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  :: Experimental study on lightning breakdown channels in the Soils
  :: 관리자 2007-06-13 17:10:00 , 조회 :6958  
  :: File download   [pdf : 1245 KB  Download: 4636]
EXPERIMENTAL STUDY ON LIGHTNING BREAKDOWN CHANNELS IN TIlE SOILS
 
Song Zeqing, He Jing iiang

Wuhan University of Hydraulic & Electric Engineering
Wuhan, Hubel, P.R. China 430072
M.R. Ro,ghtlVeer

The University of Manitoba
Winnipeg, Manitoba, Canada R3T 5V6
 
Abstract: Presenl day models do not satisfactorily describe the surge behavior of earth under the passage of lightning currents which result in a single breakdown channel. This paper presents. preliminary results of an investigation into the factors that affect lIahtning current propagation In different soils under sinale channel formation.
The results presented In this paper were obtained uslng an indoor experimental system and examine the influence of soil characteristics and source capacity and voltage on lightning channel characteristics.
  It is hoped that Ihese results will be useful in establishing models which describe both single and multiple channel formation In soils.
 
l.introductton
  According to traditional lightning theory, when lightning reaches the surface of earth, it spreads into the soil, which may be represented by an equivalent hemispherical eleerrode ofrodilts equal to that of the sparking channel. Severa! mode-Is that des.cribe the behavior of earth under impulse currems are based on this assumption [1][2]. But it is known, from the results of both indoor experiments and field data conceming outdoor lightning strikes resulting in damage to underground cables that impulse currents propagate along a single channel during the total
propagating period or for part of the total traversed distance [3] [4]. Present day models do not satisfactorily describe either siruation [I]. In order to investigate the factors which influence formation of single and multiple patins and the breakdown mechanism of soils, we have be·en carrying out cooperative research work. to date we have addressed the following nspects:
1) When an impulse c urrent generator discharges into different kinds of soils, what are the factors that decide whether the current propagates along a single or multiple channels?
2) What is the influence of moisture, compaction and resistivity?
3) What is Ihe influence of generator capacitance and voltage?
4) Finally, does the lightning current exhibit a tendency to flow along pre·formed channels?
  This paper presents results concerning single channel formation. Other results will be published in future papers.
 
2.Experimental system
  The indoor experimental system was comprised of an impulse current generator discharging into a hemispherical pot of radius 100 mm into which different soils were packed. (Figure 1).
  At the bottommost point and at a height of 45mm from the bottom, six 40mm radius round plate electrodes are located synunemcally (one at the bottom point and five along the circle, Figure 2),
The six electrodes cover about 3/4 of the area of hemispherical surface and their outputs are linked together to a point th at is earthed. The current entrance electrode is an 18 mm-diameter red placed at the center of hemisphere just above the soil suface in order to provide a sparking connection. It is held syrometricaliy with respect to the six
measurement electrodes and is also constrained so that it does not move under tile impulsive force produced by discharging.
   Magnetic links were used to detennine which elearode was hit by the lightning channel. Branch currents were estimated by proof current_angle curve of magnetic link.
  The signal across a shunt was connected to TDS220 oscilloscope (Bandwidth: IOOMHZ, Sampling: rate: lOs/S, Record length: 2500 points) to display and print the total current wOlv-eform.
  The resistivity and moisture of soi were ineasured using tile methods reponed in [5]. Soil compaction was calculated by dividing the total weight of soH by the volume of the hemisphere.
 
3.Experimental results
  Although experiments were carried out in it soil model indoors, the fonowing two observations are common to several outdoor lightning incidents which resuhed in damage 10 communication cables in the Guangdong province of China [4]. This commonality is very important since it lends credibllity to the results generated indoors using a
small model.
1) when a soil is broken down it results in holes or ditches in the soil.
2) The conditions that cause multiple channel formation are more severe than those which cause single channel formation.
 
3.1 Influence of soils characteristics
  Soil resistivily is the basic parameter tbat affects breakdown channels. Since different soils have different compaction and moisture content, their resistivities are different, and hence their breakdown voltages for single channel formation are different.
The general townendency exhibited is that the lower the resistivity, the smaner the minimum breakdown voltage for a single channel. --See Table 1
  From Tables 1 and 2 it is also seen that when moisture or compaction increased. the soils resistivity decreases and its minlmum breakdown voltage for single channel formation decreases also.
  A dry soil has a very high res istivity, and its breakdown voltage is very high, but only a little water sprinkled on the surface of soil (simulating natural r ain) causes a sig:tificant decrease in the breakdown voltage of a single channel. Table 1 shows that send wjth zero moisture content could not be broken down even when the generator
yoltage was increased to 46 kV. From data concerning dielecnic strength published in [3], it can be calculated that the break down voltage for this case is approximately 70kV. However, after only about 15 to 20 dtops of tap water were evenly sprinkled on the surface of the soil the breakdown voltage decreased to 35 kV with resulting peak
value of current of 500A
  Table I:Minimum breakdown voltage for single channel in dlfferent soils
  Table 2:Influence of compaction
 
3.2 Influence o f impulse generator capaCitance and voltage
 When the generator capccitors DC Voltage is at the minunum value for a particular soils breakdown, it results in the formation of a single channel. If the voltage is increased, only one channel forms but the value of current
increases. If the voltage is increased beyond a certain value, the current path has a tendency to split into two
branches.
  TABLE 3: Influence of gene rotor capacitance Cultivat-cd soil , 13.7%moislUre , 1 27g/㎤ 1900Ωm
When the generator capacitance is decreased, the breakdown voltage for formation of a single channel increases and the total current decreases. On the other hand, the voltage value for multiple path formation does not change appreciably despite the fact that both total and branch currents decrease. - See Table 3.
 
3.3 Location of channels:
  If the soil condition is restored to its original condition after each discharge of the generator, each plate electrode is equally likely to be the recipient of a chaMel which forms randomly. The reason is that the six electrodes are situated equidistantly from the top entrance electrode from an electromagnetic field point of view. This observation was confmned for various values of generator capacitance.
  1f the soil is left undisturbed after each discharge, the next discharge (for same value of generator voltage) propagates in the soil along or close to the channel formed upon previous application of voltage. Under such circumstances all channels form close to each other. Table 4 shows that the breakdown channels are confined to a small volume formed by the entrance electrode and plate electrodes #2 to #4.
 
  Table 4:Location of channe
3.3 Location of channels:
  Figure 3 shows a typical waveform of the total current. Because breakdown resistance varies under different condition, it affects the total current waveform.
  The Time lag Tl is the time difference between the application of a voltage sufficient to cause soil breakdown and occurrence of breakdown itself.
  For any type of soil, Time lag T1 decreases as generator voltage is increased. When the voltage applied corresponds to the minimum voltage for single breakdown channel fonnation, Tl reaches a maximum that may be several hundred microseconds. When the capacitor voltage exceeds a certain value, TI becomes shorter than 3
microseconds and does not change significantly thereafter. If other conditions are not changed the lower the resistivity of soil, the sholter the Tl.
  The duration of impulse current Td is also affected by applied voltage. As the generator capacitor voltage is increased beginning from the minimum value necessary for single channel fonnation, the duration Td shows a tendency to decrease. After the voltage exceeds a certain value, the duration changes very little. -See Table 5
 
  Table 5: Tl and Td
  With respect to some soils, when the generator voltage is increased, the wavefonn of total current exhibits a negative overshoot. --See Figure 4.
 
4.Conculsions
--Resistivity is the basic parameter that affects formation of breakdoVl1 channels. With regard to homogeneous soils the influence of moisture and compaction is reflected by resistivity changes.
--The mechanism of breakdown is different for dry soils, soils that are wetted only at the top and homogeneously wetted soils.
--In model studies breakdoVl1 is more likely to occur along or close to preformed channels if the soil is left undisturbed between discharges of the impulse current generator.
--Breakdown channel has different equivalent resistance under different condition, so it affects the total current waveform.
 
5. References
[1] AC.Liew, "Dynamic model of impulse characteristics of concentrated earths", Proc. lEE, VoL 121, No.2, Feb. 1974
[2] M.E. Almedia, "Modeling the hysteresis behavior of the transmission tower footing", 9th ISH in Graz., Australia, 1995
[3] l.A.l Pettinga, "Fulgurites useful to deternine the charge of a lightning strokes", 10th ISH in Montreal, Canada, ] 997
[4 J Xue Lei, "Lightning protection for underground communication cables", Lightning and Electrostatics, Vo1.1 No.1, 1985
  :: File download   [pdf : 1245 KB  Download: 4636]
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