CN115616462A - High-precision temperature compensation method for optical fiber current sensor - Google Patents
High-precision temperature compensation method for optical fiber current sensor Download PDFInfo
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- CN115616462A CN115616462A CN202110804561.7A CN202110804561A CN115616462A CN 115616462 A CN115616462 A CN 115616462A CN 202110804561 A CN202110804561 A CN 202110804561A CN 115616462 A CN115616462 A CN 115616462A
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Abstract
The invention discloses a high-precision temperature compensation method for an optical fiber current sensor. The system structure comprises a light source 1, an optical fiber collimator 2, a polarizer 3, a Faraday optical rotation sheet 4, a polarization beam splitter 5, a double-optical fiber collimator 6, a photoelectric detector 7, a photoelectric detector 8, a magnetic field bias device 9, a signal processing unit 10 and a glass sleeve 11. The magnetic field bias means 9 is operative to provide a bias magnetic field of known magnitude to the faraday rotator 4. The Faraday rotation angle caused by the magnetic field of the current to be measured and the Faraday rotation angle caused by the bias magnetic field are divided, so that the influence of the temperature can be eliminated, and the quotient of the Faraday rotation angle and the Faraday rotation angle is multiplied by the bias magnetic field to obtain the magnetic field size of the current to be measured after temperature compensation.
Description
Technical Field
The invention relates to the field of optical fiber sensing, in particular to a method for carrying out high-precision temperature compensation on a magneto-optical effect type optical fiber current sensor.
Background
The current sensor has important application in the aspects of monitoring, metering and the like of a power system. With the continuous improvement of the scale and the voltage grade of a power grid, the traditional electromagnetic current transformer has the defects of easy saturation, large inductance, response lag, limited frequency response, large volume, high manufacturing cost, complex insulating structure, difficulty in networking, large-scale remote monitoring and the like, and cannot meet the increasing requirements of distributed, high-precision and remote protection and measurement of the modern power grid. The optical current sensor has the advantages of no transient magnetic saturation, large dynamic range, wide frequency response, small volume, light weight, strong anti-electromagnetic interference capability, intrinsic insulation and the like. Particularly, the optical current sensor is combined with the optical fiber, and by utilizing the advantages of low optical fiber communication loss, wide bandwidth and the like, a long-distance, large-capacity and distributed current monitoring network can be realized, so that the method has important significance for improving the modernization of power grid operation and maintenance management. The optical fiber current sensor has become the first choice of a current transformer in a high-voltage high-current power system.
Currently, the mainstream schemes of the optical fiber current sensor can be roughly divided into two types, namely, an all-fiber type current sensor and a magneto-optical effect type optical fiber current sensor. The all-fiber type current sensor utilizes the intensity, polarization or phase change of light caused by the magnetic field of current when polarized light is transmitted in the optical fiber ring to realize the measurement of the current. Because the Verdet constant of the optical fiber is extremely low, the all-fiber current sensor usually needs the optical fiber with the length of hundreds of meters or even kilometers, so that the volume of an optical fiber ring is large, the optical fiber ring is easily influenced by the residual birefringence of the optical fiber and the change of the optical fiber along with the temperature, the vibration and the like, the environmental stability is poor, the measurement precision is low, and the application is limited.
The working principle of the magneto-optical effect type optical fiber current sensor is the faraday effect. The input and output of the magneto-optical crystal or magneto-optical glass are coupled by an optical fiber. When linearly polarized light passes through the magneto-optical material, the polarization direction can rotate along with the size of a magnetic field, and the measurement of the magnetic field and the current can be realized by detecting the deflection angle. Because the Verdet coefficient of the magneto-optical material is very large, the wide-range angle deflection can be realized only by the thickness of hundreds of micrometers, and therefore, the magneto-optical material has the advantages of small volume, high sensitivity, point measurement and the like. Magneto-optical fiber current sensors have become the mainstream of high-end fiber current sensors. At present, the magneto-optical type optical fiber current sensor has the main problem of temperature cross sensitivity. The accuracy of the measurement is affected by temperature, since the verdet constant of the magneto-optical material varies with temperature. In order to improve the measurement accuracy, the sensor must be temperature compensated. At present, a common scheme for performing temperature compensation on the sensor is to add an additional optical or electrical temperature sensor to measure the temperature, and then perform fitting and interpolation according to temperature calibration data of the optical fiber current sensor to realize correction of temperature influence. Although the principle of the scheme is simple and direct, the scheme has the defects of complex sensing head structure, larger error, additional temperature demodulation system, complex compensation algorithm and the like because the temperature sensor measures the temperature which is not the magneto-optical material.
Disclosure of Invention
The invention provides a high-precision temperature compensation method for an optical fiber current sensor, aiming at solving the problems in the prior art of temperature compensation of a magneto-optical effect type optical fiber current sensor.
The technical scheme adopted by the invention is as follows: a high-precision temperature compensation method of an optical fiber current sensor, and the system structure is shown in figure 1. The device comprises a light source 1, an optical fiber collimator 2, a polarizer 3, a Faraday optical rotation sheet 4, a polarization beam splitter 5, a double optical fiber collimator 6, a photoelectric detector 7, a photoelectric detector 8, a magnetic field bias device 9, a signal processing unit 10 and a glass sleeve 11. The optical fiber collimator 2, the polarizer 3, the Faraday optical rotation sheet 4, the offset beam splitter 5 and the double optical fiber collimator 6 are assembled and packaged in the glass sleeve 11 through optical cement. The magnetic field bias device 9 is composed of two parts, is sleeved outside the glass tube 11 and is symmetrically distributed on two sides of the Faraday rotation piece 4, and provides a bias magnetic field with known magnitude for the Faraday rotation piece 4. Light output by the light source 1 is changed into linearly polarized light through the optical fiber collimator 2 and the polarizer 3, the polarization direction of the light is deflected along with the size of a magnetic field after the light passes through the Faraday optical rotation sheet 4, the deflected polarized light is divided into components in two orthogonal polarization directions, namely horizontal and vertical polarization directions, through the polarization beam splitter 5 and is respectively sent to the photoelectric detector 7 and the photoelectric detector 8 to be converted into electric signals, and the deflection angle (Faraday rotation angle) of the linearly polarized light after the light passes through the Faraday optical rotation sheet 4 can be demodulated after the calculation of the signal processing unit 10. The Faraday rotation angle caused by the magnetic field of the current to be measured and the Faraday rotation angle caused by the bias magnetic field measured at the same temperature are divided, so that the influence of the temperature can be eliminated, and the quotient of the Faraday rotation angle and the Faraday rotation angle is multiplied by the bias magnetic field, so that the size of the magnetic field of the current to be measured after temperature compensation can be obtained.
Compared with the prior art, the invention has the following beneficial effects: firstly, temperature compensation can be realized by utilizing the temperature characteristic of the magneto-optical material by only introducing a bias magnetic field with proper size, so that an additional temperature sensor and a temperature demodulation system are not needed, and the complexity and the cost of the system are obviously reduced; secondly, the change of the Faraday rotation angle caused by the bias magnetic field along with the temperature reflects the temperature of the magneto-optical material, so that the compensation precision is high; thirdly, magnetic field measurement and temperature compensation are realized by measuring the Faraday rotation angle without measuring temperature, the influence of the temperature is eliminated by the ratio of the Faraday rotation angle of the total magnetic field and the bias magnetic field, and operations such as interpolation, fitting and the like on calibration data according to the temperature are not needed, so that the measurement scheme and the compensation algorithm are simple, and the complexity and the cost of the system are further reduced.
Drawings
FIG. 1 is a schematic structural diagram of the present invention.
Fig. 2 is a graph showing the magnetic field and temperature response of the fiber optic current sensor in an embodiment of the present invention.
Fig. 3 shows the measurement error of the fiber optic current sensor in the embodiment of the present invention.
Fig. 4 is a typical signal curve for ac signal measurement according to the present invention.
Reference numeral 1. A light source; 2. a fiber collimator; 3. a polarizer; 4. a Faraday rotation sheet; 5. a polarizing beam splitter; 6. a dual fiber collimator; 7. a photodetector; 8. a photodetector; 9. a magnetic field biasing device; 10. a signal processing unit; 11. a glass sleeve.
Detailed Description
The principle, embodiments and advantages of the temperature compensation of the present invention will be described in detail with reference to the following examples.
The magnetic field bias device 9 is the key of the invention for realizing temperature compensation. The device may be served by a constant magnet, a current coil or an electromagnetic induction coil. The effect of which is to introduce a bias field of known magnitudeB 0 . In thatB 0 Under the action of the Faraday rotator 4, the incident linearly polarized light generates a Faraday rotation angleθ 0 And the two satisfy the following relation:
whereinVAndLare respectively asThe verdet constant and the thickness of the faraday rotator 4. Applying a magnetic field of a current to be measuredB m Faraday rotation angle caused by total magnetic fieldθCan be expressed as
The magnetic field of the current to be measured can be obtained by dividing the formula (2) by the formula (1)
Whereinθ-θ 0 Is thatB m Induced faraday rotation angle. The influence of temperature on the fiber optic current sensor comes from the Verdet constantVVariation with temperature and thicknessLThermal expansion and contraction. Due to elimination of in formula (3)VLTherefore, the measurement result is independent of the temperature, and the temperature compensation of the optical fiber current sensor is realized. Therefore, the technical scheme of the invention only needs to measure the temperature at the same temperatureθAndθ 0 and the influence of the temperature can be eliminated without measuring the specific temperature.
According to Malus' law, faraday rotation angleθCan be obtained according to the following formula:
whereinV 1 AndV 2 respectively, electrical signals detected by photodetectors 7 and 8. By adopting double-path detection and dividing the difference of the two paths of signals by the sum of the two paths of signals according to the formula (4), the interference caused by the power fluctuation of the light source can be eliminated, and the stability and the measurement precision of the measurement signals are improved.
In this embodiment, the Faraday rotator 4 is a 450 μm thick single crystal slice of rare earth doped iron garnet, or other magnetic materials such as magneto-optical glass which can induce Faraday rotation effectAn optical material. The magnetic field bias device 9 is composed of two ring-shaped permanent magnets which are symmetrically distributed at about 1.5cm positions on both sides of the Faraday rotator 4. A magnetic field of 15 mT can be generated at the spinning wafer. Magnitude of bias magnetic fieldB 0 The distance between the magnet and the wafer is adjusted according to the angle resolution capability and the temperature compensation precision requirement of the demodulation system. In this embodiment, the minimum angle change resolvable by the demodulation system is about 0.01 o To make the temperature compensation accuracy to be +/-1 ℃, after the bias magnetic field is applied,θ 0 should have a temperature sensitivity of not less than 0.01 o V. C. According to the requirement, the required temperature and magnetic field calibration data of the optical fiber current sensor can be determinedB 0 . FIG. 2 shows the calibration results of the temperature and the magnetic field of the fiber current sensor in this embodiment, respectively applying the magnetic fieldB m Faradaic angle was measured in the temperature range of-20 ℃ to 60 ℃ at =0, 5, 10, 15, 20 mTθThe value of (c). It can be seen that at each temperatureθAre all along with the magnetic fieldBLinearly, only the slope of the change is different. The measured data under various temperatures are subjected to linear fitting, and the intersection point of the fitted straight line and the transverse axis is the magnitude of the bias magnetic fieldB 0 . In thatB m At the position of =0, the measured deflection angle isθ 0 。θ 0 The temperature dependence is shown in the inset of FIG. 2, which shows that the present embodiment hasθ 0 Has a temperature sensitivity of about 0.01366 o /. Degree.C., diagonal resolution 0.01 o The demodulation system can meet the requirement of temperature compensation accuracy of not less than +/-1 ℃. Although the present invention does not require the measurement of a specific temperature to achieve temperature compensation, if calibrated, it is determinedθ 0 And temperatureTThe invention can realize temperature compensation of the fiber current sensor and measure the real-time temperature of the optical rotation wafer. As can be readily seen from the view of figure 2,B 0 the larger the temperature sensitivity of the wafer, the higher, and hence the higherB 0 The accuracy of temperature compensation is improved. But a saturation magnetic field exists due to the Faraday rotator 4B s Sensor working normallyTime, the total magnetic field size must not exceedB s . Thus, it is possible to provideB 0 It is also not desirable to be too large, otherwise the measurement range of the sensor would be reduced. From the measurement data in fig. 2, it can be calculated that the relative measurement error of the sensor in the temperature range of-20 c to 60 c before temperature compensation is as high as 12% (± 6%).
FIG. 3 shows the signal demodulated by the signal processing unit 10 according to equation (3)B m And actually appliedB m Relative measurement error of (a). The relative measurement error of the sensor after temperature compensation is about +/-0.6%. Compared with the measurement before temperature compensation, the measurement precision is improved by 10 times. The measurement error of the embodiment reaches the standard of class 1 current transformers for measurement and class 5P current transformers for protection specified by national standard GB 1208-2006.
The invention can be used for measuring direct current and alternating current. Fig. 4 shows a typical output signal for a 50Hz ac current measurement. Faraday rotation angleθThe time-varying signal of (2) is calculated by the alternating electrical signals output by the two photodetectors according to the formula (4).θIs proportional to the magnitude of the current, the maximum value of the curveθ max And minimum valueθ min The average value of (a) is introduced by the bias magnetic fieldθ 0 . Therefore, the present invention does not require special measurement when measuring AC signalsθ 0 Only need to measureθIs derived from its maximum and minimum valuesθ 0 The magnetic field of the current after temperature compensation can be obtained according to the formula (3), and real-time continuous measurement of the alternating current signal is realized.
This example is only a partial embodiment of the present invention, and not a complete one. All other embodiments, which can be made by a person skilled in the art without any inventive step based on the embodiments of the present invention, belong to the scope of the present invention.
Claims (6)
1. A high-precision temperature compensation method for an optical fiber current sensor is characterized by comprising the following steps: the device comprises a light source, an optical fiber collimator, a polarizer, a Faraday rotation plate, a polarization beam splitter, a double-optical fiber collimator, a magnetic field bias device, a glass sleeve, two photoelectric detectors and a signal processing unit; realizing temperature compensation through the ratio of the total magnetic field to the Faraday rotation angle caused by the bias magnetic field; the temperature compensation can be realized when the direct current or alternating current magnetic field is measured, and the Faraday rotation angle of the bias magnetic field can be obtained by the average value of the maximum value and the minimum value of the time-varying signal when the Faraday rotation angle is used for alternating current measurement.
2. The method for high-precision temperature compensation of the optical fiber current sensor as claimed in claim 1, wherein: the magnetic field biasing means may be implemented by a permanent magnet, a current coil or an electromagnetic induction coil, which acts to introduce a bias magnetic field of known magnitude.
3. The method for high-precision temperature compensation of the optical fiber current sensor according to claim 1, wherein the method comprises the following steps: and eliminating the influence of temperature through the ratio of the total magnetic field to the Faraday rotation angle caused by the bias magnetic field, thereby realizing the temperature compensation of the sensor.
4. The method for high-precision temperature compensation of the optical fiber current sensor as claimed in claim 1, wherein: the method can be used for direct current measurement and alternating current measurement, and the Faraday rotation angle introduced by the bias magnetic field can be determined by the average value of the maximum value and the minimum value of the Faraday rotation angle time-varying signal during alternating current measurement, so that real-time continuous measurement is realized.
5. The method for high-precision temperature compensation of the optical fiber current sensor as claimed in claim 1, wherein: the magnitude of the bias magnetic field is determined based on the angular resolution of the demodulation system and the temperature characteristic of the Faraday rotator, and the stronger the bias magnetic field, the higher the temperature compensation accuracy, but the smaller the measurement range.
6. The method for high-precision temperature compensation of the optical fiber current sensor as claimed in claim 1, wherein: the Faraday rotator can be magneto-optical materials which can cause Faraday effect, such as magneto-optical crystal, magneto-optical glass and the like, and the main function of the Faraday rotator is to change the polarization direction of incident linearly polarized light.
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