Metal strands of OPGWs (composite fiber-optic ground wires) installed in overhead power transmission lines are sometimes melted and broken when struck by high-energy lightning. This paper presents the calculation results regarding OPGW strands melting behavior when struck by DC arcs simulating high-energy lightning. The calculations revealed that the melted volume of the strand was 26% of the strand volume before the arc test, i.e. the rate of the non-melted volume of the strand was 74%. On the other hand, the residual tensile strength of the melted strand was 69% of the other non-melted strands after DC arc test. These results suggest there is a strong correlation between the calculated non-melted volume of the strand and the measured residual tensile strength of the melted strand

Goda, Y.

Iwata, M.

Kotari, M.

Nakano, T.

Ohtaka, T.

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

doi:10.14311/ppt.2019.2.188Plasma Physics and Technology 6(2):188–192, 2019 © Department of Physics, FEE CTU in Prague, 2019CALCULATION OF OPGW STRANDS MELTING DUE TO DC ARCDISCHARGE SIMULATING HIGH-ENERGY LIGHTNING STRIKEM. Iwata∗, T. Nakano, M. Kotari, T. Ohtaka, Y. GodaCentral Research Institute of Electric Power Industry (CRIEPI), 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken240-0196, Japan∗ m-iwata@criepi.denken.org.jpAbstract. Metal strands of OPGWs (composite fiber-optic ground wires) installed in overhead powertransmission lines are sometimes melted and broken when struck by high-energy lightning. This paperpresents the calculation results regarding OPGW strands melting behavior when struck by DC arcssimulating high-energy lightning. The calculations revealed that the melted volume of the strand was26% of the strand volume before the arc test, i.e. the rate of the non-melted volume of the strandwas 74%. On the other hand, the residual tensile strength of the melted strand was 69% of the othernon-melted strands after DC arc test. These results suggest there is a strong correlation between thecalculated non-melted volume of the strand and the measured residual tensile strength of the meltedstrand.Keywords: ground wire, strand melting, DC arc, lightning, power transmission line.1. IntroductionGround wires (GWs) have been used to prevent over-head power transmission lines from being struckby lightning. Composite fiber-optic ground wires(OPGWs) have also been used for the aforementionedpurpose as well as transmitting high volumes of com-munication information. Metal strands of OPGWs aresometimes melted and broken when they are struck byhigh-energy lightning, although the main componentof strands is steel. In Japan, winter lightning hashigh energy and the lightning current can continue foran extraordinarily long time. For example, as shownin Figure 1, the measured current was not a pulsecurrent but similar to a direct current (DC) with asquare wave shape [1]. Therefore, in order to clarifythe melting and breaking characteristics of conven-tional and newly-developed OPGW strands, DC arctests have been performed and the test conditionsthat adequately simulate high-energy lightning strikeshave also been investigated [2–6], and the authorshave developed a method of calculating the strandsmelting due to DC arcs [7].It is important to clarify the melting and break-ing characteristics of OPGW strands with a smallcross-section (e.g., 60–80mm2) as thin OPGWs havebeen melted and broken by lightning strikes on actualtransmission lines [4, 8]. And, it is also importantfor electric utilities to grasp the residual strength ofOPGW which has some melted strands even if thestrands are not broken.In this paper, the calculations regarding the melting(not the breaking) behavior of 60mm2 OPGW strandswhen struck by DC arcs were performed, and thecalculation results were compared with the test resultsof the residual tensile strength of the strands afterDC arc tests.Figure 1. An example of current waveform of winterlightning in Japan [1].2. Methods of DC arc testIn this chapter, the methods and conditions of DCarc tests are summarized.2.1. OPGW specificationsFigure 2 shows the structure of the 60mm2 OPGWused in the tests. Optical fiber cables were not in-stalled in the OPGW in the tests. The strand typewas 23AC, where “AC” denotes aluminum-clad steel,while “23” indicates that the electric conductivity ofthe AC strand is 23% of that of a copper strand withan equivalent cross-section. The OPGW has six ACstrands whose shape is trapezoidal.2.2. Test methods and conditionsFigure 3 shows the setup used for the DC arc testswhich were carried out in open air. An OPGW witha length of 5m was stretched out at both ends with atensile force of 12.0 kN, which is almost equivalent tothe force applied when stringing in actual transmissionlines. A thin copper wire with a diameter of 0.2mmwas stretched between the OPGW and a counter elec-trode so that the current flowed to vaporize the copperwire and generate an arc.188vol. 6 no. 2/2019 Calculation of OPGW strands meltingFigure 2. Structure of 60mm2 OPGW used in the tests.The counter electrode was an iron rod with a di-ameter of 20mm, set above the OPGW with a gaplength of 1.5m to minimize the influence of the arc jetspouting from the counter electrode on the behaviorof an arc near the OPGW. However, the arc root onan OPGW tends to be too mobile to melt the strandsbecause of the electromagnetic force induced by thecurrent flowing through it. Accordingly, conductorswere connected to both ends of the OPGW to sym-metrically divert the arc current to the earth side sothat the arc root was hard to move. In this paper,to investigate the OPGW strands melting behaviorfundamentally, an arc was generated only on a partof one strand by winding vinyl tape onto the otherpart of strands, as shown in Figure 4(a). The lengthof the exposed (arc-producing) part of one strand was10mm.ArcOPGWCounter electrode1.5mWeightTo power supplyInsulatorWeightInsulatorTensionmeterConductor(earth side)To power supply(voltage applied side)ConductorEnlarged view10mmVinyl tapeFigure 3. Setup used for DC arc tests.The test current was supplied by full-wave recti-fying three-phase AC using silicon diodes. The ACwas generated using a short-circuit generator (15 kV,2500MVA) and a transformer. The DC output canrange between 120 kV/ 10 kA and 16 kV/ 60 kA bychanging the AC voltage and the combination ofdiodes. In this paper, the melting (not breaking)behavior of OPGW strands was investigated funda-mentally as described above. Therefore, the test cur-rent IDC (0.4 kA (400A)) and the electric charge QFigure 4. Photographs (a) before and (b), (c) afterDC arc test; in (c) vinyl tape was removed.Strand # Tensile strength (kN)1(*) 10.502 15.053 15.204 15.105 15.206 15.25Table 1. Measured residual tensile strength of strandsafter DC arc test. (*) A part of the strand #1 wasexposed and melted by the arc.(10C) were relatively low. The polarity of the cur-rent was positive, i.e., the polarity of the OPGW wasnegative.3. Test resultsFigures 4(b), 4(c) show photographs of the OPGWafter the arc test. The exposed strand was meltedbut not broken. It is important for electric utilities tograsp the residual strength of OPGW which has somemelted strands even if the strands are not broken.To measure the residual strength of strands, tensiletests of the melted strand and the other non-meltedstrand were performed individually. Table 1 showsthe residual tensile strength of six strands after DCarc test. The tensile strength of the strands #1,which has a melted part, was 10.50 kN. On the otherhand, the tensile strength of other non-melted strands#2–#6 was 15.05–15.25 kN (average value: 15.16 kN).These results show the residual tensile strength ofthe melted strand was about 69% of those of non-melted strands. In this chapter, the test results forone specimen (OPGW) were shown. The test resultsobtained under other conditions for the same kind ofspecimens showed a tendency similar to the resultsdescribed above.4. Calculation methods and conditionsCalculations were carried out using the method devel-oped in [7]. This chapter summarizes the calculationmethods and conditions.189M. Iwata, T. Nakano, M. Kotari et al. Plasma Physics and TechnologyArc conditions(e.g. Current, Duration, Polarity, Gap length)Calculation of total heattransferred to OPGW from arcCalculation of amount of OPGW meltingExposedpart ofOPGWArcTransferredheat areaTotal transferredheatOPGW conditions(e.g. Size, Type and Number of strands)Calculation of area of heattransferred to OPGW from arcFigure 5. Flowchart of calculations [7].Figure 5 shows the flowchart used to calculate themelting characteristics of OPGWs due to DC arc. Byusing the arc conditions (e.g., current, duration, polar-ity, gap length) and OPGW conditions (e.g., the size,type and number of strands), the quantity of heat andits area transferred to the OPGW from the arc werecalculated, and the amount of melting of the OPGWstrands due to the arc was calculated. Although thematerial of the central part of the aluminum-clad steelstrand was steel, the material in the calculation wasiron since it is the main component of steel and itsthermal properties are easily obtained from references.4.1. Heat transferred to OPGW from the arcand its areaThe total heat Ptotal transferred from the DC arc tothe OPGW is calculated using the following equation[7]:Ptotal = Pele + Pconv (1)where Pele is the heat transferred by electrons emissionfrom the OPGW and by electrons absorption into theOPGW. Pconv is the heat transferred by convectiondue to the arc jet spouting from the upper counterelectrode, where the convective heat from the arccolumn was not considered because the velocity of thearc column towards the OPGW was thought less thanthat of the arc jet. For the reference, the heat dueto the radiation from the arc to the OPGW, and thejoule heating in the OPGW were not considered inthis paper, because these heats were thought relativelysmaller than Pele according to the reference [9].Pele depends on the polarity of electrode as follows[10]:In the case of an anode,Pele = (Va + Vw + VT) IDC (2)In the case of a cathode,Pele = (Vc − Vw − VT) IDC (3)where Va and Vc are the anode-arc-fall and cathode-arc-fall voltages, respectively, Vw is the work functionof the electrode. VT is the voltage estimated by di-viding the enthalpy due to the electron drift by thearc current IDC. VT is given by the following equa-tion, in which Boltzmann’s constant and the value ofelectronic charge are considered [10]VT = (1.29 · 10−4)Tarc (4)where Tarc is the arc temperature near the electrode.These voltages Va, Vc, Vw, and VT vary dependingon the electrode material and the surrounding gas. Inthis paper, these voltages are the ones for the electrodematerial aluminum and iron and the surrounding gasair [7], because the OPGW strand was composedof aluminum and iron in the calculations describedabove, and DC arc tests were performed in the air asdescribed in section 2.The convective heat transferred from the arc jetsto the OPGW is calculated usingPconv = hAh (Tarc − TOPGW) (5)where h is coefficient of the heat transfer and TOPGW isthe temperature of the OPGW surface. Ah is the areaof heat transfer, and the estimation method for thearea is described below. For the reference, Pconv wasless than 1% of Ptotal because the spouting velocity ofthe arc jet was low due to the low current (0.4 kA) andthe gap length between the upper counter electrodeand the OPGW was long (1.5m).The transferred heat area Ah for Pele and Pconv wasestimated as an area touched by the arc using thefollowing equation assuming that the arc root wouldstay within the exposed area of the OPGW:Ah = MIN(IDC/JDC, Aex) (6)where MIN is a function giving the smallest valuein the brackets, JDC is the current density on theOPGW, Aex is the exposed area of the OPGW. JDCfor aluminum and iron are 60.0 and 33.8A/mm2, re-spectively, and independent of its polarity, which wereestimated using the arc current and the arc area eval-uated by means of the diameter of the arc root on theelectrode measured in [11] assuming that the cross-section shape of the arc root was circular.4.2. Simplification of the cross-sectionalstructure of OPGW strandsIn the case of an actual strand, as shown in Figures 2and 6(a), the central part is iron (the main compo-nent of steel), which is covered with aluminum. Inthe calculations, the cross-sectional structure of thestrand was simplified, as shown in Figure 6(b), to analuminum layer on an iron layer. The outer diameterD and the cross-sectional areas of the aluminum layerand iron layer in Figure 6(b) are the same as those inFigure 6(a). It was assumed that the heat transferredfrom the arc was injected into the aluminum layer,which was subsequently melted and removed, where-upon the heat from the arc was injected into the ironlayer.190vol. 6 no. 2/2019 Calculation of OPGW strands meltingFigure 6. Simplification of cross-sectional structure ofOPGW strands [7]. (a) Actual structure, (b) Simplifiedstructure.5. Calculation resultsAlthough the exposed length of strand was 10mm,the length of melted part of aluminium and steelof the strand was 8.2mm and 5.2mm, respectively,which was measured by X-ray computed tomography(X-ray CT) observation. As shown in Figure 7, thecalculated durations required to melt the aluminumlayer (8.2mm in length) and the steel layer (5.2mmin length) was 6.3ms and 81.9ms, respectively. Thearcing duration in the DC arc test was 27.3ms. As thealuminum layer was firstly melted and removed in thecalculations referring to the observation by X-ray CT,the duration for injecting the heat from the arc to thesteel layer was 21.0ms (= 27.3ms−6.3ms). Assumingthat the melted volume of the steel part increased inproportion to time, the melted volume of the steelpart was 26% (= (21.0ms/81.9ms) ∗ 100%) of thesteel volume before the DC arc test, which meansthe non-melted volume of the steel part was 74% inthis case. As described in chapter 3, the residualtensile strength of the melted strand was about 69%of those of non-melted strands. For the reference, apart of the steel of the strand was melted and removedaccording to the observation by X-ray CT. The lackof a part of the steel would lead to the decrease of thetensile strength of the strand. These results suggestthere is a strong correlation between the calculatednon-melted volume of the steel part of the strand andthe measured residual tensile strength of the meltedstrand.6. ConclusionsMetal strands of OPGW are sometimes melted andbroken when struck by high-energy lightning. It isimportant for electric utilities to grasp the residualstrength of OPGW which has some melted strandseven if the strands are not broken. This paperdescribed the calculation results regarding OPGWstrands melting behavior when struck by DC arcs sim-ulating high-energy lightning. The calculations wereperformed considering the heat due to electrons andthe convective heat transfer. As a result, the calcu-lation results revealed that the melted volume of theCal. Exp.806040200100Duration (ms)27.3AlFe6.381.988.221.06.327.3Figure 7. Comparison of the experimental arcing du-ration and the calculated duration required to melt thestrand. Cal. - Calculated duration required to meltaluminum and iron layer; Exp. - Arcing duration intest.steel part of the strand was 26% of the volume of thesteel part before the arc test, i.e. the rate of the non-melted volume of the steel part was 74%. In addition,the residual tensile strength of the strand after DCarc test was measured. The residual tensile strengthof the melted strand was 69% of the other non-meltedstrands. These results suggest there is a strong corre-lation between the calculated non-melted volume ofthe steel part of the strand and the measured residualtensile strength of the melted strand.References[1] Working Group on Lightning Protection forTransmission Systems. Lightning observation on JapanSea coast in winter. In CRIEPI Report, chapter T10.CRIEPI, Tokyo, Japan, 1989.[2] M. Boehme and K. Moeller. Arc motion duringlightning test of power ground wires with optical fibers(OPGW). In Proc. 24th Int. Conf. on LightningProtection, pages 571–575, 1998.[3] Y. Goda, S. Yokoyama, S. Watanabe, T. Kawano, andS. Kanda. Melting and breaking characteristics ofOPGW strands by lightning. IEEE Trans. PowerDelivery, 19(4):1734–1739, 2004.doi:10.1109/TPWRD.2004.832410.[4] Y. Goda, M. Shimizu, and A. Matsumoto. DC arctests of OPGW simulating high energy lightning strike.In Proc. 28th Int. Conf. on Lightning Protection, pages959–964, 2006.[5] S.-H. Xie and R.-S. Yang. Study of OPGWperformance against lightning strikes in UHVtransmission lines. In Proc. 58th Int. Wire and CableSymp., pages 393–398, 2009.[6] T. Ohtaka, M. Iwata, Y. Goda, Y. Kuzuma, M. Fujii,A. Matsuba, and K. Nagano. Strand breakingcharacteristics of OPGW 80mm2 and AC 150mm2 byDC arc. In Proc. the 2013 Annual Meeting Record IEEJ,volume 7, pages 150–151, 2013.191M. Iwata, T. Nakano, M. Kotari et al. Plasma Physics and Technology[7] M. Iwata, T. Ohtaka, Y. Kuzuma, and Y. Goda.Development of calculation method for meltingcharacteristics of OPGW strands due to DC arc. InCRIEPI Report, chapter H11003. CRIEPI, Tokyo,Japan, 2011.[8] J. Huang, X. Chen, and J. Xu. Investigation andanalysis of an OPGW lightning strike incident. In Proc.58th Int. Wire and Cable Symp., pages 389–392, 2009.[9] M. Shibuya, T. Inaba, and Y. Goda. Molten losses oncut end of rod electrodes in AC high current arcdischarge. Inst. Elect. Eng. Jpn. Trans., 113-B:327–334,1993. doi:10.1541/ieejpes1990.113.4_327.[10] Y. Kito and T. Mizuno. An experimental approach tomelting of aluminium alloy string at an arc spot formedby surging current. Inst. Elect. Eng. Jpn. Trans., 108-B:577–584, 1988. doi:10.1541/ieejpes1972.108.577.[11] Y. Goda, M. Iwata, K. Ikeda, and S. Tanaka.Behaviour of arc jet of 50 kA class high current arc in along gap. In CRIEPI Report, chapter W00034. CRIEPI,Tokyo, Japan, 2001.192

Calculation of OPGW Strands Melting due to DC Arc Discharge Simulating High-Energy Lightning Strike

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Calculation of OPGW Strands Melting due to DC Arc Discharge Simulating High-Energy Lightning Strike

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