J. Schoene

Lightning induced disturbances in buried cables - part II: experiment and model validation

This paper presents experimental results obtained at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida during the summers of 2002 and 2003. Currents induced by triggered and natural lightning events were measured at the terminations of a buried power cable, in the cable shield, and in the inner cable conductor. Measurements of the horizontal component of the magnetic field above the ground surface for both natural and triggered lightning are also presented. For distant natural lightning events, locations of ground strike points were determined using the U.S. National Lightning Detection Network (NLDN). Based on the theoretical developments presented in Part I of this paper , the field-to-buried cable coupling equations are solved in both the time domain and in the frequency domain. The obtained experimental results are then used to validate the numerical simulations provided by the relevant developed codes.

Return stroke transmission line model for stroke speed near and equal that of light

Assuming that the lightning return stroke transmission-line model is applicable, we derive an expression for the return-stroke magnetic field for an arbitrary return stroke speed and from that expression show that for a return stroke speed equal to the speed of light c the electric and magnetic field waveforms at all points in space and the current waveform are identical. Recent measurements indicate that the electric field and current waveforms are similar for about 100 ns for triggered lightning return strokes, potentially implying that the initial return stroke speed is actually near c for that time, that is, for the bottom 30 m or so of the triggered lightning channel. While for the transmission-line model the current waveform and the total electric field waveform are identical for a return stroke speed of c, we show that each of the three individual components (electrostatic, induction, and radiation) that comprise the total field varies significantly with distance.

Distribution of Currents in the Lightning Protective System of a Residential Building—Part I: Triggered-Lightning Experiments

We present the results of structural lightning protective system (LPS) tests conducted in 2004 and 2005 at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, FL. Lightning was triggered using the rocket-and- wire technique, and its current was directly injected into the LPS. The test configurations in 2004 and 2005 differed in the lightning current injection point, number of down conductors, grounding system at the test house, and the use of surge protective devices. The primary objective was to examine the division of the injected lightning current between the grounding system of the test house, and remote ground accessible via the neutral of the power-supply cable. In 2004, the mean value of the peak current entering the electrical circuit neutral in search of its way to remote ground was about 22% of the injected lightning current peak, while in 2005, it was about 59%. For comparison, more than 80% of the injected peak current was observed to enter the electrical circuit neutral in similar 1997 tests at the ICLRT in which a different test house with a different (poorer) grounding system was used (Rakov et al. 2002 [1]). An attempt to model the 2004 and 2005 experiments is presented in a companion paper.

Test of the transmission line model and the traveling current source model with triggered lightning return strokes at very close range

We test the two simplest and most conceptually different return stroke models, the transmission line model (TLM) and the traveling current source model (TCSM), by comparing the first microsecond of model-predicted electric and magnetic field wave forms and field derivative wave forms at 15 m and 30 m with the corresponding measured wave forms from triggered lightning return strokes. In the TLM the return stroke process is modeled as a current wave injected at the base of the lightning channel and propagating upward along the channel with neither attenuation nor dispersion and at an assumed constant speed. In the TCSM the return stroke process is modeled as a current source traveling upward at an assumed constant speed and injecting a current wave into the channel, which then propagates downward at the speed of light and is absorbed at ground without reflection. The electric and magnetic fields were calculated from Maxwells equations given the measured current or current derivative at the channel base, an assumed return stroke speed, and the temporal and spatial distribution of the channel current specified by the return stroke model. Electric and magnetic fields and their derivatives were measured 15 m and 30 m from rocket-triggered lightning during the summer of 2001 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. We present data from a five-stroke flash, S0105, and compare the measured fields and field derivatives with the model-predicted ones for three assumed lightning return stroke speeds, v = 1 x 10 8 m/s, v = 2 x 10 8 m/s, and v = 2.99 × 10 8 m/s (essentially the speed of light). The results presented show that the TLM works reasonably well in predicting the measured electric and magnetic fields (field derivatives) at 15 m and 30 m if return stroke speeds during the first microsecond are chosen to be between 1 × 10 8 m/s and 2 x 10 8 m/s (near 2 x 10 8 m/s). In general, the TLM works better in predicting the measured field derivatives than in predicting the measured fields. The TCSM does not adequately predict either the measured electric fields or the measured electric and magnetic field derivatives at 15 and 30 m during the first microsecond or so.

Testing of Lightning Protective System of a Residential Structure: Comparison of Data Obtained in Rocket-Triggered Lightning and Current Surge Generator Experiments

We present a comparison of data obtained during testing of lightning protective system of a residential structure in rocket-triggered lightning experiment at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, and current surge generator experiment at Rzeszow University of Technology in Poland. Three different configurations of LPS were tested in Poland with the dc grounding resistances of the entire system 4.09 Omega (LPS 1a), 1.65 Omega (LPS 1b), and 2.88 Omega (LPS 2). For LPS 1a with three ground rods the value of the peak current entering the electrical circuit neutral was about 56% of the injected current peak, and for LPS 1b with two additional ground rods, each connected by a buried horizontal conductor 5 m long, was about 16%. For LPS 2 with five ground rods interconnected by a buried loop conductor this ratio was 21%. The current waveshapes in the ground rods differed from the injected current waveshapes and the current waveshapes in other parts of the test system, especially, for poorer LPS 1a. The surge-generator results are consistent with those of triggered-lightning experiments at Camp Blanding, Florida (DeCarlo et al., 2006 [2]).

Direct Lightning Strikes to Test Power Distribution Lines—Part I: Experiment and Overall Results

The interaction of rocket-triggered lightning with two unenergized power distribution lines of about 800 m length was studied at the International Center for Lightning Research and Testing in Florida. A horizontally configured line was tested in 2000, and a vertically configured line in 2001, 2002, and 2003. The horizontally and vertically configured lines were equipped with six and four arrester stations, respectively, and, additionally, in 2003, the vertical line with a pole-mounted transformer. During the 2000, 2001, and 2002 experiments, arresters were frequently rendered inoperable by disconnector operation during triggered lightning strokes, but there was no disconnector operation during the 2003 experiment when the transformer was on the line. In all four years, there were commonly flashovers from the struck phase-conductor to the closest phase-conductor not subjected to direct lightning current injection. The self-consistency of measurements is assessed via comparison of the injected lightning current with: 1) the total current flowing to Earth through the multiple line groundings and 2) the total phase-to-neutral current flowing through the line arresters and line terminations. This paper is part one of two related papers.

Experimental Study of Lightning-Induced Currents in a Buried Loop Conductor and a Grounded Vertical Conductor

Currents induced in: (1) a 100 mtimes30 m buried rectangular loop conductor (counterpoise) and (2) a grounded vertical conductor of 7-m height by natural and rocket-triggered lightning at distances ranging from 60 to 300 m were recorded in 2005 at the International Center for Lightning Research and Testing (ICLRT). The peak values of 12 triggered lightning channel-base currents and the peak values of the induced currents in the counterpoise are strongly correlated. The first few microseconds of the current induced in the vertical conductor by triggered lightning return strokes 100 m away resemble electric field time-derivative waveforms simultaneously measured at the ICLRT. During a close natural lightning flash, five pre-first-return-stroke current pulses with peak currents up to 140 A were measured in the vertical conductor. These are apparently associated with multiple attempts of an upward-moving unconnected leader occurring in response to the charge lowered by downward-propagating leader steps.

Direct Lightning Strikes to Test Power Distribution Lines—Part II: Measured and Modeled Current Division Among Multiple Arresters and Grounds

The division of return stroke current among the arresters and groundings of two unenergized test distribution lines, one horizontally configured and the other vertically configured, was studied at the International Center for Lightning Research and Testing in Florida. The division of return stroke currents for the vertically configured line was initially similar to the division on the horizontally configured line: at the time the return stroke current reached peak value (after one microsecond, or so) the two closest arresters/grounds on both lines passed about 90% of the total current. However, the time during which the return stroke current flowed primarily through the closest arresters to the neutral conductor was significantly shorter on the vertically configured line. On that line, the arrester current was about equally divided among all four arresters after several tens of microseconds. The arrester current division as a function of time measured on the vertical line was successfully modeled using the published VI-characteristic, while the division on the horizontal line after some tens of microseconds was only successfully modeled if the residual voltage of the two arresters closest to the current injection point was reduced by 20%. Based on the triggered lightning current division observed on our line, the minimum energy absorbed in each of the two arresters closest to the strike point during a typical natural first stroke is estimated to be 40 kJ.

Lightning Currents Flowing in the Soil and Entering a Test Power Distribution Line Via Its Grounding

Current from nearby rocket-triggered lightning that flowed through the soil and into an unenergized test power distribution line was studied based on experimental data acquired in 2003 at the International Center for Lightning Research and Testing in Florida. The 15-pole, three-phase line was 812 m long, was equipped with four arrester stations, at poles 2, 6, 10, and 14, and was terminated in its characteristic impedance at poles 1 and 15. The neutral conductor of the line was grounded at each arrester station and at both line terminations. Measurements suggest that a significant fraction of the lightning current injected into the earth a distance of 11 m from pole 15 entered the line through the grounding system of pole 15. The peak value of the microsecond-scale return stroke current entering the line through the pole 15 line ground was 7% of the peak value of the return stroke current injected into the earth. The peak value of the millisecond-scale triggered lightning initial stage current and the millisecond-scale return-stroke and initial-stage charge transfer to the line through the pole 15 line ground was between 12% and 19% of the lightning peak current/charge transfer, indicating that the percentage values for the injected peak currents are dependent on the current waveshape: for microsecond-scale return stroke currents, possibly due to electromagnetic coupling effects, a smaller fraction of the current peak enters the line compared to millisecond-scale initial stage currents. In the latter case, any influence of electromagnetic coupling to the line on ground currents is expected to be negligible.

Interaction between grounding systems and nearby lightning for the calculation of overvoltages in overhead distribution lines

The aim of the paper is to analyze overvoltages in overhead distribution lines induced by nearby lightning return stroke currents taking into account also the conductive coupling between the lightning current injected into the soil and the lines grounding system. Experimental results obtained in 2003 at the International Center for Lightning Research and Testing (ICLRT) in Florida have shown that significant currents were coupled with an overhead experimental line through its grounding system by nearby rocket-triggered lightning. The paper first proposes a conductive coupling model between the lightning current injected into the soil and the nearby line grounding system, then presents the incorporation of such a model into the LIOV-EMTP computer code. The paper finally presents an experimental validation of the proposed models by making reference to results obtained during the above-mentioned triggered-lightning campaign.