全光纤电流互感器传感机理建模分析

Mechanism Analysis and Modeling on the Sensing of Fiber-optical Current Transformers

该文针对全光纤电流互感器测量精度受各种内外部因素影响的问题,建立全光纤电流互感器微元传感单元的分布参数模型,从本质上解释传感单元中线性双折射的产生机理,即极化率张量对角元不相等的作用结果;并结合光电转化的数学模型得到全光纤电流互感器的开环机理,提出提高全光纤电流互感器(fiber-optical current transformer,FOCT)测量精度的方法:采用新型传感材料或新型传感头结构及引入反馈信号构建闭环结构。采用COMSOL有限元数值分析方法,实现光场和磁场的耦合。分析双折射、被测电流、纤芯折射率、光纤的弯曲半径对测量结果的影响。研究表明,线性双折射是由传感材料的折射率变化引起,会降低测量灵敏度;同一双折射,在不同的外界条件(如被测电流)下,对测量结果的影响不同;对于不同的纤芯折射率,折射率越小,传输相同距离后,旋转角越大;微元传感单元中由弯曲半径引起的线性双折射较小,通常可采用分段补偿法,得到理想的Faraday旋转角;最后通过仿真分析与现有经典公式的比较,验证分布参数模型的有效性,为后续FOCT传感单元中的光场、磁场、温度场、应力场等复杂的多物理场耦合提供模型基础。

The measurement accuracy of all fiber current transformers is affected by a variety of internal and external factors. In this paper, an infinitesimal distributed parameter model of FOCT sensing unit was built and the mechanism of the linear birefringence in the sensing unit was revealed, that is, diagonal elements of the polarization tensor are not equal. Combined with the mathematical model of photoelectric conversion, the open loop mechanism of FOCT was obtained. And based on the theory above, two methods to improve the measurement accuracy of FOCT were putting forward. One is by using a new type of sensing materials and a new sensor head structure. The other is by using the feedback signal to construct the closed loop structure. By using COMSOL, the coupling of optical fields and magnetic fields was realized, which provided the model foundation for the coupling of complex multi- physics fields in FOCT sensor units. Besides the influence of birefringence, the measured current, the bending radius of the optical fiber and the refractive index of the fiber core on the measurement results was also analyzed. The study shows that the linear birefringence is caused by the change of the refractive index of the sensing material, and it reduces the measurement sensitivity. Under different external conditions（such as measured current）, the influence of the same birefringence on the measurement results is different. The smaller the refractive index is, the larger the rotation angle will be. The linear birefringence caused by the bending radius of the sensing element is small, and the ideal Faraday rotation angle can be obtained by the segmentation compensation method. By simulation analysis the validity of the distributed parameter model was verified.