Changfei Company Launches Rotating Fiber for Fiber Optic Current Transformer

With the vigorous promotion of the construction of smart grids during the 13th Five-Year Plan period and the steady development of the Belt and Road development strategy, the process of building a global energy Internet and the rapid development of low-energy metallurgical industries have been promoted. The rapid development of these industries has placed an increasingly urgent need for the accurate measurement of UHV and high current. The fiber-optic current transformer based on Faraday magneto-optical effect has a wide application prospect in the field of high-current measurement because of its advantages of high measurement accuracy, wide frequency band, large dynamic range, simple insulation and flexible installation. At the same time, due to the complexity of the field application environment, in addition to having a high enough measurement accuracy, the fiber optic current transformer should also have excellent temperature performance and vibration resistance. For the measurement of ultra-high current in the field of electro-metallurgy, the current transformer should also meet the requirements of flexible installation, and the measurement accuracy is sensitive to loop shape, bus eccentricity, and stray magnetic field interference. Based on the above background, Changfei Optical Fiber Cable Co., Ltd. (hereinafter referred to as Changfei Company) has introduced a special optical fiber for fiber optic current transformers - rotating fiber. As the core component of the fiber optic current transformer, the rotating fiber ensures the practical application requirements of the fiber optic current transformer.

Rotating fiber is a special polarization- maintaining fiber that introduces circular birefringence through the axis rotation of the fiber and forms elliptical birefringence together with the high linear birefringence of the fiber itself. This type of fiber has a good circular polarization retention capability when the rotation period is sufficiently small. At the same time, due to the high internal stress of the optical fiber, the polarization- maintaining fiber has good ability to resist external interference and internal defects, which can substantially improve the environmental adaptability of the fiber-optic current transformer and ensure that the transformer meets the actual application requirements. It is suitable for the application of all-fiber current transformers and is the most promising research direction.

1. Rotating optical fiber preservation mechanism  
The fundamental mode of the fiber has two polarization states, and their propagation constants (ie, wave number, β=2π•neff/λ) are β1 and β2, respectively, and the circular polarization maintains β1≈β2. The difference between the propagation constants of the two polarization states in the birefringent fiber Δβ (ie β1-β2) is large (that is, the propagation velocity differs greatly), and the delay difference between the modes (corresponding to the phase difference) increases as the propagation distance increases. Linear increase.
  

 

Using the discrete concept, a rotating fiber can be thought of as an unprocessed birefringent fiber slice that is longer than a quarter of a rotation period connecting another segment of the same fiber, but with the main axis rotated by π/2. Thus, the mode delay of these two fibers can be completely offset, as shown in Figure 1.
Such a rotating fiber is partially a linear polarization maintaining fiber, and the whole is a circular polarization maintaining fiber. The shorter the rotation period, the shorter the length of the slice, and the smaller the local maximum phase difference, the closer to the ideal circularly polarized light.

2, rotating fiber preparation  

The preparation method of the rotating optical fiber is generally divided into two types: one is the post-processing of the polarization-maintaining fiber , and the other is the in-line rotation of the polarization-maintaining stress rod. Although the former method is flexible, it is difficult to ensure the mechanical reliability of the optical fiber due to the stripping and recoating of the optical fiber coating layer. The latter method is similar to the production of a conventional polarization-maintaining fiber , which is directly drawn and coated by a preform. It not only ensures the strength of the fiber, but also has a sufficient production capacity.

Ultra-high-speed rotation of preforms and ultra-low-speed drawing of fiber optics are challenging for preforms, rotary draw towers and process control. Rotating optical fibers currently sold in the domestic market are mainly monopolized by foreign suppliers . Most of the fiber-optic current transformer products in the domestic pilot network use imported rotating fiber. However, the imported rotating optical fiber has high unit price, limited fiber length, and uncontrolled delivery batch and delivery time .

Changfei Company, as a professional fiber optic cable design and manufacturing manufacturer, has been closely following and continuously following the development of current transformer technology. Based on the major scientific instrument special led by China Institute of Metrology, the development of broadband high current measuring instrument Application", after years of accumulation on the mature fiber optic technology platform, continue the Panda-type polarization- maintaining fiber production process, and successively overcome a) high-precision straightness (geometry, center of gravity) stress preform; b) high-speed rotating drawing tower Dynamic vertical accuracy and stability; c) Process problems such as automatic control of ultra-low-speed wire drawing, forming a complete set of rotating fiber preparation and testing platform with long flying characteristics. Based on this, the first high birefringence rotating fiber SH1016-A has a 5mm rotating pitch, segment length ≥500m, and screening tension up to 100kpsi. The related products have been recognized by the industry's authoritative application units, and the performance has reached the leading level at home and abroad. The exchange and cooperation between the State Key Laboratory of Changfei Fiber Optic Cable Manufacturing Technology and the Wuhan Branch of China Electric Power Research Institute will provide a broader research platform for the special fiber to better serve the power industry.