A fiber optic sensing system for measuring lightning-induced transient ground potential distribution

Abstract

The present invention provides a fiber optic sensing system and method for measuring lightning-induced transient ground potential distribution. The system comprises a fiber optic sensor, an insulating rod, a fiber optic integrator, a camera, and an image analysis module. The fiber optic sensor includes a light-emitting diode (LED), a sensor circuit, and two soil electrodes in contact with the soil. The LED and sensor circuit are positioned at the bottom of the insulating rod's inner cavity, while the soil electrodes are positioned on the rod's outer surface. Each LED is connected to an optical fiber, which is then connected to the fiber optic integrator after exiting the insulating rod. The fiber optic integrator is connected to the image analysis module via a camera. By converting soil potential differences into light intensity signals, with distributed transmission and centralized data collection, the present invention avoids the influence of soil current and its electromagnetic field on measurement results. Furthermore, through fiber optic integration technology, the system can achieve simultaneous measurement of ground potential distribution at multiple points.

Description

Technical Field

[0001] The present invention relates to the technical field of high voltage testing, and in particular to an optical fiber sensing system and method for measuring lightning transient ground potential distribution. Background Art

[0002] When high-amplitude lightning currents enter the earth, they generate large soil potential gradients (step voltages), posing a threat to personal safety and distribution network systems with weak insulation. Because soil is a conductor, the current distribution in the ground can cause electromagnetic interference to existing voltage sensors. Furthermore, lightning currents are short-lived, making ground potential measurement extremely difficult. Currently, ground grid design relies solely on simulation.

[0003] Patent references [1-3] propose measuring ground potential distribution and combining it with theoretical calculations to determine the ground electrode potential rise. While the number of ground potential measurement points is relatively small, a theoretical ground resistance model is used to compensate for this deficiency. However, research has found that when large impulse currents dissipate through the soil, soil discharge occurs around the grounding body, which contradicts the assumption of constant soil resistivity in conventional ground resistance models. Theoretically, increasing the number of soil potential measurement points could yield more accurate lightning-induced transient ground potential distribution characteristics. However, ground potential sensing devices based on metal cables present significant electromagnetic compatibility and high costs.

[0004] [1] Ma Yutang, Zhang Bo, Zhou Fangrong, Huang Caowei, Wang Ke, He Jinliang, Zhai Bing, Peng Zhaoyu, Qian Guochao, Ding Wei. A method for measuring ground potential rise[P]. Yunnan Province: CN106526278B, 2020-06-16.

[0005] [2] Ma Yutang, Huang Caowei, Zhou Fangrong, Yu Hong, Wang Ke, Zhang Bo, He Jinliang, Ding Wei. A method for measuring the ground potential rise in converter stations[P]. Yunnan Province: CN105467192B, 2018-05-04.

[0006] [3] Liu Xiaojiang, Liu Fan, Zhen Wei. A method for measuring grounding electrode potential distribution[P]. Sichuan Province: CN106018943B, 2018-12-25. Summary of the Invention

[0007] The purpose of the present invention is to provide a fiber optic sensing system for measuring the transient ground potential distribution caused by lightning strikes and a method for measuring the transient ground potential distribution caused by lightning strikes. By converting the pulse voltage in the soil into a light intensity signal and then transmitting it through optical fiber, the influence of soil current on the measurement results is avoided. In addition, through optical fiber integration technology, synchronous measurement of multi-point ground potential distribution can be achieved.

[0008] A fiber optic sensing system for measuring lightning-induced transient ground potential distribution comprises a fiber optic sensor, an insulating rod, a fiber optic integrator, a camera, and an image analysis module. The fiber optic sensor comprises a light emitting diode (LED), a sensor circuit, and two soil electrodes in contact with the soil. The LED and sensor circuit of the fiber optic sensor are disposed at the bottom of the inner cavity of the insulating rod, and the soil electrodes are disposed on the outer surface of the insulating rod. The sensor circuit is configured to convert an electrode voltage U between the two soil electrodes into a current I of the LED in equal proportion, such that the light intensity generated by the LED is proportional to the current I. Each LED is connected to an optical fiber, which is connected to the fiber optic integrator after being led out of the insulating rod. The fiber optic integrator is connected to the image analysis module via a camera. The camera is configured to record the brightness peak of each optical fiber on the fiber optic integrator during an impulse current test. The image analysis module is configured to analyze the soil potential difference amplitude and the lightning-induced transient ground potential distribution based on the brightness peak, fiber number, and coordinates of each optical fiber recorded by the camera.

[0009] Furthermore, the image analysis module analyzes the soil potential difference amplitude and lightning transient ground potential distribution through the brightness peak, fiber number and coordinates of each optical fiber recorded by the camera. The specific steps are as follows:

[0010] The optical fiber brightness L is proportional to the soil electrode voltage , the proportionality coefficient k is determined experimentally:

[0011]

[0012] Assuming the distance between a pair of soil electrodes is d, calculate the electric field strength E at sensor No. 1 as:

[0013]

[0014] The position coordinates of sensor No. 1 are measured as (x1, y1), and the position coordinates of sensor No. n are measured as (x n ,y n ), the electric field strength is E n ;

[0015] Use the cubic spline function to interpolate the electric field intensity E(x, y) at any coordinate (x, y) position (MATLAB function interp2);

[0016] Finally, the corresponding ground potential value U(x, y) is calculated as:

[0017]

[0018] Where (x0, y0) is the coordinate of the reference potential point, and l is an arbitrary path from (x0, y0) to (x, y).

[0019] Furthermore, the fiber integrator is a fiber lens, which is used to convert the light intensity in the fiber into a plurality of point light sources on the same plane, and the brightness of these light sources can be recorded by a camera.

[0020] Furthermore, the insulating rod is lightproof, slender and strong, and is used for repeated insertion into the soil and for protecting the light-emitting diode and the optical fiber from damage.

[0021] Furthermore, the positive and negative poles of the light emitting diode are connected to two soil electrodes respectively, and a junction voltage compensation and polarity reversal circuit of the light emitting diode is provided so that the potential difference between the two soil electrodes is proportional to the brightness of the diode.

[0022] Furthermore, the fiber optic integrator is used to integrate multiple fiber optic terminals and connect multiple optical sensors to achieve synchronous measurement of multi-point ground potential. The fiber optic integrator couples the light intensity in several fiber optic sensors to an imaging plane to form an image composed of several pixels or light spots. Each pixel or light spot in the plane image corresponds to the instantaneous brightness of a light-emitting diode.

[0023] A method for measuring lightning-induced transient ground potential distribution is provided, using the above system. The method comprises the following steps:

[0024] Step 1: Configuring a surge current generating system, the surge current generating system includes a surge current generator and two grounding bodies connected to the surge current generator, the two grounding bodies including a grounding electrode and a return electrode, the grounding bodies being buried in the soil, the surge current generator being placed on the ground and insulated from the soil, the surge current generator being connected to the two grounding bodies via two metal wires, forming a current closed loop through the soil, connecting the surge current generator to a power source, charging the surge current generator via a console, then triggering a lightning surge current discharge, and recording the voltage and current waveforms generated by the surge current generator;

[0025] Step 2: Configure the fiber optic sensing system: First, number the fiber optic sensors and connect them to the fiber optic integrator in sequence. Second, apply a DC voltage to the two soil electrodes of each fiber optic sensor and observe whether the brightness change of the imaging plane of the fiber optic integrator is correct. Third, connect the fiber optic integrator to a camera and take photos of the fiber optic sensors under different voltage conditions. Use the image analysis module to analyze and calibrate the voltage measurement values ​​of the fiber optic sensors. Then, set the surface coordinate system with the grounding electrode as the origin, plan measurement points around the grounding electrode, mark their coordinates, insert the fiber optic sensors vertically into the surface measurement points in sequence, and record the head depth. Finally, set a test safety area, start the impulse current generator, and record photos and voltage measurement data.

[0026] Step 3: Repeat the above test steps, measure the soil potential difference amplitude through the optical fiber sensor, and interpolate and fit the lightning transient ground potential distribution in combination with the optical fiber number and its coordinates to obtain the ground potential distribution curve; and by changing the amplitude and waveform of the impulse current, study the changing characteristics of the lightning transient ground potential distribution curve; by changing the measurement coordinates and depth of the optical fiber sensor, study the potential distribution characteristics of the soil impact discharge area around the grounding body.

[0027] Furthermore, the distance between the two grounding bodies is greater than 10 m, and the power frequency grounding resistance is less than 10 Ω.

[0028] The present invention avoids the influence of current and its electromagnetic field in the soil on the measurement results by converting soil potential difference into light intensity signal, distributing transmission and centrally collecting the technical scheme; and through optical fiber integration technology, it can realize synchronous measurement of multi-point ground potential distribution. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 1 is a schematic structural diagram of an optical fiber sensing system for measuring transient ground potential distribution caused by lightning strikes according to an embodiment of the present invention;

[0030] Figure 2 This is a schematic diagram of the structure of the optical fiber integrator in an embodiment of the present invention.

[0031] Figure 3 This is a schematic diagram of the optical fiber measurement of transient ground potential distribution under the conditions of a lightning grounding test of the present invention;

[0032] Figure 4 This is a waveform diagram of the lightning current injected according to an embodiment of the present invention;

[0033] Figure 5 This is the 10m horizontal strip vertical grounding electrode model of the embodiment of the present invention;

[0034] Figure 6 This is a ground potential distribution diagram when the soil resistivity is 100Ω·m and there is no vertical grounding electrode in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0036] See also Figure 1An embodiment of the present invention provides a fiber optic sensing system for measuring lightning transient ground potential distribution, including: a fiber optic sensor, an insulating rod, a fiber optic integrator, a camera, and an image analysis module.

[0037] The fiber optic sensor includes a light-emitting diode, a sensor circuit, and a pair of soil electrodes in contact with the soil. The insulating rod serves as the outer shell of the fiber optic sensor, and the light-emitting diode and sensor circuit of the fiber optic sensor are placed at the bottom of the inner cavity of the insulating rod, which plays the role of inserting into the soil and protecting the fiber optic sensor. The fiber optic integrator is connected to the fiber optic sensor via optical fibers. In actual implementation, multiple fiber optic sensors are buried in different positions, so one end of the multiple optical fibers is connected to the multiple fiber optic sensors respectively, and the other ends of the multiple optical fibers are integrated into a plane inside the fiber optic integrator and imaged.

[0038] The optical fiber integrator is connected to the image analysis module via a camera, wherein the camera is used to record the brightness peak of each optical fiber on the optical fiber integrator during the impulse current test, and the image analysis module is used to analyze the soil potential difference amplitude and the transient ground potential distribution caused by lightning strikes based on the brightness peak, optical fiber number and coordinates of each optical fiber recorded by the camera.

[0039] The image analysis module analyzes the soil potential difference amplitude and lightning transient ground potential distribution through the brightness peak, fiber number and coordinates of each optical fiber recorded by the camera. The specific steps are as follows:

[0040] The optical fiber brightness L is proportional to the soil electrode voltage , the proportionality coefficient k is determined experimentally:

[0041]

[0042] Assuming the distance between a pair of soil electrodes is d, calculate the electric field strength E at sensor No. 1 as:

[0043]

[0044] The position coordinates of sensor No. 1 are measured as (x1, y1), and the position coordinates of sensor No. n are measured as (x n ,y n ), the electric field strength is E n ;

[0045] Use the cubic spline function to interpolate the electric field intensity E(x, y) at any coordinate (x, y) position (MATLAB function interp2);

[0046] Finally, the corresponding ground potential value U(x, y) is calculated as:

[0047]

[0048] Where (x0, y0) is the coordinate of the reference potential point, and l is an arbitrary path from (x0, y0) to (x, y).

[0049] In specific implementations, the fiber optic sensor is mounted at one end of an insulating rod, which serves as a rigid housing for the sensor, protecting it from damage during insertion into the soil. The sensor circuit is connected to a pair of metal soil electrodes, located on the outer surface of the insulating rod. These electrodes ensure good contact with the surrounding soil when the rod is inserted and provide electrical insulation between the two electrodes. The electrode voltage U between the two electrodes represents the potential difference between the adjacent soils. The sensor circuit converts this voltage U into a proportional current I for a light-emitting diode (LED), with the intensity of the light generated by the LED being proportional to the current I. Each LED is connected to an optical fiber, which directs the light generated by the sensor to the surface. The front end of the optical fiber is placed inside the insulating rod, protecting it from soil pressure and sparks. The rear end of the optical fiber is placed above ground in the air and connected to a fiber optic integrator via a flexible connector and standard fiber optic terminals. The fiber optic integrator is essentially a fiber lens that converts the light intensity within the fiber into several point light sources on the same plane. The brightness of these light sources can be recorded by a camera (or a high-speed camera can be used to record changes in brightness). Finally, the brightness value of each point in the photo is extracted through the image analysis module, and the voltage amplitude measured by the corresponding sensor is analyzed to obtain the transient ground potential distribution characteristics in the soil.

[0050] like Figure 3 As shown, when lightning strikes a grounding body, a large current flows through the soil, generating transient uneven ground potential distribution. The present invention uses a light-emitting diode (LED) to convert the potential difference between two adjacent points in the soil into an optical signal. This signal is then transmitted via optical fiber to a ground-based optical fiber integrator. The LED's anode and cathode are connected to two soil electrodes, respectively. A voltage compensation and polarity reversal circuit is provided for the LED, ensuring that the potential difference between the two soil electrodes is proportional to the diode's brightness.

[0051] The light signal from the LED is transmitted via optical fiber to a fiber optic integrator located on the ground. The optical fiber and LED circuit are housed within an insulating rod, with the LED located at the rod's head and two soil electrodes positioned on either side of the rod's outer surface. The rod is light-proof, slender, and sturdy, allowing for repeated insertion into the soil and protecting the sensor and optical fiber from damage.

[0052] The fiber integrator is used to integrate multiple fiber terminals and connect multiple optical sensors, enabling simultaneous measurement of ground potential at multiple points. It couples the light intensity from several fiber sensors onto an imaging plane, forming an image composed of several pixels (light points). Each light point (pixel) in the plane image corresponds to the instantaneous brightness of a light-emitting diode.

[0053] During the impulse current test, the brightness peak of the image on the fiber integrator is recorded by a camera (the camera B gate can be used to shoot), and the image analysis module is used to analyze the photos. The soil potential difference amplitude and the transient ground potential distribution caused by lightning strike are analyzed based on the corresponding fiber number and its coordinates.

[0054] An embodiment of the present invention further provides a method for measuring lightning transient ground potential distribution, which is performed using the above-mentioned optical fiber sensing system and impulse current generating system. The method includes the following steps:

[0055] Step 1: Configure a surge current generating system, which includes a surge current generator (100 kA / 200 kV) and two grounding electrodes connected to the surge current generator. The two grounding electrodes consist of a grounding electrode and a return electrode. The grounding electrodes are buried in the soil, spaced at least 10 m apart, with a power-frequency grounding resistance of less than 10 Ω. The surge current generator is placed on the ground, insulated from the soil. It is connected to the two grounding electrodes via two metal wires, forming a closed current loop through the soil. Connect the surge current generator to a power source and charge it via a control console. Then, trigger a lightning surge current discharge. Record the voltage and current waveforms generated by the surge current generator.

[0056] Step 2: Configure the fiber optic sensing system: First, number the fiber optic sensors and connect them to the fiber optic integrator in sequence. Second, apply a DC voltage (0-60 V) to the two soil electrodes of each fiber optic sensor and observe whether the brightness change of the fiber optic integrator imaging plane is correct. Third, connect the fiber optic integrator to a camera and take photos of the fiber optic sensors under different voltage conditions. Use the image analysis module to analyze and calibrate the voltage measurement values ​​of the fiber optic sensors. Then, set the surface coordinate system with the grounding electrode as the origin, plan measurement points around the grounding electrode, mark their coordinates, insert the fiber optic sensors vertically into the surface measurement points in sequence, and record the head depth. Finally, set up a test safety area, start the impulse current generator, and record photos and voltage measurement data.

[0057] Step 3: Repeat the above test steps, measure the soil potential difference amplitude through the optical fiber sensor, and interpolate and fit the lightning transient ground potential distribution in combination with the optical fiber number and its coordinates to obtain the ground potential distribution curve; and by changing the amplitude and waveform of the impulse current, study the changing characteristics of the lightning transient ground potential distribution curve; by changing the measurement coordinates and depth of the optical fiber sensor, study the potential distribution characteristics of the soil impact discharge area around the grounding body.

[0058] CDEGS software was used to simulate the ground potential distribution under the action of lightning current on a single horizontal electrode (10 m long, 6 mm radius) with a 2.5 m long vertical grounding electrode set every 5 m. The experimental lightning current amplitude was 10 kA (waveform 10 / 350 μs). Figure 4 The injected lightning current waveform is at the rightmost point (considering the worst case, the lightning current injection points are all at the corners of the grounding electrode). Figure 5 This is a simulation model of a single horizontal grounding electrode with a vertical grounding electrode. Figure 6 The ground potential distribution when the soil resistivity is 100Ω·m and there is no vertical grounding electrode is given by Figure 6 It can be seen that the ground potential is highest at the surface corresponding to the electrode, and the value decreases sharply towards the sides. The step potential distribution under different soil resistivity conditions with and without vertical grounding electrodes is studied. Table 1 shows the maximum step potential values ​​under different soil resistivity conditions when the electrode length is 10m:

[0059] Table 1 Maximum step potential values ​​(kV) at different soil resistivities when the electrode length is 10 m

[0060]

[0061] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by a person skilled in the art within the technical scope disclosed in the present invention should be included in the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be based on the scope of protection of the claims.

Claims

1. An optical fiber sensing system for measuring lightning transient ground potential distribution, characterized by: The device comprises an optical fiber sensor, an insulating rod, an optical fiber integrator, a camera, and an image analysis module; the optical fiber sensor comprises a light-emitting diode, a sensor circuit, and two soil electrodes in contact with the soil; the light-emitting diode and sensor circuit of the optical fiber sensor are placed at the bottom of the inner cavity of the insulating rod, and the soil electrodes are arranged on the outer surface of the insulating rod; the sensor circuit is used to convert the electrode voltage U between the two soil electrodes into a current I of the light-emitting diode in equal proportion, so that the light intensity generated by the light-emitting diode is proportional to the current I; each light-emitting diode is connected to an optical fiber, and the optical fiber is connected to the optical fiber integrator after being led out of the insulating rod; the optical fiber integrator is connected to the image analysis module via a camera; the camera is used to record the brightness peak of each optical fiber on the optical fiber integrator during the impulse current test; the image analysis module is used to analyze the soil potential difference amplitude and the transient ground potential distribution caused by lightning strikes based on the brightness peak, optical fiber number, and coordinates of each optical fiber recorded by the camera.

2. The optical fiber sensing system for measuring lightning transient ground potential distribution according to claim 1, wherein: The image analysis module uses the brightness peak, fiber number and coordinates of each optical fiber recorded by the camera to analyze the soil potential difference amplitude and lightning transient ground potential distribution. The specific steps are as follows: The optical fiber brightness L is proportional to the soil electrode voltage , the proportionality coefficient k is determined by experiment: ； Assuming the distance between a pair of soil electrodes is d, calculate the electric field strength E at sensor No. 1 as: ； The position coordinates of sensor No. 1 are measured as (x1, y1), and the position coordinates of sensor No. n are measured as (x n ,y n ), the electric field strength is E n ; Use the cubic spline function to interpolate the electric field intensity E(x, y) at any coordinate (x, y); Finally, the corresponding ground potential value U(x, y) is calculated as: ； Where (x0, y0) is the coordinate of the reference potential point, and l is an arbitrary path from (x0, y0) to (x, y).

3. The optical fiber sensing system for measuring lightning transient ground potential distribution according to claim 1, wherein: The optical fiber integrator is a fiber lens used to convert the light intensity in the optical fiber into a plurality of point light sources on the same plane. The brightness of these light sources can be recorded by a camera.

4. The optical fiber sensing system for measuring lightning transient ground potential distribution according to claim 1, wherein: The insulating rod is lightproof, slender and strong, and is used for being repeatedly inserted into the soil for use, and protects the light-emitting diode and the optical fiber from being damaged.

5. The optical fiber sensing system for measuring lightning transient ground potential distribution according to claim 1, wherein: The positive and negative poles of the light emitting diode are connected to two soil electrodes respectively, and a junction voltage compensation and polarity reversal circuit of the light emitting diode is provided so that the potential difference between the two soil electrodes is proportional to the light emitting brightness of the diode.

6. The optical fiber sensing system for measuring lightning transient ground potential distribution according to claim 1, wherein: The fiber optic integrator is used to integrate multiple fiber optic terminals and connect multiple optical sensors to achieve synchronous measurement of multi-point ground potential. The fiber optic integrator couples the light intensity within several fiber optic sensors to an imaging plane, forming an image composed of several pixels or light spots. Each pixel or light spot in the plane image corresponds to the instantaneous brightness of a light-emitting diode.

7. A method for measuring lightning transient ground potential distribution, characterized by: The method is performed using the system according to any one of claims 1 to 6, comprising the steps of: Step 1: Configuring a surge current generating system, the surge current generating system includes a surge current generator and two grounding bodies connected to the surge current generator, the two grounding bodies including a grounding electrode and a return electrode, the grounding bodies being buried in the soil, the surge current generator being placed on the ground and insulated from the soil, the surge current generator being connected to the two grounding bodies via two metal wires, forming a current closed loop through the soil, connecting the surge current generator to a power source, charging the surge current generator via a console, then triggering a lightning surge current discharge, and recording the voltage and current waveforms generated by the surge current generator; Step 2: Configure the fiber optic sensing system: First, number the fiber optic sensors and connect them to the fiber optic integrator in sequence. Second, apply a DC voltage to the two soil electrodes of each fiber optic sensor and observe whether the brightness change of the imaging plane of the fiber optic integrator is correct. Next, connect the fiber optic integrator to a camera and take photos of the fiber optic sensor under different voltage conditions. Use the image analysis module to analyze and calibrate the voltage measurement values ​​of the fiber optic sensor. Then, set the surface coordinate system with the grounding electrode as the origin. Plan measurement points around the grounding electrode and mark their coordinates. Insert the fiber optic sensor vertically into the surface measurement points in sequence and record the head depth. Finally, set up a safe test area, start the impulse current generator, and record photos and voltage measurement data. Step 3: Repeat the above test steps, measure the soil potential difference amplitude through the optical fiber sensor, and interpolate and fit the lightning transient ground potential distribution in combination with the optical fiber number and its coordinates to obtain the ground potential distribution curve; and by changing the amplitude and waveform of the impulse current, study the changing characteristics of the lightning transient ground potential distribution curve; by changing the measurement coordinates and depth of the optical fiber sensor, study the potential distribution characteristics of the soil impact discharge area around the grounding body.

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