Research On All Fiber Optical Current Transducer

With the development of power system, both voltage level and the transmission capacity are increasing. The stability of relay protection system becomes more and more important. Conventional electromagnetic current transducers have inevitable defects, such as insulating structural complexity, high cost, large size, etc. In contrast, all fiber optical current transducers (AFOCT) have various advantages, such as wide measurement range, high sensitivity, small size, high security and so on.This thesis gives a review of AFOCT, including its background, classification, principles, etc. Then It focus on the research of reflective AFOCT, including theoretical analysis, system design, signal processing and results analyzing.In this thesis, every component of reflective AFOCT has been represented by Jones matrix. And the evolution of the polarization state has been analyzed. The birefringence of sensing fiber and the imperfection of quarter wave plate (QWP) also have been analyzed. It's found that the scale factor deviates from ideal value with the linear birefringence increases, which can be compensated by the introduction of circular birefringence. The scale factor is nonlinearized with increasing QWP's retardation error as well as deviation of QWP's alignment. However, a certain value of linear birefringence can compensate for deviation of QWP's alignment to make a better scale factor. In addition, it has been proved that the scale factor can be stabled by using Faraday rotator mirror (FRM) and harmonic division.Based on theoretical analysis, the optical expriment has been designed and optimized in this thesis. Key components and parameters have been selected. Harmonic division has been chosen for signal processing based on analysis and comparison. The first and second harmonics have been obtained by demodulating the output signal from photodetector (PD). Then they have been processed by FPGA. The results have showed that measuring range is 2~2000A. And they also have showed good linearity, whose relative error is below 0.5% while the current is above 200A.