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Keywords:

  • magnetic field system;
  • experimental test;
  • artificially triggering lightning experiment

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

[1] A magnetic field measuring system with two rectangular loops perpendicular to each other is developed and used to detect the total horizontal magnetic field produced by lightning discharges. Two sets of antenna with low gain and high gain are designed separately in order to obtain both small and large signals. The system was tested experimentally in a high-voltage laboratory, and the results show that the system is reliable, and the waveforms detected by the system are very similar to the source current. The corresponding maximum current that can be measured by the system with low gain antenna and high gain antenna locating at a distance of 60 m from a lightning channel, is about 84.4 kA and 37.7 kA, respectively. For artificially triggered lightning flashes the magnetic field waveforms detected at 60 m from the channel reflect quite well the channel base currents. Using Ampere's law of magnetostatics, the inferred currents from the magnetic fields for three artificially triggered lightning return strokes were 39.8 kA, 29.1 kA and 43 kA, respectively, which are close to the directly measured results of 41.6 kA, 29.6 kA and 38 kA at the base of the discharge channel. The system can be a useful tool in the research of close electromagnetic environment of lightning flashes.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

[2] Lightning current and its close electromagnetic field are very important in the design of lightning protection. Artificially triggering lightning technique provides an effective way to conduct lightning research because of its predictable ground striking point. However, direct measurement of current for altitude triggered flashes is still difficult for its arbitrary striking point to some extent. Therefore, the detection of electric field and magnetic field of the triggered flashes at close distance will be very useful to infer the property of the discharges.

[3] Krider and Noggle [1975] made the first broadband antenna system to measure the magnetic field caused by distant lightning, and has been widely used in the cloud-to-ground lightning location systems. The close magnetic field in artificially triggered lightning flashes was measured by Rakov et al. [1998] with H31 Thomson-CSF and one loop antenna magnetic field measuring system. It has been inferred that the leader electric field change at close range had a positive linear correlation with the succeeding return stroke current peak at the channel base. Schoene et al. [2003] also measured close magnetic fields and their derivatives in artificially triggered lightning flashes with one loop antenna. For the external magnetic flux density may not go through the loop completely (for example the lightning channel might be parallel with the loop, as an extreme case), and the magnetic field got with one loop might be smaller than the actual one. Therefore, a magnetic field measuring system, including two loops perpendicular to each other, has been developed to measure the close magnetic field caused by the triggered lightning flashes. The system was tested in a Laboratory of High Voltage and Insulation, and a very reliable and stable result has been obtained quantitatively. Moreover, synchronous data of the current at the base of lightning discharge channel and close magnetic field at a distance of 60 m from the channel were documented during Shandong Artificially Triggering Lightning Experiment (SHATLE) [Qie et al., 2007]. The developed system, laboratory test results and in situ results for triggered lightning flashes will be presented in this paper.

2. Magnetic Field System and Its Test in Laboratory

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

2.1. Magnetic Field System

[4] A schematic diagram of the magnetic field measuring system is shown in Figure 1. The antennae include two rectangular loops (N/S and E/W labeled in Figure 1) perpendicular to each other, and the area of each loop is about 30 × 30 cm2. A low gain antenna and high gain antenna are designed separately to detect small and large magnetic field changes. Signals from the antennae are transmitted with coaxial cables, then being recorded on a multiple channel digital scope after amplification. The operational amplifier used is OPA602 manufactured by the Burr-Brown Corporation and its gain bandwidth is 3.5 MHz, and the slew rate is 35 V/μs.

image

Figure 1. Schematic diagram of the system.

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2.2. Experimental Test in Laboratory

[5] The developed system was tested in the Laboratory of High Voltage and Insulation, Wuhan University, with a 120 kA pulse current generator. Shown in Figure 2 are a schematic diagram and a picture when testing the system in the laboratory. As shown in Figure 2, left panel, the rectangular loop antenna (labeled P) was installed in parallel with a concentric current circle (labeled R and made of wire), and R equals to 1.2 m. The distance r0, between the circle and the antenna loop, equals to 1.5 m. The circle was fixed to a wooden rectangular frame and could not be seen clearly due to the background of the picture.

image

Figure 2. A schematic diagram and a picture in the laboratory.

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[6] As shown in Figure 2, when a pulse current goes through the circle R, the magnetic flux density along the axes of the circle will change, thus induced current will flow in the rectangular loop and the output voltage v of the system will be obtained after integration of the current. On the other hand, the magnetic field B at the antenna location (labeled P in Figure 2) can be calculated theoretically and is

  • equation image

Therefore, relationships between the magnetic field B at the antenna location and the output voltage v of the system could be obtained. For practical application, the magnetic field at the location of the antenna will be obtained from the output voltage of the system based on the tested results. The system can be used to measure the close magnetic field changes in artificially triggering lightning experiment in the same way, and then the discharge property can be inferred from the measured close magnetic field.

[7] During the laboratory testing, a shunt, manufactured in Germany with a current limit of 100 kA and bandwidth of 2 MHz, was used to measure the pulse current, and the developed system with frequency response from 10 kHz to 0.5 MHz, was used to measure the magnetic field caused by the current at the same time. The current and the output voltage of the system, after 12 bit A/D conversion, were both recorded on a digital scope with a sampling rate of 1 MS/s. Figure 3 shows the current waveforms, the output voltages of the system, and their relationships for the two different gain antenna systems.

image

Figure 3. Pulse currents (a, d), output voltages of the system (b, e) and their relationships (c, f). (left) Low gain antenna and (right) high gain antenna.

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[8] It can be seen from Figure 3 that the magnetic field waveforms are very similar to the current waveforms, indicating that the system reflects quite well the external magnetic flux density change. We repeated the experiment many times and very similar correspondence was obtained. With considering the fitted value of R2 in Figures 3c and 3f, it can be concluded that the developed system works stably and reliably. From expressions in Figures 3c and 3f, the magnetic field could be obtained by multiplying the output voltage by 25 and 8.6 for low gain antenna and high gain antenna, respectively. The results also indicate that the maximum current that could be measured by the system is about 84.4 kA with low gain antenna and 37.7 kA with high gain antenna locating at 60 m from a lightning discharge channel.

3. Application in Artificially Triggering Lightning Experiment

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

[9] Synchronous data of the current and close magnetic field at a distance of 60 m from the lightning channel of triggered lightning flashes have been obtained during SHATLE 2006 and 2007. Two Rogowski coils with current limits of 2 kA and 100 kA, respectively, were used to measure the currents directly at the base of the discharge channel [Qie et al., 2007]. The bandwidth of both Rogowski coils is 300 Hz–1 MHz with an error for current measurement smaller than 5%. The magnetic field was detected with the system developed. Waveforms of the second return stroke current and its corresponding magnetic field in triggered flash named 0602 obtained in SHATLE 2006 are shown in Figure 4. The peak return stroke current shown in the figure is about 29.6 kA. Two dart-leader return stroke sequences occurred in this flash, and the first peak return stroke current is about 41.6 kA.

image

Figure 4. Directly measured return stroke current (a) and magnetic field (b) in triggered flash 0602.

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[10] Based on the tested results and Ampere's law of magnetostatics, B = μ0I/2πr, currents of the triggered flash 0602 were inferred from the measured magnetic field. The inferred currents for the two return strokes are 39.8 kA and 29.1 kA, respectively, which are consistent with the directly measured results of 41.6 kA and 29.6 kA. The difference between the inferred currents and the measured ones for the two return strokes is 1.8 kA and 0.5 kA, respectively, that is only about 4.3% and 1.7% of the original value. In addition, currents obtained here are in agreement with the current range of 2.5 kA – 60 kA for triggered flashes reported by Depasse [1994].

[11] Comparing the current waveform with the magnetic field waveform, it can be found that two small pulses, labeled P1 and P2 in Figure 4a, in the current rising slope are not well reflected by the magnetic field system. One reason might be the different bandwidth in the Rogowski coils and the developed system. The high frequency of the developed system might be overestimated, for the electronic integrator and the length of co-axial cables connecting the loops to the integrator also influence this limit, and the high frequency cutoff might not be faster than 1st order. On the other hand, in artificially triggering lightning experiment the sampling rates for the current and the magnetic field were 5 MS/s and 2 MS/s, respectively, which might also account for the difference between the current and the magnetic field waveforms, although both of them are adequately sampled. However, the inferred current from the magnetic field is in good agreement with the measured results, and the whole waveforms, especially the rising part of the two are very similar. In addition, we can also see that the developed system reflected well the pulse labeled P3 in the descending part of the current, and their discrepancy will be explained in the following.

[12] Figure 5 shows the waveforms of directly measured current and detected magnetic field for triggered flash named 0701 obtained in SHATLE 2007. There was only one return stroke in this flash. The peak return stroke current for this stoke is about 38 kA. The inferred current based on the magnetic field is about 43 kA. Figure 5 shows that the system responded properly for the current ascending part but not the descending, possibly indicating large amount of low frequency components in the current, which could not be measured properly by the magnetic field system due to its low frequency cutoff caused by the loop antennae, co-axial cables, electronic integrator, etc. And this can be also used to explain the discrepancy in the descending part of the waveforms in Figure 4. Similar problem was expected in the Rogowski coils for the 300 Hz lower frequency cutoff. The 10–90% risetime of the current was 0.6 μs and the corresponding value in magnetic field was 0.8 μs, which agree well with Schoene et al. [2003], whose current risetime is 0.3–4 μs and magnetic field risetime 0.3–3.2 μs.

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Figure 5. Directly measured return stroke current (a) and magnetic field (b) in triggered flash 0701.

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4. Conclusions and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

[13] A magnetic field system with two rectangular loop antennae perpendicular to each other has been developed and used to obtain the total horizontal magnetic field during artificially triggering lightning experiment. The signals from the antennae are then amplified before being recorded on a multiple channel digital scope. The laboratory results provided a stable and quantitative relationship between the detected magnetic field and the output voltage of the system. In addition, the waveforms of the detected magnetic field are quite similar to that of the source current. The current limit is about 37.7 kA and 84.4 kA with high gain antenna and low gain antenna locating at a distance of 60 m from the lightning channel, respectively. Main advantages of the system are its low cost and can be manufactured easily, which will greatly facilitate in the research of close electromagnetic environment of triggered flashes.

[14] Results of comparative analysis of the current and the magnetic field show that the three inferred return stroke currents (39.8 kA, 29.1 kA and 43 kA), based on the detected magnetic field, are in good agreement with that (41.6 kA, 29.6 kA and 38 kA) measured with Rogowski coils. Although the system could not respond completely to the fine structure in the current, the whole waveform and most of the discharge processes were well reflected.

[15] The system responds well to the pulse current generated by the 120 kA pulse current generator in the laboratory but not well to the artificially triggered flashes, which may indicate that the triggered lightning discharge process is more complicated than that of the pulses produced by the generator.

[16] Although artificially triggering lightning experiment provides an effective way for direct measurement of lightning current, the steep risetime of the current and the reflection from the ground objectives make inferring the discharge processes extremely difficult [Rachidi et al., 2002]. More theoretical and experimental work is needed. High resolution equipments with high sampling rate recording systems should be employed.

[17] The electromagnetic environment is still another complicated problem. First, the ground is not a perfect conductor actually, which will influence the propagation of the electromagnetic wave. Second, the relief of the ground will also affect the electromagnetic wave propagation. These factors should be considered in inferring the discharge property from the measured electromagnetic fields. In near future, magnetic field will be measured at different distances from the lightning channel, and the propagation characteristics of the electromagnetic wave will be studied.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References

[18] This research was supported by the Main Direction Program of the Knowledge Innovation of Chinese Academy of Sciences (grant KZCX2-YW-206), the National Natural Science Foundation for Distinguished Young Scholars of China (grant 40325013) and the National Natural Science Foundation of China (grant 40774083). Special thanks to E. Williams, editor Dr. Tarek, Dr. Steven, Dr. Matthew, and one anonymous reviewer for their valuable suggestions and comments, which improved the quality of this paper.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Magnetic Field System and Its Test in Laboratory
  5. 3. Application in Artificially Triggering Lightning Experiment
  6. 4. Conclusions and Discussion
  7. Acknowledgments
  8. References