05 Over Voltages and Insulation Coordination

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Industrial electrical network design guide T & D 6 883 427/AE

5. OVERVOLTAGES AND INSULATION CO-ORDINATION

Different types of overvoltage may occur in industrial networks. Devices must therefore be

installed to reduce their magnitude and the insulation level of equipment must be chosen so

that fault risks are reduced to an acceptable level.

5.1. Overvoltages

An overvoltage is any voltage between one phase conductor and earth, or between phase

conductors having a peak value exceeding the corresponding peak of the highest voltage for

equipment, defined in standard IEC 71-1.

An overvoltage is said to be of differential mode if it occurs between phase conductors or

between different circuits. It is said to be of common mode if it occurs between one phase

conductor and the frame or earth.

origin of overvoltages

Overvoltages can be of internal or external origin.

internal origin

These overvoltages are caused by a given network element and only depend on the

characteristics and structure of the network itself.

For example, the overvoltage that occurs when a transformer's magnetizing current is

interrupted.

external origin

These overvoltages are caused or transmitted by elements outside the network, for example:

- overvoltage caused by lightning

- spread of HV overvoltage through a transformer to the internal network of a factory.


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classification of overvoltages

Standard IEC 71-1 gives the classification of overvoltages according to their duration and form.

According to the duration, a distinction is made between temporary overvoltages and transientovervoltages:

- temporary overvoltage: power frequency overvoltages of relatively long duration (from

several periods to several seconds).

- transient overvoltage: short-duration overvoltage lasting only several milliseconds, which

may be oscillatory and is generally highly damped.

Transient overvoltages are divided into:

. slow-front overvoltage. fast-front overvoltage

. very-fast-front overvoltage.

standard voltage forms

Standard IEC 71-1 gives the standardised wave forms used to carry out tests on equipment:

- short-duration power frequency voltage: this is a sinusoidal voltage with a frequency

between 48 Hz and 62 Hz and a duration equal to 60 s.

- switching impulse: this is an impulse voltage having a time to peak of 250 s and a time to

half-value of 2500 s.

- lightning impulse: this is an impulse voltage having a front time of 1.2 s and a time to

half-value of 50 s.

consequences of overvoltages

Overvoltages in electrical networks cause equipment degradation, a drop in service continuity

and are a hazard to the safety of persons.

The consequences can be very varied depending on the type of overvoltages, their magnitude

and their duration. They are summed up as follows:

- breakdown in the insulating dielectric of equipment in the case where the overvoltageexceeds the specified withstand

- degradation of equipment through ageing, caused by non-destructive but repetitiveovervoltages

- loss of power supply caused by the destruction of network elements


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- disturbance of control, monitoring and communication circuits by conduction orelectromagnetic radiation

- electrodynamic stress (destruction or deformation of equipment) and thermal stress(elements melting, fire, explosion) essentially caused by lightning impulses

- hazard to man and animals following rises in potential and occurrence of step and touchvoltages.

5.1.2. Power frequency overvoltages

Power frequency overvoltages are generally caused by:

- an earth fault

- resonance or ferro-resonance

- neutral conductor breakdown

- a generator voltage regulator or transformer on-load tap changer fault

- overcompensation of reactive energy following a varmeter regulator fault

- load shedding, notably when the supply source is a generator

5.1.2.1. Overvoltage caused by an earth fault

Overvoltages caused by the occurrence of an earth fault greatly depend on the neutral

earthing system of the given network.

unearthed (MV or LV) or impedance earthed (MV) neutral

Figure 5-1 shows that on occurrence of a solid earth fault, the voltage between the neutral

point and earth becomes equal to the single-phase voltage:

V VNeutral n=

Vn : nominal single-phase voltage

For a fault on phase 1, V VNeutral = 1 .

The phase-earth voltage of healthy phases thus becomes equal to the phase-to-phase

voltage:

V V V V V E Neutral2 2 2 1= + =

V V V V V E Neutral3 3 3 1= + =

whence V V VE E n2 3 3= =


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2

3

1

V3

V2

V1

earth

fault VE2 VE3VE1VNeutral ZNeutral

V2 V3

V1

Neutral

VE2 VE3

VE1 0

V1 , V2 , V3 : phase-neutral voltages

VE1 , V E2 , VE3 : phase-earth voltages

ZNeutral : earthing impedance (ZNeutral = for an unearthed neutral)

Figure 5-1: overvoltage on an unearthed or impedance earthed network

on occurrence of a phase-to-earth fault

Note 1 : for an impedance earthed neutral, the value of ZNeutral

is much greater than the value of the

transformer and cable impedances and the fault resistance, which is why V VNeutral = 1 .

Note 2 : in overhead public distribution networks, there are highly resistive faults (several k), having avalue close to or higher than the earthing impedance. In this case, a highly resistive fault will

cause an overvoltage lower than 3 Vn .


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solidly earthed neutral (HV or MV)

On occurrence of an earth fault on one network phase, a high current is generated which

circulates in the circuit formed by the fault phase, earth and neutral earth electrode

(see fig. 5-2).

At the fault point, the three-phase voltage system is disturbed. The fault phase voltage in

relation to earth is almost zero if we neglect the fault resistance. The voltages of the other two

phases in relation to earth are higher than the single-phase voltage, while remaining lower

than the phase-to-phase voltage.

VE3

V3ZT

ZT

ZT

ZC

ZC

ZC

Rf

V2

V1

fault

Re

VE1 V E2

V1 , V2 , V3 : single-phase voltages

ZT

: transformer impedance

ZC : cable impedance

Re : neutral earth electrode resistance

Rf : fault resistance

Figure 5-2: equivalent diagram of a phase-earth fault when the neutral is solidly earthed

Thus, we can define an earth fault factor k characterising the phase-earth overvoltage

occurring on the healthy phases:

V V k V E E n2 3= =

Vn : nominal single-phase voltage

The symmetrical component calculation method (see 4.2.2. of the Protection guide) can be

used to determine the value of k in relation to the positive, negative and zero-sequence

impedances:

kZ a Z a Z

Z Z Z Rf=

+ +

+ + +1

3

2

2 0

2 0

(1) ( ) ( )

(1) ( ) ( )


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In most networks, generators are sufficiently far away to take the approximation Z Z(1) ( )= 2 ; we

thus have:

( )ka Z Z

Z Z Rf= +

+ +1

2 31 0

1 0

( ) ( )

( ) ( )

Nomographs can be used to determine factor k for a zero fault resistance ( Rf = 0 ) in relation

to the ratiosR

X

( )

( )

0

1

andX

X

( )

( )

0

1

for R( )1 0= and R X( ) ( ).1 10 5= (see fig. 5.3. et 5.4.).

where:

R( )1 : positive-sequence resistance seen from the fault point

X( )1 : positive-sequence reactance seen from the fault point

R( )0 : zero-sequence resistance seen from the fault point

X( )0 : zero-sequence reactance seen from the fault point

When the fault resistance is not zero, we can see in the formula expressing k that the

overvoltage is weaker. The calculation of the overvoltage with a zero fault resistance thus

provides an excess value.

If we again use the diagram in figure 5-2, we can determine these impedances for a practicalcase:

by taking:

Z R j X

Z R j X

Z R j X

Z R j X

T T T

C C C

T T T

C C C

= +

= +

= +

= +

positive- sequence impedances

zero- sequence impedances( ) ( )

( ) ( )

0 0

0 0


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we can determine:R R RT C( )1 = +

X X XT C( )1 = +

R R R Re T C( )0

3= + +

X X XT C( ) ( ) ( )0 0 0= +

Note: A factor 3 appears before Re . The reason for this is explained in figure 4-11 of the Industrial

network protection guide.

8

7

6

5

4

3

2

1

1 2 3 4 5 6 7

8

k = 1.7

k = 1.6

k = 1.5

k = 1.4

k = 1.3k = 1.2

R

X(1)

(0)

X

X(1)

(0)

Figure 5-3: earth fault factor in relation to ratiosX

X

( )

( )

0

1

andR

X

( )

( )

0

1

for R( )1 0= and Rf = 0

8

7

6

5

4

3

2

1

1 2 3 4 5 6 7

8

k = 1.7

k = 1.6k = 1.5

k = 1.4

k = 1.3

k = 1.2

X

k = 1.5

R

X(1)

(0)

X(1)

(0)

Figure 5-4: earth fault factor in relation to ratiosX

X

( )

( )

0

1

andR

X

( )

( )

0

1for R X( ) ( ).1 10 5= and Rf = 0


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example

Let us consider a YNyn, 33 kV/11 kV transformer with a power rating of S MVAn = 24 (see

IEC 909-2 table 3 A) supplying a network with 240 mm aluminium cables the longest outgoing

feeder of which is 5 km. The neutral earth electrode resistance is 0.5 .

- transformer characteristics:

Usc = 24 2. %

R

X

T

T

= 0 046.

X

X

T

T

( ).

00 7=

we can deduce( )

X UU

ST sc

n

n

= =

=

2 3

60 242

11 10

24 101 22. .

RT = 0 056.

X T( ) .0 0 85=

Note: the value of Usc is extremely high in relation to the transformers feeding a network with alimiting resistor earthed neutral. The transformer here is a United Kingdom transformer adaptedto the solidly earthed neutral system.

The short-circuit voltage has been chosen high on purpose so as to minimise the short-circuit

current. Indeed, if Usc is high, the valueR

X

( )

( )

0

1

is minimised ( )since X X XT C( )1 = + , which

decreases the overvoltage factor (see fig. 5-3 and 5-4).

- cable characteristics:

RL

SkmC = =

=

0 036 1000

2400 15

.. /

X kmC = 0 1. /

We assume that X X kmC C( ) . /0 3 0 3= = .

Note: the value of X C( )0 is highly variable (from 0.2 to 4 X( )1 ) depending on what the cable is made

of and the return via the earth (remote earth, screen or earthing conductor).


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For a solid fault (Rf = 0 ) at the transformer terminals:

R RT( ) .1 0 056= =

R R Re T( ) . . .0 3 3 0 5 0 056 1 56= + = + =

X XT( ) .1 1 22= =

X X T( ) ( ) .0 0 0 85= =

whence R X( ) ( ).1 10 05 0=

R

X

( )

( )

.0

1

1 28=

X

X

( )

( )

.0

1

0 70=

Figure 5-3 shows that k is between 1.4 and 1.5.

For a solid fault (Rf = 0 ) 5 km away from the transformer:

R R RT C( ) . . .1 0 056 0 15 5 0 81= + = + =

R R R Re T C( ) . . . .0 3 3 0 5 0 056 0 15 5 2 31= + + = + + =

X X XT C( ) . . .1 1 22 0 1 5 1 72= + = + =

X X XT C( ) ( ) ( ) . . .0 0 0 0 85 0 3 5 2 35= + = + =

whence R X( ) ( ).471 10=

R

X

( )

( )

.0

1

1 34=

X

X

( )

( )

.0

1

1 37=

Figure 5-4 shows that k is between 1.2 and 1.3.


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TN earthing system

The current of an earth fault circulates in the protective conductor. The neutral earth electrode

resistance is thus not used to determine the zero-sequence impedance

(see fig. 5-5).

V3

ZT

ZT

ZT

ZC

ZC

ZC

V2

V1V VM2

V V3

VMZPERe

V1 , V2 , V3 : single-phase voltages

ZT : transformer impedance

ZC : cable impedance

ZPE : protective conductor impedance

VM : potential of exposed conductive parts (masses) in relation to earth

Re : neutral earth electrode resistance

Figure 5-5: equivalent diagram of an earth fault in a TN earthing system

We are interested in the overvoltage of the healthy phases in relation to the exposed

conductive part, which determines whether or not an insulation fault may occur on the other

load: k

V V

V

V V

VMM

n

M

n=

=

2 3.


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For a transformer or a cable in low voltage, we can take the zero-sequence impedance to beapproximately equal to the positive-sequence impedance: Z ZT T( )0 = and Z ZC C( )0 = .

We thus have Z Z Z Z T C PE ( )0 3= + +

Z Z ZT C( )1 = +

whence( )

ka Z

Z Z Z

a Z

Z Z ZM

PE

T C PE

PE

PE T C

= + +

= + +

13

31 for a solid fault ( Rf = 0 )

a ej

=2

3

: rotation operator of 120

The overvoltage will be maximum when ZT is negligible compared with Z ZPE C+ , which is the

case for a long length cable.

Thus ka Z

Z ZM

PE

PE C

+

1

kM will be maximum when the protective conductor cross-sectional area is as small as

possible, i.e. equal to half the phase conductor cross-sectional area; thus R RPE C= 2 .

For an aluminium cable cross-sectional area smaller than 120 mm, the reactance can be

neglected compared with the resistance, which thus gives us:

Z

Z Z

R

R R

PE

PE C

PE

PE C+

+=

2

3since R RPE C= 2

whence k aM 12

3

k jM +

1

2

3

1

2

3

2

kM 1.45

We can show that for a cable with a large cross-sectional area (> 120 mm), the overvoltage

will be lower than in the case of a small cross-sectional area.


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TT earthing system (see fig. 5-6)

V3

ZT

ZT

ZT

ZC

ZC

ZC

V2

V1If

If

VN

load 1 load 2

RM1VM1 RM2Re

Re : substation earth electrode resistance

RM1 : load 1 and fault load earth electrode resistance

RM2 : load 2 earth electrode resistance

VM1 : load 1 and fault load phase-to-earth voltage

Figure 5-6: equivalent diagram of an earth fault in a TT earthing system

We want to know the overvoltage of the healthy phases in relation to the exposed conductive

part, which determines whether or not an insulation fault may occur on the other load:

kV V

V

V V

VM

M

n

M

n

=

=2 3

In low voltage, the neutral and load earth electrode resistances are very high in relation to the

transformer and cable impedance (ZT and ZC are roughly several tens of m).

We can thus write that the fault current is:

IV

R Rf

e M

=+

1

1

and ( )Z Z IT C f+ 0

The exposed conductive part of load 1 is connected to phase 1 by the fault (zero impedance).

The voltage of one healthy phase of this load in relation to the frame is V V2 1 or V V3 1

(since ( )Z Z IT C f+ 0 ) , whence kM = =3 1 73. .


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The exposed conductive part of load 2 is at the same potential as the remote earth.

The voltage of one healthy phase of this load in relation to the exposed conductive part istherefore V VNeutral2 or V VNeutral3 :

V V V R I V R V

R RV a V

R

R RV

a R

R RNeutral e f

e

e M

e

e M

e

e M2 2 2

12 2 2 1 = = +

= +

= +

whence ka R

R RM

e

e M

= +

1

for R R kM e M= =, 1 32.

for R R kM e M> 2

IrN : current in the neutral earthing resistor during the fault

IC : currents in the network phase-earth capacitances (see 4.3 ofProtection guide)

V3

V2

V1

C C C

Ph 3

Ph 2

Ph 1

If

CB

ZNrN

or

ZN : neutral earthing impedance (or rN )

C : phase-earth capacitance

If : fault current

CB : circuit-breaker

V V V1 2 3, , : single-phase voltages

Figure 5-32: phase-earth fault clearance

C

If

CB

ZNrN

or

Xnet

~

IrN

IC

Xnet : network reactance

C : fault phase earth capacitance

ZN or rN : neutral earthing impedance (or resistance rN )

If : fault current

Figure 5-33: fault circuit on occurrence of a phase-earth fault


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High resistance earthing with restrike in the

fault or circuit-breaker, case of industrial

networks for which I to ArN < 20 30(see Protection guide - 10.1.1.).

The overvoltage depends on the ratio II

rn

C

Limiting resistor earthing with restrike in the

fault or circuit-breaker, case of public

distribution networks for which IrN is equal

to several hundred to 1 000 A. The overvoltage

depends on the ratio rX

N

d

0.5 1 1.5 2 2.5

460

300

260240

200

100

healthy

phases

neutral

fault

phaseI

I

rN

C

%

transient voltage as

% of the nominal

single-phase voltage

peak value

3 6

250

150

100

50

healthy

phases

neutral

faultphase

200

4 5 8 10 20 30 50 70 90

%


% of the nominal


peak value

r

X

N

(1)

I V rrN n N = / : current in the neutral resistor duringthe fault

I C VC n= 3 : vectorial sum of current in the phase-earth capacitances

If I IrN C 2 , the overvoltage does not exceed 240 %

rN : neutral point earthing resistance

X( )1 : network positive-sequence reactance

Reactance earthing, case of public distribution networks for which IXN is equal to 1 000 to

several thousand amps

Case without restrike in the circuit-breaker Case with restrike in the circuit-breaker

0

100

theoretical limitswithoutdamping

200

300

400

2 4 6 8 10 12 14 16

C

N

A

B

%


% of the nominal


peak value

X

X

N

(1) 0

100

theoretical limitswithoutdamping

200

300

400

2 4 6 8 10 12 14 16

C

N

A

B

500

%


% of the nominal


peak value

X

X

N

(1)

X( )1 : network positive-sequence reactance

XN : neutral point earthing reactance

A : earth fault phase

B C, : healthy phases

N : voltage at reactance terminals

Figure 5-34: transient overvoltages depending on the type of neutral earthing

during a phase-earth fault


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Reactance earthing: (network reactance)

Voltage at circuit-breaker terminals Voltage at the terminals of the reactance

Resistance earthing: (network reactance)

Voltage at circuit-breaker terminals Voltage at the terminals of the resistor

: arc extinction voltage

Figure 5-35: transient voltage on circuit-breaker opening during a permanent phase-earth fault


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5.1.4. Atmospheric overvoltages

general

The earth and the electrosphere, the conductive area in the atmosphere (about 50 to 100 km

thick), constitute a natural spherical capacitor which is charged by ionization, thus producing

an electric field directed towards the ground of roughly several hundred volts/metre

Since air is not very conductive, there is thus an associated permanent conduction current, of

roughly 1 500 A for the entire earth's globe. Electrical balance is ensured during discharges by

rain and strokes of lightning.

The formation of storm clouds, masses of water in the form of aerosols, is accompanied by

charge separation electrostatic phenomena: the positively charged light particles are driven by

the rising air currents, and the negatively charged heavy particles fall because of their weight.

At the base of the cloud, there may also be islets of positive charges where heavy rains are

located.

On an overall macroscopic scale, a dipole is created.

When the breakdown withstand limit gradient is reached, a discharge is produced inside the

cloud or between clouds or between the cloud and the ground. In the latter case, it is referred

to as lightning.

The cloud-ground electric field can reach 15 to 20 kV/metre on flat ground. But the presence of

obstacles deforms and locally increases this field by a factor of 10 to 100 or even 1 000

depending on the form of the obstacles (also called the "peak effect"). The atmospheric air

ionizing threshold is thus reached, i.e. roughly 30 kV/cm, and corona effect discharges are

produced. When these discharges are located on fairly high objects (tower, chimney, pylon)

they may divert lightning to this objects.

classification and characteristics of strokes of lightning

Strokes of lightning are classed according to the origin of the discharge (or leader) and their

polarity.

Depending on the leader origin, the stroke of lightning may be:

- either descending from the clouds to the ground in the case of fairly flat land- or ascending from the ground to the clouds in the case of mountainous land.

Depending on the polarity the following distinctions between lightning strokes are made:

- negative when the negative part of the cloud is discharged, which represents 80 % of casesin temperate countries

- positive when the positive part of the cloud is discharged.


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form and magnitude of the lightning wave

The physical phenomenon of lightning corresponds to a source of impulse current the actual

form of which is highly variable: it consists of a front rising up to the maximum magnitude of

several miscroseconds to 20 s followed by a decreasing tail of several tens of s (see

figure 5-36).

Figure 5-36: oscillogram of a lightning current

The magnitude of strokes of lightning varies according to a log-normal distribution law. We can

thus determine the probability of a given magnitude being exceeded (see figure 5-37). We can

see, for example, that for the average curve (IEEE), the probability of exceeding a magnitude

of 100 kA is 5 %. This means that 95 % of lightning strokes have a magnitude less than

100 kA.

Figure 5-37: probability of exceeding positive and negative lightning stroke magnitudes,

according to IEEE (experimental statistic)


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Similarly, the steepness of the wave front varies according to a log-normal distribution law. Let us

determine the probability of exceeding a given front steepness (see fig. 5-38). We can see that the

probability of exceeding a front steepness of 50 kA/s of a negative stroke of lightning is 20 %.

Figure 5-38: probability of exceeding the front steepnesses of positive and negative

lightning currents according to IEEE (experimental statistic)

standard wave form

The lightning impulse wave form given by IEC 71-1 is a 1.2/50 s wave (see fig. 5-39):

- rise time to the maximum value of 1.2 s- time to half-value of 50 s.

Figure 5-39: standard lightning impulse voltage wave form (IEC 71-1)


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lightning density

On a world-wide scale, 63 billion discharges are recorded on average each year which

corresponds to 100 discharges per second. In France, this figure varies from 1.5 to 2 million

lightning strokes per year.

We then define the lightning density as being the number of days per year on which

thunder has been heard in a place.

In France, the average value of is 20 with a variation range going from 10 in channel

coastal regions up to over 30 in mountainous regions.

The value of may be much higher and reach 180 in tropical Africa or Indonesia.

lightning strike density

The lightning strike density represents the number of lightning strikes per km2 per year,

whatever their current value levels.

In France, varies between 2 and 6 lightning strikes/km2/year depending on the region.

lightning impact mechanism

The lightning impact mechanism begins with a leader from a cloud which approaches the

ground at a low speed. When the electric field is sufficient, sudden conduction is established

giving rise to the lightning discharge.

An experimental practical approach has enabled the relation linking the current of the

lightning strike to the distance between the starting point (leader position) and discharge point

(point of impact connected to the earth) to be found:

or according to the authors.

: striking distance, in m

: lightning current, in kA


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By applying an electro-geometrical model to a vertical rod with a height (see fig. 5-40-a),

we can show that there are two distinct zones:

- zone 1 : this is located between the ground and the parabola which is the locus of the

equidistant points of and the ground; the instant the flash occurs, any leaderlocated in this zone will touch the ground since it is nearer to this than to

- zone 2 : this is located above the parabola; the instant the flash occurs, any leader locatedin this zone will be picked up by point on the vertical rod as soon as thedistance between and the leader is less than the striking distance .

Figure 5-40-a: diagram of different protection zones offered by a vertical rod

For a lightning current with a value of , and thus a given striking distance, the distancex between

the point of impact on the ground and the point where the rod is fixed to the ground (called the rod

pick-up radius) is:

if

if

The rod pick-up radius is thus all the greater the more intensive the lightning stroke.

For very weak currents, the pick-up radius becomes less than the height of the rod which is then

able to pick up the current along its length. This has been experimentally proved.


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application to equipment protection using a lightning conductor

The lightning conductor diverts lightning to itself in order to protect equipment. Its principle is

based on the striking distance; tapered rods are placed at the top of equipment to be

protected, they are connected to the earth by the most direct path (the lightning conductorssurrounding the structure to be protected and interconnected with the earthing network).

The electrogeometric model allows the zone to be protected to be determined using the fictive

sphere method.

The point of impact of the lightning is determined by the object on the ground the closest to the

leader starting distance d. Everything happens as if the leader was surrounded by a fictive

sphere with a radius d moving with it. For good protection, the fictive sphere rolling on the

ground reaches the lightning conductor without touching the objects to be protected

(see fig. 5-40-b).

Protection against direct lightning strikes is approximately good in a cone the top of which is

the top of the lightning conductor and the half-angle at the top is 45 .

leader

d= critical striking distance

fictive

sphere

protectedzone

(cone)

lightningconductor

45

Figure 5-40-b: determining the zone protected by a lightning conductorusing the "fictive" sphere method


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direct lightning strike (on phase conductors)

When lightning strikes the phase conductor of a line, the current ( )i t is shared out in equal

quantities on either side of the point of impact and is spread along the conductors. These have

a wave impedance Z the value of which is between 300 and 500 . This impedance is thatseen by the wave front, is independent of the length of the line and of a different type from the

impedance at 50 Hz.

This results in a voltage wave of:

( )( )

U t Zi t

= .2

which spreads along the line (see fig. 5-41).

U i

t

i

U Zi

=2

Figure 5-41: lightning strike on a phase conductor


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Depending on the magnitude of the lightning current, two cases may occur:

full impulse propagation

If the maximum voltage U ZI

maxmax=

2

is below the sparkover voltage Ua of the insulator

string, the entire (full) wave spreads along the line.

chopped impulse propagation

In the case where U Uamax , as a first approximation, insulator sparkover occurs at the valueof Ua , and a phase-earth fault occurs at 50 Hz due to the arc being maintained. The lightning

that is propagated is thus broken at the maximum value corresponding to Ua .

The lightning current causing this flashover, for a given line, is called the critical current IC

such that:

IU

ZC

a= 2

For lines, the order of magnitude of IC is:

- 5.5 kA at 225 kV, which corresponds to a probability of exceeding the magnitude accordingto the IEEE method of 95 % (see figure 5-37)

- 8.5 kA at 400 kV, which corresponds to a probability of exceeding the magnitude accordingto the IEEE method of 92 % (see figure 5-37).

In medium voltage, flashover is systematic in the case of a stroke of lightning occurring due to

the small distances in the air of the insulator string. This flashover of the insulator gives rise to

a phase-earth fault current, called a follow current, which is held at the power frequency of 50

Hz until it is cleared by the protections.


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indirect lightning strikes (lightning protection rope or pylons)

When lightning strikes the line protection rope, part of the current flows through the pylon since

the protection rope is connected to it (see fig. 5-42).

This results in a potential rise at the top of the pylon the value of which depends on the self

inductance L of the pylon and the resistance R of the earth electrode:

( ) ( )( )

U t k R i t Ldi t

dt= +

k : ratio of the current shunted into the pylon by the incident current

i

Uk. i

L

R

k . i

lightning strike

protection rope

U k R I Ldi

dt= +

Figure 5-42: lightning strike on a protection rope

The voltage U may reach the impulse sparkover voltage of the insulators and cause a

breakdown. This is "back-flashover". Part of the current is then propagated along the affected

phase(s) towards the users. This current is in general greater than that of a direct lightning

strike.


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In extra high voltage (> 220 kV), back-flashover is unlikely (the flashover level of the insulators

is high), which is why it is useful to install protection ropes thus limiting the number of service

interruptions. But below 90 kV back-flashover occurs even if the value of the earth electrode

resistance is low (< 15 ); the usefulness of protection ropes is thus limited (more frequent

service interruptions).

induced impulse

A stroke of lightning that falls anywhere on the ground behaves like an electromagnetic field

radiation source.

The steeper the rising front of the lightning current the greater the radiation.

For front steepnesses of 50 to 100 kA/s, the effects of this field will be felt several hundreds

of metres, if not kilometres, away.

The magnetic field H at a point located at a distance of r from a circuit through which a

current Iflows, is given in the relation:

HI

r=

2

This field creates induced voltages in the neighbouring circuits which are able to reach

dangerous values both for equipment and persons.

case of a loop

Let us consider the loop formed by the supply cable and the telecommunication link in figure

5-43, with a surface S and located 100 m from the lightning impact which has a current rising

front steepness of 80 kA/s.


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The induced voltage is given in the relation:

ed

dtS

dB

dtS

dH

dt= = =

0

= 074 10 : magnetic permeability of the vacuum

nowdH

dt r

dI

dtA m s= =

=

1

2

1

2 10080

10

10127 10

3

66

/ /

whence e kV= =4 10 120 127 10 197 6

A phase-earth overvoltage of 19 kV thus occurs on the loop. This has a very short duration(

1 s ) but can cause insulation breakdown.

To avoid this risk, the surfaces of the loops must be reduced.

telecommunication link computer

supply cable

printer

circuit

loop

surface = 120 m

earth

lightning impulse

phase-earth insulation

subjected to 19 kV ( 1 s)

100 m

front steepness = 80 kA/s

magnetic field

Figure 5-43: circuit loop


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impulse wave transference in a transformer(see IEC 71-2 - appendix A)

In lightning impulse conditions, the transformer behaves like a capacitive divider with a ratio of

s 0.4 . It is equivalent to a capacitance Ct (see figure 5-44-a).

lightning wave

U1

U sU0 1

Ctequivalent

U sU0 1

U1 : impulse voltage on the high voltage terminal

U0 : no-load voltage transferred

Figure 5-44-a: impulse wave transference in a transformer

U0 represents the no-load overvoltage, i.e. when the secondary outgoing terminals are not

connected to any cables or lines. This overvoltage is generally not acceptable by the

transformer.

In reality, the transformer is connected to a network with a capacitiance Cs

. This plays the

role of a voltage divider with the transformer capacitance Ct (see fig. 5-44-b).

U sU0 1=

Ct

Cs U2

U2 : voltage transferred to the secondary with a network

Figure 5-44-b: transformer with its equivalent network

The voltage transferred to the secondary is thus:

UC

C Cs Ut

t s2 1= +


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The values of Ct are generally between 1 and 10 nF. The capacitance of a cable is close to

0.4 nF/m. Thus, several tens of metres of cable will greatly reduce the overvoltage transferred

to the secondary.

In general, the network is sufficiently widespread for the overvoltage transferred not to raiseany difficulties.

However, in the case of a short cable, e.g. between a specific transformer and a load (arc

furnace, etc.), the overvoltage transferred may be unacceptable for the equipment on the low

voltage side.

To reduce the magnitude of the impulse transferred, it is possible to:

- use a surge arrester with a lower sparkover voltage on the high voltage side

- install a surge arrester on the low voltage side between each phase and earth

- increase the capacitance between each phase and earth on the low voltage side.

5.1.5. Propagation of overvoltages

Overhead lines and cables constitute a propagation media for any overvoltage wave likely to

occur on a network.

For high frequencies (case of switching and lightning overvoltages), the line is characterised by

its so-called "characteristic" or "wave" impedance:

ZL

Cc

L : line inductance

C : line capacitance

We can see that this impedance is independent of the length of the line.

The speed of the wave propagation on an overhead line is close to the speed of light:

c = 3 108 m/s (300 m/s)

for cables, it is equal to vc

r

=

r : relative permittivity of the cable insulating material


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The value of v is close to 150 m/s.

This gives us an idea of the way a lightning wave spreads along a conductor. Figure 5-45

shows how a lightning wave spreads along an overhead line in relation to time and space.

t (to x constant)

V front : 200 kV / s

2 s

400 kV

developmentin time

x ( to t constant)

V front : 0.66 kV / m

600 m = 300 x 2 s

400 kV

spread in

space

Figure 5-45: diagram showing how a lightning wave spreads along an overhead line

in relation to time and space

Let us closely examine the phenomenon that is produced at a point M, where a change of

impedance exists, separating two circuits with characterstic impedances of Z1 and Z2

(see fig. 5-46).

i1'

MZ1 Z2

i1

v1 v2

i2

v1'

Z Z1 2, : upstream and downstream characteristic impedances

v i1 1, : incident wave upstream of M

v i2 2, : wave transferred downstream of M

v i1 1' ', : wave reflected upstream of M

Figure 5-46: propagation of a wave at a change of impedance point M


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Upstream of M , we have:

v Z i1 1 1= and v Z i1 1 1' '= (1)

immediately downstream of M :

v Z i2 2 2= (2)

at point M :

v v v2 1 1= +' and i i i2 1 1= +

' (3)

We can thus deduce:

( )v v v v Z i v Z i i2 1 1 1 1 1 1 1 2 1= + = = ' '

whencev

Z

Z Zv2

2

2 112= +

In particular:

- for a line short-circuited to earth, Z2 0= ; we can deduce from this that v2 0= and v v1 1' =

- for a conductor without a change of impedance, Z Z2 1= ; we can deduce from this that

v v2 1= and v1 0' =

- for an open line, Z2 = ; we can deduce from this that v v2 12= and v v1 1' = .

To conclude, at the point of change of impedance, the maximum voltage value may reach

double the incident wave. This is the case of a line feeding a transformer as its impedance in

relation to the lightning wave is very high in relation to the characteristic impedance of the line.


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5.2. Overvoltage protection devices

5.2.1. Principle of protectionThe protection of installations and persons against overvoltages is greatly improved when

disturbances flow to earth, and this is done as close as possible to the sources of disturbance.

This requires low impedance earth electrodes to be implemented.

Thus, three overvoltage protection levels can be distinguished:

1st protection level

The objective is to avoid a direct impact on structures by catching the lightning and directing it

towards designated flow points, via:

- lightning conductors, whose principle is based on the striking distance; a rod placed at thetop of a structure to be protected captures the lightning and evacuates it through theearthing network (see fig. 5-40-b)

- meshed or Faraday cages

- lightning protection ropes (see fig. 5-42).

2nd protection level

Its aim is to ensure that the basic impulse level (BIL) of the substation components has not

been exceeded.

In HV, this type of protection is established using elements ensuring that the lightning wave

flows to earth, such as:

- spark-gaps- HV surge arresters.

3rd protection level

Used in LV as an extra protection for sensitive equipment (computers, telecommunication

devices, etc.).

It uses:

- series filters- overvoltage limiters- LV surge arresters.


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5.2.2. Spark-gaps

operation

The spark-gap is a simple device made up of two electrodes, the first connected to the

conductor to be protected and the second connected to earth.

At the place where it is installed in the network, the spark-gap constitutes a weak point where

overvoltages can flow to earth and thus protects the equipment.

The sparkover voltage of the spark-gap is set by adjusting the distance in the air between the

electrodes so as to obtain a margin between the impulse withstand of the equipment to be

protected and the impulse sparkover voltage of the spark-gap (see fig. 5-47). For example,

B = 40 mm on French public EDF 20 kV networks.

bird proof rod

earth electrode phase electrode

45

electrode

holder

B

rigid

anchoring chain

device for adjusting B

and locking the electrode

45

Figure 5-47: MV spark-gap with birdproof rod


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advantages

The main advantages of spark-gaps:

- their low price- their simplicity- the possibility of setting the sparkover voltage.

drawbacks

- The sparkover characteristics of the spark-gap are highly variable (up to 40 %) dependingon the atmospheric conditions (temperature, humidity, pressure) which modify the ionizationof the dielectric medium (air) between the electrodes.

- the sparkover level depends on the overvoltage.

- spark-gap sparkover causes a power frequency phase-to-earth short circuit owing to the arcbeing maintained. The short circuit lasts until it is cleared by the switching devices (thisshort circuit is called a follow current). This means that it is necessary to install shunt circuit-breakers or rapid reclosing system on the circuit-breaker located upstream. Because of this,the spark-gaps are unsuitable for the protection of an installation against switchingovervoltages.

- the sparkover caused by a steep front overvoltage is not instantaneous. Due to this delay,the voltage actually reached in the network is higher than the chosen protection level. Totake this phenomenon into account, it is necessary to study the voltage-time curves of the

spark-gap.

- sparkover causes the appearance of a steep front broken wave which could damage thewindings of the transformers or motors located nearby.

Although still used in certain public networks, spark-gaps are currently being replaced by surge

arresters.


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5.2.3. Surge arresters

To overcome the drawbacks of spark-gaps, different models of surge arresters have been

designed with the aim of ensuring better protection of installations and good continuity of

service.

Non-linear resistor type gapped surge arresters are especially found in HV and MV

installations which have been in operation for several years. The current tendency is to use

zinc oxide surge arresters which provide better performance.

definitions

Surge arrester discharge current

The surge or impulse current which flows through the arrester after a sparkover of the series

gaps.

Surge arrester follow current

The current from the connected power source which flows through an arrester following the

passage of discharge current.

Surge arrester residual voltage

The voltage that appears between the terminals of an arrester during the passage of discharge

current.

5.2.3.1. Non-linear resistor type gapped surge arresters (see IEC 99-1)

operating principle

In this type of surge arrester, a variable resistor (varistor), which limits the current after the

passage of the impulse wave, is associated with a spark gap.

After evacuation of the impulse wave to earth, the surge arrester is only subjected to the

network voltage and the follow current is limited by the varistor.

The arc is systematically extinguished after the 50 Hz wave of the single-phase-to-earth fault

current has reached zero.

Owing to the variation of the resistance, the residual voltage is maintained close to the

sparkover level. Indeed, this resistance decreases with the increase in current.

Various techniques have been used to make non-linear resistor type gapped arresters. The

most conventional method uses a silicon carbide (SiC) resistor.

Some surge arresters also have voltage grading systems (resistive or capacitive dividers) and

arc blowing systems (magnets or blow-out coils).


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characteristics

Variable resistor type surge arresters are characterised by:

- the rated voltage, which is the maximum specified value of the power frequency rms voltagepermitted between its terminals for which the surge arrester is designed to functioncorrectly. This voltage can be continuously applied to the surge arrester without thismodifying its operating characteristics.

- the sparkover voltages for the different wave forms (power frequency, switching impulse,lightning impulse, etc.).

- the impulse current evacuation capacity.

5.2.3.2. Zinc oxide (ZnO ) surge arresters

operating principle

Figure 5-48 shows that, unlike the non-linear resistor type gapped surge arrester, the zinc

oxide surge arrester is only made up of a highly non-linear variable resistor.

The resistance goes from 1.5 M at the duty voltage (which corresponds to a leakage currentbelow 10 mA) to 15 during discharge.

Following the passage of the discharge current, the voltage at the terminals of the surgearrester become equal to the network voltage. The current which flows through the surge

arrester is very weak and is stabilised around the value of the earth leakage current.

Because of the high non-linearity of the ZnO surge arrester a high current variation causes a

low voltage variation (see fig. 5-49).

For example, when the current is multiplied by 107, the voltage is only multiplied by 1.8.


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connecting spindle

flange

(aluminium alloy)

exhaust pipe and

overpressure device

in the upper and

lower flanges

fault indication

plate

exhaust pipe

flange

ring clamping

device

overpressure device

prestressed ti ghtness

device

rubber seal

compression spring

porcelain enclosure

thermal shield

spacer

washer

rivet

elastic stirrup

blocksZn O

Figure 5-48: example of the structure of a ZnO surge arrester in a porcelain enclosure

for 20 kV networks

600500

400

300

200

100

.001 .01 .1 1 10 100 10000

peak kV

I

U

1000

SiC

linear

Z On

SiC : non-linear resistor type gapped surge arrester made up of a silicon carbide resistorZnO : zinc oxide surge arrester

linear : U curve proportional to I

Figure 5-49: characteristics of two surge arresters having the same 550 kV/10 kA protection level


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characteristics

ZnO surge arresters are characterised by:

- the steady-state voltage which is the permitted specified value of the power frequency rmsvoltage that can be continuously applied between the terminals of the surge arrester

- the rated voltage which is the maximum power frequency rms voltage permitted between itsterminals for which the surge arrester is designed to operate correctly in the temporaryovervoltage conditions defined in the operating tests (a power frequency overvoltage of 10seconds is applied to the surge arrester - see IEC 99-4)

- the protection level defined at random as being the residual voltage of the surge arresterwhen it is subjected to a given current impulse (5,10 or 20 kA according to the class), with awave form of 8/20 s

- steep front current impulse (1 s), lightning impulse (8/20 s), long duration impulse, andswitching impulse withstand

- nominal discharge current.

Table 5-4 gives an example of the characteristics of a phase-to-earth ZnO surge arrester for

a 20 kV public distribution network (with tripping on occurrence of the first fault).

Maximum steady-state voltage (phase-earth) 12.7 kV

Rated voltage 24 kV

Residual voltage for nominal discharge current < 75 kV

Nominal discharge current (8/20 s wave) 5 kA

Impulse current withstand (4/10 s wave) 65 kA

Table 5-4: example of the characteristics of a ZnO surge arrester for a 20 kV network


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choice of zinc oxide surge arresters in HV

The general method for choosing a zinc oxide surge arrester in HV consists in determining its

characteristic parameters using the network data, at the place where it will be installed.

The parameters characterising the surge arrester are:

- UC , steady-state voltage

- Ur , rated voltage

- Ind , nominal discharge current

- discharge class and energy capacity

- mechanical characteristics.

The data relative to the network are:

- Um , highest phase-to-phase voltage applied to equipment

- TOV temporary overvoltages (appearing on occurrence of an earth fault or load sheddingon the public distribution network).

The choice of the surge arrester involves making a compromise between the equipment

protection levels and the energy capacity of the surge arrester.

The protection level must be as low as possible for the equipment withstand. This involves the

lowest voltage rating possible and thus greater difficulty withstanding temporary overvoltages.

determining UC and Ur

simplified method using equipment characteristics

The voltages UC and Ur may be directly determined using the highest voltage for the

equipment Um :

UU

Cm3

U Ur C= 1 25.


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more accurate method using temporary overvoltages

The simplified method has a drawback as it does not take into account the real requirements

of the network which are generally lower thanUm

3

.

The temporary overvoltages likely to occur in a network are of two types:

- overvoltages due to a phase-earth fault the clearance time of which depends on theprotection system (see table 5.1 - the earth overvoltage factor is equal to 1.73 for unearthedor impedance earthed networks)

- overvoltage due to load-shedding on the public distribution network, of the order of 15 %but able to reach 35 % in some networks.

The temporary overvoltage value to be taken into account is the product of the earth faultovervoltage and load shedding factors.

- specific case

If one of the temporary overvoltages lasts over 2 hours, it is considered to be a steady-state

condition for the surge arrester and thus UC is chosen to be equal to this overvoltage and

U Ur C= 1 25.

- general case

A surge arrester's capacity to withstand temporary overvoltages is given in relation to an

equivalent voltage lasting 10 seconds ( )U s10 expressed in the following equation:

U TOV T

s1010

=

where 0 02.

T : overvoltage duration

TOV : overvoltage value

This formula allows the 10 second overvoltage which would cause the same stress on the

surge arrester to be calculated for each temporary overvoltage.

The duration of the temporary overvoltage must be between several seconds and two to threehours (U TOV s10 0 97= . for T s= 2 and U TOV s10 114= . for T hours= 2 ).

The rated voltage of the surge arrester will be chosen to be above or equal to the maximum

value of the equivalent 10 second voltages: ( )U Ur s max 10 .

We will take UU

Cm3


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nominal discharge current Ind

In practice, for the voltage range 1 52kV U kV m , two values of Ind are available: 5 kA and

10 kA.

The value I kAnd = 10 is chosen for areas with a high lightning density.

discharge class and energy capacity

These are determined by testing or comparison with identical projects.

mechanical characteristics

The IEC 99-4 and 99-5 standards fix the allowable pressure limit (expressed in "kA") which

must be met for the three-phase short circuit at the surge arrester terminals.

The surge arrester characteristics will also be checked in relation to:

- the ambient temperature

- the altitude

- the level of pollution

- the mechanical resistance to the wind, seismic stress, frost.

surge arrester protection level

The protection level of the surge arrester at the installation point corresponds to the residual

voltage ( )Ursd at its terminals when its nominal discharge current flows through it.


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5.2.3.3. Installation of HV and MV surge arresters

In HV and MV electrical networks, surge arresters are installed at the entrance to the

substation to ensure protection of the substation transformer and equipment. This protection

only works if the protection distance and the installation rules are respected.

protection distance

The wave propagation phenomenon studied in 5.1.5. shows that at the point of reflection

(e.g. MV/LV transformer), the overvoltage reaches double the value of the incident wave.

The surge arrester peaks at a sparkover voltage Ursd (equal to the residual voltage for ZnO

surge arresters).

If it is located a considerable distance away, the maximum voltage at the location of theequipment to be protected will thus be 2 Ursd . Now, the equipment impulse withstand is

generally lower than 2 Ursd .

To overcome this drawback, the surge arrester is installed at a shorter distance away than the

"protection" distance D . The surge arrester then undergoes the sum of the incident wave and

the reflected wave. It is thus sparked for an incident wave below Ursd .

Assuming that at the equipment termination point, the wave is totally reflected, we can show

that the overvoltage in relation to the equipment is limited to U U rD

vrsd= + 2

rdV

dt= : rise front steepness of the voltage wave, kV/s

v : wave propagation speed, m/s

For a lightning impulse withstand voltage Ul , the surge arrester must therefore be located at

a distance D such that:

U rD

v

Ursd l + 2

whence DU U

rvl rsd

2


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Numerical application:

Let us consider the example illustrated in figure 5-50:

U kVl = 125 , case of an MV/LV transformer complying with IEC 76.3

U kVrsd = 75 , residual voltage of the surge arrester

r kV s= 300 / , voltage wave rise front steepness

v m s= 300 / , for an overhead line (speed of light)

we then have D

125 75

2 300300

D m 25

The surge arrester must therefore be installed less than 25 m away from the transformer for

the overvoltage not to exceed the lightning impulse withstand value.

lightning impulse

overhead line

surge arrester

transformer

A BD

ZCZC

Figure 5-50: protection distance of a surge arrester protecting a transformer fed

by an overhead line


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5.2.4. protection of LV installations

general

LV installations are protected against overvoltages by installing devices in parallel; 3 types of

devices are used:

- overvoltage limiters located on the secondary of MV/LV transformers (only in an ITearthing system); they only provide protection against power frequency overvoltages

- low voltage surge arresters installed in LV switchboards or incorporated in loads

- surge diverters designed to protect telephone networks, LV terminal boxes and loads.

The main technologies used are:

- zener diodes

- gas discharge tubes

- zinc oxide varistors.

Zener diodes have the drawback of only ensuring the protection of a precise point in the

network. The gas discharge tube requires the addition of a varistor to prevent follow current.

Variable resistor-type surge arresters are currently the most cost-effective solution owing to

their simplicity and reliability


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LV surge arrester installation rules

The equipment is only protected properly if certain installation rules are followed:

- rule 1The length of the connection between the surge arrester and its disconnecting circuit-breaker

must be below 0.5 m.

disconnectingcircuit-breaker

L < 50 cm load to beprotected

Figure 5-51: diagram of connections

- rule 2

The outgoing feeders of the protected conductors must be connected to the terminals of the

surge arrester and its disconnecting circuit-breaker.

- rule 3

The loop surfaces must be reduced by tightly grouping together the incoming, phase, neutral

and PE wires.

- rule 4

the incoming wires of the surge arrester (polluted) must be moved away from the protected

outgoing wires (healthy) in order to avoid any possible electromagnetic coupling.

- rule 5

The cables must be flattened against the metal structures of the box in order to reduce frame

loops and thus benefit from a reduction in disturbances.


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connection layout according to the earthing system

In figures 5-52-a and 5-52-b the connection layouts of the LV surge arrester are shown for

different earthing systems.

main earthterminal

(entrenched

loop)

load earthelectrode

electrical switchboard

disconnecting

circuit-breaker

equipment

to be protected

PE

LV neutral

earth electrode

PE

surge arrester

RCD

Ph1

Ph2

Ph3

N

TT earthing system


disconnecting

circuit-breaker

equipment

to be protected

PE

LV neutralearth electrode

main earthterminal

(entrenched

loop)

loadearth

electrode

PE

overvoltagelimiter

PIM

surge

arrester

Ph1

Ph2

Ph3

N

IT earthing system

Figure 5-52-a: connection layout of an LV surge arrester for TT and IT earthing systems


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main earth

terminal(entrenched

loop)

load earth

electrode


equipment to

be protected

surge arrester

LV neutral

earth electrode

PEN

disconnecting

circuit-breaker

Ph1Ph2

Ph3

PEN

TNC earthing system

main earth

terminal(entrenched

loop)

load earth

electrode


disconnecting

circuit-breaker

equipment to

be protected

PE

LV neutral

earth electrode

PE

surge arrester

PE

Ph1

Ph2

Ph3N

TNS earthing system

Figure 5-52-b: connection layout of an LV surge arrester

for TNC and TNS earthing systems


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5.3. Insulation co-ordination in an industrial electrical network

5.3.1. General

Co-ordinating the insulation of an installation consists in determining the insulationcharacteristics necessary for the various network elements, in view to obtaining a withstand

level that matches the normal voltages, as well as the different overvoltages.

Its ultimate purpose is to provide dependable and optimised energy distribution.

Optimal insulation co-ordination gives the best cost-effective ratio between the different

parameters depending on it:

- cost of equipment insulation

- cost of overvoltage protections

- cost of failures (loss of operation and destruction of equipment), taking into account theirprobability of occurrence.

With the cost of overinsulating equipment being very high, the insulation cannot be rated to

withstand the stress of all the overvoltages studied in paragraph 5.1.

Overcoming the damaging effects of overvoltages supposes an initial approach which consists

in dealing with the phenomena that generate them, which is not always very easy. Indeed, ifusing the appropriate arc interruption techniques the switchgear switching overvoltages can be

limited, it is impossible to prevent lightning strikes.

clearance (see fig. 5-53)

This term covers two notions:

- gas clearance (air, SF6, etc.), which is the shortest path between two conductive parts.

- creepage distance: this is also the shortest path between two conductors, but following theouter surface of a solid insulating material (e.g. insulator).


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The clearance is directly related to the withstand of the equipment to different overvoltages.

creepage

distancedistance

in air

distance

in air

Figure 5-53: air clearance and creepage distance

overvoltage withstand

The overvoltage withstand depends on the type of overvoltage applied (magnitude, wave form,

frequency and duration, etc.).

It is also influenced by external factors such as:

- ageing

- environmental conditions (humidity, pollution)

- variation in air or insulating gas pressure.

withstand voltage

Electrical equipment is characterised by its withstand voltage to different types of overvoltages.

We can thus distinguish:

- the power frequency withstand voltage

- the switching impulse withstand voltage

- the lightning impulse withstand voltage.


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power frequency withstand voltage

This corresponds to the equipment withstand to power frequency overvoltages likely to occur

on the network and the duration of which depends on the network operating and protection

mode.

The equipment withstand is tested by applying a sinusoidal voltage with a frequency of

between 48 Hz and 62 Hz for one minute. The test is valid for nominal network frequencies of

50 Hz and 60 Hz (see IEC 71-1).

switching impulse withstand voltage

This characterises the equipment withstand to switching impulses (only for equipment with a

standard voltage above or equal to 300 kV).

The equipment test (see IEC 60-1) is performed by applying a wave with a front time of 250 s

and a time to half-value of 2500 s.

lightning impulse withstand voltage

This characterises the equipment withstand to the 1.2 s / 50 s lightning voltage wave.

This withstand voltage concerns all voltage ranges, including low voltage.

examples of equipment withstand (see table 5-5)

Highest voltage for the

equipment

Um (kV)(1)

(r.m.s. value)

Standard short-duration

power frequency withstand

voltage (kV)

(r.m.s.)

Standard lightning impulse

withstand voltage (kV)

(peak value)

3.6 10 2040

7.2 20 4060

12 28 6075

9517.5 38 75

95

24 50 95125145

36 70 145170

52 95 250

72.5 140 325

(1) Um is the highest rms value of the phase-to-phase voltage for which the equipment is specified .


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Table 5-5: standard withstand voltages for 3.6 kV < Um < 72.5 kV


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5.3.2. Reduction in risks and overvoltage levels

The risks of overvoltages, and consequently the danger they represent for persons and

equipment, can be greatly reduced if certain measures of protection are taken:

- limiting substation earth electrode resistances in order to reduce power frequencyovervoltages

- reducing switching overvoltages by choosing suitable switchgear (interruption in SF6)

- making lightning impulses flow to earth by a first clipping operation (surge arrester or spark-gap at the entrance to the substation) with limitation of the earth electrode resistances andpylon impedances

- limiting the residual voltage from the first clipping operation by HV surge arrester which istransferred to the downstream network by providing a second protection level on the

transformer secondary

- protection of sensitive equipment in LV (computers, telecommunications, automaticsystems, etc.) by connecting series filters and/or overvoltage limiters to it.

5.3.2.1. Rise in potential of LV exposed conductive parts following an MV fault in the

transformer substation

In this paragraph, we propose to study overvoltages in LV caused by an earth fault on the MV

side in an MV/LV substation, and the measures to be taken in order to protect equipment and

persons, in compliance with IEC 364-4-442.

The values of rises in potential of the substation or LV installation exposed conductive parts

depend on the values of the earth electrode resistances, the fault current values and the

earthing system.

earthing in transformer substations

A single earth electrode must be installed in a transformer substation, to which must be

05 Over Voltages and Insulation Coordination

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