Study Of Solenoid-type High Current Sensor Based On Magnetic Coupling And Fiber Bragg Gratings

Electric current value, as one of the basic parameters in power systems, has reached to hundreds, thousands or even tens of thousands amperes in power plants, substations, AC/DC transmission systems, electrolysis and electroplating systems, locomotives’ power supply systems, nuclear physics, etc. Monitoring current value directly and accurately in real time, is not only related to products’ quality, energy conservation, or even systems’ success or failure, but related to the safety of people’s lives and properties. The traditional current transformers have appeared to be inadequate sometimes in the current situation that vast capacity and extremely high voltage level exist in power systems. Fiber optical current sensors, born with explosion resist ability, immunity to electromagnetic interference, excellent security and other outstanding advantages, have attracted more and more attention. It is reported that products of all-fiber current sensors have come into service. Besides all the advantages of fiber optic sensors, fiber Bragg grating (FBG) sensors have their superiorities of wavelength demodulation, ability of multiplexing and distributed measurement as well. Electric current sensors based on FBGs have also got a series of studies in recent years, but not even a product of this kind of sensors appears so far. A new method of measuring high current with magnetic coupling and FBGs is put forward in this paper. Magnitude of current is converted into the intensity of electromagnetic force between magnetic material blocks, leading to the changing of the FBGs’ resonance wavelengths. The sensor is designed with a simple structure, small size and stable performance.The following contents are the work completed and results achieved in this paper:(1) problems and shortcomings of the traditional current transformers are analyzed, and the urgency and necessity of studying a new type of fiber optic current sensors are indicated;(2) magnetic coupling theory is derived when magnetic field applied to ferromagnetic materials, and, combined with FBG theory, an current sensing method based on magnetic coupling and FBGs is put forward;(3) the solenoid-type FBG current sensing model is designed, and the theoretical relationship between the resonance wavelength shift of the FBG and the current value is derived;(4) the sensor’s key components are simulated and the results coincide well with the theory, then the structure and parameters are determined;(5) experiments with direct current and alternating current are performed to prove the theory, and further experiments with temperature stability compensation are performed. The sensor’s error indicates that the accuracy of the of the sensor prototype reaches5.0grade, which can be applied in relay protection field. With more accurate design and machine, its accuracy grade shall be increased.The sensor is an important exploration to measure current with FBGs. Both the emulational and experimental results are coinciding well with the theory, and the sensor can work for applications in some situations. The method would be a reference for studying new FBG-based current sensors.