1

2

INTRODUCTION

Protection of transmission lines and Power apparatus from the chief causes of over voltages is necessary

Lightning Overvoltages Switching Overvoltages

Lightning Overvoltages – Natural Phenomenon of charging and Discharging of clouds Switching Overvoltages – connection and disconnection of circuit breakers due to interruption of faults – short duration overvoltages – Temporary overvoltages

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Comparison between overvoltages

Magnitude of Lightning OVs not depends on Line design and Operating VoltagesMagnitude of Switching OVs proportional to line design and Operating Voltages.Probability of Both OVs occurring together, is very small &negligible for insulation designFrom 300 kV to 765 kV, both OVs to be consideredAbove 700 kV, Switching OVs be the chief condition for design purpose

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5

Introduction

Switching surges generated inside the system reached the same order of magnitude equivalent to lightningLasts for longer durationsDepends on Normal Voltage of the systemInsulation Design is based upon the magnitude of these surgesOrigin is making and breaking of circuits

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Characteristics

De-energizing of transmission lines, cables shunt capacitors, banks, etc.,Disconnection of unloaded transformers, reactors, etcEnergization or reclosing of lines and reactive loadsSudden switching off of loadsShort circuits and fault clearancesResonance phenomenon like Ferro-resonance, arcing grounds, etc

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Switching in EHV,UHV Systems

Situations of Overvoltages of the order of 2-3.3 pu, duration of these OVs from 1 to 10ms Interruption of Low Inductive Currents (Current Chopping) By High Speed CBs. It occurs when the transformers or reactors are switched off.Interruption of small capacitive currents, such as switching off of unloaded lines etc.Ferro Resonance condition. It occurs when poles of CB do not close simultaneouslyEnergization of long EHV or UHV lines

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Overvoltages study

Mathematical modeling of a system using digital computersScale modeling using transient network analyzersBy conducting field tests to determine the expected maximum amplitude3 of the overvoltages and their duration at different points on the line.

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Wave shapes of switching surges

a. Recovery Voltage after fault clearing

b. Fault Initiation

c. Overvoltage at the line end after fault clearing

d. Energization of long transmission line

e. Overvoltage at line end during (d)

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Some of the overvoltages

SI No

Type of OperationOver

voltage pu

1 Switching an open ended line with:a) Infinite bus as source with trapped

charges on lineb) Infinite bus as source without

trapped chargesc) De-energizing an unfaulted line

with a restrike in the circuit breaker

d) De-energizing an unfaulted line with a line to ground fault

4.1

2.6

2.7

1.3

11

Contd.,

SI No

Type of OperationOver

voltage pu

2 a) Switching a 500 kV line through an auto transformer, 220 kV/500 from the LV side

b) Switching a transformer terminated line

c) Series capacitor compensated line with 50% compensation

d) Series capacitor compensated line with shunt reactor compensation

2.0

2.22.2

2.6

3 High speed reclosing of line after fault clearance

3.6

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Situations of lower magnitude Overvoltages

Short duration-(0.5-5ms), Mag-(2-2.5 pu) Single pole closing of CBInterruption of fault current when the LG or LL fault is clearedResistance Switching used in circuit breakersSwitching lines terminated by transformers Series capacitor compensated linesSparking of the surge diverter located at the receiving end of the line to limit the lightning overvoltages

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Factors

Effect of line parameters, series capacitors and shunt reactors on the magnitude and duration of the transientsThe damping factors needed to reduce magnitude of OvervoltagesThe effect of single pole closing, restriking and switching with series resistors or circuit breakers on the overvoltages, andLightning arrestor spark over characteristics

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Power Frequency Overvoltages

Sudden loss of loadsDisconnection of inductive loads or connection of capacitive loadsFerranti Effect, Unsymmetrical faults, andSaturation in transformers

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Sudden Load Rejection

It causes the speeding up of generator prime moversVoltage rise is given by

Where,xs – Reactance of the generator (sum of transient reactance of generator and transformer)xc – Capacitive reactance of the line at open end at increased frequency, E’ – Voltage generated before the over-speeding and load rejectionf – Instantaneous increased frequencyf0 – Normal frequency

c

sx

x

f

fE

f

fv

001

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Contd.,

Increase in V-> 2 pu with 400 kV linesVoltage at sending end is affected by line length, SC MVA at sending end bus and Reactive Power Generation of lineShunt reactors may reduce the voltage to 1.2 to 1.4 pu

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Ferranti EffectLong uncompensated lines

exhibit voltage rise at receiving end

V2=V1/cos βlWhere V1, V2 are the sending

and receiving end voltages respectively

l=length of the line

β=Phase constant of the line

= [(R+jωL) (G+jωC)/LC]1/2

60 per 100 km line at 50 Hz frequency

ω=Angular frequency

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Approximated solution

Capacitance is concentrated in middleSolution yields

C

LX

XVV

2112

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Ground Faults and Their Effects

SLTG cause rise in voltage in other healthy phasesSolid ground systems, voltage rise is less than the line to line voltageEffectively ground systems,

X0/X1< 3.0 and R0/X1 < 1.0rise in voltage of healthy phases

does not usually exceed 1.4 pu

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Saturation Effects

When overvoltage is applied to transformers, their magnetizing currents increase rapidly, full rated current at 50% overvoltageThese are not sinusoidal3rd,5th and 7th harmonics be 65%,35%,and 25% of rated current with fundamental frequency to OV 1.2 pu

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Control of Switching Overvoltages

Energization of transmission lines in one or more steps by inserting resistance and withdrawing them afterwards,Phase controlled closingDrainage of trapped changes before reclosingUse of shunt reactorsLimiting switching surges by suitable surge diverters

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Charge Formation

Positive & Negative charges in the clouds separated by heavy air currents with ice crystals in upper part and rain in lower part.Charge separation – height of clouds – 200m – 10 km with their charge centres at a distance of 300m to 2kmCharge inside cloud 1 – 100 CPotentials of Clouds 107 – 108 VField Gradients 100 V/cm – 10 kV/cmEnergy 250 kWhMax gradient at ground level 300 V/cmFair weather gradients 1 V/cm

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The Thundercloud

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How Lightning Starts

Lightning starts in cloud

Around 0°C - that’s typically 15,000 ft above groundBreakdown starts in high-field regionBranching discharge moves up and down

++++++++++++++++++++++

++++++++++++++++++++++

----------------------------------------

----------------------------------------

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Leader Propagation

Ground flashes almost always start with downward (usually) stepped leader from high charge regionSteps 10-100 m longPauses between stepsLowers charge to earth Negative in > 95% of ground flashes

Breakdown zone

--

- -- -

- -

--

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Connecting Leaders

Launched upward by electric field of stepped leader as it approaches earthOccur at many locations near descending flashMost are unsuccessfulOne or more connect with downward leader to provide final channel to earthNot often seen, but frequently heard

- -- -

- - --

---

++

++

+ +

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Connecting Leaders

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Return Stroke

Large current impulse flows to groundLarge electromagnetic pulse radiatedLeader charge neutralized

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Subsequent Strokes

"Dart" leaders launched from cloudFollow path of first return strokeTap new cloud chargesCause subsequent return strokesOften depart from old path

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Video Stills of Multi-Stroke Flash

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Multiple Stroke Flashes

Typically 2-4 strokes per flashStroke intervals 5 -100 millisecondsReach ground at 1 to 5 pointsSevere flashes have >4 strokesContinuing currents likely

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Multiple Ground Channels

Multiple ground channels are common

Root branchingSimultaneous leader branchesSuccessive strokes may depart from "main" channel

Three major channels for every two flashes

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Multiple Ground Channels

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Currents & Voltages

Cloud charging current a few ampsCloud voltages 50 MV to 500 MVLeader currents 10 A to 1000 A Return stroke currents 5kA to 500 kA

Approximately log-normal distribution with 30 kA to 40 kA median

0 50 100 150 200 250 300

Microseconds

0

20

40

60

80

100

120

Kiloamperes

Return Stroke Current

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Cloud Model

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Simpson’s Theory

Three regions A, B & CIn region A, due to high velocity air, it breaks falling raindrops causing +ve charge in cloud & -ve charge in air.Region B –vely charged by air currentsRegion C ,only ice crystals exist due to low temperature.

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Simpson Theory

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Reynolds and Mason Theory

Thunder clouds are developed at heights from 1 km to 14 km above ground levelTemp – 0oC @ 4 km Ht, reaches -50oC @ 12 km HtWater droplets freeze below -40oCThundercloud consisting supercooled water droplets moving upwards and large hail stones moving downwardsSplinters moved up by air Cts and carry +ve charge to upper region.Hail stones that travel downwards carry an equivalent –ve charge to the lower regions in the cloud

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Mason Theory

Water-> H++ OH-ion density α temperatureHigher concentration of ions in lower regionsH+ ions are much lighter, they diffuse much faster all over the volume.Lower portion will have a net negative charge density, Upper portion will have a net negative charge densityOuter shells of freezed water droplets coming into contact with hail stones acquire +ve charge

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Reynolds Theory

Based on exp resultsHail packets get –vely charged when impinged upon by warmer ice crystalsCharging polarity changes if temperature conditions reversesRate of charge generation to disagree with practical observations relating to thunderclouds

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Rate of Charging of Thunder clouds

Consider λ – factor depends on conductivity of medium E – Electric Field Intensity ν – Velocity of separation of charges ρ – Charge density in the cloud

)exp(1 tE

Edt

dE

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Contd.,

Let Q and QS be separated and Generated Charges respectively ε – Permittivity of the medium A – Cloud Area h – Height of Charged region

M-> Electric Moment of thunder storm

)]exp(1[)]exp(1[

;

t

M

t

hQQ

A

QsE

Ah

Qg

sg

o

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Values obtained…

The average values observed for thunder clouds are:Time constant=1/λ = 20 sElectric Moment = M = 110 C-km Time for first lightning flash to appear, t=20 s The velocity of separation of charges, ν = 10 to 20 m/sSubstituting these values Qg = 1000C

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Modeling of Lightning

During charge formation, cloud considered as a non conductorAt charging process, local breakdown takes place within the cloudDielectric – cloud – air – groundLightning stroke -> Current source Io, Source Impedance Zo

ο

ο

ο

οο

Ζ

Ζ1

ΖΙV

ΖΖ

ΖΖΙV

ΙΖ;V

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Contd.,

Source Imp – 1-3 k ΩSurge Imp - <500 Ω

OH lines – 300 - 500 ΩGround wires – 100 - 150 ΩTowers – 10-50 Ω

Z/Zo is neglected; V=IoZ If lightning stroke current is 10 kA strikes a line of 400Ω surge imp, it cause over voltage of 4 MV.

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Lightning StrokesWhen EFI exceeds BD value(10kV/cm), An electric Streamer with plasma starts towards the ground velocity of 1/10th times that of the light, progress about 50mAfter 100μs, streamer starts again.Total time required for stepped leader to reach the ground 20 ms.Branches from initial leader is also formedLightning consists of separate discharges starting from a leader discharge and culminates in return strokes/main dischargesVelocity of leader stroke (first discharge) = 1.5 X 107 cm/sSucceeding strokes velocity 108 cm/sReturn strokes 1.5 x 109 to 1.5 x 1010 cm/s

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Return Strokes

Currents at return strokes about 1-250 kAReturn Strokes vanishes before it reaches to cloudDia – 1-2 cmCorona envelop 50 cm

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Propagation Of Stepped Leader Stroke From The Cloud

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Development Of Main Or Return Stroke

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Lightning and Overhead Lines

Direct strikes affect all voltage systemsProblems decrease with insulation levelFlashover when lightning strikes phase conductorAlso back flashover when tower or shield wire struck

Indirect strikes affect distribution and sub-transmission systems

Induced voltages up to 300 kV

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Striking Distance

Major influencesHeight of structureCharge on lightning leaderSlenderness of structureRandom effects

Striking Distance

Capture Radius "Rc"

Charge "Q"Current "I"

Height "H"

--

----

-

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Striking Distance

Can be inferred from photographs

Point of last downward branchUpward connecting leader pathApparent junction

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Voltages from Direct Strikes to Overhead Lines

Stroke to conductorConductor has surge impedance of about 400 ohmsAverage return stroke current 30 kAConductor voltage = 400 x 15,000 V = 6 MV

Stroke to towerTower has footing resistance of 30 ohmsTower voltage = 30 x 30,000 V = 900 kV

Shielding and grounding provide effective protection

Especially for higher voltage systems

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Transmission Lines & Lightning

CharacteristicsShielded construction High insulation levelsGood tower groundingEffective protection

Well coordinated fast switchgear

ResultExcellent lightning performancePermanent damage rareFew flashovers quickly cleared by protection

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Shielding Effectiveness

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Shielding Failure

Likely low current strokes

Less leader charge Smaller striking distanceFlashover less probable

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Lightning Protection using Ground wires

Ground Wire conductor run parallel to the main conductor of line supported on the same tower and earthed at every equally and regularly spaced towers.It is run above main lineShielding angle θs ≈ 300was considered adequate for tower heights of 30m or less.Number of shielding wires is depends upon the type of tower usedPresent trend in fixing the tower heights and shielding angles is by considering the “flashover rates” and failure probabilities.

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Shielding Arrangement of Ground Wires

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Typical Lightning Stroke Voltage on a Line without ground wire

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Ground rods and Counter-Poise wires

Instantaneous Potential to which tower top can rise is

VT=I0ZT / (1+(ZT/ZS))Where, ZT, ZS surge impedances of tower and ground wire. If ZT (tower footing resistance), is reduced , surge voltage developed is also reduced.It can be done by ground rods and counterpoise wires at legs of the tower.

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Ground Rods15mm dia, 2.5-3m long driven into the ground50m long for hard soilsMade by galvanized iron or copper bearing steelDesign specifications depends on desired tower footing resistance.10 rods of 4m long and spaced 5m apart, effective resistance is reduced to 10%

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Counter Poise Wires

Wires buried in the ground at a depth of 0.5 to 1 m, running parallel to the conductors connected to legs of tower50 – 100 m longMore effective then rods and resistance is reduced to as low as 25 Ohms

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Protective Devices

Expulsion GapsProtector tubesRod gapsSurge diverters or lightning arresters

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Expulsion Gaps and Protector Tube Mounting

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Volt time characteristics of rod gap

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MV Surge Arresters

November ‘07

Brian McGowanGlobal MVSA Product Manager

IEEE C62.11IEC 60099-4

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Introduction – over-voltage sources

•Overvoltages in the system can result in Significant damage

•Surge arresters limit excessive over-voltages that occur on the electrical power system from damaging the substation, plant and equipment.

•Main sources of overvoltages•Lightning •Switching

Circuit Breaker Operation

Traffo

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Sources of Overvoltage

Overvoltages can be due to Lightning - direct or indirectSwitching

Dist. Lines - operating voltage is lower more vulnerable to the effect of induced lightning overvoltagesInduced lightning overvoltages result from electromagnetic coupling between lightning strike channel and overhead line

71

System Overvoltages

Continuous TOVTemporary

Earth FaultsFerranti – capacitive charging current (eg. line energisation)Ferroresonnance (Resonant V due to interaction of power system L & Cap)Sudden Loss of Load

TransientSlow Fronted (Switching, Line energisation)Fast Fronted (Lightning – )Very Fast Fronted (GIS –operation)

1/20µs

8/20µs

30/60µs

Up to 8 hrs

Overvoltage

Power freq Voltage

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System Overvoltages in perspective

Durationµs ms s

Lightning overvoltages

Switching overvoltages

Temporary overvoltages

p.u. voltage

1

2

3

4

5

6

VFT

System voltage

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Typical Waveform - (IEC 60060-1)

8 / 20µs - Lightning waveform1 / 20µs - Steep front lightning waveform30 / 60µs - Switching current impulse

IL

1

0.5

0

8

20

Time (ms)

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Insulation Coordination (IEC 60099-5)

Is lightning voltage at transformer higher than BIL?If it is, can it be reduced?, or is it better to increase transformer insulation level?

Lightning strikes Overhead Line

Lightning voltage appears at Transformer

11/0.415kV75kV BIL

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Overvoltages / Insulation Coordination

TOV U ref U rated MCOVUsys

Lightning Over-voltage

Switching Over-voltage

Basic Insulation Level

Lightning Over-voltage Protection Level

Switching Over-voltage Protection Level

KVpeakkVrmsIEEE C62.11

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Fast transients

Lightning (uS)Switching (mS)Abnormal (mS – S)

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Fast transients

Lightning (uS)

Strokes to groundInduce voltage

Strokes to shieldInduce voltage

Direct stroke to phase

e

E ( back flashover)

e

Lightning strikes induces over voltages in a line by several mechanisms

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Fast transients

Lightning Transmission Line Design issues

Shield placementto eliminateDirect Stroke

Geometry to provideAdequate insulation

Grounding to minimize Induced voltage

Surge arrester

79

Fast transients

Switching

80

Fast transients

Design issues (Non Self Restoring Insulation)

Insulation coordination

Time

kVEquipment Withstand

Prospective Surge(Limited by design, surge arresters,…)

Margin