed which based on the 3×3 coupler, and its feasibility is investigated by simulation experiment. It turned out that an accurate and stable splitting ratio of the coupler is the key to achieving this structure.2. A technology, which is called the reverse method used to create all-optical 1/4 plate, is improved. Compared with the traditional method of welding method and speed spun, the production process of it is simple and there is need to use of an elliptical core polarization maintaining fiber. It can be made by ordinary fiber welding machine as well, which reduces the difficulty in making an all-fiber 1/4 wave plate. The test results show that its extinction ratio is less than 0.8d B and the degree of polarization is up to 90%. At the same time, the mathematical model has been built to study the effects of the error in phase delay and welding point of the quarter-wave plate on AFOCS. It concludes that the error of twist angle q must be less than 0.023 rad and the phase delay error d ￠D must be less than 0.046 rad when linear birefringence is zero and the accuracy the system is 0.2.3. By analyzing the shortcomings of the current methods of signal processing, a new demodulation approach is presented, and a mathematical model is built by MATLAB to simulate its feasibility.4. A prototype of AFOCS system has been built, and the initial prototype development has been completed, while the system signal output has been measured under the condition of different modulation amplitude and frequency. Ultimately, the best modulation voltage(1.76V) and frequency(23.845 k Hz) of the system have been determined. At last, the current has been measured and the signal is collected. A program is written to demodulate the signal. The maximum error of measurement results is 6.492%. The error sources of the measured results are analyzed, and the vibration effect has been improved with good results.