“lightning 101”

IEEE T&D – Lightning 101

“Lightning 101”

Presented tothe IEEE Towers, Poles and Conductors Subcommittee

by members of the IEEE Lightning and Insulator Subcommittee

IEEE/PES Technical Committee Meeting, OrlandoJanuary 11, 2010


“Lightning 101”Section A – Introduction

Presented by William A. Chisholm, Kinectrics / UQAC

Secretary, IEEE Transmission and Distribution Committee

Presented to the IEEE Towers, Poles and Conductors Subcommittee



What Is Lightning?

A Conducting Channel of Air Plasma!Electrical energy as equivalent to gasoline

1 US Gallon gasoline = 132 MJoule = 33.56 kWhNegative Lightning Flash I2t = 6,000 to 550,000 A2s6,000 A2s x 20 Ω = 120 kJouleGlobal Lightning energy = 1 transmission line

PowerEnergy is concentrated into100 μs120 kJoule / 100 μs ≈ 1.21 GW

And more relevant to overhead lines,

A Powerful Source of Transient Current!

http://upload.wikimedia.org/wikipedia/en/a/a9/Backfu2.jpg


What Is Lightning?

A Powerful Source of Transient Current!- Concave front observed on flashes to towers- 2 μs Ramp Approximation at Peak of Wave


What Is Lightning?Widely Variable Source of Current!

- First strokes range from 2 to 200 kA- Subsequent Strokes smaller, faster- Logarithm of distribution is normally

distributed

n

aa

aP⎟⎠⎞

⎜⎝⎛+

=

ˆ1

1)(


Lightning Current Peak Amplitude

1.00 10.00 100.00 1000.00

0.050.10

0.501.002.00

5.0010.00

20.0030.0040.0050.0060.0070.0080.00

90.0095.00

98.0099.0099.50

99.9099.95

99.99

( ) 6.2*

*

31/1

1)(

P

PPI

IIP+

=≥

[kA]

IEEE:

Ip ≤ 20kAIp = 61.1 kAδln Ip = 1.33

Ip > 20 kAIp = 33.3 kAδln Ip = 0.605

Cigré:95% > 10 kA

5% > 96 kA


What Is Lightning Protection?

Electrically Conducting Path: Interception to GroundBuildings: Air terminal(s), down conductors, grounding systemOverhead Lines: Continuous overhead groundwires (OHGW), also known as shield or static wiresTakes advantage of internal conducting elementsGets current in and out of structures without ionization damageLasts as long as the building or structure


History of Lightning Protection

Roman Goddess of Lightning:• Offerings to Fulgora protect crops from the destruction of

thunderstormsChurch Bells with “Fulgora Frango” imprint:• Theory: Sound from ringing bells will destroy lightning• Practice: From 1750 to 1784, there were 386 flashes to

church steeples; 103 church bell ringers died while trying to dispel lightning.

- FISCHER, J. N. (1784): Beweis, daß das Glockenläuten bei Gewittern mehr schädlich als nützlich. München. - A Proof that the Ringing of Bells during Thunderstorms May Be More Dangerous than Useful, Munich, 1784



Franklin and others established electrical nature:• “Giving light; Colour of the light; Crooked direction,

Swift motion. • Being conducted by metals.• Crack or noise in exploding. Subsisting in water or ice. • Rending bodies it passes through. Destroying animals.• Melting metals. Firing inflammable substances.• Sulphreous smell.



Franklin lightning and grounding protection:• Iron Rod, 6 to 8 feet above highest part of building• Brass wire, sharpened, “the Size of a common Knitting-

needle”• “Rod and Point at each End, with a middling Wire

along the Ridge”• “The lower end of the rod should enter the earth so

deep as to come at the moist part, perhaps two or three feet; and if bent when under the surface so as to go in a horizontal line six or eight feet from the wall, and then bent again downwards three or four feet, it will prevent damage to any of the stones of the foundation.”



Two-wire telegraph, telephone systems• Initially two-wire systems, later protected with spark gaps

Single-wire earth return systems• Discovered that the earth itself can be a return conductor, with

constant surge impedance in a practical 500-800 Ω range• Blitzplatte of K. A. Steinheil, 1846

Telegrapher’s Equations• Heaviside Transmission Line Model• Wave propagation, impedance Z=60 ln (2h/r)



Grounding of power system secondaries• Grounding mandatory only for lightning arresters• New York Board of Fire Underwriters deadline for removal of all

grounds on electric circuits: October 1, 1892• The 1901 National Electric Code (NEC) permitted grounding,

“provided that under normal conditions of service there will be no passage of current over the ground wire”

Four-Terminal Measurements of Earth Resistivity ρ• Two outer rods used to inject current from ac (crank driven) source;• Two inner rods read out corresponding potential difference• Equations give apparent resistivity as function of probe separation



Characterization of Lightning using OscilloscopesBewley measures surge impedance of buried wiresPractical surge arresters for station equipment

Damage to aircraft from charge ablation sets metal skin thicknessExperiments initiated with concrete-encased

foundation (Ufer) electrodes



Improved surge arrester materials (Doped Zinc Oxide)Improved polymer insulator materials (fiberglass, EPDM/Silicone Rubber)

Ability to track lightning from its electromagnetic pulse using wide area networks (Krider, Noggle, Uman)Characterization of downward lightning flashes to

instrumented towers (Berger, Anderson and Eriksson)


“Lightning 101”Section B – Common Factors

in Lightning Protection

Presented by William A. Chisholm, Kinectrics/UQACSecretary, IEEE Transmission and Distribution

Committee




Risk Assessment: Direct Strokes

Number of Flashovers per Year is the Product of:

• Ground Flash Density, flashes / km2/year• Attractive Width of Line, m• Length of line, km• Probability of exceeding critical current,

P=1/(1+(Icrit/31 kA)2.6)


Risk Assessment Process

Lightning Incidence to Ground (Flash Density)

Establish critical current that causes flashover• Direct stroke to phase: Insulator BIL divided by surge

impedance in corona• Direct stroke to overhead groundwire: Insulator BIL divided by

footing resistance, adjusted for coupling• Direct stroke to mast at substation: Insulator BIL divided by

station resistance


Global Map, Total (Cloud + Ground) Flashes

http://thunder.msfc.nasa.gov/images/HRFC)AnnualFlashRate_0.5.pngRecommend: Ng=0.33 Ntotal


Global Map, Total (Cloud + Ground) Flashes

http://thunder.msfc.nasa.gov/images/HRFC)AnnualFlashRate_0.5.pngUnits: Flashes per km2 per year. Recommend: Ng=0.33 Ntotal

CG20

10

5

3

2

1

0.3

0.2


Lightning Location Network (NALDN)


Direct Stroke Termination Model

•Da is attractive width to each side of the line (m);•L is line length (km);•Ng is ground flash density (flashes/km2/year).

Ng



Direct stroke incidence model• Distribution Lines: small critical current, accept

flashovers for all direct strokes• Transmission lines: critical current well above

median 31 kA, intercept large fraction of direct strokes with overhead groundwires

• Stations: critical current well above 100 kA, deal with shielding failures and coupling issues


Direct Stroke Termination Model


2 Da

L


Direct Stroke Termination Model - Rizk

45.019hDa=Average for all currents:


Direct Stroke Termination Model - Rizk

10002

#

19 45.0

ga

a

NLDhD

⋅⋅⋅=

=


If # normalized to L=100 km, denominator becomes 10.



Lightning Incidence to Ground (Flash Density)

Establish critical current that causes flashover• Direct stroke to phase: Insulator CFO divided by

surge impedance in corona• Direct stroke to overhead groundwire: Insulator CFO

divided by footing resistance, adjusted for coupling and volt-time upturn

• Direct stroke to mast at substation: Insulator CFOdivided by station resistance


What Does an Insulator Do?

Maintains an Air Gap to Establish CFOSeparates Line from Ground

length of air gap depends primarily on system voltage, modified by desired safety margin, contamination, etc.

Resists Mechanical Stresses“everyday” loads, extreme loads

Resists Electrical Stressessystem voltage/fields, overvoltagesCFO = Critical Flashover (50% probability)

Resists Environmental Stressesheat, cold, UV, contamination, etc.


Insulator Types

For simplicity will discuss lightning protection in terms of three broad applications:

Distribution lines (thru 69 kV)

Transmission lines (69 kV and up)

Substations (all voltages)


Insulator Types – Distribution Lines

Pin type insulators• mainly porcelain• growing use of polymeric (HDPE – high density polyethylene)• limited use of glass (in US at least)

Line post insulators – porcelain, polymericDead end insulators – polymeric, porcelain, glass Spool and Strain insulators – porcelain,

polymericFiberglass StandoffsWood Crossarm or Pole, not shorted out by bond

to insulator base


Insulator Types – Distribution Lines


Insulator Types – Transmission Lines

Suspension insulators • new installations mainly polymer• porcelain and glass discs now used less frequently

Line Post insulators • mainly NCIs for new lines and installations• porcelain much less frequent now


Insulator Types – Transmission Lines


Insulator Types – Substations

Post insulators • Primarily porcelain• NCIs growing in use at lower voltages (~161 kV

and below)Suspension insulators • NCIs (primarily), ceramicCap and Pin Apparatus insulators • “legacy” type• Poor puncture performance, great for icing with

silicone coating


Insulator Types – Substations


Design Criteria – Mechanical (from Insulators 101…)

An insulator is a mechanical support!

• Its primary function is to support the line mechanically

• Electrical Characteristics are an afterthought.

• Will the insulator support your line?

• Determine The Maximum Load the Insulator Will Ever See Including NESC Overload Factors.


Design Criteria – Mechanical (from Lightning 101…)

An insulator is a mechanical support!

• Its primary function is to support the line mechanically

• Electrical Characteristics are an afterthought.

• Will the insulator support your line?

• Determine the Critical Current that will cause Electrical Flashover of the insulator based on its CFO

electrical

electrically

Mechanical


Design Criteria - Mechanical

Other Considerations

• Suspensions and Deadends – Only apply tension loads- Available in any desired length

• Line Posts – Mainly cantilever load- For given core size / strength, increase in dry arc gives decrease in cantilever strength

- Combined Loading Curve for Cantilever, Transverse and Longitudinal Load – see Insulators 101

• Braced Line Posts – tension and cantilever members


Standing on Line Posts is a BAD Idea!…Insulators 101 course


Design Criteria - Electrical

Dry Arcing Distance –(Strike Distance) – “The shortest distance through the surrounding medium between terminal electrodes….” 1

1 – IEEE Std 100 - 1992


Design Criteria - ElectricalDefine System Maximum Line-to-Ground VoltageDetermine Leakage Distance Required – mm per kVConsider Switching Over-voltage Requirements on EHV systems Accept the Critical Flashover Level (CFO) given by the Dry Arc distance

Calculate Critical Current from CFO, divided by the impedance of the structure (phase, OHGW or mast), adjusted for coupling and the volt-time curve

Make design, construction or follow-up adjustments if estimate is unsatisfactory


Electrical RatingsANSI C2 Insulation Level Requirements

ANSI C2-2007, Table 273-1

Higher insulation levels required in areas where severe lightning, high atmospheric contamination, or other

unfavorable conditions exist

Nominal Phase-to-Phase Voltage (kV)

Rated Dry Flashover Voltage (kV)


Design Criteria - ElectricalDefine System Maximum Line-to-Ground VoltageDetermine Leakage Distance Required – mm per kVConsider Switching Over-voltage Requirements on EHV systems Accept the Critical Impulse Flashover Level (CFO) given by the Dry Arc distance

Calculate Critical Current from CFO, divided by the impedance of the structure (phase, OHGW or mast), adjusted for coupling and the volt-time curve

Make design, construction or follow-up adjustments if estimate is unsatisfactory


Design Criteria - ElectricalWhere do I get these values?• Manufacturer’s Catalog

Critical Impulse Flashover (CFO)• Negative Polarity for shielding failures• Positive Polarity for lightning to tower• If only Basic Impulse Level (BIL) is available, assume this

is 90% of Critical Impulse Flashover (CFO)As a default, you can assume 540 kV per (m dry arc distance) for full lightning impulseThis increases to 822 kV/m for 270-m span, higher for shorter spans, lower for longer spans+CFO = (400 + 710 t -0.75) kV/m


“Lightning 101”Section C – Distribution Line

Protection

Presented by John McDaniel, National GridIEEE Chairman, Lightning Working Group




Risk Assessment: Induced Overvoltages

Indirect stroke coupling model (Lightning Electromagnetic Pulse, LEMP)

• Fast transient voltage, needs only poor grade insulation to withstand (including wood path)

• Causes problems with coupling into control circuits, coaxial cables and sensitive electronics

• Big factor in protection of distribution lines


Indirect Stroke Influence Model

y


Indirect Stroke Influence – Side View


Effects of Nearby Lightning – Std 1410

Rusck simplified formula:

Assumptions:a. Single conductor, Infinitely long lineb. Perfect (zero resistivity) groundc. Step current waveshape

Ω== 30/4/1 00 oZ εμπh Height of phase over ground (m)Io Peak Current (kA)v return stroke velocity, c/3

Model is simple and correct, but assumptions are weak.

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

⎟⎠⎞

⎜⎝⎛−

+=2

00

211

12

11

cvc

vy

hIZU max


Effects of Nearby Lightning – Std 1410Flashovers per 100 km per year

for GFD = 1/km2/year

Critical Lightning Impulse

Flashover Level (kV)



125 kV CFO (115 kV BIL):Design Vulnerable to Induced Overvoltages



300 kV CFO (275 kV BIL):Design Resists Induced Overvoltages


Using Conductivity in Rusck Model

h

h’=h

Perfectly Conducting Ground:Image Method

h’>h

Imperfect Ground:Image Method

σ7.4

+= hheff

σ in mS/m, heff and h in m


FCC M3 Conductivity Map Provides σ

σ of 2 mS/m = ρ of 500 Ωm4 mS/m = 250 Ωm8 mS/m = 125 Ωm15 mS/m = 67 Ωm


Ground Effect on Induced FlashoversFlashovers per 100 km per year

for GFD = 1/km2/year

Critical Lightning Impulse Flashover Level (kV)


Ground Effect on Induced Flashovers


Value typical for Central USA

Flashovers per 100 km per year for GFD = 1/km2/year




Value typical for Appalachians, Rockies

Flashovers per 100 km per year for GFD = 1/km2/year



Higher Insulation (> 300 kV CFO) needed for Design Resistant to Induced Overvoltage

with 1 mS/m


How to achieve CFO >300 kV

Eliminate weak-link structures• Insert insulation in guy wires• Review framing of guyed poles

Add wood in series with insulators• Consider isolated bonding of leakage current across ceramic

insulators is a pole-fire concern

Add fiberglass in series with insulators• Poles or crossarms

Or …Don’t, add surge arresters (SA) instead


Arrester Effect on Induced Flashovers

LIOV-EMTP code (http://www.liov.ing.unibo.it)

σg = 1 mS/m


Arrester Effect on Induced Flashovers

Arresters every 200 m (650’) equivalent to 420 kVfor Design Resistant to Induced Overvoltage

σg = 1 mS/m


“Lightning 101”Section D – Transmission Line Protection

Presented by William A. Chisholm, Kinectrics/UQACSecretary, IEEE Transmission and Distribution

Committee




Critical Current for Stroke to TowerInitial Estimate: CFO divided by Resistance of Stricken Tower

• Geometric term from overall surface area A of foundations, radius g• Contact resistance if the actual metal / concrete area fills only a small

fraction of the total contact patch to soil, L being path length• F=2 for almost all electrodes, ρ is soil resistivity (Ωm)

Correction 1: Short Duration of Overvoltage• Adjacent towers help out, share current within a few μs• The show is over” when reflections from adjacent towers arrive

Correction 2: OHGW to Phase Coupling• A fraction Cn (20-35%) of the voltage on the OHGW will be picked up by

the parallel phase conductors• The voltage across the insulators will be reduced by (1-Cn)

Correction 3: Parallel Impedance of OHGW

⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎟⎠

⎞⎜⎜⎝

⎛⋅

+⎟⎟⎠

⎞⎜⎜⎝

⎛=+

WireContactGeometric AF

ALA

gg

RR ln18.11ln12

2

πρ


Performance Goals for Transmission

Typically lines have at least 1 m of dry arc distance• CFO of 540 kV for full standard impulse, no adjacent towers• CFO of 822 kV for adjacent towers 270 m away (2 μs @ 0.9 c)

Typically towers with four foundations have base radius rx, ry, rz of 3 m, g=5.2 m, A = 108 m2, Rgeom=ρ/30Resistivity ρ varies from tower to towerCentral US, ρ=100 Ωm; Appalachians 1000 ΩmIn Central US, critical current for two OHGW (Cn=0.3)

is well above 200 kA and probability of failure is lowIn Appalachians, Icrit=35 kA; P(I>Icrit) = 41%



Doubling dry arc distance will double the critical current

• 230-kV lines, 2 m dry arc, have critical current of 70 kA rather than 35 kA in area with ρ=1000 Ωm

• Probability 11% rather than 41%, outage rate four times better

Two, rather than four, foundations (H-frame), 3 m deep, 7 m apart, 0.5 m radius, will have a resistance of ρ/17 rather than ρ/30

• 115-kV line performance still excellent for ρ=100 Ωm, P(I>Icrit) <1%• 115-kV line performance poor for ρ=1000 Ωm, P(I>Icrit) =76%• 230-kV line performance degrades from 11% to 35% in 1000 Ωm



Greater Separation from Phase to OHGWs Reduces Coupling of Voltage onto Phases

• This is bad – it increase the insulator voltage• Two OHGW at 40 m above ground, ± 5 m• Coupling at 25 m: Cn = 0.3, P(35 kA) = 41%• Coupling at 15 m: Cn = 0.17, P(30 kA) = 52%• Bottom Phase more prone to flashover



Shield Wire located Below Phases Improves Coupling• Underbuilt Ground Wire (UBGW)• Could be OPGW with lightweight armor, easy access• Two OHGW at 40 m above ground, ± 5 m; Third at 10 m• Coupling at 25 m: Cn = 0.4, P(42 kA) = 32%• Coupling at 15 m: Cn = 0.33, P(37 kA) = 38%


Dual Voltage Line, OHGW + UB OPGW

At ENbW in Germany, it was not possible to take the line out of service to replace an existing OHGW with an optical fiber groundwire (OPGW)Phase motion restrained by long-rod ceramic post insualtorsThe OPGW was successfully installed below the bottom phases


Compact Lines, OHGW+UBGW

COPEL added UBGW to control of tower base potentialsMore effective for lightning too.

138 kV with Horizontal Posts 230 kV with Braced Posts



Arresters limits voltage on protected insulator, and also convert protected phase into UBGW

• Most effective partial application on unshielded lines is on the top phases, giving equivalent performance to OHGW in the same positions

• Most effective partial application on lines that already have OHGW is often on bottom phases


TLSA Application (AEP 138 kV, 1980s)R. E .Koch , J. A. Timoshenko, J. G. Anderson, C. H. Shih, “Design of Zinc Oxide Transmission Line Arresters for Application on 138 kV Towers”, IEEE Trans PAS V. 104 No.. 10, Oct 1985, pp. 2675

Arrester limits power-follow current,Gap does not re-ignite.

Externally Gapped Line Arrester (EGLA)


TLSA on Unshielded 230-kV (Nalcor)


TLSA on Unshielded 138-kV (FPL).. With UBGW too, for managing tower base voltage


“Lightning 101”Section E – Standards




IEEE, NFPA and IEC StandardsConsensus standardsStandards writing bodies must include representatives from

materially affected and interested parties.Public review Anybody may comment. Comments must be evaluated, responded to, and if found to be appropriate, included in the standard . Right to appeal

By anyone believing due process lacking.Objective is generally to ensure that Standards are developed in an environment that is equitable, accessible, and responsive to the requirements of various stakeholders*.

* The American National Standards Process, ANSI March 24, 2005


Lightning Protection of Structures

Each country has its own standards, such as NFPA 780 in USA and CSA B72 in Canada

US MIL-HDBK-419A, Grounding, Bonding and Shielding for Electronic Equipments and Facilities

There is some work towards adopting the IEC 62305 standard throughout Europe

• It is relatively complicated and expensive• It involves risk analysis and management• It is more of a guide than a standard• It is vastly superior to “fire protection” for buildings that have

internal electronic components


Lightning Protection of Lines

IEEE P1410/D6 has recently been balloted with high approval, resolving comments this week.

• Computer method for calculating induced overvoltage effects• Expanded treatment of multi-component insulation

IEEE 1243 revision is being initiated.• More consideration being given to line surge arresters versus

overhead groundwires• Coordinated with C62 Surge Arrester Application Guide


Lightning Protection of Stations

IEEE 998 is undergoing revision.• Considering update, inclusion of attractive radius models with

height dependence rather than fixed rolling sphere radius


Questions?

A long-standing tradition of technical committee meetings is that the TPC Chair buys a free beverage for everyone who asks a question that can be answered by the presenters, so .. Ask Away!


Dr. William A. (Bill) Chisholm is a well-known expert in electric power reliability problems involving adverse weather including lightning, winter pollution and low wind conditions.

IEEE Fellow in 2007.

Led/leading IEEE Standards 1243 and 1410 for improving lightning protection of transmission and distribution lines.

Associate at Kinectrics (former Ontario Hydro Research Division) in Transmission and Distribution group.

Principal author of EPRI Transmission Line Reference (Red Book), 200 kV and Above, Chapter 6 (lightning protection) and main technical contributor to the upcoming Grey Book (lightning and grounding).

Spent 2007-2008 at the University of Quebec at Chicoutimi, co-writing a book for and teaching lightning protection.

Columnist (Transient Thoughts) for INMR Magazine.

About your Presenters


John McDaniel graduated from Michigan Technological University with a B.S.E.E. and M.S.E.E. in Electrical Engineering, majoring in power systems. John is a Reliability Engineer in Distribution Field Engineering at National Grid.Previously, he was Sr Engineer – Planning in Detroit Edison’s Distribution Planning department and in several positions in Engineering and Operations at Commonwealth Edison.

He is a Senior Member of the IEEE and his professional activities include work with the IEEE Distribution Sub-Committee and IEEELightning and Insulator Sub-Committee. John chair’s the IEEE/PES Working Group on the Lightning Performance of Distribution Lines and is the Vice Chair of the Lightning & Insulator SC, the Distribution Subcommittee and the Distribution Reliability Working.

He is also a corresponding member of CIGRE WG C4.4 (Lightning) and CIRED Joint WG C4.4.05 (Protection of MV and LV networks against lightning).

About your Presenters

“lightning 101”

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