JP2006046938A - Feeble earth-current detection method and its system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a feeble earth-current detection method and its system, capable of contributing to highly accurate detection of earthquake earth current which is a premonitory phenomenon of an earthquake, by detecting a feeble earth current with high sensitivity or to quantitative grasping of the electromagnetic environment. <P>SOLUTION: Polarization fluctuation caused by a current flowing in an overhead ground line 14, of an optical signal transmitted through an optical fiber 23 stored in the overhead ground line 14 of a power transmission line is converted into an electrical signal by a polarization detecting part 24, and a signal showing the feeble earth current is separated from other noises by an ultra-long wave band ELF component measuring part 26 from an output signal from the polarization detecting part 24. Hereby, the feeble earth current is measured with high accuracy, by utilizing the fact that the polarization fluctuation of the optical signal transmitted through the optical fiber 23 is changed by the earth current flowing in the overhead ground line 23. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a weak ground current detection method, a seismic ground current detection method, and a system therefor, and is particularly useful when applied to applications that need to detect a weak ground current with high sensitivity, such as a ground current abnormality caused by an earthquake precursor. is there.

  It has been reported worldwide that various electromagnetic phenomena are detected in the weeks and months before the earthquake. In response to this, a method of detecting an abnormality immediately before an earthquake from an abnormality of a mechanical measurement amount such as expansion or displacement of the crust is mainly employed at present. However, it is often the case that an abnormality in the mechanical measurement amount can be detected near the surface of the earth just before about half a day to just before a few days. For this reason, sufficient disaster prevention activities cannot be adopted.

  Therefore, if an early abnormality can be detected by the electromagnetic measurement method, the effect on disaster prevention cannot be measured. Incidentally, the existence of electromagnetic phenomena associated with earthquakes has been reported worldwide, and in particular, it has been reported that electromagnetic abnormal phenomena are observed earlier in the weeks and months before the earthquake.

  One of the earthquake prediction methods based on the observation of electromagnetic phenomena is a method of directly observing the earth current, earth potential and the electromagnetic field around the earth. The other is a method of indirectly detecting an abnormality of the crust on the propagation path from the propagation abnormality of navigation radio waves and broadcast radio waves.

  However, in these methods, sufficient sensitivity cannot be obtained stably due to artificial noise or the like, and the apparatus becomes complicated. In addition, there are problems such as large restrictions on the installation of observation points. That is, the detected electromagnetic anomaly is small in quantity, and the detection signal is often masked by artificial noise or the like by a commercial power supply and cannot be detected.

  In addition, methods for detecting ground current anomalies using power transmission equipment are being studied. For example, a method of detecting a ground current abnormality from a neutral ground current abnormality of a power transmission substation is an example. However, in this case, the detection performance is limited, and detailed detection of crustal anomalies has not been achieved. The cause is considered to be that it is difficult to detect a weak ground current signal due to noise caused by leakage of a transmission current and load fluctuations and discharge noise such as lightning.

  In addition, the following nonpatent literature can be mentioned as a well-known literature regarding the detection of earth current.

  As mentioned above, it is believed that the crust in the earthquake preparation stage is subjected to high pressure, and electromagnetic abnormalities such as electrification and earth current changes are formed due to changes in electrical conductivity and melting of some rock bodies. However, since these abnormal earth currents flow in a wide range of the crust, the amount of detection is extremely small and generally difficult to detect. In particular, in a power transmission facility, it is difficult to detect a weak ground current due to a noise current due to a leakage of a transmission current or a load current and a discharge noise such as lightning. In addition, even when using an optical fiber in an overhead ground wire, the ground current flowing through the overhead ground wire is larger than the detected current in the power system current and noise, and is further polarized due to lightning discharge current and cable fluctuations caused by wind. Fluctuating noise is generated, preventing detection of weak ground current.

  In addition, investigations of electromagnetic environments have been conducted in which weak electromagnetic waves are detected with high accuracy to quantitatively grasp environmental electromagnetic waves. Currently, this survey is carried out with an observation device installed at each survey point. Therefore, there is a problem that the greater the number of survey points, the more labor is required for the survey.

Aichi, Horii, Kurono, Denki High Voltage Study Group HV-00-4, 2000 Kurono, Morimura, Horii, Electromagnetic environment study group EMC-02-10, 2002

  In view of the above prior art, the present invention uses the fact that the polarization rotation amount of the light wave propagating in the optical fiber accommodated in the overhead ground wire of the transmission line is related to the ground current flowing through the overhead ground wire. Sensitive ground current detection method, seismic ground current detection method and system capable of contributing to highly accurate detection of seismic ground current of earthquake precursory by detecting sensitive ground current in sensitivity and quantitative understanding of electromagnetic environment The purpose is to provide.

  As a first solution of the present invention, in order to avoid the influence of strong transmission current and load fluctuation, the ground current signal is not directly detected from the neutral grounding current in a substation or the like, but the overhead on the transmission tower is By adopting a method that indirectly detects the ground current flowing through the ground line from the polarization fluctuation of the optical signal of the optical fiber accommodated in the overhead ground line, noise from the power system such as transmission current and load fluctuation is reduced. It is assumed that an optical fiber detection method that can be greatly reduced is adopted. Then, with this solution alone, high-sensitivity detection cannot be achieved due to power transmission noise and lightning discharge noise flowing through the overhead ground wire, and also optical fiber detection noise due to the fluctuation of the overhead ground wire. The six means of solution are added to achieve high sensitivity.

  Of these, the second solution is not to detect the ground current change directly in the low frequency range from the detected signal after detecting the polarization fluctuation of the light, but to remove the noisy low frequency range. A means for removing noise contained in the detection signal is detected by detecting the fluctuation spectrum of the ground current phenomenon in a narrow band in the carrier frequency range.

  As a third means, in order to remove the double harmonic wave, the sub-harmonic wave, and their intermodulation noise components of the strong transmission current remaining in the detection signal, the detection frequency is selected avoiding those frequencies. Here, as one frequency selection method, a prime frequency such as 17,223 Hz is selected as a detection frequency for a transmission current of 50 or 60 Hz. In addition, the detection band is selected to be 1 Hz or less because the frequency interval of the intermodulation frequency component is 1 Hz, and a method of detecting a ground current change with high sensitivity by excluding noise from the power system is adopted.

  Further, as a fourth solution, there is a case where the artificial noise is mixed in the detection signal due to the artificial noise that exists in the vicinity of the observation device. However, a means for smoothing and removing the artificial noise is adopted by averaging with a sufficiently long integration circuit. Specifically, the integration time is 60 to 200 seconds (preferably 60 to 150 seconds) with respect to the observation time of about one day. Here, it is necessary to employ an integration time that is sufficiently longer than the fluctuation period of the artificial noise and sufficiently shorter than the earth current change period to be detected, but the period of the crustal movement is often sufficiently longer than 150 seconds. I understand. In order to detect a sudden and rapid fluctuation phenomenon, the observation is performed in a balanced manner with the removal of artificial noise using an average of 6 seconds.

  In addition, as a fifth solution, ground current observation is performed for each power transmission path using an optical fiber housed in an overhead ground wire of a power transmission facility network spread nationwide, and a plurality of abnormality detection signals are acquired. Thus, the correlation component and the non-correlation component are separated by the correlation detector, and the latter is often caused by noise and is removed. Further, the detection performance is improved by an improvement means characterized in that the phase change is detected by the synchronous detection method and the analysis performance such as movement and spread of the detection target is improved.

  As a sixth solution, an abnormal signal generation region is estimated from a difference in detection signals depending on a path direction.

The specific configuration is as follows.
1) Utilizing the fact that the polarization state of the optical signal transmitted through the optical fiber accommodated in the overhead ground wire of the power transmission line is changed by the current flowing through the overhead ground wire, the polarization state of the optical signal After the polarization fluctuation, which is a change in frequency, is converted into an electric signal, it is filtered and the weak ground current is detected with high accuracy by separating the signal representing the weak ground current from other noises. .

As a result, by using the fact that the polarization fluctuation of the optical signal flowing through the optical fiber accommodated in the overhead ground wire of the power transmission line changes due to the ground current flowing through the overhead ground wire, the weak ground current can be accurately obtained. It can be measured.
2) In the weak ground current detection method described in 1) above,
An earthquake precursor is detected based on the signal representing the weak ground current.

As a result, information useful for earthquake prediction can be obtained based on the ground current via the ground current abnormality.
3) an optical fiber housed in the overhead ground wire of the power transmission line;
A polarization detecting means for forming an electrical signal representing a polarization fluctuation caused by a current flowing through the aerial ground wire of the optical signal transmitted through the optical fiber;
It has a filtering means for separating a signal representing a weak ground current and other noise from the output signal of the polarization detecting means.

As a result, by using the fact that the polarization fluctuation of the optical signal flowing through the optical fiber accommodated in the overhead ground wire of the power transmission line changes due to the ground current flowing through the overhead ground wire, the weak ground current can be accurately obtained. It can be measured.
4) In the weak ground current detection system described in 3) above,
The filtering means detects a signal representing a weak ground current in a frequency band of 10 to 500 Hz from the electrical signal converted by the polarization detecting means.

As a result, it is possible to eliminate noise based on the power transmission system, vibration noise of the overhead ground wire due to wind, and lightning discharge noise.
5) In the weak ground current detection system described in 3) or 4) above,
The filtering means further selects, as a center frequency, a frequency that is a prime number with respect to a fundamental frequency of transmission current, harmonics and subharmonics generated by nonlinear characteristics of transmission and power utilization equipment systems, and their intermodulation frequency components. In addition, the detection band is selected to be smaller than the frequency interval of the intermodulation frequency component.

As a result, the basic frequency of the transmission current is 50 or 60 Hz, but the harmonics, subharmonic waves and their intermodulation frequency components generated by the non-linear characteristics of the transmission and power utilization equipment system, the detection frequency of 17 Hz, 223 Hz, etc. By selecting the prime frequency and amplifying the detection band smaller than 1 Hz, which is the frequency interval of the intermodulation frequency component, it is possible to obtain a ground current fluctuation signal with high sensitivity and to prevent interference from the power transmission system. Noise can be reduced.
6) In the weak ground current detection system described in any one of 3) to 5) above,
In order to smooth and remove noise, the filtering means has an integration time that is sufficiently longer than an artificial noise generation period and shorter than a fluctuation period of a signal representing the weak ground current, which is about 60 seconds. It includes 200 seconds of integration means.

As a result, in addition to power system noise, commercial power use equipment installed in the vicinity of the observation device, nearby static noise, automobile ignition noise, and artificial noise generated by other equipment are Can be smoothed out by an integration means having an integration time that is sufficiently longer than that and is sufficiently shorter than almost the fluctuation period of the observed seismic ground current.
7) In the weak ground current detection system described in 6) above,
The integration means is characterized in that it is set for several seconds to 6 seconds.

As a result, when observing subtle changes in crustal deformation, it is possible to maintain a balance with the effects of improving lightning noise and artificial noise.
8) In the weak ground current detection system according to any one of 3) to 7) above,
The filter means includes a narrowband amplifying means and a synchronous detection means, digitally processing using a time reference clock obtained from the reference signal generating means, and clocking a reference signal having the same frequency as that of the narrowband amplifying means. It is formed by a circuit, and by detecting synchronously with this signal as a reference, two components of an in-phase component and a quadrature component of the detection signal are detected, and a phase change of the signal is detected.

As a result, detailed ground current analysis can be performed by detecting a phase change of the signal.
9) The weak ground current detection system described in 8) above is installed for each transmission line route to construct a plurality of observation points, and the detection method at each observation point is based on a time reference clock by GPS. The in-phase component and the quadrature component are detected at each observation point by synchronous detection, and collected at the analysis center to detect the change state of the source of the ground current abnormality due to the earthquake precursor phenomenon from each phase change state. To do.

As a result, a broadband seismic current can be detected collectively, which contributes to more accurate earthquake prediction.
10) The weak ground current detection system described in 8) above is installed for each transmission line route to construct a plurality of observation points, and the detection method at each observation point is based on the time reference clock by GPS. The in-phase and quadrature components are detected at each observation point by synchronous detection, and collected at the analysis center to extract the correlation and non-correlation components between the detection signals at each observation point, and distinguish the signal and noise. It is characterized in that the uncorrelated noise component is removed.

  As a result, it is possible to improve the S / N ratio by collectively detecting broadband seismic currents and taking into account the correlation between the detection signals.

  According to the present invention having the above-described configuration, the necessary frequency component is extracted by filtering after converting the polarization rotation amount of the optical fiber in the transmission line overhead ground wire into an electrical signal, so that noise is removed. Therefore, it becomes possible to measure the ground current with much higher sensitivity than the conventional method. For this reason, it becomes possible to apply to various measurements such as detection of a small change in earth current that is a sign of an earthquake. In addition, since existing power transmission equipment can be used to observe earthquake precursors with high accuracy, the effects of disaster prevention and the role played by earthquake countermeasures for lifelines such as power transmission lines are significant. is there.

  In other words, compared to the conventional method of measuring directly from the neutral point ground current of the transmission line, noise from the transmission system is remarkably reduced, and furthermore, a method using polarization fluctuation of several Hz or less of the conventional overhead ground optical fiber. In contrast, since a method of detecting in a narrow band at a frequency of 10 to 500 Hz is adopted, mixed noise is removed, and the sensitivity can be increased from 1000 times to 10,000 times compared to the conventional method. Further, it is possible to detect a change in the ground current that can be detected up to a weak value of several tens of μA.

  Furthermore, it becomes possible to realize a new earth current measuring device for various purposes using a power transmission line, for example, an investigation of the distribution state of environmental electromagnetic waves. In other words, in the past, observation equipment was installed at each point and weak electromagnetic waves were measured. However, it is possible to measure transmission lines that are spread all over the country as antennas, dramatically improving measurement efficiency. The effect of doing.

  Embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a configuration diagram of power transmission line equipment and a conceptual diagram of ground current measurement using this. As shown in the figure, each terminal of the winding 11 of the three-phase transformer is connected to three power transmission lines 12 (typically, only one is shown) 12. The common neutral point 13 of the transformer winding is grounded. The overhead ground wire 14 is suspended at the top of the transmission line tower in order to protect the transmission line from lightning failure. The aerial ground wire 14 has a structure in which a plurality of conductive wires are spirally wound, and is hollow and has an appropriate flexibility. A plurality of optical fibers for communication are accommodated in the hollow portion of the overhead ground wire 14. In addition, the structure which wound the optical fiber around the conducting wire is applicable.

FIG. 1 shows a schematic diagram in which an optical signal is transmitted from left to right. In addition to the current due to the earth potential difference between the towers, the neutral point unbalanced current due to load fluctuations, a lot of current flows through the overhead ground wire 14 such as the lightning induced current induced in the overhead ground wire. The earth current of this i section is indicated by I _gi 15. These values generally differ from section to section.

The earth current according to this embodiment is related to abnormal earth currents and electromagnetic waves in the crust generated by the seismic source region 16 that appeared in the middle of the transmission path, and some of them cause a potential difference E _i 17 between the steel towers, thereby A ground current change is formed in the overhead ground wire 14 of each section. In the figure, 18 shows an example of the abnormal potential distribution in the crust with respect to the distance.

  The ground currents due to various factors as described above flow through the overhead ground wire 14 in an overlapping manner. At this time, it flows around the optical fiber by the winding structure of the overhead ground wire 14. For this reason, the magnetic field of the direction along an optical fiber is formed. This magnetic field causes polarization rotation of the lightwave signal, termed the Faraday effect. The amount of polarization rotation is approximately proportional to the amount of ground current flowing through the overhead ground wire 14 and the transmission distance. Therefore, the total polarization rotation amount (variation) of the entire observation path is detected at the receiving end. The amount of polarization fluctuation due to the seismic ground current to be detected is included in this, and is detected based on the difference in the nature of the polarization fluctuation. Details are as follows.

  FIG. 2 shows a connection between a light source composed of a semiconductor laser 21 installed on the transmission end side of an optical fiber according to an embodiment of the present invention and a polarization detection unit 24 on the reception end side via an optical fiber 23 in the overhead ground wire 14. is doing. As a detection signal of the ground current, an output signal 25 obtained by converting the polarization fluctuation into an electric signal by the polarization detection unit 24 is input to the narrow ultra-long wave band (ELF) component measurement unit 26 for high sensitivity. Detect earth current. With the introduction of this device, the detection sensitivity of ground current can be increased by a factor of 1000 to 10,000.

FIG. 3 is a block diagram showing a detailed structure of the polarization detector shown in FIG. As shown in the figure, in this polarization detecting unit 24, the output light of the optical fiber 23 is converged by the light receiving lens 31, and then branched by the branch circuit 32 so that only the polarized light in a certain direction is passed through. Polarization fluctuation is detected from a kind of polarizer detection system. Each set of detection systems, such as a photodiode 34 that detects the output of the polarizer 33, is used to obtain the polarization fluctuation amount by obtaining three polarization component outputs 35, 36, and 37 of S ₁ , S ₂ , and S _3. To detect. The uppermost detection system 38 is used to detect the total power P ₀ 39 of light and to standardize the polarization component output and convert it into an angle.

FIG. 4 shows the correspondence between the detection outputs S ₁ , S ₂ , S ₃ of the polarization detector 24 and the polarization fluctuation amount from the reference time.

  FIG. 5 is a block diagram showing a detailed configuration of the ultra-long waveband component measuring unit 26 shown in FIG. As shown in the figure, the ultra-long wave band component measurement unit 26 includes a narrow band amplification unit 51, a detection unit 53, an integration circuit 55 for 6 seconds, and an integration circuit 57 for 150 seconds. 1 to 37 are provided corresponding to one set.

  That is, the input signal of the ultra-long wave band component measuring unit 26 is the output 25 from the polarization detecting unit 24, but as shown in FIG. Thru 37 are sent out, so that they are respectively input to three processing systems.

  The narrow band amplifying unit 51 in the ultra-long wave band component measuring unit 26 amplifies a prime frequency that is mutually prime with a transmission current frequency component such as 17 Hz or 223 Hz in a frequency band of 10 to 500 Hz (ultra-long wave band ELF). In addition to the frequency, the amplification band is a narrow band of less than 1 Hz, for example, an amplifier having a high amplification degree of 150,000 times. A plurality of ultra-long wave band component measuring units 26 can be used in parallel for each frequency to be detected.

  The output signal 52 of the narrow-band amplifier 51 is a ground current detection signal from which noise due to various low-frequency factors is removed in addition to power system noise. Since this output signal 52 is a narrow band signal of the selected frequency, its amplitude component is detected by the detection unit 53, and the amplitude 54 of the ground current signal is detected.

  Furthermore, since minute fluctuations that change rapidly with time due to lightning discharge current and artificial noise remain and are detected, an integration circuit 55 is added to remove them.

  Here, it is clear from observations that the earth current due to crustal activity has fewer components that change more quickly than noise, and that there are many components with a long fluctuation period of several seconds to several hours. On the other hand, it has been clarified that the fluctuation due to artificial noise or the like has many components that are sufficiently faster than about 6 seconds to 150 seconds. For this reason, the 6-second average value is output 56 by the integrating circuit 55. An average value of 150 seconds with a high noise removal effect can be created by an integrating circuit 57 that averages 25 6-second average values, and an output 58 is obtained.

  As described above, artificial noise can be smoothed out and only the ground current signal component based on the crustal abnormality can be detected with high sensitivity. In FIGS. 3 and 5, even if one or two of 35 to 37 are omitted for simplification of the apparatus, a certain level of signal can be obtained although the sensitivity is inferior.

  FIG. 6 is a characteristic diagram schematically showing the relationship between the spectrum 61 of the ground current to be detected, the low frequency noise spectrum 62, the noise 63 of the power system, and the narrowband amplification characteristic 64.

  Although it is generally known that the noise spectrum 63 of the power system is strong at the power frequency, its harmonics, and subharmonic, recently, non-linear control devices are widely used in the power system, and due to their non-linear characteristics. Each frequency component generated by the cross modulation action is further increased. In this embodiment, since the detection target related to the earthquake precursor phenomenon is a weak ground current signal, it is necessary to consider even a small cross modulation component. For this reason, it is necessary to consider the noise spectrum generated due to the higher order of nonlinearity, and it must be considered that there is a difference in intensity but exists for each 1 Hz frequency. Further, the noise 63 of the power system in FIG. 6 schematically shows that these frequency components are seen as noise that changes due to power load fluctuations and spreads on the frequency axis. Note that the amplitude of the intermodulation component generated at the prime frequency is small because of the high-order coupling.

  Here, the earth current spectrum 61 to be detected is characterized by various high-speed and low-speed changes / fluctuations and fluctuations of the crust, and is distributed as fractal fluctuation noise up to a wide frequency. Its spectrum intensity (per area, power density per hertz) is small, but the total power is enormous.

  In general, when detecting a ground current change with a source in a remote deep crust near the ground surface, the detected signal level is much weaker than the power system noise. However, although it is a narrow band observation at a specific frequency, strong power system noise can be eliminated by the ground current detection system according to this embodiment, and a weak ground current component can be measured. If the detection signal is detected, the low frequency fluctuation component can be reproduced, so there is no problem in observation.

  FIG. 7 is a block diagram showing the configuration of FIG. 5 in more detail. This figure shows a processing system for one of the polarization component outputs 35 to 37, but actually, three similar processing systems are provided as in the case of FIG.

  The narrowband amplifier 71 is a narrowband amplifier composed of a digital signal processing circuit type filter operated by the clock of the clock circuit 72, and its output 73 is branched and supplied to two synchronous detectors 74 and 75. The synchronous detector 74 is supplied with a carrier wave having the same frequency as the center frequency of the narrow band filter from the clock circuit 72, and the synchronous detector 75 is supplied with a clock via a delay circuit 76 that delays the phase by 90 degrees at that frequency. Is done. Integration circuits 77 and 78 are connected to the output sides of the synchronous detectors 74 and 75, respectively, and after performing time averaging for about 6 seconds, they are output as outputs 79 and 80. Here, the output 79 is called an in-phase output, and the output 80 is called a quadrature output. These vectors are combined to produce an amplitude output, which is the same as the output 56 in FIG.

  In the configuration shown in FIG. 7, the phase information of the signal is obtained from the in-phase and quadrature components, and information such as whether the change of the signal (ground current) is getting faster or slower, approaching or moving away, etc. Because it is obtained, it is an effective means to elucidate the earthquake phenomenon.

  In addition, by synchronizing the clock circuit 72 with the time reference signal 70 such as GPS, it is possible to detect and process more detailed signals compared to detection signals at different observation points.

  FIG. 8 shows a configuration for obtaining information such as how the crustal phenomenon changes and moves by comparing the phase change of the detection signal between remote observation points. A, B, C,... Indicate observation points. In each observation device 81 or the like, the GPS receiver 82 synchronizes the clock circuit 83 serving as the time reference for each observation point A, B, C,. The reference clock signal 84 is obtained. The output 86 of the sensor system 85 that detects polarization fluctuations using the optical fiber 23 of the overhead ground wire 14 described with reference to FIGS. The synchronous detection circuit 87 is driven to obtain an in-phase component 88 and a quadrature component 89.

  As described above, signals from the earthquake preparation area are acquired as in-phase and quadrature two-component data at each observation point, transferred to the analysis center 90 via a communication line, and the detection signals at each observation point are integrated and ranked. Analyze the distance shift from the phase difference and the direction of its travel, etc., and grasp the estimation and change of the location of crustal anomalies in detail.

  The analysis center 90 detects the correlation component and non-correlation component of the signal between the observation points by digital signal processing, removes the unrelated non-correlation component as noise, and further improves the signal quality and analyzes the phenomenon. Evaluation is possible. As digital signal analysis methods, in addition to principal component analysis and independent component analysis, many methods such as nonlinear analysis methods have been developed recently, and the effect of multi-signal analysis can be expected.

  In the ground current detection system according to the present embodiment, the reason why the observation frequency is selected to be a very long wave band of 10 to 500 Hz is as follows. That is, it is known that the earth magnetic field fluctuation is strong in the 1 Hz or less mHz band, and the fluctuation noise of the electronic circuit for amplification increases as the frequency becomes lower than 100 Hz. Further, unnecessary low-frequency fluctuation signals are generated due to mechanical detection characteristics due to distortion of the optical fiber 23 caused by the fluctuation of the power transmission line due to wind, but it is known that they are remarkable at 15 Hz or less. On the other hand, the noise due to lightning discharge increases sharply at a frequency of 500 Hz or higher, and is prominently present up to the broadcast wave band.

  Considering these, it can be seen that 10 to 500 Hz as an evaluation frequency of the polarization output signal is a frequency suitable for observation of a weak ground current by removing various noises. Note that, as discussed in FIG. 6, the ground current to be observed also changes and fluctuates, and has a continuous power spectrum distribution that is approximately inversely proportional to the frequency, so that the intensity of the detection signal exists over a wide frequency range.

FIG. 9 shows various observed data waveforms for comparison. The first column (1) at the top of the figure shows the east-west in the atmosphere at a certain point by a coil sensor using the ELF band 223 Hz which is not directly related to the present invention in order to compare the observation data according to the present invention. This shows magnetic flux density fluctuation observation data (vertical axis is picotesla / root hertz). Β _{e in} the second column (2) indicates high-sensitivity observation data obtained by averaging the polarization fluctuation output of the aerial ground optical fiber for 60 seconds after removing noise from the ultralong wave component measuring unit according to the present invention. The reason why the integration time is set to 60 seconds instead of 150 seconds is to compare with 1 minute data by the conventional methods (3) and (4). The vertical axis is normalized in radians, with the highest one representing 10 ^-4 radians (rad). When the ultralong wave component measuring unit according to the present invention was not used, the detection level of (3) was about 1/20 radians. It can be seen that the lowest level of observation indicated by β _{e in} the second column according to the present invention is about 10 ⁻⁵ radians. This value indicates the residual noise of the observation system and indicates the limit of observation.

Thus, it has been shown that the present invention has increased the sensitivity from 1000 times to 10,000 times from 1/20 radians to about 10 ^-5 radians.

In addition, β _s and β _m in the third column (3) and the fourth column (4) indicate the amount of variation obtained by observing the optical polarization variation output according to the conventional method in the low-speed variation measurement system, and β _s is an interval of 1 second. The variation, β _m , indicates the variation at 1 minute intervals.

In FIG. 9, strong changes are observed in β _e , β _s , and β _m on the 13th, which is considered to be a windy day and the optical fiber 23 vibrates with the wind. Regarding the relationship between β _s , β _m and weather, there are examination results in the observation period of the preceding 22 months, and the following has been found. That is, when the maximum wind speed is 1 m / s on an average for one hour, the fluctuation in β _s is approximately 0 to 0.2 rad / sec. The fifth column (5) shows the hourly average maximum wind speed during the observation period. Fluctuation noise is also related to the wind direction and fluctuates strongly with respect to the northeastern wind. It seems to be related to northwest to southeast, which is the traveling direction of the transmission line. In addition, β _m is related to the amount of solar radiation, and the increase in the daytime can be attributed to the fiber temperature change. The sixth column (6) at the bottom shows the amount of solar irradiation heat (megajoule per square meter), while (4) β _m shows a correspondence with the amount of solar irradiation heat.

The polarization fluctuation amount β _e observed through the ELF component measurement unit is similar to β _s from β _m , but is not similar to β _s on the 10th and 15th days, and the wind It is also different from the change. After 15th, changes similar to (1) are observed. Therefore, it is considered that β _e detects a current component corresponding to the ELF magnetic flux density. When the value of β _e is evaluated, the ground wire ground current is 10 ⁻⁵ radians, which is on the order of about several tens of μA. On the 16th and 17th, the ground current of about 100 μA was present at the maximum level. It was clarified from the observation that the earth current diverted to the ground line because the earth current flows in a distributed manner. Moreover, it became clear from the above evaluation that high-sensitivity earth current observation of several 10 to 100 μA is possible.

  The present invention can be used in industrial fields such as earthquake prediction that requires accurate detection of weak ground currents and observation of electromagnetic field distribution in the environment.

It is a conceptual diagram which shows the principle of the earth current detection by the overhead ground wire optical fiber (OPGW) which concerns on this invention with the outline | summary of power transmission equipment. It is a block diagram which shows the earth current detection system which concerns on embodiment of this invention. It is a block diagram which shows the detailed structure of the polarization | polarized-light detection part in the block diagram shown in FIG. It is explanatory drawing which shows the definition of polarization fluctuation amount. It is a block diagram which shows the detailed structure of the ultra-long wave band ELF component measurement part in the block diagram shown in FIG. It is a characteristic view which shows the relationship between a ground current spectrum, the noise of a power system, low frequency noise, and a narrow-band amplification characteristic. It is a block diagram which shows the detection circuit of an in-phase and a quadrature component with the synchronous detection by GPS time reference | standard. It is a block diagram which shows the detailed observation system of the earthquake preparation area using the in-phase and quadrature component by the synchronous detection from another observation point. It is a graph which shows an example of the observation data concerning an embodiment of the invention.

Explanation of symbols

14 Overhead ground wire 23 Optical fiber 24 Polarization detection unit 26 Ultra-long wave band component measurement unit 51 Narrow band amplification unit 53 Detection unit 55 Integration circuit 57 Integration circuit 71 Narrow band amplification unit 72 Clock circuit 74 Synchronous detector 75 Synchronous detector 82 GPS receiver

Claims (10)

  A change in the polarization state of the optical signal by utilizing the fact that the polarization state of the optical signal transmitted through the optical fiber accommodated in the overhead ground wire of the power transmission line is changed by the current flowing through the overhead ground wire. The weak ground current is detected with high accuracy by converting the polarization fluctuation to an electric signal and then filtering to separate the signal representing the weak ground current from other noises. Current detection method. The weak ground current detection method according to claim 1,
An earthquake ground current detection method, comprising: detecting an earthquake precursor based on a signal representing the weak ground current. An optical fiber housed in the overhead power line of the power transmission line,
A polarization detecting means for forming an electrical signal representing a polarization fluctuation caused by a current flowing through the aerial ground wire of the optical signal transmitted through the optical fiber;
A weak ground current detection system comprising filtering means for separating a signal representing a weak ground current and other noise from an output signal of the polarization detection means. The weak ground current detection system according to claim 3,
The weak ground current detection system, wherein the filtering means detects a signal representing a weak ground current in a frequency band of 10 to 500 Hz from the electrical signal converted by the polarization detection means. In the weak ground current detection system according to claim 3 or claim 4,
The filtering means further selects, as a center frequency, a frequency that is a prime number with respect to a fundamental frequency of transmission current, harmonics and subharmonics generated by nonlinear characteristics of transmission and power utilization equipment systems, and their intermodulation frequency components. In addition, a weak ground current detection system, wherein the detection band is selected to be smaller than the frequency interval of the intermodulation frequency component. The weak ground current detection system according to any one of claims 3 to 5,
In order to smooth and remove noise, the filtering means has an integration time that is sufficiently longer than an artificial noise generation period and shorter than a fluctuation period of a signal representing the weak ground current, which is about 60 seconds. A weak ground current detection system comprising an integration means for 200 seconds. The weak ground current detection system according to claim 6,
The weak ground current detection system characterized in that the integration means is set to several seconds to six seconds. The weak ground current detection system according to any one of claims 3 to 7,
The filter means includes a narrowband amplifying means and a synchronous detection means, digitally processing using a time reference clock obtained from the reference signal generating means, and clocking a reference signal having the same frequency as that of the narrowband amplifying means. It is formed by a circuit, and by detecting synchronously with this signal as a reference, the two components of the in-phase component and the quadrature component of the detection signal are detected, and the phase change of the signal is detected. Ground current detection system.   A weak ground current detection system according to claim 8 is installed for each transmission line route to construct a plurality of observation points, and a detection method at each observation point is set as a time reference clock by GPS (Global Positioning System). The in-phase and quadrature components are detected at each observation point by synchronous detection with reference to, and collected at the analysis center to detect the change state of the source of ground current anomaly due to the earthquake precursor phenomenon from each phase change state. A seismic ground current detection system. The weak ground current detection system according to claim 8 is installed for each transmission line route to construct a plurality of observation points, and the detection method at each observation point is synchronized with a time reference clock by GPS as a reference. The in-phase and quadrature components are detected at each observation point by detection, collected at the analysis center, and the correlation and non-correlation components between the detection signals at each observation point are extracted. An earthquake ground current detection system characterized by eliminating noise components.

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