Study on characteristics of Fiber Optic Current Transformer and the impact on reliability of protection

Fiber Optic Current Transformer (FOCT) realizes current measurement by using the Faraday magneto-optical effect. With advantage of unsaturation, wide dynamic range and immunity to electromagnetic interference etc, FOCT is widely used in smart substation and HVDC transmission, so the accuracy of measurement and reliability of long-term operated have become the focus of attention[1-3]. In this paper based on the structure and principle of FOCT, the AC and DC transient response characteristic are analyzed ; the accuracy characteristic and influence factors of FOCT are researched(include Verdet constant, turns of fiber coil, particle nature of light, optical components aging, ambient temperature, magnetic field in space etc), especially impact of peak current on its accuracy, and the influence of fiber stress in its field installation process, correctness of polarity on its accuracy is also studied; furthermore the problem of protection reliability caused by characteristics of digital sampling, fiber installation and polarity is analyzed base on analysis of accuracy and transient characteristics of FOCT, then points out some strategy to deal with it; Finally the measures to improve reliability of FOCT are proposed based on FOCT operation and defects in practical engineering applications.