This article explores the measurement of electric current using optical fibers, primarily through the Faraday effect, also known as the magneto-optic effect. Fiber-Optic Current Sensors (FOCS) offer high accuracy, modularity, and easy installation. They provide isolation of the measuring part from the primary technology and can measure inside transformers, including winding temperature. The paper summarizes published results, approaches, and principles for defining electrical values using fiber optic technology.
The measurement of electrical current using fiber-optic technology is based on Faraday's magneto-optical effect, discovered in 1845. This effect involves the rotation of the polarization plane of light in the presence of a magnetic field. The use of this phenomenon for measuring electrical quantities has only become feasible with advancements in fiber optic technology. The rotation angle (β) of the polarization plane can be calculated using the following equation:
β=V⋅B⋅d\beta = V \cdot B \cdot dβ=V⋅B⋅d
where:
Fiber-Optic Current Sensors (FOCS) leverage the Faraday effect to measure electric current. These sensors are highly accurate and can detect current ripple and transients over a wide bandwidth. They offer several advantages, including:
Metallurgical Industry: FOCS are used in electrolytic processes for obtaining precious metals, where they measure direct current (DC) up to 500 kA. Their compact size and high accuracy make them ideal for this application.
High-Voltage Direct Current (HVDC) Lines: FOCS are employed to measure and protect high-voltage DC lines, which are crucial for long-distance electricity transmission. Their immunity to magnetic overload and electromagnetic interference ensures reliable performance even under high short-circuit currents.
Transformer Monitoring: FOCS can be integrated into transformers to measure internal currents and temperatures, helping to prevent overheating and optimize performance.
Substation Control and Protection: Due to their small dimensions and weight, FOCS can be integrated into existing substation equipment like switches and bushings, providing accurate current measurements for control and protection.
Recirculating Architecture: An advanced FOCS design that uses a recirculating fiber loop to enhance current sensitivity. A 2×2 optical switch creates the loop, with a single-mode fiber as the sensing head. Sensitivities of 11.5 degrees/A for 1000 meters of fiber and 21.2 degrees/A for 500 meters of fiber have been experimentally achieved.
Spun Highly Birefringent Fiber: This approach reduces the size of the sensing coil, making it suitable for high-voltage watt-hour meters. The special spun fiber resists bending, improving measurement performance. The FOCS using this fiber achieved a measurement error of ±0.2% across a temperature range of -40°C to 70°C.
Plasma Current Measurement: FOCS technology has been adapted for measuring plasma currents in fusion reactors, such as the International Thermonuclear Experimental Reactor (ITER). The sensor measures the rotation of the polarization state in the presence of a magnetic field, achieving high accuracy in extreme conditions.
Temperature Drift Compensation: Research on temperature drift shows a linear relationship between temperature and FOCS error. Understanding this relationship allows for effective temperature compensation, improving sensor accuracy in varying temperatures.
Vibration Sensitivity Reduction: Improvements in fiber design and sensor configuration can mitigate errors caused by mechanical vibrations, enhancing the stability and accuracy of FOCS in dynamic environments.
HVDC Applications: FOCS technology is used to measure high DC currents up to 500 kA, offering significant advantages over traditional current transducers, such as smaller size, lighter weight, and higher accuracy.
Interferometric Techniques: Advanced interferometric designs, such as Sagnac interferometers, are used in FOCS to improve accuracy and sensitivity. These designs minimize errors caused by linear birefringence and environmental factors.
Fiber-Optic Current Sensors (FOCS) represent a significant advancement in the measurement of electric current. Leveraging the Faraday effect, FOCS offer high accuracy, wide bandwidth, and immunity to electromagnetic interference. They are particularly valuable in applications requiring precise current measurements, such as the metallurgical industry, HVDC lines, transformer monitoring, and substation control. The development of advanced FOCS designs and compensation techniques continues to enhance their performance and reliability, making them an essential tool in modern electrical measurement and monitoring systems.
By summarizing the current state of FOCS technology and highlighting key advancements, this article underscores the importance and utility of fiber-optic sensors in the accurate measurement of electrical currents. The ongoing research and development in this field promise further improvements and broader applications, solidifying the role of FOCS in ensuring the efficiency and safety of electrical systems worldwide.
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