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Tab 6 - OHTL Protection Against Lightning

Distribution System Engineering Course – Unit 10

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6-2

Overhead Line Clearances

Stressed by:

-Power frequency voltages (60 Hz)

-Switching surges (250/2500 μs)

-Lightning surges (1.2/50 μs)

In the USA, all clearances must comply

with the NESC minimum clearance

requirements Siemen

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6-3

Self-Restoring Insulation (Air Gaps)

n Can restore its strength after an electrical breakdown

n Insulation breakdown is statistical in nature

n major impact on power system

- design

- operation (reliability)

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6-4

n Parameters Influencing the Flashover Voltage of Air Gaps

- The statistical nature of the electrical breakdown process

- The waveshape and polarity of the applied voltage

- The geometrical configuration of the gap

- The proximity of the gap to grounded structures

- Atmospheric conditions

Self-Restoring Insulation (Air Gaps)

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6-5

IEEE Standard Lightning Impulse Test Wave

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6-6

Impulse Voltage Testing

n Series of impulse levels illustrating definitions

associated with impulse voltage testing

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CFO determination from Impulse Voltage Testing

6-7

Applied Crest

Voltage

(kV)

#S Number of

Shots

#F Number of

Flashovers

#F/#S (%)

1000 80 53 66

950 80 47 59

900 80 40 50 [CFO]

850 80 32 40

800 80 27 34

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6-8

Probability of Flashover vs. Peak Impulse Voltage

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6-9

CFO and BIL concepts

n Lightning Standard Waveshape (1.2/50 μs)

n CFO – critical flash overvoltage (V50%)

• Crest value associated to a Flashover Probability of 50%

• Self-restoring insulation

n BIL - Basic Lightning Impulse Level (V10%)

• Flashover Probability = 10%

• BIL = CFO (1-1.3σ) where 0.02< σ < 0.03Siemen

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6-10

CFO and BSL concepts

n Switching Standard Waveshape (250/2500 μs)

n CFO – critical flash overvoltage (V50%)

• Crest value associated to a Flashover Probability of 50%

• Self-restoring insulation

n BSL - Basic Switching Impulse Level (V10%)

• Flashover Probability = 10%

• BSL = CFO (1-1.3σ) where 0.05< σ < 0.07Siemen

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6-11

Standard Deviations – Rod Gaps

Rod-to-Rod Rod-to-Plane

Type of TestTypical s Range of s Typical s Range of s

PowerFrequency 0.02 0.01-0.06 0.02 0.015-0.045

Impulse 0.03 0.015-0.07 0.04 0.02-0.08

SwitchingSurge 0.04 0.025-0.09 0.05 0.03-0.11

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6-12

Atmospheric Conditions

n Relative Air Density (RAD)

- Temperature

- Barometric pressure (dominant)

n Humidity

n Precipitation (wet or dry conditions)

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6-13

Standard Atmospheric Conditions

n Temperature

- 77° F (25°C)

- 20°C for IEC standards

n Barometric pressure

- 29.92 inches of mercury (760 mm Hg)

- 101.3 kPa (760 mm Hg) for IEC

n Humidity

- 0.6085 inches of Hg (15.5 mm Hg) – partial pressure of water vapor

- 11 g water / m3 air for IEC

n Precipitation

- Wet condition definition: 1 to 1.5 mm rain/minuteSiemen

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6-14

n.cond.stdnon

.cond.std RADCFO

CFO -=

Where n = 1 for short gaps (switching surges)

n = .7 for long air gaps (switching surges)

and n = 1 for lightning

RAD Formula:

T273h)*0.086-(760*0.386RAD

+=

Atmospheric Correction Factors (RAD)

RAD=1 when h=0 m and T=200C

T typically decreases 6.50C for every 1000m of increase in altitudeSiemen

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6-15

Leakage Distances, Striking Distances and CFO

Leakage

Distance

Striking

Distance

CFO Siemen

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6-16

CFO (insulator unit)

CFO

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6-17

CFO (insulator string)

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6-18

BIL Correction Example

n A 230kV line section located at a geographical region where RAD=1 (Sea Level and 200Celsius air

temperature) has insulator strings designed for a 1050kV BIL.

n To keep a consistent insulation coordination criteria, for another line section located at a

geographical region where RAD= 0.908 (1000m altitude and 13.50Celsius) it would be necessary to

design insulator strings for a BIL of 1156 kV, because this would be equivalent to a 1050kV BIL at

RAD=1 (standard atmospheric conditions). As shown below.

n This correction is similarly applicable to CFOkV

RADBILstdBILcorr 1156

908.01050

===

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Insulation Coordination for Lightning

Part 1 – Protection Against Direct Strokes

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6-20

Lightning Strikes a Power Line

n When lightning hits a conductor at

midspan, the current splits in half and

initiates traveling waves of very high

currents and voltages which moves

away from the stricken point.

For example:

- Lightning current: I kA

- Conductor surge impedance: Zs ohm

- Voltage: (I/2) * Zs kV

I/2

I

I/2

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6-21

230 kV overhead line typical characteristics

Surge impedance : 360 ohms

Insulator string BIL : 1050 kV

Lightning critical current calculation

V=(Ic/2)*Zs

Ic = (2*V)/Zs = (2*1050)/360 = 5.8 kA

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6-22

R

Shield Wire

Phase Conductor

Shield wires can intercept most of the lightning strikes when the shield angle is

small

R = 8 I0.65

R - Striking distance in meters

I - First stroke peak current in kA

For example:

I = 5.8 kA

R = 25.1 m

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6-23

About 19

Degrees

Shielding Angle

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6-24

A Shielding Failure Without a Flashover

n The lightning stroke bypasses the shield wire

and hits a phase conductor.

n Built to 765 kV insulation levels, operated at

345 kV.

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Insulation Coordination for Lightning

Part 2 – Protection Against Backflashovers

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6-26

Flashover due to potential

rise of ground wire

Back Flashover

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6-27

Simplified

Equivalent Model

Tower Impedance

I Shield Wire

Grounding Resistance

Phases

Gaps

Shield Wire

Phases

Ground Rod

A simplified equivalent circuit for backflashover analysis

For example:

current to ground = 20 kA

R = 50 Ohm

Voltage = 20 x 50 = 1000 kVSiemen

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6-28

Niagara Mohawk

115 kV Line

Backflashovers can occur on multiple phases

- Lightning hits the shield wire of a double

circuit line

- All three phases of each circuit flashed over

• Not all of the flashovers occurred on the

same tower.

- Several insulators show streamers

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6-29

115 kV Tower Geometry

22.9m

2.3m

2.8m

2.3m

2.4m

3m

3m

44º

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6-30

Impact of Grounding and CFO on Backflashovers (115 kV design)

0102030405060708090

100

0 100 200 300Ground Resistance (ohms)

Per

cent

age

Flas

hove

rs

0246810121416

Flas

hove

rs/1

00 k

m/y

r

CFO = 500 kVCFO = 750 kVCFO = 1000 kV

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6-31

Lowering Grounding Resistance

n Reduces backflashovers on lines with shield wires

n What is a good value of footing resistance?

- Generally < 10 ohms

- Depends on

• line type

• insulation level

• 1 vs 2 shield wires

• other parameters

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6-32

Effect of Line Surge Arresters Installed at the Towers

• No flashovers

• Surge arresters on left

circuit

Lightning Hits the Shield Wire

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6-33

Effectiveness of Line Surge Arresters

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6-34

Line Surge Arresters Installed at the Towers

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6-35

Line Surge Arresters Installed at the Towers

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