Chapter 5Chapter 5

Overvoltages and Insulation Overvoltages and Insulation Co-ordinationCo-ordination

Insulation Co-ordinationInsulation Co-ordination Insulation coordinationInsulation coordination : The correlation of the insulation of : The correlation of the insulation of

electrical equipment with the characteristics of protective devices electrical equipment with the characteristics of protective devices such that the insulation is protected from excessive overvoltages.such that the insulation is protected from excessive overvoltages.

Conventional approachConventional approach: Protection margin must be : Protection margin must be sufficient sufficient

Statistical approach:Statistical approach: - Overvoltage distribution: the - Overvoltage distribution: the stressstress

- Insulation breakdown probability: - Insulation breakdown probability: the the strengthstrength..

Conventional Insulation Approach Statistical Approach

Typical Standard Insulation LevelsTypical Standard Insulation Levels

Power System Overvoltages and Power System Overvoltages and Insulation Levels Insulation Levels

Surge arrester: protection level

Temporary Overvoltages (TOV’s)Temporary Overvoltages (TOV’s)

TOV’s: Load RejectionTOV’s: Load Rejection

Ferranti effect on a lineFerranti effect on a line: end voltage > beginning : end voltage > beginning voltagevoltage

-- VV00 > E : > E :

-- Voltage increases at end of lineVoltage increases at end of line-- Remedy: Series capacitors or shunt reactor compensationRemedy: Series capacitors or shunt reactor compensation

L (transformer)

VoC (cable)

I

E

VL

Vo= E - VL

VL

E I

TOV’s: Earth FaultsTOV’s: Earth Faults

Earth faults: effect of neutral earthingEarth faults: effect of neutral earthing Sound phase voltages to earth depend on Sound phase voltages to earth depend on

neutral earthing impedance, Zneutral earthing impedance, ZNN

E

F

B

A

B

C

A

AC

N

E,F

ZN

N

ZN=0

ZN=

ZN

Earth Fault on Phase C

Switching OvervoltagesSwitching Overvoltages

Fault clearingFault clearing Transformer magnetising currentTransformer magnetising current Capacitance switching Capacitance switching Energizing of unloaded transmission Energizing of unloaded transmission

lines, lines, Travelling waves.Travelling waves.

Switching Overvoltages: Fault Switching Overvoltages: Fault ClearingClearing

-- Arc across CB and fault Arc across CB and fault while fault current, I, flows.while fault current, I, flows.

-- When current ceases at When current ceases at current zero, a high current zero, a high frequency transient voltage frequency transient voltage occurs (2Voccurs (2Vmm))

-- Solution: Tripping resistorsSolution: Tripping resistorsVcb

I

Vm

Im

Lightning Lightning

The Nature of LightningThe Nature of Lightning-- Charge separation Charge separation in in cloudsclouds-- Downward leader Downward leader develops from clouddevelops from cloud-- Leader is invisible, zig-Leader is invisible, zig-zags, 100 A zags, 100 A - - Positive leaders form Positive leaders form from sharp points on from sharp points on earth.earth.-- Final discharge, 20 - Final discharge, 20 - 100 kA100 kA

Stepped downward leaderStepped downward leader

Moves in 30 m steps, Moves in 30 m steps, zigzagszigzags

Current approx. 100 ACurrent approx. 100 A Not visible to naked eyeNot visible to naked eye Induces positive charge Induces positive charge

on projecting objectson projecting objects Final discharge , “return Final discharge , “return

stroke”, typically 50 kA.stroke”, typically 50 kA.

Lightning ProtectionLightning Protection Rolling sphere conceptRolling sphere concept Striking distance 40 - 130 mStriking distance 40 - 130 m Ground Flash DensityGround Flash Density Shielding AngleShielding Angle Overhead ground wire to intercept Overhead ground wire to intercept

leaderleader Importance of Grounding Importance of Grounding

Impedance Impedance Attractive radius:Attractive radius:

RRAA= 0.84 I= 0.84 I0.740.74 h h0.60.6

RRAA = 14 h = 14 h 0.6 0.6 (for I =35 kA) (for I =35 kA)

rmast

building

Lightning ProtectionLightning Protection

Leader

Lightning Over-voltagesLightning Over-voltages

Direct StrokeDirect Stroke V = I ZV = I Zc c /2/2

ZZcc= = (L/C)(L/C)

L henry/mL henry/m

C farad/mC farad/m

Typically:Typically:

ZZcc= 350 ohm= 350 ohm

i/2

i

i/2

Lightning OvervoltagesLightning Overvoltages

Step or touch potentialStep or touch potential Indirect: inducedIndirect: induced Capacitively or inductivelyCapacitively or inductively

Distance from object0

i

+

-++

+ +

--

--

Lightning OvervoltagesLightning Overvoltages

The current flowing The current flowing through the earthing through the earthing impedance raises the impedance raises the tower potential to cause tower potential to cause a flashover from the a flashover from the tower to the phase tower to the phase conductorconductor

Importance of a low Importance of a low

tower footing resistance.tower footing resistance.

L

R

i

i

back flashover

V=0

V = Ri +Ldi/dt (high)

V: medium voltage

earthed tower

Insulation CoordinationInsulation Coordination

Correlation of the Correlation of the insulation strengthinsulation strength of of electrical equipment with the electrical equipment with the characteristics of characteristics of protective devicesprotective devices such such that the insulation is protected from that the insulation is protected from excessive excessive overvoltagesovervoltages

50 % Impulse flashover Voltage50 % Impulse flashover Voltage Time to flashover (on front/ tail, withstand) Time to flashover (on front/ tail, withstand)

Overvoltage Protection Overvoltage Protection

Rod and horn gapsRod and horn gaps: Flash over with : Flash over with overvoltage - Fault current flows - Outage- Poor overvoltage - Fault current flows - Outage- Poor protection for short , high impulsesprotection for short , high impulses

Silicon carbide gapped lightning arrestersSilicon carbide gapped lightning arresters: : Nonlinear resistor in series with gap - gap Nonlinear resistor in series with gap - gap flashes over - current and voltage limited by flashes over - current and voltage limited by resistor - power arc goes out at current zero - resistor - power arc goes out at current zero - magnetic blow out coils.magnetic blow out coils.

Overvoltage Protection Overvoltage Protection

Gapless ZnO arresters: Gapless ZnO arresters: High resistance High resistance below knee point - no gap required.below knee point - no gap required.

Well defined knee pointWell defined knee point Suitable to protect transformers against Suitable to protect transformers against

steep pulses. steep pulses. Placing of ArrestorsPlacing of Arrestors: As close to : As close to

protected apparatus as possible.protected apparatus as possible.

Impulse TestsImpulse Tests The purpose is to The purpose is to

generate a voltage generate a voltage impulse to simulate a impulse to simulate a ligtning overvoltageligtning overvoltage

Typical standard 1,2/ Typical standard 1,2/ 5050s wave.s wave.

Typical circuit to Typical circuit to generate impulsegenerate impulse

Capacitor C1 Capacitor C1 charged and charged and discharges when discharges when spark gap G flashes spark gap G flashes over.over.

Volt

age

Time

Laboratory Impulse TestingLaboratory Impulse Testing In impulse testing each test In impulse testing each test

produces a single impulse produces a single impulse that can result in either a that can result in either a flashover or a withstand.flashover or a withstand.

In order to quantify the In order to quantify the flashover voltage a number flashover voltage a number (say 10) tests are done at (say 10) tests are done at each voltage level in order each voltage level in order to obtain the 50% to obtain the 50% (probability) flashover (probability) flashover voltage.voltage.

The withstand level is The withstand level is defined as that voltage defined as that voltage where only 3 flashovers where only 3 flashovers occur during a series of 15 occur during a series of 15 consecutive impulses.consecutive impulses.

critical flashover voltage (50 %)

no flashover

flashover on front

time

100 %

0 %

50 %

Impulse voltageV50

ConclusionConclusion

Insulation Coordination encompasses Insulation Coordination encompasses all aspects of the power system and all aspects of the power system and attempts to ensure uninterrupted attempts to ensure uninterrupted supply of power under the worst supply of power under the worst overvoltage and environmental overvoltage and environmental conditions.conditions.

Surge arresters play an important Surge arresters play an important role in reaching this goal.role in reaching this goal.