advances in lightning protection and grounding systems for power systems

8/14/2019 Advances in Lightning Protection and Grounding Systems for Power Systems

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Conference Papers

Paper No04 A 3 0-7803-8298-6/04/ 20 00 020041EEE

Advances in L ightning Protection andGroun ding Systems for Pow er Systems

M.M. Drabkln Lightning Eliminators Consultants Inc.Roy B. Carpenter Jr. Lightning Eliminators & Consultants Inc.

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Advances in Lightning Protection and Grounding Systems for Power SystemsM M Drabkm and Roy B Carpenter, Jr.Lightning Eliminators & Consultants, Inc

Boulder, Colorado,USA

Abstract: Power lines outages caused by lightningpresent a serious engineering problem. Design ofoverhead power lines with an acceptable level oflightning protection for regions with high density oflightning flashes and high specific resistivity of the soilcan be a challenging task. In many cases, shield wiresand lightning arresters are widely used means for thispurpose, but do not provide the total compliance withthe required number of line outages due to lightning andmay be not cost effective. This paper describes analternative method of lightning protection for overheadpower lines. The enhanced zone of protection providedby multi-point lightning protectors installed on he polesinstead of shield wires will cover most of the span.Implementation of chemically charged grounding rodstogether with the multi-point protecton will decreasethe number of back flashovers causing the trip off of theline or eliminate them, depending on final earthingresistance..1 IntroductionWith the growing importance of the quality of electricpower supplied to all categories of customers, industrial,commercial and residential, development of protectivesystems and devices becomes increasingly critical inimproving reliability of electric power. Considerableattention is given to lightning protection of power lines.to achieve acceptably low lightning outage rates.

- Lightning cloud-to-ground discharges can causeovewoltage on an overhead power line by either directstroke into the line or by stroke into the ground or intothe nearby object in close vicinity of the line. Thehigher the nominal system voltage, the less sensitive theline is to the induced overvoltages.In the case of a direct lightning stroke into a powerline there are three possibilities to consider, i.e., strokeinto phase conductor, stroke into a pole, or a stroke intoan overhead shield wire (OHSW) in the span. A strokeinto a phase conductor usually trips off a power linebecause a relatively low magnitude of the lightningcurrent (usually less than 1 kA) s sufficient to produceovervoltage exceeding the lines BIL. This is a commoncase for power distribution lines, which are builtwithout OHSW. Sub-transmission and transmissionpower limes are usually built with one or two OHSW.Direct stroke into OHSW requires substantially higher

magnitude of the lightning current to produce backflashover. The total number of the power h e nsulationflashovers due to lightning is the sum of flashoverscaused by indirect strokes and by direct strokes intophase conductors, into poles or into the OHSW close topole and in vicinity of the middle of a span. Flashoversdue to the direct lightning strokes to the OHSW present90 to 95 percent of the total number of the lineflashovers for operating voltages of 13.4 kV or less.

2 Lightning s t rokes into a po l eWhen lightning strikes into a pole of the shielded powerline the lightning current at the point of attachment isdivided into three parts: current flowing downwardthrough the pole into the pole foot resistance, q dcurrents flowing to the adjacent poles via the OHSW.Assuming 400 Ohms as a typical value for the OHSWsurge impedance, the share of the current through thepole will be 50, 80 95 and 97 5 percent of the totallightning current for the pole footing resistances of 100,50, 1 and 5 Ohms correspondently.

The voltage applied to the line insulation during thelightning current flow through the pole into the footgrounding resistance will be equal to (I) :

AV = Vp - Vph.. 11)where: V, is the voltage at the pole and

V,,., is the voltage at a phase conductorThe voltage V, appeared at the pole consists of threecomponents and can be calculated according to the 2 )

V, = I bdIddt MldIddt 2)where: I, the lightning current through the pole, or

Z - the pole footing surge impedance,MI he mutual inductance with the lightningII - peak lightning current

ground wirehe inductance of the pole, or ground wirechannel.

The voltage VPhhconsists of the working voltage, thevoltage induced by the charge in the lightning channel,and the voltage induced by lightning current in theOHSW. Vph.. can be calculated according to (3)

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where: V - he system maximum working voltage,E, - the average electric field at the groundlevel produced by the charge of thelightning channel,

h, height of the pole,k, - coefficient of coupling between theOHSW and the phase conductor

A back flashover will occur if the voltage AV willexceed the impulse voltage strength of insulation.Table I presents the results of the sample calculationaccording to I ) , 2) and 3) for a case of a 134 kVtransmission line. The lightning current was assumed tobe equal to 20 kA peak and with front duration equal toI ps poles average height 20 m, an average lightningelectric field 10 kV/m. The calculation wereperformed for three values of the pole foot groundingsurge impedances - 100,50, 1 and 5 Ohms.

electrode. It is comprised of either various types ofclays, carbon based materials, or blends of both. Achemically charged grounding electrode is a perforatedcopper tube, 2 to 2.5 inches in diameter, which is filledwith a mixture of metallic salts. While the electrode isin the earth, the salts go into solution and seep out of theholes. Adding salts to soil increases its electrolytecontent, thus reducing its resistivity and lowering theresistance of the grounding electrode.

By surrounding a conventional grounding rod with aconductive backfill, the resistance of the rod can bereduced by approximately50%.Still much better resultscan be obtained by using a combination of conductivebacldill and a chemically charged grounding electrode.

The measurements of the grounding resistance of acombination of the chemically charge grounding rodwith the conductive backfill mixture were conductedunder direction of the National Electrical GroundingResistance Project (NEGRP). Five locations, Chicago,Table1: Lightning stroke into a pole of I38 kV power line , Dallas. Las Vkgas, Virg nia and Upstate New York,have been chosen and grounding electrodes of 27different manufacturers have been installed. Theresistance measurements have been conducted manytimes during the year during five-year test. According tothe published NEGRF eports, the chemically charge

The hack flashover and the following line trip off willdefinitely occur in the case of 100and 50 Ohms footingsurge impedance, because the stress on the lineinsulation will exceed the BIL of the line (typically650kV).3 Reduction of poles grounding resistanceThe sample calculations presented in Table show thatthe voltage drop across the pole grounding resistancegreater than O Ohms is the dominant component of thetotal overvoltage caused by the direct lightning strokethe pole. Therefore, reduction of the groundingresistance will decrease the number of the line trips offdue to direct strokes into the polesIn regions with high specific soil resistivity ( rockyor sandy grounds) the pole grounding resistance is oftenhas values more than 50 or even in excess of 100 Ohms.Reducing it to the desirable low value using traditionalgrounding technique is often not possible consideringcost and available space. In such cases the installation ofthe line arresters is usually chosen as an alternativesolution [I].

There is, however, available the non-traditionalgrounding technique including the use of the conductivebackfill materials, chemically charged groundingelectrodes and combination of both. A conductivebacMiU material is a highly conductive earth substitutethat can be placed around any type of the grounding

rods had the lowest grounding resistance at every testconducted. The measured values of resistance of thechemically charged rods had also the lowest seasonalvariations: the LEC CR-IO roved to bebest of all.

Because this non-traditional grounding techniquecreates much more efficient grounding electrodes, theydo not require large area required by the conventionalgrounding rods or counterpoise. For example, in [ I themedian earth specific resistivity was 531 Ohm-meter,and the median pole fwting resistance was 128 Ohmsafter the footing resistance was improved by drivingeither two or four additional conventional ground rods.The further improvement was limited due to theunavailabilityof space and high rock content in soil.

The cost of non-traditional grounding methods issubstantially lower than the cost of line lightningarresters. Using [ I ] as an example to compare costs,lightning arresters for three phases at one pole wouldcost about USD 7,500, while using two chemicallycharged ground r o d s in conductive backfill would costabout USD 1,300. The installation cost of lightningarresters includes time for a skilled lineman plus theprobable necessity of bucket truck or other liftingequipment. The installation cost of the non-traditionalgrounding includes the time for a semi-skilled labor plusthe use of a truck-mounted auger. Therefore, theinstallation cost also favors the non-traditionalgrounding methods. Overall, when combining bothmaterial and installation cost, the financial impactgreatly favors improving lightning performance of

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power lines by .lowering footing resistance rather thaninstalling the lightning arresters. The higher specific soilresistivity, the greater is the cost saving.4. Multi-point lightning protector

Another cost effective way of reducing the numberof the lightning induced into a power line tripouts isthe use of the multi-point lightning protectors (AnIon Plasma Generator, IPG) nstalled on the polesof a power lines. The typical IPG is shown onFirmre 1

I \ \ \ \

~ Rgwe I : Multi-point lightning prO(ect0~

IPG consists of many long thin metal splines wthsharpened tip welded to the central rod and bended insuch a manner as to produce a hemisphere.

Having many points instead of just one as in thecase of a conventional lightning rod substantiallyincreases the device ability of intercepting the lightningstrokes. Because of enhancement of the external electricfield developed under influence of the charges inthunderstorm clouds, the corona current initiated fromthe IPG starts at relatively low value of the externalelechic field. A space charge produced by the IPG hassufficient time to propagate for tens of meter inside thegap between the IPG and the thundercloud cell.

The appearance of the local space charge with radiusof several meters or even several tens of meter causesrhe redistribution of the electric field along the air gap.The space charge smoothes the electric field distribution

at vicinity of the IPG pushing the most of the voltageinto the part of the air gap free of the space charge.

The space charge produced by the IFG reduces theelectric field below the IPG location, which leads toincreasing the zone of protection compared to the zoneof protection provided by a lightning rod installed at thesame height. The comparative calculation of theprotection for a lightning rod and IPG was performed byapplication of the rolling sphere method, which iswidely used for this pupose as , for an example, in [Z].

The radius of the protection zone at the ground levelfor the IPG can be calculated according to (4)

R,pC,o= J h f R;,) Nk) h 24)where: h height of the installed IPG or lightning

rodd,, triking distance, determined by the pe k ofthe lightning current in return strokeN number of splines (points) of the 1PGk shielding coefficient,R..o adius of the protection zone at the groundlevel for the lightning rod of height h,calculated according to 5)

R r . ~ d M 5 )Installation of IPG of the appropriate height on the

poles of a power lines can be considered as a soundalternative to using OHSW. The combination of theIPG with lowering footing resistance by the non-conventional grounding methods can be one of the leastexpensive and very technically effective method ofachieving the total compliance of the power linelightning performance with the acceptable number oftrips off due to lightning. For the power lines with theaverage length of the span about 150 m the IPG caneffectively replace the OHSW without any reduction inthe provided level of the line shielding from directlightning stroke. Of course, other aspects of the powersystem performance have to be considered in makingsuch a design decision.4 ConclusionThe lowering of the footing resistance byimplementation of the non-traditional groundingmethods as well as installation on poles of the IPG canresult in reduction of power line trips off due tolightning. The best technical and economical results canbe achieved by combination of these proposed methods.

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This concept has been demonstrated at several locationswithin the USA and Ireland.5 References[ I ] T.A.Shon. et al., ApplicationofSurge Arrestors to I15 kVCircuit 1996 I UP S ConferenceRoceedings, pp. 276-282IEEE Std 1243 -1997 IEEE Guide for Improving the LightningPerformanceof Transmission Lines[ l

Author address: Mark M. Drabkin, Ph. D, P.E., andRoy B . Carpenter, Jr., Chief Technologist, LightningEliminators & Consultants, Inc., 6687 Arapahoe Rd.,Boulder, Colorado, 8030 3, USA

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advances in lightning protection and grounding systems for power systems

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