Lightning current measuring device and lightning current measuring method

Abstract

A lightning current measuring device is provided with: a polarized light separation element which separates light output from a sensor fiber into horizontal and vertical components having orthogonal planes of polarization, a Faraday rotation angle calculation unit calculating a Faraday rotation angle through arc-sine processing of a digitized signal of the difference between the horizontal and vertical components converted to a signal through photoelectric conversion after separation by the polarized light separation element, amplifiers which amplify the horizontal and vertical components converted to a signal through photoelectric conversion after separation by the polarized light separation element, a Faraday rotation angle calculating unit which calculates a Faraday rotation angle based on a digitized signal of the difference between the amplified horizontal and vertical components, and current value conversion units converting current values based on the calculated Faraday rotation angles.

Claims

1. A lightning current measuring device which is provided with a sensor fiber, inputs linearly polarized light to the sensor fiber, and detects magnitude of Faraday rotation applied to the linearly polarized light by a magnetic field due to a current to be measured flowing through a conductor provided around the sensor fiber to measure the current to be measured, the lightning current measuring device comprising: polarized light separation means for separating output light from the sensor fiber into a horizontal component and a vertical component having orthogonal planes of polarization; first calculation means for calculating a Faraday rotation angle through arc-sine processing of a digitized signal of difference between a horizontal component signal and a vertical component signal converted from the horizontal component and the vertical component through photoelectric conversion after separation by the polarized light separation means; amplification means for amplifying the horizontal component signal and the vertical component signal converted through the photoelectric conversion after the separation by the polarized light separation means; second calculation means for calculating a Faraday rotation angle based on a digitized signal of difference between the horizontal component signal and the vertical component signal amplified by the amplification means; first conversion means for converting a current value from the Faraday rotation angle calculated by the first calculation means; second conversion means for converting a current value from the Faraday rotation angle calculated by the second calculation means; and offset adjustment means for adjusting an offset of the horizontal component signal and the vertical component signal converted from the horizontal component and the vertical component separated by the polarized light separation means based on an amount of offset by setting, as the amount of the offset, a reference level of the horizontal component signal and the vertical component signal in the Faraday rotation angle in a state where the current to be measured does not flow, wherein the second calculation means calculates the Faraday rotation angle based on the difference between the horizontal component signal and the vertical component signal amplified by the amplification means with the offset adjusted by the offset adjustment means.

2. The lightning current measuring device according to claim 1, further comprising: discount means for discounting the horizontal component signal and the vertical component signal being converted from analog to digital, amplified by the amplification means with the offset adjusted by the offset adjustment means with an amplification factor of the amplification means, wherein the second calculation means calculates the Faraday rotation angle based on difference between the horizontal component signal and the vertical component signal discounted by the discount means.

3. The lightning current measuring device according to claim 1, wherein a signal representing the horizontal component and a signal representing the vertical component input to the first calculation means are converted from analog signals to digital signals at a first sampling frequency, a signal representing the horizontal component and a signal representing the vertical component input to the second calculation means are converted from analog signals to digital signals at a second sampling frequency, and the first sampling frequency is faster than the second sampling frequency.

4. The lightning current measuring device according to claim 1, further comprising: combination means for combining waveform of the current converted by the first conversion means and waveform of the current converted by the second conversion means based on a threshold value determined in advance.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a configuration diagram of a lightning current measuring device according to an embodiment of the invention.

(2) FIG. 2 is a diagram showing a horizontal component Px and a vertical component Py of light intensity.

(3) FIG. 3 is a diagram showing the relationship of the difference between the horizontal component Px and the vertical component Py of the light intensity and a Faraday rotation angle.

(4) FIG. 4 is a flowchart showing the flow of small current processing according to this embodiment.

(5) FIG. 5 is a diagram showing changes of signals Px and Py due to the small current processing according to this embodiment.

(6) FIG. 6 is a configuration diagram of a lightning current measuring device according to a modification example.

(7) FIG. 7 is a diagram showing the magnitude of a lightning current.

DESCRIPTION OF EMBODIMENTS

(8) Hereinafter, an embodiment of a lightning current measuring device and a lightning current measuring method according to the invention will be described referring to the drawings.

(9) FIG. 1 is a configuration diagram of a lightning current measuring device 10 according to this embodiment.

(10) As shown in FIG. 1, the lightning current measuring device 10 includes a light source 12, a sensor fiber 14, optical circulator 16, a polarized light separation element 18, a Faraday rotator 20, a signal processing unit 22, and a signal analysis unit 24.

(11) The sensor fiber 14 is arranged so as to revolve around a conductor 26 through which a current to be measured flows. As the sensor fiber 14, preferably, a lead-doped lead glass fiber which is an optical fiber having a characteristic of a large Verdet constant for determining the magnitude of a Faraday effect can be used. As the conductor 26, for example, the main wing of an aircraft is used. With the use of the flexible sensor fiber 14, it is possible to measure a current even if the conductor through which the current to be measured flows is a thick conductor, has a complicated shape, is provided at a narrow place (for example, a narrow strain portion of equal to or less than several mm), or the like.

(12) The Faraday rotator 20 is attached to one end of the sensor fiber 14, and a reflection part (mirror) 28 is formed of a thin metal film by vapor deposition or the like at the other end of the sensor fiber 14. The Faraday rotator 20 and the polarized light separation element 18, and the polarized light separation element 18 and the optical circulator 16 are connected by optical fibers, and the optical circulator 16 is connected in a direction in which light supplied from the light source 12 through a light transmission fiber 30 is transmitted toward the sensor fiber 14.

(13) The signal processing unit 22 has two light receiving elements 32x and 32y as an input unit, the one light receiving element 32x is connected to a terminal from which transmitted light from the sensor fiber 14 of the optical circulator 16 is output by a light receiver fiber 34x, and the other light receiving element 32y is connected to the polarized light separation element 18 by a light receiver fiber 34y.

(14) Light emitted from the light source 12 enters the polarized light separation element 18 through the light transmission fiber 30 and the optical circulator 16. Light is converted to linearly polarized light in which the vibration direction of an electric field is aligned in one direction (the principal axis direction of the polarized light separation element 18) by the polarized light separation element 18, and is input to the Faraday rotator 20. The Faraday rotator 20 is made of a permanent magnet and a ferromagnetic garnet which is ferromagnetic crystal magnetically saturated by the permanent magnet, and applies a Faraday rotation of 22.5 degrees in a single unidirectional trip to light transmitted through the ferromagnetic garnet. Linearly polarized light emitted from the Faraday rotator 20 is input to the sensor fiber 14 and is subjected to the Faraday rotation by a magnetic field generated around the current to be measured flowing through the conductor 26 in the revolving portion of the sensor fiber 14, and the plane of polarization thereof rotates at a Faraday rotation angle proportional to the magnitude of the magnetic field.

(15) Light propagating through the sensor fiber 14 is further reflected by the reflection part 28, passes through the revolving portion again, is subjected to the Faraday rotation, and is input to the Faraday rotator 20. The Faraday rotation of 22.5 degrees is further applied when light passes through the Faraday rotator 20 again, whereby the Faraday rotation of a total of 45 degrees in a round trip is applied by the Faraday rotator 20. That is, in the lightning current measuring device 10, an optical bias is set to 45 degrees. Light passing through the Faraday rotator 20 is guided again into the polarized light separation element 18 and separated into a horizontal component and a vertical component (see FIG. 2) which are two polarization components having orthogonal polarization directions (the principal axis direction of the polarized light separation element 18 and the direction perpendicular thereto).

(16) Light of the separated horizontal component is received by the light receiving element 32x through the optical circulator 16 and the light receiver fiber 34x and is converted to a signal Px (analog signal) proportional to the light intensity of the horizontal component through photoelectric conversion. Light of the vertical component is received by the light receiving element 32y through the light receiver fiber 34y and is converted to a signal Py (analog signal) proportional to the light intensity of the vertical component through photoelectric conversion.

(17) The signal processing unit 22 includes A/D converters 36x1 and 36y1 which convert an analog signal to a digital signal, and small current processing units 38x and 38y.

(18) The signal Px output from the light receiving element 32x is input to the A/D converter 36x1 and the small current processing unit 38x. The small current processing unit 38x includes an offset unit 40x, an amplifier 42x, an A/D converter 36x2, and a discount unit 44x.

(19) The A/D converter 36x1 converts the signal Px from an analog signal to a digital signal at a first sampling frequency. The signal Px converted to a digital signal by the A/D converter 36x1 is input a Faraday rotation angle calculation unit 46A in the signal analysis unit 24.

(20) The signal Px input to the small current processing unit 38x is subjected to the small current processing described below in detail, is converted from an analog signal to a digital signal at a second sampling frequency by the A/D converter 36x2, and is then input to a Faraday rotation angle calculation unit 46B in the signal analysis unit 24.

(21) The signal Py output from the light receiving element 32y is input to the A/D converter 36y1 and the small current processing unit 38y. The small current processing unit 38y includes an offset unit 40y, an amplifier 42y, an A/D converter 36y2, and a discount unit 44y.

(22) The A/D converter 36y1 converts the signal Py from an analog signal to a digital signal at the first sampling frequency. The signal Py converted to a digital signal by the A/D converter 36y1 is input to the Faraday rotation angle calculation unit 46A in the signal analysis unit 24.

(23) The signal Py input to the small current processing unit 38y is subjected to small current processing described below, is converted from an analog signal to a digital signal at the second sampling frequency by the A/D converter 36y2, and is then input to the Faraday rotation angle calculation unit 46B in the signal analysis unit 24.

(24) The Faraday rotation angle calculation unit 46A calculates a Faraday rotation angle through arc-sine processing of the difference between the horizontal component of the light intensity represented by the signal Px and the vertical component of the light intensity represented by the signal Py. A current value conversion unit 48A converts a current value from the Faraday rotation angle calculated by the Faraday rotation angle calculation unit 46A.

(25) The Faraday rotation angle calculation unit 46B calculates a Faraday rotation angle based on the difference between the horizontal component of the light intensity represented by the signal Px subjected to the small current processing and the vertical component of the light intensity represented by the signal Py subjected to the small current processing. A current value conversion unit 48B converts a current value from the Faraday rotation angle calculated by the Faraday rotation angle calculation unit 46B.

(26) A waveform combination unit 50 in the signal analysis unit 24 combines the waveform of the current converted by the current value conversion unit 48A and the waveform of the current converted by the current value conversion unit 48B based on a threshold value determined in advance.

(27) Next, the operation of the lightning current measuring device 10 will be described.

(28) The measurement by the lightning current measuring device 10 is triggered, for example, when an external magnetic field which occurs with the generation of actual lightning is detected, or the timing of discharging by simulating lightning when lightning is generated through an experiment.

(29) A lightning current which is a current to be measured is changed, for example, from 100 kA to 100 A, when the lightning current is small, the Faraday rotation angle is small, and when the lightning current is large, the Faraday rotation angle is large.

(30) As shown in FIG. 3, when the Faraday rotation angle is small, the difference between the horizontal component and the vertical component of the light intensity and the Faraday rotation angle are in a substantially proportional relationship (broken line), and when the Faraday rotation angle is large, the relationship of the difference between the horizontal component and the vertical component of the light intensity and the Faraday rotation angle is expressed by a sine function (solid line).

(31) That is, when measuring a large current included in a lightning current, it is preferable to obtain a current value through the arc-sine processing of the difference between the horizontal component and the vertical component of the light intensity.

(32) Specifically, if an electric field is E and the Faraday rotation angle is .sub.F, the light intensity of the horizontal component represented by the signal Px is expressed by Expression (1) described below, and the light intensity of the vertical component represented by the signal Py is expressed by Expression (2) described below. The difference S between the horizontal component and the vertical component of the light intensity is expressed by Expression (3) described below.

(33) $\begin{array}{cc}{S}_x=E^2{\cos }^2\left(\frac{}{4}-2{}_F\right)& \left(1\right)\\ S_y=E^2{\sin }^2\left(\frac{}{4}-2{}_F\right)& \left(2\right)\\ S=S_x-S_y=E^2{\sin }^{2}4{}_F& \left(3\right)\end{array}$

(34) The Faraday rotation angle .sub.F is expressed by Expression (4) described below based on Expression (3).

(35) $\begin{array}{cc}{}_F=\frac{1}{4}{\mathrm{Arcsin}}^2\left(\frac{S}{E^2}\right)& \left(4\right)\end{array}$

(36) A lightning current I is expressed by Expression (5) using the Faraday rotation angle .sub.F.

(37) $\begin{array}{cc}I=\frac{I_o}{/2}.Math.{}_F& \left(5\right)\end{array}$

(38) In Expression (5), I.sub.O is a current which becomes a reference at a predetermined Faraday rotation angle, and is specified by the property of the optical fiber. For example, in a lead-doped optical fiber, when the Faraday rotation angle is 45, the current value becomes 100 kA.

(39) In this way, the lightning current measuring device 10 calculates the Faraday rotation angle through the arc-sine processing of the difference between the horizontal component and the vertical component using Expressions (1) to (4) by the Faraday rotation angle calculation unit 46A, and converts the current value from the calculated Faraday rotation angle by the current value conversion unit 48A, whereby it is possible to measure a large current in a lightning current with excellent accuracy.

(40) Next, small current processing which is performed in order to measure a small current included in a lightning current with excellent accuracy will be described referring to FIGS. 4 and 5.

(41) FIG. 4 is a flowchart showing the flow of the small current processing according to this embodiment, and the small current processing which is performed in each of the small current processing units 38x and 38y is the same. FIG. 5 is a diagram showing changes of the signals Px and Py due to the small current processing.

(42) If the signal Px is input to the small current processing unit 38x, first, in Step 100, a Px baseline (Pxb) which becomes the amount of offset is subtracted from the signal Px by the offset unit 40x, thereby removing the offset of the signal Px (FIG. 5(b)). As shown in FIG. 5(a), the Px baseline is a reference level before a lightning current is generated. With this, it is possible to remove temperature dependence of the Faraday rotator 20, a ripple component of the light source 12, or the like.

(43) Next, in Step 102, the signal Px with the offset removed is amplified with a predetermined amplification factor Gain by the amplifier 42x as shown in Expression (6) described below and becomes a signal Fx (FIG. 5(c)). As the amplifiers 42x and 42y, an amplifier having a wide amplifiable band (0.1 Hz to several 10 MHz) is used.
Fx=(PxPxb).Math.Gain(6)

(44) Next, in Step 104, the signal Fx is converted from an analog signal to a digital signal by the A/D converter 36x2.

(45) Next, in Step 106, as shown in Expression (7) described below, the signal Fx which has become a digital signal is discounted by the discount unit 44x by dividing the signal Fx by the amplification factor Gain and adding the Px baseline Pxb to calculate a fine variation Px (FIG. 5(d)).

(46) $\begin{array}{cc}{\mathrm{Px}}^{}=\mathrm{Pxb}+\frac{\mathrm{Fx}}{\mathrm{Gain}}& \left(7\right)\end{array}$

(47) In this way, in the process of converting an analog signal according to a small current to a digital signal, the small current processing amplifies the analog signal according to the small current, converts the amplified analog signal to a digital signal, and then discounts the digital signal with the amplification factor used for amplification, whereby it is possible to reduce quantization errors.

(48) Next, in Step 108, as shown in Expression (8) described below, the calculated fine variation Px is standardized by the discount unit 44x by dividing the fine variation Px by the Px baseline Pxb (FIG. 5(e)), the signal Px after standardization is output to the Faraday rotation angle calculation unit 46B, and the small current processing ends.

(49) $\begin{array}{cc}\mathrm{Px}=\frac{{\mathrm{Px}}^{}}{\mathrm{Pxb}}& \left(8\right)\end{array}$

(50) With the standardization shown in Expression (8), when an initial state where a lightning current is not generated is 1, it is possible to clarify to what extent the Faraday rotation angle is changed.

(51) The small current processing for the signal Py is the same as the small current processing for the signal Px, a Py baseline (Pyb) which becomes the amount of offset is subtracted from the signal Py by the offset unit 40y, and then, the signal Py is amplified with a predetermined amplification factor by the amplifier 42y. The small current processing for the signal Py performs A/D conversion by the A/D converter 36y2, then calculates a fine variation Py by the discount unit 44y, and outputs the signal Py standardized by dividing the fine variation Py by the Py baseline Pyb to the Faraday rotation angle calculation unit 46B.

(52) The above-described small current processing is performed on the signals Px and Py, whereby it is also possible to measure a Faraday rotation angle of a small current with a small amount of displacement of the Faraday rotation angle.

(53) The lightning current measuring device 10 calculates the Faraday rotation angle based on the difference between the horizontal component and the vertical component of the light intensity subjected to the small current processing by the Faraday rotation angle calculation unit 46B, and converts the current value from the Faraday rotation angle by the current value conversion unit 48B, whereby it is possible to measure a small current included in a lightning current with excellent accuracy.

(54) In the Faraday rotation angle calculation unit 46B, the Faraday rotation angle may be calculated with the difference between the horizontal component and the vertical component of the light intensity and the Faraday rotation angle in the proportional relationship, or the Faraday rotation angle may be calculated with the relationship expressed by the sine function.

(55) When calculating the Faraday rotation angle with the proportional relationship, the Faraday rotation angle is calculated by multiplying the difference between the horizontal component and the vertical component of the light intensity by a predetermined coefficient. When calculating the Faraday rotation angle with the relationship expressed by the sine function, the Faraday rotation angle is calculated using Expressions (1) to (4).

(56) As described above, a lightning current includes a pulsed large current, and a small current which is maintained for a longer time than the large current. That is, the large current has a high frequency, and the small current has a low frequency.

(57) Accordingly, the lightning current measuring device 10 makes the first sampling frequency for digitizing the input input to the Faraday rotation angle calculation unit 46A corresponding to the large current faster than the second sampling frequency for digitizing the signal input to the Faraday rotation angle calculation unit 46B corresponding to the small current. That is, the first sampling frequency of the A/D converters 36x1 and 36y1 is faster than the second sampling frequency of the A/D converters 36x2 and 36y2, and requested time resolution for measuring the large current at the high frequency is satisfied.

(58) With this, the lightning current measuring device 10 can perform a measurement with higher accuracy according to a large current and a small current included in a lightning current.

(59) The waveform of the current converted by the current value conversion unit 48A and the waveform of the current converted by the current value conversion unit 48B are combined based on the threshold value determined in advance by the waveform combination unit 50.

(60) Since the waveform of the current converted by the current value conversion unit 48A is the waveform obtained by the processing according to the large current included in the lightning current, accuracy of the waveform corresponding to the small current in the waveform of the current converted by the current value conversion unit 48A is not higher than accuracy of the waveform corresponding to the large current. Since the waveform of the current converted by the current value conversion unit 48B is the waveform obtained by the processing according to the small current included in the lightning current, accuracy of the waveform corresponding to the large current in the waveform of the current converted by the current value conversion unit 48B is not higher than accuracy of the waveform corresponding to the small current.

(61) For this reason, the waveform of the current converted by the current value conversion unit 48A and the waveform of the current converted by the current value conversion unit 48B are combined based on the threshold value determined in advance, whereby a waveform from a high frequency domain to a low frequency domain of a lightning current can be simply generated.

(62) The threshold value determined in advance is determined by, for example, a current value or a time, specifically, a current value (for example, 1 kA) between the large current and the small current, a time (for example, 5 msec) between the large current and the small current, or the like.

(63) As described above, the lightning current measuring device 10 according to this embodiment includes the polarized light separation element 18 which separates output from the sensor fiber 14 into the horizontal component and the vertical component having orthogonal planes of polarization, the Faraday rotation angle calculation unit 46A which calculates the Faraday rotation angle through the arc-sine processing of the digitized signal of the difference between the horizontal component and the vertical component converted to a signal through the photoelectric conversion after separation by the polarized light separation element 18, the amplifiers 42x and 42y which amplify the horizontal component and the vertical component converted to a signal through the photoelectric conversion after separation by the polarized light separation element 18, the Faraday rotation angle calculation unit 46B which calculates the Faraday rotation angle based on the digitized signal of the difference between the horizontal component and the vertical component amplified by the amplifiers 42x and 42y, the current value conversion unit 48A which converts the current value from the Faraday rotation angle calculated by the Faraday rotation angle calculation unit 46A, and the current value conversion unit 48B which converts the current value from the Faraday rotation angle calculated by the Faraday rotation angle calculation unit 46B.

(64) Accordingly, the lightning current measuring device 10 performs processing corresponding to a large current and processing corresponding to a small current simultaneously, whereby it is possible to measure a lightning current of a DC component of a high dynamic range including a large current and a small current.

(65) Table 1 described below shows the comparison of an existing current measuring device and the lightning current measuring device 10 according to this embodiment.

(66) TABLE-US-00001 TABLE 1 Lightning Current Measurement High-Speed Rise Current Continued Measurement of (1 sec) (1 sec) Narrow Place Large Current Small current or Electromagnetic Electric Current Measuring Device (100 kA) (400 A) Thick Conductor Noise Insulation Rogowski Coil A C C C C Current Transformer A C C C C Existing Optical Fiber B B A A A Current Sensor (1) (Lead Glass Fiber Sensor, Light Intensity Detection, Use for AC, Analog Processing) Existing Optical Fiber C A C A A Current Sensor (2) (Quartz Glass Fiber Sensor, Sagnac Interferometer, Use for DC + AC, Digital Processing) Lightning Current Measuring A A A A A Device according to this Embodiment

(67) As shown in Table 1, in each existing current measuring device, when a measurement of a lightning current is intended, there are unsuitable items; however, the lightning current measuring device 10 according to this embodiment satisfies each item and is suitable for a measurement of a lightning current.

Modification Example

(68) FIG. 6 shows a modification example of the lightning current measuring device 10.

(69) As shown in FIG. 6, the lightning current measuring device 10 includes A/D converters 36x and 36y on the downstream side of the light receiving elements 32x and 32y. Accordingly, the configuration of the lightning current measuring device 10 is further simplified.

(70) The lightning current measuring device 10 may perform the same processing as the offset removal performed in the small current processing units 38x and 38y for the signals Px and Py input to the Faraday rotation angle calculation unit 46A, that is, signals corresponding to the large current.

(71) Although the invention has been described based on the above-described embodiment, the technical scope of the invention is not limited to the scope of description in the above-described embodiment. Various changes or improvements can be made to the above-described embodiment without departing from the spirit and scope of the invention, and embodiments including the changes or the improvements still fall within the technical scope of the invention.

(72) For example, in the above-described embodiment, although a form in which the conductor 26 is an aircraft has been described, the invention is not limited thereto, and a form in which an automobile, a wind power generation device, a cable, a building, or the like is used as the conductor 26 may be made.

REFERENCE SIGNS LIST

(73) 10: lightning current measuring device 14: sensor fiber 18: polarized light separation element 26: conductor 36x1: A/D converter 36x2: A/D converter 36y1: A/D converter 36y2: A/D converter 40x: offset unit 40y: offset unit 42x: amplifier 42y: amplifier 46A: Faraday rotation angle calculation unit 46B: Faraday rotation angle calculation unit 48A: current value conversion unit 48B: current value conversion unit