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Controlling Lightning Induced Outages on …

Controlling Lightning Induced Outages on overhead LinesByKevin Mara, Protection Overview Why do we need Lightning protection? 20% of all distribution Outages are caused by Lightning . Reliability Cost What is Lightning ? How to establish good Lightning protection of Lightning Wave Shape Follows Ohms law V = I x R Strike has current flowing and path has impedance Voltage is caused by current flowDefinition of Lightning Wave Shape Standard Direct Lightning Strike Current Wave 8/20 s 8 microseconds rise time to 90% peak 20 microseconds to half of peak value Standard Direct Lightning Strike Voltage Wave s microseconds rise time to 90% peak 100 microseconds to half of peak value Standards used in surge testing and ratings of equipmentTypical Wave Shape used to Define Lightning for Laboratory TestsPeak Current AmplitudeMedianlo g10I33834 C urrent Am plitudeNum ber o f ValuesData MedianP ro b a b ility o f Exceeding

Controlling Lightning Induced Outages on Overhead Lines By Kevin Mara, P.E. 770-425-8100

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1 Controlling Lightning Induced Outages on overhead LinesByKevin Mara, Protection Overview Why do we need Lightning protection? 20% of all distribution Outages are caused by Lightning . Reliability Cost What is Lightning ? How to establish good Lightning protection of Lightning Wave Shape Follows Ohms law V = I x R Strike has current flowing and path has impedance Voltage is caused by current flowDefinition of Lightning Wave Shape Standard Direct Lightning Strike Current Wave 8/20 s 8 microseconds rise time to 90% peak 20 microseconds to half of peak value Standard Direct Lightning Strike Voltage Wave s microseconds rise time to 90% peak 100 microseconds to half of peak value Standards used in surge testing and ratings of equipmentTypical Wave Shape used to Define Lightning for Laboratory TestsPeak Current AmplitudeMedianlo g10I33834 C urrent Am plitudeNum ber o f ValuesData MedianP ro b a b ility o f Exceeding

2 100kALog-Norm al Approxim ationNumber of Strokes per Flash12345678910 or moreFrequency of Occurrence (%)45149884332 4 Cumulative (%) Exceeding (n-1) Strokes100554132241612 9 6 4 Strokes Per FlashNRECA Lightning Protection GuideGround Flash Density Number of Lightning flashes per unit density (strikes/km2/year) Flashes per year per square mile Determined by multiplying Ground Flash Density by Lightning Detection Network has actual recorded values. Ground Flash Density ChartLightning Strikes to the LineTwo different ways:1. It can strike an object in close proximity, resulting in an Induced strike2. It can be a DIRECT strike on the line Direct Strikes to Line Causes a surge current that splits an goes both ways on the line This causes a high voltage and flashover at the poles V=IR Impedance of a distribution line 456 to 228 ohms First stroke current > kA Voltage will exceed 1,000 kV 99% of direct Lightning strike will cause distribution line flashovers overhead Guy Wire can intercept the direct strikesNeed to be well grounded to be usefulNumber of Direct Strike N = Flash Collection Rate (flashes/1 mile/yr.)

3 Ng =Ground Flash density (GFD) in flashes/km2/yr. H = pole height (meters) 62128N Strikes a Function of Pole Height and GFDS hielding of Power LinesAdjacent Trees can shield power lines from direct Lightning strikesNS= N(1- Sf )Where NS= Flash Collection Rate for Shielded Line (flashes/1 mile/yr.).N = Flash Collection Rate (flashes/1 mile/yr.).Sf = Shielding FactorShielding Factors For example, a row of 66-foot tall trees within 50 feet of a pole line with 40 foot poles a shielding factor of which is 100% shielding ( no direct strikes to the power line). Tall trees even 164 feet (50 m) from the distribution line provide a high level of shielding. 60% shielding Results in may Induced Lightning strikesInduced Voltage Just because Lightning does not strike a line directly does not mean a flashover will not occur An underbuilt distribution line will exhibit Induced voltage if built underneath a high voltage transmission line.

4 Similarly, Lightning can induce voltages onto the distribution system when the Lightning strike is near the distribution line A 30kA Lightning strike 200 feet from an infinite long line can induce 175kV and the same Lightning strike 100 feet from the same infinite long line can induce 350 kV. Most voltages are less than 300 kVDistribution Line Insulation Level Recommended method per IEEE 1410 for determining the probability of a flashover is to use the critical impulse flashover voltage (CFO), rather than BIL voltage level at which there is a 50% chance of a flashover and 50% chance of a withstand CFO is not additive Each additional element has a reduction in CFO Based on extensive testingCFOadd sec = x CFOinsCFOadd third = x CFOinsAdded CFO of third componentDescriptionType 3 CFOins (kV) (kV/ft)Description and (kV/ft)

5 Wood pole64 Fiberglass pole125 Wood pole72 Wood crossarm76 Fiberglass pole122 Fiberglass crossarm76 Fiberglass standoff96 Wood pole27 Wood crossarm49 Fiberglass crossarm76 Fiberglass standoff96 Wood pole27 Wood crossarm49 Fiberglass crossarm76 Fiberglass standoff9612kV Ceramic Pin4725kV Ceramic Pin5412kV Polymer String6325kV Polymer String117 CFO1,2 of Primary InsulationAdded CFO of second componentPolymer Insulator12 kV 25 kV120 kV 140 kVNOTE 1 All values are CFO levels obtained in standard wet tests. NOTE 2 Values are the minimum of the negative and positive polarity values. NOTE 3 Insulators are shown as examples only.

6 Refer to Manufacturer's data for more exact Strain Insulator152 kV/ftWood Pole: 20 Fiberglass Standoff: 6012 kV 25 kV140 kV 260 kVHorizontal Ceramic Insulator String 2x102mm (12 kV) 3x102mm (4")(25 kV)165 kV 250 kVCeramic Pin Type InsulatorANSI 55 3 (12 kV) ANSI 55 5 (25 kV)105 kV 120 kVHorizontal Polymer String Insulator CFO of Various RUS AssembliesGrounded Guy18-inch Guy Insulator CFO (kV)Wood Path in * * * Using polymer suspensionCFO of a with a guy Pole top pin 105 kV for insulator Guy ft from the bottom of the pin ( kV) Total 195 kV Insulated guy links can increase the CFO Guy Insulator ft x 152kV/ft = 228 kV 47kV for Insulator Wood x 20 kV/ft = 25 kVTotal is 300 kV when using guy strain insulatorFraming of Structure Impacts CFO RUS specifications Changed spacing on guy attachments Require use of guy insulator links Training ground conductor so it is not near phase associated hardware Extend from neutral position horizontally to the down guy Arrester on pole eliminates need for high CFO Grounding conductor near phase associated hardwareDistribution Line Insulation Level Increasing the CFO will improve the Lightning performance for indirect Lightning strikes Supplement CFO with arresters How much CFO is enough?

7 Induced Voltage The magnitude of the Induced surge is dependent on the soil conductivity. Use 300 kV CFO where High soil conductivity (low impedance) Consider 420 kV CFO where Low soil conductivity (high impedance)Use of Arresters and CFO for Direct StrikesArresters and Induced Voltages Spacing arresters reduces path of the voltage surge Reduces in impedance because of shorter distance Every pole is expensive So how far apart should the arresters be spaced?Arresters can greatly reduce flashovers from Induced Lightning surgesProtecting Switches, Reclosers, and Line Fuses In areas with high ground flash density (GFD), switches that are normally in the open position should be protected by arresters at both sides of the switches Line fuses have a relatively low CFO Equip tap fuses with arrestersSpacing of Lightning Arrestors Certain structures will be control points in that these points will be equipped with arresters Reclosers, Switches, Transformers, etc.

8 Suggest spacing of 1,500 feet Shown in the graph 300 kV CFO plus spacing of 1,500 feetApplication of Lightning at or near the crest of a hill with no shielding (no adjacent trees) in an open field with no at or near the bottom of a hill with no with shielding on only one with shielding on two in low area with adjacent to transmission linesApplication Rules for Lightning Arrestors Three-phase lines: Control point is only a structure that has arresters on all three phases Single phase lines: Density of consumers/density of transformers helps to provide the number of Lightning arresters to meet the 1,500 foot spaces between arrestersSummary Lightning cases as much as 20% of Outages Lightning can induce 300 kV on to a line Control overvoltage by using CFO and Arresters CFO is maintained by framing the pole Guy insulator links Use of wood in separation Arrester spacing 1,500 feet Spacing of 750 feet twice as expensive No much gain in reductio


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