ect151_overvoltage coordination by MohamedSheba


									                                       n° 151
                                       and insulation
                                       coordination in
                                       MV and HV
D. Fulchiron

Having graduated from the Ecole
Supérieure d’Electricité in 1980, he
joined Merlin Gerin in 1981 working
in the High Power Testing Station
(VOLTA) until 1987.
He then joined the technical
department of the Medium Voltage
Division in which he is currently
project manager. His involvement in
secondary distribution equipment
studies has led him to examine
insulation coordination more

The author would like to thank:
Florence Bouchet
A student at the Supelec Institute
who, as trainee, helped to produce
this document.
Jean Pasteau
Member of the Technical
Management who provided his
competence as expert contributing
to the revision of standard IEC 71.

E/CT 151, first issued February 1995
Cahier Technique Merlin Gerin n° 151 / p.2
overvoltages and insulation coordination                                             Insulation coordination is a discipline
                                                                                     aiming at achieving the best possible
in MV and HV                                                                         technico-economic compromise for
                                                                                     protection of persons and equipment
                                                                                     against overvoltages, whether caused
                                                                                     by the network or lightning, occurring
                                                                                     on electrical installations.
                                                                                     It helps ensure a high degree of
                                                                                     availability of electrical power.
contents                                                                             Its value is doubled by the fact that it
                                                                                     concerns high voltage networks. To
                                                                                     control insulation coordination:
1. Overvoltages                          Power frequency overvoltages        p. 4    s the level of the possible overvoltages
                                         Switching overvoltages              p. 5    occurring on the network must be
                                         Lightning overvoltages              p. 8
                                                                                     s the right protective devices must be
2. Insulation coordination               Definition                          p. 11
                                                                                     used when necessary;
                                         Clearance and voltage withstand     p. 11
                                                                                     s the correct overvoltage withstand
                                         Withstand voltage                   p. 12   level must be chosen for the various
                                         Insulation coordination principle   p. 13   network components from among the
3. Overvoltage protective devices        Dischargers                         p. 14   insulating voltages satisfying the
                                         Surge arresters                     p. 14   particular constraints.
4. Standards and insulation              HV insulation coordination          p. 17   The purpose of this "Cahier Technique"
coordination                             as in IEC 71                                is to further knowledge of voltage
5. Coordination applied to               Breakdown consequences              p. 20   disturbances, how they can be limited
electrical installation design                                                       and the standards to ensure safe,
                                         Reduction of overvoltage            p. 20
                                                                                     optimised distribution of electrical
                                         risks and level
                                                                                     power by means of insulation
6. Conclusion                                                                p. 21   coordination. It deals mainly with MV
Appendix 1: propagation of overvoltage                                       p. 21   and HV.
Appendix 2: installing a surge arrester Maximum safety clearance             p. 22
                                        Cabling the surge arresters          p. 22
Appendix 3: electricity standards                                            p. 23
Appendix 4: bibliography                                                     p. 24

                                                                                     Cahier Technique Merlin Gerin n° 151 / p.3
1. overvoltages

These are disturbances superimposed          cable, earthing of an overhead                                                Xo
on circuit rated voltage. They may           conductor by branches, equipment                               where k =
                                             fault, ...), the phase in question is                                         Xd
s between different phases or circuits.      placed at earth potential and the                              Xd is the direct reactance of the
They are said to be differential mode;       remaining two phases are then                                  network seen from the fault point, and
s between live conductors and the            subjected, with respect to earth, to the                       Xo the zero sequence reactance.
frame or earth. They are said to be          phase-to-phase voltage                                         Note that:
common mode.                                 U = V        3.                                                s if the neutral is completely unearthed,

Their varied and random nature makes         More precisely, when an insulation fault                       Xo = ∞: Sd = 31/2 = 3 ;
them hard to characterise, allowing only     occurs on phase A, an earth fault                              s if the neutral is completely earthed,
a statistical approach to their duration,    factor, Sd, is defined by the ratio of the                     Xo = Xd: Sd = 1;
amplitudes and effects. The table in         voltage of phases B and C with respect                         s if, as in the general case,
figure 1 presents the main                   to earth, to network phase to neutral                          Xo ≤ 3 Xd: Sd ≤ 1.25.
characteristics of these disturbances.       voltage.
In point of fact, the main risks are         The following equation is used to                              Overvoltage on a long off-load line
                                             calculate Sd:                                                  (Ferranti effect)
malfunctions, destruction of the
equipment and, consequently, lack of                                                                        An overvoltage may occur when a long
                                                        3 (k 2 + k + 1)                                     line is energised at one of its ends and
continuity of service. These effects may     Sd =
occur on the installations of both                          k + 2                                           not connected at the other. This is due
energy distributors and users.
Disturbances may result in:
s short disconnections (automatic            overvoltage                     MV-HV over             term                steepness            damping
                                             type                            voltage                                    of frequency
reclosing on MV public distribution
                                             (cause)                         coefficient                                front
networks by overhead lines);
s long disconnections (intervention for      at power frequency              ≤    3                 long                power                low
changing damaged insulators or even          (insulation fault)                                     >1s                 frequency
replacement of equipment).                   switching                       2 to 4                 short               medium               medium
Protective devices limit these risks.        (short-circuit                                         1 ms                1 to 200 kHz
Their use calls for careful drawing up of    disconnection)
consistent insulation and protection         atmospheric                     >4                     very short          very high            high
levels. For this, prior understanding of     (direct lightning stroke)                              1 to 10 µs          1,000 kV/µs
the various types of overvoltages is
vital: such is the purpose of this           fig. 1: characteristics of the various overvoltage types.
power frequency                                                                                                    B
overvoltages                                                                                                                         VCT
                                                                                      earth fault                             VBT
This term includes all overvoltages with
frequencies under 500 Hz.                                                                                              VAT
Reminder: the most common network
frequencies are: 50, 60 and 400 Hz.
                                                        VB                              VC
Overvoltage caused by an insulation
                                                                         N                                         the phase-earth voltage of fault-free
fault (see fig. 2)
                                                                                                                   phase is raised to the value of the
An overvoltage due to an insulation                                          VA                                    phase-to-phase voltage:
fault occurs on a three-phase network
when the neutral is unearthed or                                                                                   VBT =     3 VBN
impedance-earthed.                                                                                                 VCT =     3 VCN
In actual fact, when an insulation fault
                                             fig. 2: temporary overvoltage on an unearthed neutral network in presence of an insulation
occurs between a phase and the frame
or earth (a damaged underground

Cahier Technique Merlin Gerin n° 151 / p.4
to resonance which takes the form of a       parallel-connected on C. However,                 switching overvoltages
voltage wave increasing in linear            more generally, the powers involved               Sudden changes in electrical network
fashion along the line.                      are fairly low (1/2 C V2 with low C) and          structure give rise to transient
In point of fact, where:                     only likely to damage fragile equipment.          phenomena frequently resulting in the
s L and C refer to line inductance and       It is up to the equipment designer to             creation of an overvoltage or of a high
total capacity respectively;                 evaluate and limit this risk.                     frequency wave train of aperiodic or
s Us and Ue are the voltages at the                                                            oscillating type with rapid damping.
open end and at line entrance, the
                                             Ferromagnetic resonance, depending                Normal load switching overvoltage
overvoltage factor equals:
                                             on variable L, may occur for a wide               A «normal» load is mainly resistive, i.e. its
Us             1                             frequency band.                                   power factor is greater than 0.7. In this
                     2                       A similar demonstration can be made
Ue                                                                                             case, breaking or making of load currents
         1 −                                 for parallel resonance.                           does not present a major problem. The
                 2                           A load connected to the circuit acts as           overvoltage factor (transient voltage
This overvoltage factor is around 1.05       a reducing resistance and prevents                amplitude/operating voltage ratio) varies
for a 300 km line and 1.16 for a 500 km      maintenance of resonance conditions.              between 1.2 and 1.5.
line. These values are more or less the
same for HV and EHV lines.
This phenomenon is particularly
common when a long line is suddenly
Overvoltage by ferromagnetic
resonance                                                                                                                 i
In this case the overvoltage is the result           e                                                                   Cω
of a special resonance which occurs
when a circuit contains both a capacitor
                                                                               C                              i
(voluntary or stray) and an inductance
with saturable magnetic circuit (e.g. a
transformer). This resonance occurs                                                                               e
particularly when an operation (circuit
opening or closing) is performed on the
network with a device having poles
either separate or with non-
simultaneous operation.                                         diagram                                  vectorial representation
The circuit shown in the diagram in
figure 3, with connected in series a
saturable core inductance, L, and the
network capacitance, C, makes it
easier to understand the phenomenon.
The following three curves can then be                                                                                    i
                                                                                                                  Uc =
drawn: Uc = f(i), UL = f(i) and                                                                                          Cω
(UL - 1 / C ω i) = f(i);
s the first one is a straight sloping line
1 / C ω;                                                                                                           U L= L ω i
s the second one presents a saturation
s and the third one displays two
operating points (O and B) for which
voltage at the terminals of the                                                                                             -Lωi
LC assembly is zero, and two other                                                                                       Cω
stable operating points, M and P; N is
an unstable point of balance.
The voltages at the terminals                         E
of L and C (point P) are high. Move                   e
from M to P may be due only to a                                   M                       N             P
transient temporarily raising voltage e
to a value greater than E.
                                                      O                                         B                               i
These overvoltages (see the diagram in
figure 3) present a risk of dielectric       fig. 3: ferromagnetic resonance principle..
breakdown and a danger for any loads

                                                                                               Cahier Technique Merlin Gerin n° 151 / p.5
Overvoltages caused by making and            The energy trapped in the circuit varies              If C2 is only made up of stray
breaking of small inductive currents         according to the type of impedances                   capacitances with respect to frames,
This type of overvoltage is caused by        involved, mainly resistive and inductive.             the value of V may present a risk for
three phenomena:                             Small inductive currents (see fig. 4)                 equipment insulation (circuit-breaker or
current pinch-off, rearcing and              present a load with a high inductance                 load).
prearcing.                                   which, when the arc is extinguished,                  The generator circuit has an equivalent
                                             will have an energy given by:                         behaviour, but its inductance is
The diagram in figure 4 shows a
                                              1                                                    generally much smaller and the
network supplying a load through a               L2 I2 .                                           voltages occurring at the terminals of
circuit-breaker. It contains:                 2                                                    C1 are thus far lower.
s a sinusoidal voltage source with an        The L2 C2 circuit is now in the slightly
                                                                                                   s rearcing
inductance, L1 and a capacitance, C1,        damped, free oscillation state, and the               This occurs when the pinching-off
s a circuit-breaker, D, which cannot be      peak value V of the voltage occurring                 phenomenon described above causes
dissociated from its stray elements,         at the terminals of C2 is approximated                an input-output overvoltage to occur at
Lp1 and Cp1,                                 by the energy conservation                            the terminals of the circuit-breaker
s an inductive load, L2, the distributed     hypothesis:                                           unable to be withstood by the latter: an
capacitance of which cannot be                1             1                                      arc then occurs. This simplified
overlooked, symbolised by a                      L2 I2 =      C2 V 2 .
                                              2            2                                       explanation is complicated by the
capacitor, C2,
s finally, a line inductance, L0,
generally negligible.
                                                        first parallel oscillation loop
s current pinch-off                                              L1
The arc occurring on breaking of low                                                                   D
currents, in particular less than circuit-
breaker rated current, takes up little
space. It undergoes considerable
cooling due to the circuit-breaker’s
                                                                         C1                                                      C2          L2
capacity to break far higher currents.
It thus becomes unstable and its                                                             Cp1
voltage may present high relative
variations, whereas its absolute value                                                                  L0
remains far below network voltage                  second oscillation
(case of breaking in SF6 or vacuum).               loop

These e.m.f. variations may generate         fig. 4: equivalent circuit for the study of overvoltages caused by inductive current breaking
oscillating currents (see fig. 4) of high    where:
frequency in the adjacent capacitances,      Cp1: circuit-breaker capacitance,
both stray and voluntary. The amplitude      Lp1: circuit-breaker inductance.
of these currents can become non-
negligible with a 50 Hz current and
reach 10 % of its value.                                                                      current in
Superimposition of the 50 Hz current
and of this high frequency current in the
circuit-breaker will result in the current
moving to zero several times around                    "pinched-off"
the zero of the fundamental wave                       current
(see fig. 5).
The circuit-breaker, little affected by                             possible
these low currents, is often capable of                             extinguishing
breaking at the first current zero
occurring. At this moment, the currents
in the generator and load circuits are
not zero. The instantaneous value, i, of                                                                        50 Hz wave
the 50 Hz wave on arc extinguishing is
                                             fig. 5: superimposition of a high frequency oscillating current on a power frequency current.
known as the «pinched-off current».

Cahier Technique Merlin Gerin n° 151 / p.6
presence of the stray elements               oscillation of existing parallel circuits   oscillation is mainly centered around
presented above.                             (surging discharge of stray                 the 50 Hz wave peak value. The
In actual fact, following current breaking   capacitances) and reflections on            maximum voltage value observed is
and rearcing, three oscillating              impedance failures, and hence in the        then around twice the 50 Hz wave peak
phenomena occur simultaneously at            appearance of high frequency currents,      value.
                                             with respect to 50 Hz, through the arc.
the respective frequencies Fp1, Fp2 and                                                  In the case of faster operating devices,
Fm:                                          If device operation is slow compared        arcing does not systematically occur
s in the loop                                with this phenomenon, the arcing
                                                                                         around the peak value: the overvoltage,
D - Lp1 - Cp1:                               current may be made to move through
                                                                                         if any, is thus lower.
                                             zero by superimposition of the high
                 1                           frequency current and the incipient         If a capacitor bank is put back into
Fp1 =                                        50 Hz current.                              operation very soon after it has been
         2 π      Lp1 Cp1                                                                disconnected from the network, its
                                             Extinguishing of the arc, according to
of around a few MHz.                         equipment characteristics, will then        residual load voltage is between zero
s in the loop                                result in a behaviour similar to that       and the 50 Hz wave peak voltage.
D - C1 - Lo - C2:                            described for the phenomena above.          Arcing between contacts occurs around
                                             However, since dielectric withstand         a peak of opposite polarity (breakdown
          1    C1 + C2                       between contacts decreases with             under a stress twice peak voltage).
Fp2 =                                        closing, the successive overvoltages        The oscillation described above occurs
         2π    Lo C1 C2
                                             decrease right up to complete closing.      with a double initial pulse. The
of around 100 to 500 kHz.                    This phenomenon is extremely                maximum voltage value observed may
s throughout the circuit,                    complex. The resulting overvoltages         then be close to three times the 50 Hz
                                             depend, among other factors, on:            peak voltage.
         1           L1 + L 2
Fm =                                         s circuit-breaker characteristics           For safety reasons, capacitor banks are
        2π     L1 L 2 5 (C1 + C2 )           (dielectric properties, capacity to break   always fitted with discharging resistors
                                             high frequency currents, ...),              able to eliminate residual voltages with
of around 5 to 20 kHz.                       s characteristic cable impedance,           time constants of around one minute.
Multiple rearcing then occurs                s load circuit natural frequencies.         Consequently, an overvoltage factor
(chopping) until it is stopped by            Overvoltages, extremely hard to             of 3 corresponds to very specific cases.
increasing contact clearance. This           calculate, cannot generally be
rearcing is characterised by high                                                        s energising of off-load lines or cables
                                             predetermined since they involve            Slow closing of a device on this type of
frequency wave trains of increasing          uncalculable elements which vary from
amplitude. These overvoltage trains                                                      load causes, in this case also, arcing
                                             site to site. They also require a
upstream and downstream from the                                                         around the 50 Hz peak: the voltage
                                             sophisticated mathematical model of
circuit-breaker can thus present a           the arc chute.                              step applied to one end of the line or
considerable risk for equipment                                                          cable will spread and be reflected on
containing windings.                         Prearcing overvoltages particularly
                                                                                         the open end (see appendix 1).
                                             affect, in HV and MV, off-load
This phenomenon must not be                  transformers on energising and motors       Superimposition of the incident step
confused with «reignition» which is the      on starting (see Merlin Gerin "Cahier       and the reflected step results in a
reappearance of a power frequency            Technique" n° 143).                         voltage stress twice the applied step,
current wave and thus a breaking                                                         give or take the dampings, and
failure on the current wave zero.            Overvoltage caused by switching on
                                             capacitive circuits                         assuming that the 50 Hz can be likened
s prearcing                                                                              to DC for these phenomena.
                                             Capacitive circuits are defined as
When a device closes (switch,
                                             circuits made up of capacitor banks,
contactor or circuit-breaker), there is a
                                             and off-load lines.
moment when dielectric withstand                                                                         L
between contacts is less than applied        s energising of capacitor banks
voltage. In the case of rapidly closing      When capacitor banks are energised,
devices, with respect to 50 Hz,              normally without initial load, and in the
behaviour depends on the phase angle         case of slow operating devices, arcing      e                                            C
during operation.                            occurs between the contacts around
                                             the 50 Hz wave peak.
An arc is then created between the
contacts, and the circuit witnesses a        Damped oscillation of the LC system in
voltage pulse due to the sudden              figure 6 then occurs. The frequency of      fig. 6: schematic diagram showing a
cancellation of voltage at the device        this oscillation is generally far higher    capacitor operating circuit.
terminals. This pulse may result in          than power frequency, and voltage

                                                                                         Cahier Technique Merlin Gerin n° 151 / p.7
As this type of behaviour is related to        Note that the rising front of lightning            U then reaches values of several
the distributed inductances and                strokes chosen by standards is 1.2 µs              million volts, which no line can
capacitances of the conductors                 for voltage and 8 µs for current.                  withstand. At a point in the line, for
considered, overhead lines present, in         A distinction is often made between:               example at the first pylon the wave
addition, a phase-to-phase coupling            s «direct» lightning strokes striking a            meets, voltage increases until
making modelling relatively complex.           line;                                              clearance breakdown occurs (insulator
This reflection phenomenon must be             s «indirect» lightning strokes, falling            string). According to whether or not
taken into consideration particularly in       next to a line, on a pylon or, which               arcing has occurred (depending on the
(EHV) transmission lines, as a result of       comes to the same, on the earth cable              value of the current injected into the
the small relative difference between          (this cable, earthed, connects the tops            line), the wave which continues to
operating voltage and insulating               of pylons and protects live conductors             propagate after the pylon is said to be
voltage.                                       from direct lightning strokes).                    broken or full.
s capacitive circuit breaking                  Direct lightning strokes                           For various network voltages, arcing
Breaking of capacitive circuits normally       This results in the injection of a current         does not occur below the critical current
presents few difficulties. In point of fact,   wave of several dozens of kA in the line.          indicated by the straight line in figure 10.
as capacitances remain charged at the          This current wave, which may cause                 For networks with a voltage less than
50 Hz wave peak value, after the arc is        conductors to melt by propa-gating on              400 kV, virtually all direct lightning
extinguished at current zero, voltage is       either side of the point of impact (see            strokes result in arcing and an earth
resumed at the equipment terminals at          fig. 9) results in an increase in voltage U        fault.
50 Hz with no transients. However, one         given by the formula:
                                                                                                  In actual fact, it is estimated that only
alternation after breaking, the device is                   i                                     3 % of overvoltages, observed on the
subjected to an input output voltage            U = Zc
                                                           2                                      French 20 kV MV public network,
twice peak voltage.                            where i is the injected current and                exceed 70 kV and are thus ascribable
If it is unable to withstand this stress       Zc the characteristic line zero sequence           to direct lightning strokes. Moreover, as
(e.g. opening not yet sufficient),             impedance (300 to 1,000 ohms).                     a result of attenuation of the voltage
reignition may occur. This is followed,
provided the circuit so allows (single-
phase or connected neutral circuit) by
voltage inversion at capacitor terminals,                                                                           +5 Vc
raising them to a maximum load of
three times peak voltage (see fig. 7).
The current breaks yet again and a
new reignition may take place with a
value five times peak voltage at the
next alternation.
Such behaviour may result in
considerable escalation and must be
avoided by choosing equipment which
prevents reignition.
                                                                    20 ms                         1/2 T
                                                    V                                                                                U, I
lightning overvoltages                                                                 Vc
A storm is a natural phenomenon well
known to all, and which is both
spectacular and dangerous.
On average 1,000 storms break out
each day throughout the world.
In France, (see fig. 8), they cause each                I
year 10 % of fires, the death of
40 people and 20,000 animals and
50,000 electricity or phone cuts.
Overhead networks are those most
affected by lightning overvoltages and                                         breaking
Lightning strokes are characterised by                                                                      -3 Vc
their polarisation: they are generally
negative (negative cloud and positive          fig. 7: voltage escalation on separation of a capacitor bank from the network by a slow
ground). Roughly 10 % have reversed            operating device.
polarity, but these are the most violent.

Cahier Technique Merlin Gerin n° 151 / p.8
                                                                                                                                wave throughout its propagation along
                                                                                                                                the line, maximum overvoltages (very
                                                                    10                                                          rare) at the entrance of a substation or
                                                                                                                                building are estimated at 150 kV
                                                                                                                                in MV.
                                      5                                       20
                                          10                                                                                    It should be remembered that the
                                                          15                                                                    highest impulse withstand of 24 kV
                                  5            10
                                                                                                                    25          equipment is 125 kV.
                                                                                                                                Indirect lightning strokes
                                                          10                                                                    When indirect strokes fall on a support
                                                                                                                                or even simply next to a line, high
                                                               15                                                               overvoltages are generated in the
                           10                                                                                                   network.
                       5                                                                                       30
                                                                                                                                Indirect strokes, more frequent than
                                                                                                                                direct ones, may prove almost as
                                                                    25                                                          s if lightning falls on the pylon or the
                                      10                                                                   30                   earth cable, the current flowing off
                                                                                                                                causes an increase in metal frame
                                                           30                                                                   potential with respect to earth
                                                                                                                                (see fig. 11). The corresponding
                                                                                                                                overvoltage U may reach several
                                                                                                          30                    hundreds of kV.
                                                                                                                                            i     L di
                                                                                                                                U = R         +
                                                                          20                15            25        25                      2     2 dt
                                                                                                                                where R is the earth connection steep
                                                                                                                                wave resistance and L is the
                                               25                                                                               inductance of the pylon and/or the
                                                                                                                                earthing conductor.

    5   10       15   20   25     30 days
                                                                                                                                          i       L di
fig. 8: isokeraunic levels in continental France (graduated in annual mean number of stormy                                     U = R         +
days).                                                                                                                                   2        2 dt
Source: Météorologie Nationale.                                                                                                                                                i

                                                                                                                                                              U      i/2
U = Zc i/2                                                      likelihood (%)
                                                                                       225 kV
                                  U             i                                                                               L
                                                                     90                          400 kV

                                                    i/2              70
                                                                                                          750 kV
                                                                     50                                     1,100 kV

                                                                                                                     1,500 kV
                                                                          3        5         10       20 30     50
                                                                                        lightning stroke strenght (kA)

                                                                fig. 10: statistical distribution of the strength               fig. 11: when lightning falls on the earth
fig. 9: when lightning strikes directly, the                    of direct lightning stokes and minimum                          cable, current evacuation causes an
current wave propagates on either side of                       arcing strengths as a function of network                       increase in the potential of the pylon metal
the point of impact.                                            voltage level.                                                  frame with respect to earth.

                                                                                                                                Cahier Technique Merlin Gerin n° 151 / p.9
When this voltage reaches the arcing          on LV lines, induced overvoltages are              For example, in the minutes preceding
voltage of an insulator, an «arcing           generally less than 7 kV.                          a lightning stroke, when a cloud
return» occurs between the metal              A statistical observation, retained by             charged at a certain potential is placed
structure and one or more of the live         the French electrotechnical committee,             above a line, this line takes on a
conductors.                                   revealed that 91 % of overvoltages                 charge of opposite direction (see
In the case of network voltages greater       occurring at LV consumers did not                  diagram in figure 13).
than 150 kV, this arcing return is            exceed 4 kV and 98 % did not exceed                Before the lightning strikes, thus
unlikely. The quality of pylon earth          6 kV (see fig. 12). This accounts, for             discharging the cloud, an electric field,
connections plays an important role.          example, for the connection circuit-               E, thus exists between the line and the
From 750 kV onwards, there is virtually       breaker manufacturing standard which               ground which can reach 30 kV/m.
no more risk of arcing return, thus           stipulates an impulse withstand of 8 kV.           Under the effect of this field, the line/
                                                                                                 earth capacitor is charged to a potential
justifying the installation of earth cables   Electrostatic overvoltages
                                                                                                 of around 150 to 500 kV according to
on EHV lines. Below 90 kV, these              Other types of atmospheric discharges
                                                                                                 how high the line is from the ground.
cables only provide efficient protection      exist. Indeed, although the majority of
if the pylon earth connection is              induced overvoltages are                           Unenergetic breakdown may then
excellent.                                    electromagnetic in origin, some are                occur in the least well insulated
                                              electrostatic and concern in particular            components of the network.
s if lightning falls next to the line, the
energy flowing off to the ground causes       unearthed networks.                                When arcing occurs between the cloud
a very fast variation of the                                                                     and the earth, since the electric field
electromagnetic field. The waves                                                                 has disappeared, the capacitances
induced on the line are similar in shape                                number of                discharge.
and amplitude to those obtained by                                      overvoltages
direct lightning. They are mainly                                 30
characterised by their very steep front                           25
(around one micro-second) and their                               20
very fast damping (whether or not                                 15                                       +            +       +
aperiodic) (typical characteristics of                                                                 -            -               -
these waves as in standard IEC 60:
front time: 1.2 µs and tail time: ≈ 50 µs).
s when the voltage wave resulting from
a lightning stroke passes through a
                                              -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 kV
                                                                 atmospheric overvoltage
                                                                                                   +                +                   +
MV / LV transformer, transmission                                levels
mainly occurs by capacitive coupling.
The amplitude of the overvoltage thus         fig. 12: statistical distribution of atmospheric
                                              overvoltage amplitude drawn up from two                          E
transmitted, observed on the secondary
                                              observation campaigns (183 between 1973
winding on the LV side, is less than
                                              and 1974, and 150 in 1975), hence
10 % of its value on the MV side
                                              duplication of curves.                             fig. 13: origin of an electrostatic overvoltage.
(generally less than 70 kV). Therefore,

Cahier Technique Merlin Gerin n° 151 / p.10
2. insulation coordination

The first electrical networks (Grenoble-    reminder will be given of the definitions             clearance and voltage
Jarrie 1883) were technologically           of clearance and withstand voltage.
extremely rudimentary and at the mercy                                                            withstand
of atmospheric conditions such as wind                                                            Clearance
and rain:                                                                                         This term covers two notions: «gas
s wind, by causing inter-conductor
                                            overvoltage      overvoltage                          clearance (air, SF6, etc...) and
clearance gaps to vary, was                 factor           types                                «creepage distance» of solid insulators
responsible for arcing;
                                                                                                  (see fig. 15):
s rain encouraged current leaks to
                                                                                                  s gas clearance is the shortest path
These problems resulted in:                                                                       between two conductive parts;
                                                                                                  s creepage distance is also the
s use of insulators;
s determination of clearances;
                                                                                                  shortest path between two conductors,
s earthing of metal frames of devices.                >4     electrostatic                        but following the outer surface of a solid
                                                                                                  insulator (this is known as creepage).
                                                                                                  These two clearances are directly
definition                                              4                                         related to the concern with overvoltage
The purpose of insulation coordination                                                            protection, but do not have identical
is to determine the necessary and                                                                 withstand.
sufficient insulation characteristics of            2 to 4   switching
the various network components in                                                                 Voltage withstand
order to obtain uniform withstand to                                                              This varies in particular according to
normal voltages and to overvoltages of                                                            the type of overvoltage applied
                                                     ≤ 3     power frequency
                                                                                                  (voltage level, rising front, frequency,
various origins (see fig. 14). Its final
objective is to ensure safe, optimised                                                            time....).
distribution of electrical power.                       1
                                                                                                  Moreover, creepage distances may be
By «optimised» is meant finding the                                                               subjected to ageing phenomena,
best possible economic balance                                                                    specific to the insulating material in
between the various parameters              fig. 14: various voltage levels present on            question, causing deterioration of their
depending on this coordination:             MV-HV networks.                                       characteristics.
s cost of insulation;
s cost of protective devices;
s cost of failures (operating loss and
repairs) in view of their probability.
The first step towards removing the                                                                    clearance
detrimental effects of overvoltages is to
confront the phenomena generating
them: a task which is not always
simple. Indeed, although equipment                creepage
switching overvoltages can be limited             distance
by means of suitable techniques, it is                                            air clearance
impossible to have any effect on
It is thus necessary to locate the point
of least withstand through which the
current generated by the overvoltage
will flow, and to equip all the other
network elements with a higher level of
dielectric withstand.
Before presenting the various technical
                                            fig. 15: air clearance and creepage distance.
solutions (methods and equipment), a

                                                                                                  Cahier Technique Merlin Gerin n° 151 / p.11
The main influencing factors are:              s nonlinearity, mentioned above, in the
s environmental conditions (humidity,          clearance/voltage relationship;                                                                50Hz
pollution, UV radiation);                      s dispersion, which means withstand                  breakdown
s permanent electrical stresses (local         must be expressed in statistical terms;              voltage
                                               s unbalance (withstand varies                        (peak kV)                     2 mm
value of electric field).
Gas clearance withstand also depends           according to whether wave polarity is
on pressure:                                   positive or negative);                       100
s variation of air pressure with altitude;     s passage through a minimum curve                                                   SF6
s variation of device filling pressure.        value of the withstand voltage as a           80
                                               function of front time. When the gap
                                               between electrodes increases, this            60
withstand voltage                              minimum value moves to increasingly                                                 air
In gases, insulation withstand voltage is      higher front times (see fig. 17). On          40
a highly nonlinear function of clearance.      average it is around 250 µs which
For example, in air, a root mean square        accounts for the choice of standard test      20
voltage stress of 300 kV/m is                  voltage rising front (standard tests as in
acceptable under 1 m, but can be               IEC 60: application of a wave of front           0
reduced to 200 kV/m between 1 and                                                                   1    2      3         4       5     6    7
                                               time 250 µs and half-amplitude time
4 m and to 150 kV/m between 4 and              2,500 µs).                                                                     absolute pressure (bar)
8 m. It should also be pointed out that                                                     fig. 16: SF6 and air breakdown voltage as a
                                               Lightning overvoltage withstand
this clearance is practically unaffected                                                    function of absolute pressure.
by rain.                                       In the case of lightning, withstand is
                                               characterised by far greater linearity
This macroscopic behaviour is due to           than for the other stress types.              withstand voltage U50 (MV)
the lack of uniformity of the electric field
between electrodes of all shapes and           Dispersion is present in this case also
not to intrinsic gas characteristics. It       with a positive polarity withstand (the      4
would not be observed between flat             «+» applied to the most pointed
electrodes of «infinite» size (uniform         electrode) inferior to that of negative      3
field).                                        polarity.                                                                                   13 m
                                                                                            2                                               7m
Creepage distances of busbar                   The two following simple formulas                                                            4m
supports, transformer bushings and             enable withstand to a 1.2 µs/50 µs
                                                                                            1                                              2m
insulator strings are determined to            positive polarity impulse of an air gap to
obtain a withstand similar to direct air       be evaluated for EHV and MV networks:
clearance between two end electrodes                        d                                   1         10          100         1,000     front
                                               s U50 =
when they are dry and clean. On the                        1.9                                                                              time (µs)
other hand, rain and especially wet            U50 = voltage for which breakdown                        locus of minimums
pollution considerably reduce their            likelihood is 50 %;                          fig. 17: line showing minimum withstand
withstand voltage.                                         d                                values as a function of front time of impulse
                                               s Uo =                                       applied in positive polarity.
Power frequency withstand                                 2.1
In normal operating conditions, network        Uo = withstand voltage
voltage may present short duration             where d is clearance in metres                U50
power frequency overvoltages (a                U50 and Uo are in MV.                          kV
fraction of a second to a few hours:
                                               A large number of experimental studies                   d=8m
depending on network protection and
                                               have made it possible to draw up             4,000
operating mode). Voltage withstand
                                               precise correspondence tables between
checked by the standard one-minute
                                               clearance and withstand voltage,                         6m
dielectric tests is normally sufficient.                                                    3,000
                                               taking into account a variety of factors
Determination of this category of
characteristics is simple, and the             such as front and tail times,
                                               environmental pollution and insulator                    4m
various insulators are easy to compare.                                                     2,000
For example, figure 16 provides a              type.
comparison of voltage withstands               To give an example, figure 18 shows
                                               the variations in voltage U50 as a           1,000
between air and SF6 as a function of                                                             10                 100            1,000       T2 µs.
pressure.                                      function of clearance and tail time T2 for
Switching impulse voltage withstand            a positive peak-plane interval.              fig. 18: U50 as a function of time T2
Clearances subjected to switching              Moreover, table T in figure 19 shows         decreasing at half-amplitude.
                                               that withstand voltage does not depend       Intervale between the positive peak and the
impulses have the four following main
                                                                                            plane: d = 4 - 6 - 8 m.
properties:                                    on rising front time.

Cahier Technique Merlin Gerin n° 151 / p.12
Tcr             7              22               overvoltage     overvoltage types           protection level            withstand level
(µs)                                            factor
T2              1,400          1,500                                                                  overvoltages
(µs)                                                                                                  cleared           MV equipment
U50             2,304          2,227                            electrostatic                                           insulation
σ               370            217
                                                       2 to 4   switching                  surge arresters
fig. 19: influences of time up to the peak on
dielectric withstand of a positive peak-plane                   power frequency
interval = d = 8 m.                                     ≤ 3
insulation coordination
principle                                       fig. 20: insulation coordination: correctly protection level and equipment withstand as a function
                                                of probable overvoltages.
Study of insulation coordination of an
electrical installation is thus the               environment;
                                                s                                                   As regards lightning, a compromise
definition, based on the possible               s equipment use.                                    must generally be found between
voltage and overvoltage levels on this                                                              insulation level, protection level of
installation, of one or more overvoltage        Study of these «conditions» determines
                                                the overvoltage level to which the                  arresters, if any, and acceptable failure
protection levels. Installation equipment                                                           risk.
                                                equipment could be subjected during
and protective devices are thus chosen
                                                use. Choice of the right insulation level           Proper control of the protection levels
accordingly (see fig. 20).
                                                will ensure that, at least as far as power          provided by surge limiters requires
Protection level is determined by the           frequency and switching impulses are                thorough knowledge of their
following conditions:                           concerned, this level will never be                 characteristics and behaviour: this is
s installation;                                 overshot.                                           the purpose of the following chapter.

                                                                                                    Cahier Technique Merlin Gerin n° 151 / p.13
3. overvoltage protective devices

Dischargers and surge arresters are                                                               Moreover, after arcing, ionisation
the devices used to clip and limit high                                                           between the electrodes maintains the
amplitude transient overvoltages. They                                      birdproof rod
                                                                                                  arc which is then supplied by network
are normally designed so that they can         earth electrode                  phase electrode   voltage and may give rise (according
deal with lightning overvoltages.                                                                 to neutral earthing) to a power
                                                                                                  frequency retaining current. This
dischargers                                                       45°           45°               current is a full earth fault and requires
                                                                                                  intervention of the protective devices
Used in MV and HV, they are placed in
                                                                                                  placed at the front of the line (e.g.
particularly exposed network points and
                                                                                                  rapid reclosing circuit-breaker or shunt
at the entrance to MV/LV substations.
Their function is to create a weak point
controlled in network insulation so that      electrode                     B                     Finally, arcing causes the appearance
any arcing will systematically occur just     holder                                              of a steep front broken wave which
there.                                                                                            could damage the windings (transfor-
The first and oldest protective device is                                                   Ø     mers and motors) placed nearby.
the point discharger. It consisted of two                                                         Although still used in networks,
points facing each other, known as            rigid anchoring                                     dischargers are today increasingly
electrodes, one connected to the              chain                                               replaced by surge arresters.
conductor to be protected and the other       device for adjusting B and
to the earth.                                 locking the electrode                               surge arresters
The most common current models use
                                              fig. 21: a MV discharger with birdproof rod         Arresters have the advantage of having
the same principle but contain two
                                              e.g.on EDF 24 kV networks, B ≈ 25 mm.               no retaining current and of preventing
«horns» to elongate the arc, simplify
restoration of dielectric qualities by
deionising the arcing gap and, in
certain cases, to extinguish the arc.           kv                                                    point discharger e = 350 mm
In addition, some models are fitted with         900                                                  tests performed in 1.2 / 50 wave
a rod, between these two electrodes,                                                                    arcing points
designed to prevent untimely «short-             800
circuiting» by birds (see fig. 21) and                                                                theoretical voltage curve line:
their electrocution.
The gap between the two electrodes
enables adjustment of protection level.                                                               before              after
                                                 600                                                  arcing              arcing
Although this device is simple, fairly
efficient and economical, it has many
drawbacks:                                       500
s arcing voltage is considerably                                                                      voltage/time characteristic
dispersed and depends to a large                 400
extent on atmospheric conditions:
variations of more than 40 % have                300
been observed;
s arcing level also depends on
overvoltage amplitude (see fig. 22);             200
s arcing delay increases as overvoltage
decreases.                                       100
In these conditions, an impulse voltage
may cause arcing of a device with a
withstand voltage greater than that of                 0    0.5         1       1.5    2    2.5   3     3.5     4                             µs
the discharger, for the simple reason
                                              fig. 22: behaviour of a point discharger in standard lightning impulse, as a function of peak
that this device has a smaller arcing
delay (e.g. cables).

Cahier Technique Merlin Gerin n° 151 / p.14
the network from being short-circuited      s its impulse current evacuation                   Absence of air gap means that ZnO
and then de-energised after arcing.         capacity, i.e. its energy dissipation              arresters are permanently conductive,
A variety of models have been               capacity. Absorption capacity is                   but under protected network rated
designed: water stream arrester, gas        generally given by withstand to                    voltage, have a very small earth
arrester… Only the most common types        rectangular current waves.                         leakage current (less than 10 mA).
are presented in the paragraphs below.      Zinc oxide (ZnO) arresters                         Their operating principle is very simple,
These arresters are used on HV and          Made up only of varistors, they are                based on the highly nonlinear
MV networks.                                increasingly replacing nonlinear                   characteristic of ZnO varistors.
Nonlinear resistance arresters and          resistance arresters and air gap                   This nonlinearity is such that resistance
air gap protectors                          protectors (see fig. 23).                          decreases from 1.5 M Ω to 15 Ω
This arrester type connects in series air
gap protectors and nonlinear
resistances (varistors) able to limit           connecting spindle
current on occurrence of a surge.
Once the discharging current wave has
flown off, the arrester is only subjected                                                                             flange
to network voltage. This voltage                                                                                      (aluminium alloy)
maintains an arc on the air gap
protector, but the corresponding
current, known as the «retaining                                                                                      elastic stirrup
current» flows through the resistance
whose value is now high. It is thus                                                                                   rivet
sufficiently low not to damage the air          exhaust pipe and
gap protector and to be cleared when            overpressure device
                                                in the upper and                                                      ZnO blocks
the current moves to zero for the first
                                                lower flanges
time (the arc is naturally extinguished).
Nonlinearity of resistances maintains a                                                                               washer
residual voltage which appears at the
terminals of the device, close to arcing
level, since resistance decreases as            fault indication
current increases.                              plate
A variety of techniques have been
used to produce varistor arresters and                                                                                spacer
air gap protectors. The most classical
kind uses a silicon carbide (SiC)
resistance.                                                                                                           thermal shield
                                                exhaust pipe
Some arresters also contain voltage
distribution systems (resistive or                                                                                    porcelain enclosure
capacitive dividers) and arc blowing
systems (magnets or coils for magnetic                                                                                compression spring
This type of arrester is characterised          flange
                                                                                                                      rubber seal
s its extinction voltage or rated
voltage, which is the highest power
frequency voltage under which the                                                                                     prestressed tightness
arrester can be spontaneously de-
energised. It must be greater than the
highest short duration power frequency
overvoltage which could occur on the                                                                                  overpressure device
network;                                        ring clamping
s its arcing voltages according to wave         device
shape (power frequency, switching
                                            fig. 23: example of the structure of a ZnO arrester in a porcelain enclosure for the EDF 20 kV
impulse, lightning impulse....); they are
statistically defined;

                                                                                               Cahier Technique Merlin Gerin n° 151 / p.15
between operating voltage and voltage                s the protection level, defined arbitrarily    fact, humidity is the main cause of
at rated discharging current                         as the residual voltage of the arrester        failure identified on the ZnO arresters.
(see fig. 24).                                       subjected to a given current impulse           The outside of these arresters is
The advantage of these arresters is                  (5, 10 or 20 kA according to class),           generally made up of silicon polymer
their increased limitation and reliability           8/20 µs wave;                                  providing environmental resistance and
compared with silicon carbide arresters.             s rated discharging current;                   reconstitution of sufficient creepage
                                                     s impulse current withstand.                   distances. Their internal composition
Improvements have been made in
                                                     (this refers to the need for withstand to      and silicon enclosures mean that these
recent years, in particular in the thermal
                                                     long waves causing considerable                arresters can be placed in far more
and electrical stability field of ZnO
                                                     energy dissipation and not to the need         positions with optimisation of imple-
pellets on ageing.
                                                     to flow off such currents in operation).       mentation (e.g.: horizontal mounting).
Thus in 1989 only two failures were
observed on 15,000 surge arresters of                Enclosure                                      In addition to EDF specifications such
this type installed by EDF after eighteen            Zinc oxide arresters are available:            as HN 65S20 / IEC 99-1, a variety of
months’ experimentation. No changes                  s in porcelain enclosures for nearly all       French standards apply to arresters,
were noted in the characteristics,                   operating voltages;                            e.g. the NF C 65-100 for HV installation
checked by tests.                                    s in synthetic enclosures (glass fibre         devices.
                                                     plus resin) for distribution networks.
ZnO arrresters are characterised by                                                                 In conclusion, these various arrester
(see fig. 25):                                       The second technique, more recent,             types are used for protection of
s their maximum permanent operating                  has produced arresters which are far           equipment, transformers and cables. In
voltage;                                             lighter, less vulnerable to vandalism          this case, practically all the arresters
s their rated voltage which may be linked,           and with better live part protection           used are zinc oxide ones which are
by analogy with silicon carbide arresters,           against humidity since they are                gradually replacing horn gaps and
to withstand to temporary overvoltages;              completely compound-filled. In point of        silicon carbide arresters.
                                                                                                    The purpose of this evolution is
                                                                                                    increased accuracy of protection levels
  peak kV                                                                                           to guarantee insulation coordination to
    U                                                                                               a even higher degree.
                                                                                                    Readers interested in implementation
  600                                                                                               of arresters can refer to appendix 2.
  400                                          ZnO
                                                                                                    s maximum permanent             12.7 kV
  200                                                                                               s   rated voltage               24 kV
                                                                                  linear            s residual voltage at rated
                                                                                                    discharging current             < 75 kV
  100                                                                                               s rated discharging current
                                                                                                    (8/20 µs wave)                  5 kA
                                                                                                    s impulse current withstand
                                                                                                    (4/10 µs wave)                  65 kA

                        .001     .01      .1         1     10      100   1,000 10,000         I     fig. 25: example of characteristics of a ZnO
                                                                                                    arrester meeting specification EDF
fig. 24: characteristics of two arresters with the same level of protection 550 kV/10 kA peak kV.   HN 65S20.

Cahier Technique Merlin Gerin n° 151 / p.16
4. standards and insulation coordination

For many years now the International              shapes producible in laboratories and         acceptable as regards operating safety.
Electrotechnical Commission (IEC) has             having shown satisfactory equivalence.        Moreover, the gradual replacement of
been concerned with the problem of                Moreover, two new concepts are dealt          dischargers by arresters enables
HV insulation coordination.                       with in this standard:                        reduction of the safety margin which
                                                  s longitudinal insulation (between the        had become superfluous between
Insulation coordination is dealt with in
                                                  terminals of the same phase of an open        arrester protection level and equipment
two main documents:
                                                  device);                                      specified insulating voltage.
s IEC 664 for LV;
                                                  s consideration of altitude and of            Determining insulation levels
s IEC 71 for HV.
IEC 71 is divided into two parts, the             installation ageing.                          The standard does not stipulate
second part forming an exhaustive                 This draft-standard distinguishes             invariable withstand voltages valid in all
application guide.                                internal insulation, external insulation      cases, but enables insulation
«Product» standards, including:                   and two voltage ranges:                       coordination studies to be carried out in
s IEC 694 «common clauses for                     s internal insulation covers everything       a number of stages:
equipment»;                                       not in ambient air (for example, liquid       s definition of relationships between

s IEC 76 «transformers»;                          insulation for transformers, SF6 or           network type and choice of its
s IEC 99 «surge arresters»;                       vacuum for circuit-breakers);                 insulations.
comply with IEC 71 as regards specific            s external insulation refers to air           The purpose is to establish the
withstand voltages.                               clearances.                                   characteristics of the maximum
                                                  s range l: from 1 kV to 245 kV inclusive      possible permanent voltages and the
                                                  s range ll: above 245 kV.                     foreseeable temporary overvoltages as
HV insulation coordination                                                                      a function of:
                                                  For each of these, implementation of
as in IEC 71                                      insulation coordination varies slightly.
                                                                                                s network structure and its rated

One of the objectives of this standard                                                          voltage,
                                                  A table of standardised rated withstand       s the neutral earthing connection
which should come into force in 93 is to
                                                  voltages exists for each range. These         diagram,
explain and break down the various                tables have been drawn up according           s the substations and rotating
factors for achievement of withstand              to various criteria, and, although mostly     machines present on the line,
voltages. This approach encourages                empirical up to now, have been                s the type and position of surge
search for optimisation and even                  confirmed, with a few reservations, by        limitation devices, if any,
reduction in voltage withstand levels.            experience. Indeed, it cannot be denied       and according to considerations
Standard IEC 71 proposes conventional             that the levels laid down, which have         common to all overvoltage classes
modelling of actual stresses by wave              not been changed for years, are fully         defined by the standard (see fig. 26).

overvoltage                 low frequency                                transient
class                       permanent           temporary                slow front           fast front           very fast front
                                                                         Tp                   T1                   Tf
                                  Tt                  Tt                       T2                     T2                    Tt

                                                                                                                   100 > Tf > 3 ns
shape range                 f = 50 or 60 Hz     10 < f < 500 Hz          5,000 > Tp > 20 µs   20 > T1 > 0.1 µs     0.3 > f1 > 100 MHz
(frequency, rising front,                                                                                          30 > f2 > 300 kHz
term)                       Tt ≥ 3,600 s        3,600 ≥ Tt ≥ 0.03 s      20 ms ≥ T2           300 µs ≥ T2          3 ms ≥ Tt
standardised shape          f = 50 or 60 Hz     48 ≤ f ≤ 62 Hz           Tp = 250 µs          T1 = 1.2 µs          (*)
                            Tt (*)              Tt = 60 s                T2 = 2,500 µs        T2 = 50 µs
standardised                (*)                  short duration          switching            lightning            (*)
withstand test                                   power frequency         impulse test         impulse test
(*) to be specified by the relevant product Committee

fig. 26: representative overvoltage shapes and tests considered by draft-standard IEC 71.

                                                                                                Cahier Technique Merlin Gerin n° 151 / p.17
s coordination of network insulation          For operating voltages under 245 kV,                 designers is the sizing of external
Once these data have been collected,          the power frequency test and the                     insulations. Whereas use of SF6 for
the corresponding coordination                lightning impulse test are the ones                  insulation and of the vacuum or SF6
withstand voltage must be determined          normally chosen.                                     for the breaking gap means that
for each overvoltage class taking into        s the final choice is made from
                                                                                                   internal dielectric withstand is clearly
consideration the required performance        standardised levels (see fig. 28) from               determined and not dependent on
and, generally, the acceptable                all the rated voltages.                              environmental conditions (climate,
insulation failure rate. The value                                                                 altitude, degree of moisture,
                                              An example:                                          pollution,....).
obtained is specific to the network
                                              Figure 29 presents a calculation of this
studied and to its situation and is the                                                            Rated insulation levels to be retained:
                                              kind taken from the application guide of
lowest withstand voltage to the                                                                    s 50 kV at power frequency meets
                                              the draft-revision of publication IEC 71.            rated withstand voltage at low
overvoltage in question that the              It shows the insulation coordination
network has to have in its operating                                                               frequency permanent overvoltages
                                              study for a substation characterised by              (32 kV) and at more than 81 % rated
conditions.                                   the highest voltage for the equipment                withstand voltage at slow front
To choose the components of a                 Um = 24 kV.                                          transients (61 kV by equivalence);
network, their specified withstand            This example mainly deals with                       s 125 kV chosen as a technico-
voltages must be defined.                     external insulation, as the chief problem            economic compromise for fast front
Determination of coordination withstand       facing installation and network                      transients, results in:
voltages consists in setting the
minimum values of the insulation
withstand voltages satisfying
                                               s origin and classification of constraining            network analysis                  (§ 4.02)
performance criterion when insulation is
                                               voltages                               (§ 3.16)
subjected to the representative                s protection level of voltage limiting
overvoltages in operating conditions.          devices                                (§ 3.20)
Determination of specified insulation          s insulation characteristics                            representative voltages
withstand voltages consists in                                                                        and overvoltages Urp              (§ 3.18)
converting the coordination withstand
voltages into appropriate standardised         s insulation characteristics
test conditions. This is achieved by           s performance criterion               (§ 3.21)         choice of insulation satisfying
                                               s statistical distribution                 (+)         the performance criterion         (§ 4.03)
multiplying the coordination withstand
                                               s inaccuracy of initial data               (+)
voltages by factors compensating the           (+) effects combined in a
differences between actual insulation          coordination factor Kc                (§ 3.24)
operation conditions and standardised                                                                 coordination withstand
withstand test conditions.                                                                            voltage Ucw                       (§ 3.23)

Rated insulation level is chosen by            s atmospheric correction

selecting the most economical series of        factor ka                             (§ 3.26)
                                               s all equipment tested                      (*)        application of factors taking into
standardised insulation withstand              s production dispersion                     (*)        consideration the differences
voltages, sufficient to prove that all the     s installation quality                      (*)        between standard test conditions
specified withstand voltages are               s ageing in operation                       (*)        and operating conditions          (§ 4.04)
satisfied.                                     s other unknown factors                     (*)
                                               (*) effects combined in a safety
The study chart for final determination        factor Ks                             (§ 3.27)
of rated insulation is shown in figure 27.                                                            specified withstand voltage Urw (§ 3.26)
This chart covers the two factors,
altitude and manufacturing dispersion,         s test conditions                  (Chapter 5)
defined in the draft-standard, by the          s test conversion factor Kt           (§ 3.29)         choice of standardised
term of corrective factor.                     s standardised withstand                               withstand voltage      (§ 4.05 & 4.09)
                                               voltages                       (§ 4.06 & 4.07)
s rated  withstand voltage or insulation       s Um ranges                           (§ 4.08)
level is the same as specified withstand
voltage for overvoltages which can be
tested, i.e.:                                  standardised or rated insulation level: all of Uw                                 (§ 3.32 & 3.33)
s power frequency test,
s switching impulse test,                     fig. 27: organisation chart for determining rated and standardised insulation levels.
s lightning impulse test.                     Notes: between brackets, paragraphs of IEC 71 where the term is defined or the action
s the equivalence factors proposed by
                                                       data to be condidered.
standard IEC 71 mean generally that
                                                       actions to be performed.
only two withstand voltages need be
                                                       results obtained.
specified out of the 3 considered.

Cahier Technique Merlin Gerin n° 151 / p.18
s either acceptance of a failure rate
greater than that already taken as an              highest voltage               standardised short                     standardised withstand
hypothesis,                                        for equipment                 duration withstand voltage             voltages to lightning
s or addition of arresters to the                  Um                            at power frequency                     impulses
installation to ensure the latter is not           kV rms                        kV rms                                 kV rms
stressed beyond this level.                        3.6                           10                                     20/40
For high and extra high voltages, the              7.2                           20                                     40/60
insulation coordination procedure is the           12                            28                                     60/75/95
same, but equipment insulation is
                                                   17.5                          38                                     75/95
generally qualified by its switching
impulse and lightning impulse                      24                            50                                     95/125/145
withstands.                                        36                            70                                     145/170
                                                   52                            95                                     250
                                                   72.5                          140                                    325
                                                   123                           (185)/230                              450/550
                                                   145                           (185)/230/275                          (450)/550/650
                                                   170                           (230)/275/325                          (550)/650/750
                                                   245                           (275)/(325)/360/395/460                (650)/(750)/850/950/1050

                                                   fig. 28: standardised insulation levels for root mean square voltage networks between 1 and
                                                   245 kV (there is a similar table for voltages greater than 245 kV).

rated withstand voltage                  at short duration                                                                      to lightning
kV                                       power frequency                                                                        impulses
rated withstand voltage                                                         74                       108                    141
to lightning impulses
equivalence factor                                                              1.06                     1.06
slow front   fast
rated withstand voltage                  28               32                    42                       61
to an overvoltage at short
duration power frequency
equivalence factor                                                              0.6                      0.6
slow front   50 Hz
specified withstand voltage
withstand voltage                        28               32                    70                       102                    141
altitude correction                                                             1.13                     1.13                   1.13
dispersion factor                        1.15             1.15                  1.05                     1.05                   1.05
coordination withstand voltage           24               28                    59                       86                     119*
(case of equipment subjected
to atmospheric pressures)
overvoltages in operation
conventional representative              short duration                         250-2,500 µs impulse                            1.2-50 µs impulse
impulse shape                            50 Hz power frequency (1 mn)           2 % arcing
conventional representative              24               28                    59                       86
amplitude (kV)                                                                  (2.6 p.U.)               (3.86 p.U)
overvoltage categories                   phase /          phase to              phase /                  phase to               phase / frame and
                                         frame            phase                 frame                    phase                  phase to phase
                                         temporary                              slow front                                      fast front
                                         at power frequency                     (switching)                                     (lightning)
*: this value comes from the following             Calculation example:
criteria:                                          for a slow front coordination withstand voltage of 59 kV
- arrester protection level: 80 kW                 s slow front specified withstand voltage = 59 kV x 1.05 x 1.13 = 70 kV
- arrester/equipment clearance: 8 m                s equivalent rated withstand voltage at standard short term frequency = 70 kV x 0.6 = 42 kV
- safety factor: 1.05                              s equivalent rated withstand voltage to lightning impulses = 70 kV x 1.06 = 74 kV.

fig. 29: example of an insulation coordination study for a 24 kV network, with external insulation equipment (taken and adapted from the draft-
revision of IEC 71).

                                                                                                      Cahier Technique Merlin Gerin n° 151 / p.19
5. coordination applied to electrical
installation design

The high operating voltage involved           for energy distributors (invoice losses),     ferromagnetic resonance. To prevent
increases the economic importance of          industrial consumers (production              this, the capacitances must be reduced
this study.                                   losses) and people (safety).                  by approaching, for example, the
Three criteria justify this statement:        In LV                                         transformer energising equipment.
s increase of number of customers or
                                              In practice, the lower the operating          Connection of a load prior to energising
of distributed power;                         voltage, the more limited the                 is useful since this load acts as a
s increase of failure cost (cost of           consequences of breakdown in power            reducing resistance which can prevent
equipment to be replaced);                    distribution terms. However                   resonance.
s the smaller relative part of the            development of electronic equipment           Earthing the neutral is also a solution
coordination study in total installation      and systems is responsible for a large        for phase/earth resonances.
cost.                                         number of incidents further to                Overvoltage caused by capacitive
                                              overvoltages. In point of fact,               current breaking
breakdown consequences                        disturbance withstand level is not            The solution is to prevent successive
                                              always specified or is not coordinated        reignitions by increasing contact
Dielectric failure (breakdown or arcing)
                                              with the level corresponding to its           separation speed and using a good
can cause:
                                              installation.                                 dielectric (vacuum or SF6).
s tripping of the protective devices in
the best possible case;                       However, these systems play an                Overvoltage caused by closing off-
s destruction of equipment in the worst       increasingly large role in the integrity of   load lines
possible case;                                installations, production and                 This is prevented on transmission
s interruption of operation each time a       management, and the economic                  networks by progressive energising,
failure occurs.                               consequences for the company                  obtained by adding insertion
                                              concerned can be serious.                     resistances to the circuit-breaker.
In HV, the resulting power failure can
affect an entire town, a region or an         Coordination of «withstands» is thus          Overvoltage caused by lightning
iron and steel plant, and causes:             vital, even in LV...                          stroke
s a risk of network destabilisation;          .... and the use of arresters should be       There are three possibilities:
s a loss of energy billed for the energy      generalised. Today they are highly            s installation of earth cables to prevent
distributor;                                  recommended for LV consumers                  direct impulses (see chapter 1);
s production loss for industrial              supplied by overhead lines.                   s installation of protective devices at
consumers;                                                                                  vulnerable points (dischargers or,
s a risk for people (e.g. in hospitals)       reduction of overvoltage                      preferably, arresters), (see appendix 2);
and for computer data.                                                                      s creation of good quality earth
                                              risks and level                               connections (see chapter 1).
To avoid such incidents, studies must
                                              Simple solutions to the various
be conducted for each new installation
                                              overvoltages looked at in chapter 1 can
to provide consistent and optimised risk
                                              be considered as from the initial project
protection.                                   of installation.
One solution is to increase installation
                                              Overvoltage due to ferromagnetic
insulation level by increasing                resonance
clearances. However, this results in
                                              The only means of removing this
considerable increase in cost: doubling       completely is for 1/C ω to be greater
these clearances means multiplying            than the slope at the origin of L ω i.
eight times volumes and costs.                However, other solutions can be
Oversizing is therefore unacceptable          considered, in particular in MV where
in HV, which accounts for the
                                              s an unbalance between the 3 phases
importance of optimising HV equipment.
                                              can occur in the case of protection by
In MV                                         phase by phase controlled switch. The
The consequences of insulation faults         greatest simultaneity possible must be
on MV networks are the same, on a             sought on closing the 3 network phases
lesser scale, as those in HV.                 (omnipole equipment);
The consequences of the resulting             s closing an off-load transformer may
electricity failures can also be serious      be the transient phenomenon causing

Cahier Technique Merlin Gerin n° 151 / p.20
6. conclusion

Insulation coordination aims at finding     rules out the possibility of absolute              alongside optimisation of economy and
the right balance between equipment         solutions.                                         electrical operating stresses.
reliability from a dielectric standpoint,   Although the modellings chosen may                 Increasing use of arresters, partly due
on the one hand, and their sizing and       appear somewhat arbitrary at first sight,          to improvement of their characteristics
thus cost, on the other.                    they have been confirmed by                        and reliability, contributes to greater
                                            experience.                                        control of protection levels.
The presentation made in this
document shows the complexity of the        More detailed information can be found             Consideration of this aspect by
                                            in the publications quoted for readers             international standard committees, both
parameters involved in such an
                                            wishing to examine the subject in                  generally and as regards product
                                            greater depth. The progress made in                recommendations, is proof of the
Moreover, the statistical aspect of         knowledge of phenomena now ensures                 importance of the subject and of its
behaviour to transient overvoltages         increased installation reliability                 associated advantages.

appendix 1: propagation of overvoltage

Whatever the origin of the overvoltage,                                                         Propagation speed approaches that of
it will propagate along the line or cable                                                       the velocity of light, i.e. approximately
                                                     L            R
making up the network.                                                                          3 x 108 m/s. This speed can also be
This propagation support can be                                                                 said to be equal to 300 metres per
modelled by using values per length                                                             microsecond, thus providing an
unit of inductance and resistance in                                                            estimation of the distribution along the
                                                                      C              1/G
longitudinal and of capacitance and                                                             conductor of a very short term wave
conductance in transverse (see fig. 30).                                                        front (see fig. 31).
The impedance, in sinusoidal state, is                                                          The theory of guided propagation
then given by:                                                                                  makes it possible to establish that,
                                            fig. 30: modelling of a propagation support.        when a wave propagating along a
          L ω + R
 Z =
         C ω + G
At the high frequencies generally
associated with overvoltages, the                     evolution           V
                                                      in time                                    front: 300 kv/µs
inductive and capacitive terms become
                                                                      600 kv
preponderant. The impedance known
as "characteristic impedance" then                                                 2 µs
Zc =                                                 distribution
                                                                                                                t (at constant x)
          C                                                               V
                                                     in space                                                 front: 1 kv/m
The resistive and conductive terms
correspond to losses causing wave                                                                            600 m
attenuation during its propagation.
The magnitudes of the characteristic
impedances are:                                                                                                  x (at constant t)
s EHV lines: 300 to 500 ohms;
s HVA lines (overhead); approximately
                                            fig. 31: representation in time and space of a lightning wave.
1,000 ohms,

                                                                                               Cahier Technique Merlin Gerin n° 151 / p.21
conductor arrives at a point of               zero transmitted wave and a reflected                This case results in a high stress at the
impedance change, partial reflection          wave with a factor -1;                               reflection point and in its vicinity
and transmission are observed.                s Za = Zc (homogeneous conductor):                   (vicinity in the space distribution sense
                                              transmission equals 1 and reflection                 mentioned above).
If Zc is the characteristic impedance of
                                              zero;                                                The expression «doubling of the
the first conductor and Za that of the
                                              s Za = infinite (line open): voltage at the          voltage wave», frequently used, may
second, the transmission and reflection
                                              reflection point is given by the                     lead to confusion by letting people think
coefficients are given by:
                                              superimposition of the incident wave                 that the reflected wave is twice as large
T = 2 Za/(Za + Zc) et R = (Za-Zc)/(Za +
                                              and the reflected wave with a                        as the initial wave. It is only at the
Zc)                                           factor + 1. Its maximum value will then              reflection point that the maximum value
The limit values of these coefficients        be equal to twice the peak of the                    observed is twice the value of the
correspond to simple physical cases:          incident wave. Although there is no                  incident wave, since this is the only
s Za = 0 (line closed at the frame):          propagation in the medium Za, the                    point where the incident wave and the
voltage at the point in question is thus      border value is still given by T which               reflected wave join their peaks.
zero at all times: this corresponds to a      also equals 2.

appendix 2: installing a surge arrester

maximum safety clearance                      As its conduction level (if approximated             Note:
                                              to the protection level) is 75 kV,                   The coefficient 2 does not mean that
Wave reflection and propagation (see
appendix 1) mean that surge arresters         intervention is only possible by the                 peak voltage is doubled but refers to
only limit overvoltages at their              superimposition of the reflected wave                superimposition of the incident wave
terminals.                                    on the incident wave.                                and the reflected wave (see fig. 32).
                                              The reflected wave must have reached
The clipped wave retains the dv/dt of
                                              a value of 75 - 62.5 = 12.5 kV.
its rising front and could develop, by                                                             cabling the surge arresters
reflection, at the opening point, a           The difference between the incident
                                                                                                   A current wave flows off to the earth
voltage twice that of limitation voltage.     value (62.5 kV) and the reflected value
                                                                                                   when an arrester is used for limitation.
As equipment withstand voltage is             (12.5 kV), i.e. 50 kV, corresponds to the
                                                                                                   This results from application of the
generally lower than twice the residual       wave front distributed on the return
                                                                                                   voltage wave to the characteristic line
voltage of the arrester, there is a           journey between the arrester and the
maximum clearance not to be                   open point. The return distance is thus
exceeded between the arrester and the         no more than 50 m, i.e. a maximum                                u
                                                                                                   line: I =      .
substation equipment.                         protection clearance of 25 m.                                    Zc
s lightning wave: 300 kV/ µs;
s hence a voltage gradient on the line
on passage of the rising front                 direction of displacement
of 1 kV/m;                                     of the lightning wave
s MV substation: impulse withstand of          instants... T0            T1
                                                            wa             wa
125 kV;                                                        ve             ve
                                                                    at             at
s surge arrester: residual voltage:                                      T0             T1
                                                                                                        2 x U0 = maximum stress reached
75 kV.                                                                                                  at T1 ⇒ T1 the most severe instant
The maximum stress at the open point
will be generated by reflection of the                                                                  Up = arrester protection level
wave peak limited by the arrester. This
stress will have twice the value of this                                                                U0 = value of the incident wave
peak.                                                                                                   reached on the arrester at T0
In order to keep to the limit of 125 kV of
                                                                                                        u = value of the reflected wave
the equipment, the arrester must                                                                        reached on the arrester at T0
therefore act at the latest when the                                                                    (U0 + U = Up ⇒ start of conduction)
incident wave in its position                                                                                  d
          125                                  locations:                               arrester       equipment
equals:          = 62.5 kV
                                              fig. 32: propagation and reflection in the presence of an arrester.
(instant T0 in figure 32).

Cahier Technique Merlin Gerin n° 151 / p.22
                                                                                              A voltage drop, mainly inductive and
                                                                                              which may be high, then occurs in the

                                                                                              earthing circuit.

                                                                                              s current wave: 1 kA/ µs;

                                                                                              s earth down-cable inductance:
                                                                                              s hence UL = 1 kV/m.
                                                                                              If this voltage is not to be added to
                                                                                              residual voltage, the proposed
                                                                                              equipment must be bypassed at the

                                                                                              arrester terminals as regards the
                                                                                              «lightning phenomenon».
    if L < 25 m: a surge limiter placed on the pole is sufficient                             In practice this consists in connecting
    if L 25 m: a second limiter must be placed at the transformer terminals                   as shown in figure 33. If the HV/
                                                                                              equipment link is not made on the
fig. 33: position of surge limiters on a substation supplied by an overhead-underground
network.                                                                                      arrester, conductor length must be as
                                                                                              short as possible (see fig. 34).



  wrong cabling                         right cabling

fig. 34: arrester cabling principle: load-arrester connections must be as short as possible

appendix 3: electricity standards

There are three levels of standards:               s the CENELEC (European
The following three organisations act at           Electrotechnical Standardisation
international, European and French                 Committee) produces the «EN»
level respectively:                                standards and groups 18 countries,
s the IEC (International                           mostly European. Application of the
Electrotechnical Commission) produces              standards it votes is mandatory.
the «IEC» standards with the                       s the UTE (Union Technique de
participation of 60 countries. Standards           l’Electricité) produces the French «NF»
are accepted only if opposition is less            standards.
than 20 %. Its application may give rise
to waivers in certain countries.

                                                                                              Cahier Technique Merlin Gerin n° 151 / p.23
appendix 4: bibliography

s IEC 60: Hight voltage test techniques.
s IEC 71-1: Insulation co-ordination:
definition, principles and rules.
s IEC 71-2: Insulation co-ordination:
application guide.
s IEC 99: Surge arresters.
Merlin Gerin "Cahier technique"
s HV industrial network design,
Cahier Technique n° 169
s Behaviour of the SF6 MV circuit-
breakers Fluarc for switching motor
starting currents,
Cahier Technique n° 143
Other publications
s Techniques de l’ingénieur : chapter
on «Gaz Isolants».
s Les propriétés diélectriques de l’air et
les très hautes tensions.
(EDF Publication).
s Principles and procedures of the
insulation co-ordination.
s Dimensionnement des parafoudres
MT pour le réseau EDF (1988).

                                              Réal.: Illustration Technique - Lyon
Cahier Technique Merlin Gerin n° 151 / p.24   DTE - 02-95 - 2500 - Impr.: Clerc

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