The abnormal temperature rise is the precursor of the defective composite insulator in power transmission line. However no consolidated techniques or methodologies can on line monitor its internal temperature now. Thus a new method using embedding fiber Bragg grating (FBG) in fiber reinforced polymer (FRP) rod is adopted to monitor its internal temperature. To correctly demodulate the internal temperature of FRP rod from the Bragg wavelength shift of FBG, the conversion coefficient between them is deduced theoretically based on comprehensive investigation on the thermal stresses of the metal-composite joint, as well as its material and structural properties. Theoretical model shows that the conversion coefficients of FBG embedded in different positions will be different because of non-uniform thermal stress distribution, which is verified by an experiment. This work lays the theoretical foundation of monitoring the internal temperature of composite insulator with embedding FBG, which is of great importance to its health structural monitoring, especially early diagnosis.
The most important part of composite insulator is the joint of the composite rod and the metal end-fitting because most of mechanical faults take place here. Thus it is necessary to on-line monitoring of the joints. This paper theoretically analyzes the response behavior of the embedded fiber Bragg grating (FBG) in the joint based on our preliminary analysis of the stress distribution of the composite rod. Comprehensive considering the thermal stress and the material thermal coefficient, the reflection spectrum of FBG embedded in three typical locations is simulated. The simulation result shows that the wavelength shift of the FBG embedded in the joint is about 180pm larger than those embedded out of joint when the temperature rises 10°C. This result has the directive significance for the embedding position and the embedded way. It can be used to monitor the mechanical strength changing with temperature, to improve the manufacture technology of the joint, and to detect the change of electrical property.
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