OVERVOLTAGES & INSULATION COORDINATION

Lecture 6ELEC-E8409 High Voltage Engineering

No standard yet(equipment type specific)

0.1 µs

1.2 µs 50 µs 100 µs

250 µs 2500 µs

10 ms

0.1 s

OVERVOLTAGES

0.1 s

Overvoltage Shape Test Voltage

Temporary (Sustained) Overvoltage- earth fault- load disconnection- resonance and ferro-resonance- open phase connection

Slow-Front (Switching) Overvoltage- connecting load to network- faults and re-closure- disconnection of load current

Fast-Front (Lightning) Overvoltage- lightning (induced, back surge, straight hit)

Very-Fast-Front Overvoltage- arc interruption and restriking

OVERVOLTAGES

1

2

3

4

5

6

10-6 10-4 10-2 100 102 104t [s]

U [p.u.]Lightning Surges (Fast-Front Overvoltage)

Switching Surges(Slow-Front Overvoltage)

Temporary Overvoltages(Sustained Overvoltage)

Temporary Overvoltages

a.k.a. Sustained Overvoltages

Earth FaultLoad DisconnectionResonanceOpen-Phase (Asymmetric) Connection

EARTH FAULT

Most common cause for temporary overvoltages

• Overvoltage caused between healthy phase and earth

Magnitude depends on earthing type:

• a.) Isolated neutral• b.) Resonant earthed

(Peterson coil)• c.) Direct earthing

Magnitude given as earth fault

coefficient k• Ratio of peak phase-earth

voltage Up to normal operation voltage U

• Network is “effectively earthed” when k ≤ 1.4

a.a. b.b. c.c.

(magnitude ≈ 1.8 p.u.)

LOAD DISCONNECTION

The network supplies voltage U1 to the beginning of the line (network represented as short circuit impedance Zk and single phase emf)

The line is represented by a π-equivalent circuit U2 is the voltage at the end of the line

0

1

2

3

0 500 1000s [km]

U [p.u.]c2 (Sk = 5)

c1 (Sk = 5)c1 (Sk = 10)c1 (Sk = 50)c1 (Sk = 500)

c2 (Sk = 10)c2 (Sk = 50)c2 (Sk = 500)

Increase in voltage at beginning of line: s

ZZ

ZZ

c

w

k

L

k

tanh1

11

Increase in voltage at end of line: s

cccosh

12

Voltage increase greatest when load is inductive and thegrid’s short circuit power is small (Zk is large – weak grid)

kk Z

US2

Difference between c1 and c2 is the Ferranti phenomenon:• Capacitive current increases no-load line voltage as it

approaches the open end of the line

network line load

U1 U2Yj2Yj1

Zk Zj

ZL~~~

cjgljrZw

cjgljr

RESONANCE

Oscillations of higher amplitude at certain frequency (determined by R, C,and L components in the circuit)

Reactance X = opposition to alternating current (caused by the build up of electric or magneticfields in an element). Total reactance is the sum of capacitive and inductive reactance, X = XC + XL

Different resonance states and nonlinear components can cause temporary overvoltages

cL XX C

L

1

LCf

21

X > 0X > 0Reactance is inductive

(opposes change in current)

X = 0 X = 0 Impedance is purely

resistive (Z = R + jX)

X < 0X < 0Reactance is capacitive

(opposes change in voltage)

As frequency increases, inductive reactance XL increases while capacitive reactance XC decreases At a particular frequency these two reactances are equal in magnitude but opposite in sign

DC (low f): Capacitor is open circuit (charges balanced with applied voltage = no current)AC: Capacitor only accumulates a certain amount of charge before polarity changes and

charges dissipate. Increasing frequency (less charges) decreases opposition to current

FERRO-RESONANCE

Oscillations caused by series connection of capacitance and nonlinearinductance- causes temporary overvoltages concurrent with harmonic distortion

Nonlinearity effect – inductance decreases as current increases when saturation occursgiving resonant conditions at basic frequency (or multiples). Resonance may cause thevoltages and currents to jump from one state to the other and current overloading of thetransformer may lead to thermal damage.

Voltage transformer connected to an unearthedsystem UL

UCUV

Resonant circuit formed by the earthcapacitance of the system and thenonlinear inductance of the voltagetransformer connected between phaseand earth

Operation should be where the difference in UL and UC is equal and in the samedirection to UV

• P1: stable operation when dUL/dI > dU/dI. Circuit returns to P1 after short current surges (e.g. switching operations).

• P2: if critical value dUL/dI = dU/dI is exceeded, the circuit shifts to P2. Stable operation not possible at P2. If thecurrent increases slightly above I2, UV must be greater than that required to maintain the increasing current, thus, thecurrent increases further.

saturation increases (inductance decreases) and current increases until it reaches a value where the circuit becomescapacitive (current changes direction by 180° once P2 is exceeded). Operation shifts to P3.

• P3: stable operation (although current and voltage is significantly higher than normal operation)

Jump-resonance – transition from P1 to P3 is called jumping (FI: kippaus)

I

U

UL

UC

UL - U C I

U

I1 I2I3

P1 P2

P3

UV

|UL|

–|UC|U = |UV – UC|

Inductive (UL > UC)XL(lin) > XC

UV = UL – UC

Capacitive (UC > UL)XL(sat) < XC

UV = UC – UL

UV

UC

UL

UV

U

I

P1 P2 P3

UL UCUV

ULUC

UV

i–Bsat

t

t

+Bsat

P1P1P2P2 P3P3 P1P1P2P2 P3P3 P1P1P2P2 P3P3

1. 2. 3. 4. 5.

1.1.

2.2.

3.3.

5.5.

The capacitor has just charged and the magneticflux density has reached to -BS. Due to thevoltage difference between UV and UC, voltageUL increases forming a voltage-time area(displayed in grey). As this area becomes larger,the magnetic flux of the iron core decreases.

Flux density reaches +BS. Inductancedecreases. A rapid increase in current follows.

2 – 3. 2 – 3. Inductor is saturated and large current flows inthe circuit. As a consequence of the currentsurge, the capacitor charge changes from –UCto +UC.

The inductor’s voltage becomes negative. Sincethere is no current flowing in the circuit, thecapacitor’s voltage UC is constant.

The increasing U,t-area changes the flux from+Φ to –Φ.

4.4. Saturation limit is onec again achieved

4 – 5. 4 – 5. Inductor is saturated during which currents arelarge.

Process repeats itself from point 1.

Induction law:

UdtAA

B 1

For continuous operation, jumping occurs during each half cycle:

Line capacitance and no-load impedance of a transformer mayproduce resonance at harmonic frequencies, resulting in overvoltages

• MV – overvoltages not significant• LV – insulation may fail and cause thermal damage

One or two phases are

disconnected

One or two phases are

disconnected

Broken Conductor

Burnt Fuse

Switchgear Malfunction

ASYMMETRIC CONNECTION (OPEN PHASE)

Slow-Front Overvoltages

a.k.a. Switching Overvoltages

Load ConnectionApplied Voltage and Re-closureFaultsDisconnection of Load Current

Switching operations can causesignificant stress between switchgearterminals (contact gap)

Magnitude and shape of overvoltagedepends on switchgear used forcurrent disruption and networkproperties (L, C, load)

network configuration and instantaneousvalue of voltage and current at themoment when switchgear is opened or closed

Most often is a result ofstate change in network,and occurs due to:

•Faults– short circuit,earth fault, loaddisconnection, asynchronousoperation

•Switching operation–opening or closing the circuit

Previously called Switching Overvoltage

Busbar Short Circuit

Line Fault

f.f.

e.e.

d.d.

c.c.

b.b.

a.a.

Disruption of Capacitive Current

Voltage Applied to No-Load Line

Disruption of Small Inductive Load

Asynchronous network

LOAD CONNECTION

Standard procedure in network (causes a slow-front overvoltage)

Peak value of overvoltage depends on instantaneous voltage at the moment of switch closure

• Maximum peak value of overvoltage is 2.0 p.u.• Angular frequency ω caused by connection is typically ~ 100 Hz

CONNECTING A CAPACITOR

Asynchronous closure

Earthing technique Resonance

Superpositionedoscillations over

2,0 p.u.

Similar peak value as capacitor (2.0 p.u.)• Steeper overvoltage• Voltage stress concentrated at the beginning of the winding (not evenly distributed)

CONNECTING A MOTOR

NO-LOAD LINE VOLTAGE APPLICATION

Applying voltage to ano-load line is one ofthe major causes forovervoltages at highoperating voltages(≥245 kV).• Applied voltage creates a

travelling wave whichdoubles the voltage once itreflects back from the end ofthe line

2U

U

3U

U-U

NO-LOAD LINE VOLTAGE APPLICATION

0 4 8 12 16 20-3

-2

-1

0

1

2

3

1

2

Ideal Zk = 0, no losses Real Zk ≠ 0, losses included

no trapped charge (disconnected from network for some minutes)

with trapped charge -1,0 p.u. (when re-closed, trapped charge is seen as an opposite sign voltage)

1.1.

2.2.

Zk = 0

u(t) = cos ωt uR

t = s/c ≈ 1.43 mss = 430 km

~

1.1.

2.2.

u r[p

.u.]

t [ms]

Voltage at end of line after closure:

FAULTS

Typically, slow-front overvoltages related to faults do not exceed:

Onset of fault: umax < 2k – 1 k = earth fault coefficient

Removal of fault: umax = 2

• The onset of a fault and its removal both cause transients

Different network failures can cause overvoltages (earth fault is most common)

Fault causesvoltage drop

Circuit breaker is tripped (opened) to remove voltage

drop

Trippingaction resultsin overvoltage

Network side voltage oscillates and settles eventually at the

value determined by the network supply voltage

DISCONNECTION OF LOAD CURRENT

When a switch is opened, arcing may occur over the gap between terminals

• Arc is permanently extinguished at zero current, followed by avoltage transient. Voltage drop in the network caused by current(arcing) oscillates and attenuates and voltage settles at thecontinuous operation level.

Basic Situation:

Immediately after the arcing isextinguished (zero current), thevoltage over the contact gap isformed by the supply networkand the load side potentialdifference (recovery voltage ur)

It takes some time for ionization in

the contact gap to disappear and for

the switch to regain itsinsulating properties

Zu(ωt)uR~

i(ωt)

1.0

u, i [p.u.]

-1.0

RECOVERYVOLTAGE

ωt1

i(ωt)1.0

u, i [p.u.]

-1.0

ωt1

i(ωt)

usupply(ωt)

RECOVERY VOLTAGE

uload = 0, ωt > ωt1i(ωt)

ωt1RECOVERY VOLTAGE

-1.0

1.0

u, i [p.u.]

usupply(ωt)

ωt

uload = 0, ωt > ωt1

i(ωt)ωt

uload = -1, ωt > ωt1

usupply(ωt)

DISCONNECTION OF LOAD CURRENT

Load Z = only RESISTANCE R

• No phase difference between supply voltage and load current

• If current is disrupted at first zero point, load side voltage remains at zero

• Recovery voltage increases along normal sinusoidal fluctuation of supply voltage

Load Z = only INDUCTANCEjωL

• 90° phase shift between voltage and current

• When current is disrupted at zero level, voltage is at peak value (= onset recovery voltage)

• The initial steepness of the recovery voltage is large but the peak value is still the same as the supply voltage

Load Z = only CAPACITANCE 1/ jωC

• 90° phase shift between current and voltage

• When current is disrupted at zero level, charge equal to voltage at moment of disconnection remains in capacitor (load)

• Recovery voltage increases from zero to twice the peak value of the supply voltage

Zu(ωt)ur~

i(ωt)

ul

DISCONNECTION OF LOAD CURRENT

Most common cases of restriking occur when:

If the recovery voltage exceeds

the voltage withstand

strength of the contact gap

RestrikeMagnitude of

overvoltage depends on moment of

occurrence

Reignition• Voltage at both

terminals have same polarity small overvoltage

Restrike• Voltage at

terminals have opposite polarity large overvoltage

Disconnecting CAPACITIVE currentDisconnecting small INDUCTIVE current

uc

uc

uc

û sin (ωt)

uR

uR

i

t

t

û sin (ωt) uct

it

uR

b.b.a.a.

• Once arcing is extinguished, a trapped charge (value = peak voltage)remains in the capacitor (a.)

• If switch cannot regain insulating properties fast enough (recoveryvoltage higher than withstand voltage), restriking occurs at peak ofsupplied voltage (b.)

• Restriking creates another transient which can provide an evengreater trapped charge to the capacitor

DisconnectingCapacitiveCurrent• No-load line or cable

disconnection from the grid

• Disconnection of storage capacitor

û sin (ωt)uR~ CS

C

uc

L L, C and Cs define oscillation frequency of transient. Ifrestriking continues repetitively at the peak value of thesupplied voltage, an extremely large overvoltage is created.

(small inductive current = cos φ < 0.5 and significantly smaller than the breaker’s breaking capacity)

u(t) = √2 U cos (ωt)~ C1 uL

LS

C2 L2

2 22max 0 0

2

Lu u iC

Peak voltage at C2:

DisconnectingSmall InductiveCurrent• Disconnecting transformer

no-load current• Disconnecting HV motor

starting current• Disconnecting reactor

current

Energy remains in the load inductance L2 and causes the LC circuit (formed by L2 and C2) to oscillate.

Disconnection may occur before current has reached zero level because the breaker’s breaking capacity significantly exceeds the magnitude of the current to be interrupted.

0 0 and are the current and voltage at the moment of interruption. i u

i0

u0

uL

i

t0 ti0

u0

uL

i

t0 t

uL

1.u1

u2

WITHOUT restriking WITH restriking

i0 = current at moment of interruption (chopping current)u0 = voltage at moment of interruptionu1 = peak value of oscillations with restrikingu2 = peak value of oscillations without restriking1. = increase in dielectric strength of contact gap

Transient voltages increase rapidly and restriking often occurs considerably earlier before voltage has reached

its maximum value

Restriking occurs several times until the withstand

strength of the contact gap exceeds the recovery

voltage stress

In this case, restrikinglimits overvoltages

Disconnecting Small Inductive Current:

Fast-Front Overvoltages

a.k.a. Lightning OvervoltagesDirect Strike to ConductorBack FlashoverInduced Overvoltages

Typically caused by lightning:• Direct strike to

conductor

• Back flashover via grounded components

• Induced by nearby stroke

Previously called Lightning Overvoltage

Lightning is a very large leaderdischarge

Requires strong up-flow of air mass andhigh humidity of the rising air Cold and warm air masses meetAir heated by the sun rises up

Not all factors and mechanisms forthe formation of thunder clouds fullyunderstood

LIGHTNING

War

m A

irW

arm

Air

Col

d A

irC

old

Air

+

–

++ +

++

++

+– – –

––

––

–

– – –– – ––– –––

––––– –– –––

–– – ––––

– – –

– ––

––

++

++

++

++

++

+

+

– ––– ––

–– – –

––

+

+ – – –– ++ + ++++++++

+ ++ +

+

SNOWSNOW

ICEICE

WATER,VAPOR

WATER,VAPOR

Direction of MotionDirection of Motion

++

+24+24

0044

-32-32

-48-48

1010

1414

00

°C°Ckmkm

Induced Charges at GroundInduced Charges at Ground

Particles (ice, snow) inside cloudcollide due to the strong up-flowof air and are charged.

Negative charges – heavy particles(snow/hail) accumulate at mid sectionand lower area of cloud

Negative charges – heavy particles(snow/hail) accumulate at mid sectionand lower area of cloud

Positive charges– small ice crystals atthe top of the cloud. Usually there isalso a small positive charge area at thebottom of the cloud.

Positive charges– small ice crystals atthe top of the cloud. Usually there isalso a small positive charge area at thebottom of the cloud.

Potential difference inside the cloud can be around gigavolt.

Potential difference inside the cloud can be around gigavolt.

Lightning discharge begins where the charges increase the electric field above thebreakdown strength of air (~1 MV/m for air inside a water droplet)

• the lightning flash can consist of numerous subsequent strokes traveling along thesame channel and also branching discharges which terminate in air

5 - 6.) Subsequent strokes can form

from other negatively charged areas in the cloud

using the same discharge channel

4.) When the two discharges meet, a main stroke (return stroke) travels from ground

to cloud discharging the negative charge area (where

the leader started) in the cloud

3.) Breakdown strength of air is exceeded and a leader channel starts

from the ground towards the opposite

charged discharge

2.) Destination of the lightning

stroke determined ~ 100 - 150 m from ground

1.) Leader discharge

begins from cloud towards ground

Negative Lightning Discharge (cloud to ground)

––

– – – – –– – – – –– – – –– – –– – –

+ + + + + + + + +

––––– –– –––––––– ––––

–

–– –––– –

– – – – –– – – – –– – – –– – –– – –

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

–––

–––

–

–

––

–

–––

–––––

––––

–– –

– – – – –– – – – –– – – – –– – – –– – –– – –– – –

+ + + + ++

+ +++

+

++

+ ++

+ + + + –

+ + + +

– –––

– –––––––

–––– –– –– – ––– – –– –

–

+ + + + + +

+++ ++––

––– –––––– –– –– – –

––– – –– –

–

––

–––

–––

––––––

––

–

––

++

++

++ + ++ + +

+++

+ + + +

– – – – –– – – – –– – – –– – –– – –

1.1. 2.2. 3.3.

6.6.4.4. 5.5.

0.2

0.4

0.6

0.8

1.0

1.2

0

–10 –5 0 5 10

5%

95%

50%

t [µs]

i [p.u.]

0.2

0.4

0.6

0.8

1.0

1.2

0

–5 0 5 10

5%

95%

50%

i [p.u.]

t [µs]Main discharge Post discharges

Milder slope

Milder slope

Larger currentLarger current SteeperSteeper Smaller

currentSmaller current

Distribution areas of shape of downward negative discharges

DIRECT STRIKE TO CONDUCTOR

t [µs]

400

800

1200

16002200 m1300 m

620 m0 m

0 1 2 3 40

U [kV]Typically Zw= 250 – 500 Ω• Overvoltage ≈ MV• Corona causes

losses which attenuate and flatten overvoltage

Propagating overvoltage into both directionsalong the conductor

iZu w21

iZu w21

BACK FLASHOVER

Flashover from grounded component to phase conductor•backward flashover

called back flashover

Flashover from grounded component to phase conductor•backward flashover

called back flashover

Highest probability when lightning current

is high or poor earthing

conditions (large earth impedance)

Highest probability when lightning current

is high or poor earthing

conditions (large earth impedance)

Flashover occurs between the

phase with the largest opposite

voltage relative to the lightning overvoltage

Flashover occurs between the

phase with the largest opposite

voltage relative to the lightning overvoltage

If voltage u exceeds the voltagewithstand between the groundedcomponent and the live phaseconductor:

Lightning strike to grounded line components (pole or lightning shield wire) Reflections from neighbouring poles and the pole itself significantly alter the voltage waveform

u0

u

t

τT = propagation time in pole

τ1 = propagation time to adjecent poleZ1 Z1

ZT2 = 0ZT1 = 0ZT

Rf

τT

τ1 τ1

(lightning impulse assumed as step)

INDUCED OVERVOLTAGELightning can also cause overvoltage when it does not directly hitthe conductor, but in the vicinity of a conductor or equipment.The overvoltage is then caused by the electromagnetic induction ofthe main discharge current.- Induction does not mean by its usual sense since the discharge channel and theconductor on which the voltage is induced are almost perpendicular to one another.

k = propagation speed of discharge current (constant ≈ 1.2 – 1.3)Z0 = 1/4π √(µ0/ɛ0) = 30 Ω (constant)i = peak lightning currenth = height of conductord = distance of stroke from conductor

Induced voltages are typically smaller (200 – 300 kV) and slower (front time c. 10 µs)

Lightning current cause a rapidly changingmagnetic field into the lc loops of the lineinducing a voltage: d

hkiZuind 0

h

i

d

Very-Fast-Front Overvoltages

VERY-FAST-FRONT OVERVOLTAGES

E.g. air insulated switching station:• Steep transients attenuate quickly• Only dangerous to equipment located close to the

disconnector (heats wiring, causes internal resonance)

Overvoltage caused by arc

interruption and restriking when

opening disconnector

(GIS faults, switching of motors and transformers with short connections to

switchgear, certain lightning conditions)

Tens of restrikingsduring opening• Each restrike

generates high frequency oscillations

• Oscillation frequency typically 100 kHz – 50 MHz

• Discharge currents can reach 2 – 3 kA.

Typical for (HV) disconnector operation

–500

500

1500

01 2 3 4 5

t [µs]

i [A]1000

a.k.a. Very Fast Transients (VFT): in practice restricted to transients withfrequency above 1 MHz

Shape of VFT (IEC71-1): time-to-peak < 0.1 µs, total duration < 3ms, superimposed oscillations withf ranging 30 MHz – 100 MHz.

INSULATION COORDINATION

OVERVOLTAGE PROTECTION

Spark Gap

Surge Arrester

(FI: venttiilisuoja)

Protection levels:1. Avoid direct impact of overvoltage by directing it towards designated

routes (lightning conductors, shield wires, and Faraday cages)

2. Ensure basic impulse level BIL (withstand level) is not exceededusing HV protection elements:

3. Extra protection for sensitive equipment (LV filters for computers andtelecommunication)

Surge Arresters

Spark Gap with Non-linear ResistorMagnetic Blow-out ArresterMetal-Oxide Varistor

OVERVOLTAGE PROTECTION

Decrease magnitude of overvoltage in networkDecrease magnitude of overvoltage in network

Surge Arrester

Traditionally located at substation• Protects only most important equipment – transformers, GIS

used in areas (FIN) where lightning density is low (intensified protection not necessary)

• Placed at all incoming lines to substation on line-side of feeder circuit breaker all equipment has some level of protection protection level decreases with distance between surge arrestor and protected device

Also located at poles• Decrease back flashover in areas of high lightning density and poor earthing

conditions (not economically feasible in Finland)

NONLINEAR RESISTANCE TYPE ARRESTER

Ideal• When voltage exceeds peak

operating voltage, the arresterbecomes conductive (weakresistor) allowing the surgeenergy to be dischargedwithout increasing voltage overthe protected device.

• Immediately after excessenergy is discharged, thearrestor regains itsinsulting state

Reality• Limited energy discharge

capacity (only applicable to relativelyshort duration overvoltages)

• Discharge of overvoltage is notimmediate

• Leakage current is presenteven in insulating mode

1

2

1

3 3

1 1

11

4 4

44

5 5

55

NONLINEAR RESISTANCE TYPE ARRESTER

Disk Spark Gap with Nonlinear Resistor

(silicone-carbide gap type)

Metal-Oxide VaristorMagnetic Blow-Out

Arrester(active gap surge arrestor, expulsion type)

Magnetic Blow-Out Arrester

(active gap surge arrestor, expulsion type)

Magnetic Blow-Out Arrester

(active gap surge arrestor, expulsion type)

1. Nonlinear resistor, 2. Disc spark gap, 3. Active spark gap, 4. Blow-out coil, 5. Shunting resistor

1

2

NONLINEAR RESISTOR TYPE WITH GAPS

Disk spark gap (2) in series with SiC resistor (1) encased in a porcelain shell

Dividing the spark gap into sections decreases breakdown voltage scatter and flattensthe steep transient resulting from flashover.

The nonlinear resistor limits the earth fault current so that arcing is extinguished byitself:

high currents low resistance

low current high resistance

As voltage over the arrestor exceeds sparkover(striking) voltage us, the spark gap is ignited.

Surge current ip grows to a value determined by theovervoltage magnitude

Residual voltage ur (maximum voltage over arrestorduring operation) is determined by the discharge currentand nonlinear resistor magnitude

After the overvoltage has passed, the arrestor remainsconductive and follow-through current ij (fed by thepower frequency voltage) is present until the spark gap isextinguished (voltage becomes zero)

t i

u

ijip

urur

u

us

u1us

ip

u1 = overvoltage peak (without arrestor)u = normal operating voltage ur = residual voltage us = sparkover (striking) voltageij = follow-through current ip = surge current peak

parallel resistancecoil

spark gap formed when piling elements (rings) together

metal electrode

ring

parallel resistance & coil

MAGNETIC BLOW-OUT ARRESTER

MAGNETIC BLOW-OUT ARRESTER

The high frequency surge current flows through the parallelresistance [2] of the coils [3] and causes the spark gap [1] to ignite.

After this, normal operating frequency current passesthrough the coils causing the magnetic field to “blow” the arc inthe spark gap further.

• As a result, arcing voltage increases and hence, current through thearrestor and voltage over nonlinear resistor [4] (residual voltage)decreases.

When the overvoltage has been discharged through the arrestor,power frequency voltage still feeds follow-through current.

• Due to the nonlinearity of the resistor,current decreases much faster than voltage andarcing over the spark gap is extinguishedbefore voltage reaches zero.

Since extinction does not require zero level voltage,this overvoltage protection works also for DCt i

u

METAL-OXIDE VARISTOR

ZnO + other metal oxides: single core of ZnOcovered by a metal oxide surface layer

Cylindrical mass element connected in series orparallel inside porcelain/polymer shell

Resistive properties are so nonlinear that sparkgaps can be left out (e.g. R(normal operation) = 1.5 MΩ,R(discharge) = 15 Ω)

Area 1: ZnO penetrating current decreases radically undervoltage threshold value (high resistivity). Poorly conductivesurface layer determines magnitude of current.• At small currents the resistance of the ZnO element decreases as

temperature increases (negative thermal coefficient).• Sufficient cooling needed to assure that the arrestor does not

become unstable (thermal run-away) and break.

Area 2: Tunnel effect – more current penetrates throughsurface layer into ZnO core.Area 3: Tunnel effect throughout entire material.Magnitude of current determined by core. Resistivity ofmaterial is very small.

30

60

100

200

500

10-9 10-6 10-3 100 102Area 2Area 1 Area 3

25°C 150°C ZnO

SiC

200°C

J [A/mm2]

E[V

/mm

]

Nonlinearity of ZnO vs. SiC

No rapid voltage changes

No breakdown voltage scatter

Insignificant back current

t i

u

METAL-OXIDE VARISTOR

ARRESTOR SELECTION

The arrestor must be selected so that the margin between protection level of arrestor and the device’s withstand level is large enough.

Urp = representative overvoltageUcw = voltage withstand level of devicekc = protection factor

rpccw UkU

Margin exists only if arrestor is infinitely close to the protected apparatus

Otherwise, must consider:• Voltage increase in line caused by propagating overvoltage (superposition of traveling waves)• Voltage drop caused by surge current at earthing conductor and arrestor connection (coupling)

The protection level must be set high enough to avoid arrestor operation under normalcontinuous operating voltage but also low enough to avoid overvoltages above thewithstand level

Protection level Withstand levelU

Safety Margin

ARRESTOR PLACEMENT

Protected device (T) is at a distance D from the arrestor (A)• The front of the voltage pulse is linear• Inductance of earthing circuit assumed insignificantly small

Effective Protection Level:

up = rated protection level of arrestorΔu1 = inductive voltage loss at earth and joint couplingΔu2 = voltage increase between arrestor and protected deviced1 = length of arrestor connectiond2 = length of arrestor earthingl = inductance of joint and earthing conductor (~1 μH/m)D = distance between arrestor and protected deviceS = steepness of linear impulse voltagev = propagation speed of impulse voltage

distance

u

up(eff)

A

d2

d1

T

D

(d1 + d2)l ∆i∆t

2SD/v2∆u

∆t∙v

∆u1

∆u2

up

vSD

tildduuuuu ppeffp

22121

ARRESTOR PLACEMENT

E.g. A 1500 kV/ μs steep propagating wave is approaching a transformer along a 123 kVline. The voltage withstand level of the transformer is 550 kV. The arrestor is located10 m away from the transformer and has a protection level of 380 kV. Voltage dropΔu1 caused by joint and earthing coupling (d1, d2) is assumed to be 20 kV.

vSD

tildduuuuu ppeffp

22121

kV 500 V 500000m/s 10300

)m 10)(V/s 101500(2V 1020V 10380 6

933

Distance and junction results in a 32% increase inprotection level• Safety margin reduced from 170 kV to 50 kV• Protection factor reduced to kc = Ucw/Urp = 550/500 = 1.1

Effective protection level less than withstand level oftransformer OK

If S = 2250 kV/μs, withstand level is exceeded. To protect against steep impulses

• bring arrestor closer• select arrestor with

lower protection level up

ARRESTOR PLACEMENT

g) GIS, RMU protection - arrestors at all line outputs

a) Transformer Protection

b – e) Cable ProtectionShort cables (30 – 50m): Arrestors at end of cable (c)Longer cables: Risk of back flashover. Arrestors at both ends of cable or use lightning

shield wire and minimize earthing resistance. Important to groundarrestor and cable sheath to same point (b)

f) Protection of important line-side measuring equipment

d > 500 m

500 – 600 m

0.1µF

b) Connection to overhead line via cable:Capacitor not needed when distance is over500 m

a) Straight connection to overhead line:Typically 500m distance between arrestorswith protective capacitor (reflections)

a b

Phase-earth and phase-phase protection:a) 6 separate arrestorsb) 4 arrestor group

GENERATORS AND MOTORS

Spark Gap

d

d/2d/2

SPARK GAP

Surge arresters are more expensive andrequire monitoring (arrester can fail) Cheaper and simpler solution for protecting smaller

pole transformers is to use a spark gap• at most 240 kVA, 24 kV transformer (FIN)• transformer must withstand spark gap overvoltage and steep voltage transient

Simple device consisting of two electrodes –one connected to the conductor to beprotected and the other to ground.

Spark gaps form a weak point enabling overvoltages to flow toearth instead of to the protected device.

Breakdown voltage can be adjusted

SPARK GAP

60 mm80 mm

100 mm120 mm

0.2 0.4 0.6 0.80 10

100

200

300

400

t / µs

kVu

0.2 0.4 0.6 0.80 10

100

200

300

400

t / µs

60 mm

80 mm

90 mm

120 mm

kVu

Double gap

Single gap

500 kV/µs: Direct lightning stroke to conductor1000 – 2000 kV/µs: Back flashover (rare)

Voltage-Time Curve: Voltage-Time Curve:

SPARK GAPWet Test

Dry Test

Inter-electrode distance d of spark gap:• Large enough to avoid breakdown by temporary

overvoltages and small transients• Small enough to protect against fast-front

transient voltages (lightning)

Problems with spark gaps: Gap operation causes an earth fault

Short zero voltage period needed to remove fault (requires fast reclosing system)

Polarity dependence

Weather conditions Temperature, humidity, and pressure affect ionization

Large operating voltage spread Up to 40%, also dependent on overvoltage shape, i.e. steepness

Spark gap implementation:• Reasonable number of atmospheric overvoltages• Short outages allowed

0 4020 60 80 100 120 140 160d / mm

10

20

40

80100

double gap single gap

0 4020 60 80 100 120 140 160d / mm

10

20

40

6080

100

60kVU

99 % protection level (U50 + 2.3s)

1 % ignition level (U50 – 2.3s)

kVU

1 % ignition level (U50 – 2.3s)

99 % protection level (U50 + 2.3s)

SUMMARYOvervoltages:

Insulation coordination:

• Temporary (sustained) • Slow-front (switching)• Fast-front (lightning)• Very-fast-front

• Surge arrestors• placement

• Spark gaps