Controlling Lightning Induced Outages on

Overhead LinesBy

Kevin Mara, P.E.770-425-8100

Lightning 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 scheme.

Definition of Lightning Wave Shape

• Follows Ohms law– V = I x R

• Strike has current flowing and path has impedance

• Voltage is caused by current flow

Definition of Lightning Wave Shape

• Standard Direct Lightning Strike Current Wave– 8/20µs– 8 microseconds rise time to 90% peak– 20 microseconds to half of peak value

• Standard Direct Lightning Strike Voltage Wave– 0.25/100µs– 0.25 microseconds rise time to 90% peak– 100 microseconds to half of peak value

• Standards used in surge testing and ratings of equipment

Typical Wave Shape used to Define Lightning for Laboratory Tests

Peak Current Amplitude

M e d i a n l o g 1 0 I

3 3 8 3 4 k A 2 . 5 % 3 0 k A 0 . 3 2

P e a k C u r r e n t A m p l i t u d e

N u m b e r o f

V a l u e sD a t a

M e d i a n

P r o b a b i l i t y o f E x c e e d i n g

1 0 0 k A

L o g - N o r m a l A p p r o x i m a t i o n

Number of Strokes per Flash 1 2 3 4 5 6 7 8 9

10 or

more

Frequency of Occurrence (%) 45 14 9 8 8 4 3 3 2 4Cumulative (%) Exceeding (n-1) Strokes 100 55 41 32 24 16 12 9 6 4

Strokes Per Flash

NRECA Lightning Protection Guide

Ground 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 2.59

• Lightning Detection Network has actual recorded values.

• Ground Flash Density Chart

Lightning Strikes to the LineTwo different ways:

1. It can strike an object in close proximity, resulting in an INDUCED strike

2. 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 > 4.4 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 useful

Number of Direct Strike

• N = Flash Collection Rate (flashes/1 mile/yr.). • Ng =Ground Flash density (GFD) in flashes/km2/yr.• H = pole height (meters)

62128N N

6.0

gh

Direct Strikes a Function of Pole Height and GFD

Shielding of Power Lines

Adjacent Trees can shield power lines from direct lightning strikes

NS= 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 Factor

Shielding 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 1.0 which is 100% shielding (i.e. 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 strikes

Induced 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.

• 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 kV

Distribution 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 testing

CFOadd sec = 0.45 x CFOins

CFOadd third = 0.20 x CFOins

Added CFO of third component

Description Type 3  CFOins (kV) DescriptionCFOadd.sec 

(kV/ft)

Description and CFOadd.third 

(kV/ft)Wood pole 64

Fiberglass pole 125

Wood pole 72Wood crossarm 76Fiberglass pole 122

Fiberglass crossarm 76Fiberglass standoff 96

Wood pole 27Wood crossarm 49

Fiberglass crossarm 76Fiberglass standoff 96

Wood pole 27Wood crossarm 49

Fiberglass crossarm 76Fiberglass standoff 96

12kV Ceramic Pin 4725kV Ceramic Pin 54

12kV Polymer String 6325kV Polymer String 117

CFO1,2 of Primary Insulation Added CFO of second component

Polymer Insulator 12 kV        25 kV

120 kV            140 kV

NOTE 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. Refer to Manufacturer's data for more exact values.

Guy Strain Insulator

152 kV/ft

Wood Pole: 20     Fiberglass 

Standoff: 60

12 kV        25 kV

140 kV            260 kV

Horizontal Ceramic Insulator 

String 

2x102mm (12 kV)       

3x102mm (4")(25 kV)

165 kV            250 kV

Ceramic Pin‐Type Insulator

ANSI 55‐3 (12 kV)       

ANSI 55‐5 (25 kV)

105 kV            120 kV

Horizontal Polymer String 

Insulator 

CFO of Various RUS Assemblies

Grounded Guy

18-inch Guy

Insulator CFO (kV)

Wood Path in

feetA1.1 285 2.5A1.1 √ 195 1.25A1.1 √ 300 2.5A5.1* √ 181 1.5A5.1* √ 321 1.5C1.11 285 2.5C1.11 √ 185 1.25C1.11 √ 300 1.25* Using polymer suspension

CFO of a C1.11 with a guy• Pole top pin

– 105 kV for insulator– Guy 1.25 ft from the bottom

of the pin (1.25x72kV=125 kV)

– Total 195 kV• Insulated guy links can

increase the CFO– Guy Insulator 1.5 ft x 152kV/ft

= 228 kV– 47kV for Insulator– Wood 1.25 x 20 kV/ft = 25 kV

Total is 300 kV when using guy strain insulator

Framing 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 hardware

Distribution Line Insulation Level

• Increasing the CFO will improve the lightning performance for indirect lightning strikes

• Supplement CFO with arresters• How much CFO is enough?

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 Strikes

Arresters 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 surges

Protecting 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 arresters

Spacing of Lightning Arrestors

• Certain structures will be “control points” in that these points will be equipped with arresters– Reclosers, Switches, Transformers, etc.

• Suggest spacing of 1,500 feet– Shown in the graph– 300 kV CFO plus spacing of 1,500 feet

Application of Lightning Arrestors

1. Pole at or near the crest of a hill with no shielding (no adjacent trees)

2. Pole in an open field with no shielding3. Pole at or near the bottom of a hill with no shielding4. Pole with shielding on only one side5. Pole with shielding on two sides6. Pole in low area with shielding7. Poles adjacent to transmission lines

Application 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 arresters

Summary

• 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 reduction