IEC TR 61000-2-14:2006

Electromagnetic compatibility (EMC) - Part 2-14: Environment - Overvoltages on public electricity distribution networks

IEC TR 61000-2-14:2006 Describes the electromagnetic environment with respect to the voltages in excess of normal that are found on electricity supply networks operating at low and medium nominal voltages and that can be impressed on equipment connected to those networks, without considering further effects (e.g. amplification or attenuation) within an installation. Since these overvoltages have the potential to hinder the functioning of electrical and electronic equipment, they fall within the definition of electromagnetic disturbance in the field of EMC. Various categories of overvoltage are described, based on relative magnitude, duration and energy content. This Technical Report describes the phenomena of overvoltages, it does not specify compatibility levels and does not directly specify emission and immunity levels.
The report describes the various phenomena and processes that cause overvoltages, including the transfer into the networks concerned of overvoltages that originate in or traverse other networks and installations, including higher voltage networks and the installations of electricity users. The effects of overvoltages on equipment are outlined. Some case studies of overvoltage events are presented. Recommendations are made regarding the general technical approach to mitigating the risk of equipment being hindered from operating as intended by the effects of overvoltages. (It is not the function of IEC publications to assign responsibility for mitigating measures to any of the parties involved.)
The purpose of this report is to ensure that this important category of electromagnetic disturbance is included in the description of the environment in Part 2 of IEC 61000. For that purpose, only a brief description is provided of the various overvoltages and their causes and effects. A much more detailed treatment can be found in IEC 62066. A UIE publication - Guide to quality of electrical supply for industrial installations, Part VI: Transient and temporary overvoltages and currents - has a similar content. Measurement methods are specified in IEC 61000-4-30.

Compatibilité électromagnétique (CEM) - Partie 2-14: Environnement - Surtensions sur les réseaux de distribution publics

IEC/TR 61000-2-14:2006 décrit l'environnement électromagnétique en ce qui concerne les tensions supérieures aux valeurs normales qui apparaissent dans les réseaux d'alimentation électrique fonctionnant aux tensions nominales basses et moyennes, et dont peuvent faire l'objet les matériels connectés à ces réseaux, sans tenir compte de leurs effets (amplification ou atténuation, par exemple) à l'intérieur d'une installation. Ces surtensions risquant d'empêcher le fonctionnement du matériel électrique et électronique, elles entrent dans le cadre de la définition des perturbations électromagnétiques dans le domaine de la CEM. Différentes catégories de surtensions sont décrites, en fonction de l'amplitude relative, de la durée et du contenu énergétique. Le présent Rapport technique décrit les phénomènes de surtension. Il ne précise pas les niveaux de compatibilité ni ne spécifie directement les niveaux d'émission et d'immunité. Le rapport décrit les différents phénomènes et processus à l'origine de surtensions, y compris le transfert dans les réseaux concernés de surtensions qui prennent leur source ou traversent d'autres réseaux et installations, y compris les réseaux à haute tension et les installations des usagers. Les effets des surtensions sur le matériel sont présentés. Certaines études de cas relatives à des événements de surtension sont présentées.
Des recommandations sont formulées en ce qui concerne l'approche technique générale permettant de limiter le risque de difficultés de fonctionnement lié aux effets des surtensions (il ne revient pas aux publications de l'IEC d'attribuer la responsabilité des mesures d'atténuation à l'une des parties impliquées.) Le présent rapport a pour objet de vérifier que cette catégorie importante de perturbations électromagnétiques est incluse dans la description de l'environnement dans la Partie 2 de l'IEC 61000. À cet effet, seule une brève description des différentes surtensions et de leurs causes et effets est fournie. Un traitement beaucoup plus détaillé peut être consulté dans l'IEC 62066. Une publication UIE (Guide to quality of electrical supply for industrial installations, Part VI: Transient and temporary overvoltages and currents – disponible en anglais seulement) présente un contenu similaire. Les méthodes de mesure sont spécifiées dans l'IEC 61000-4-30. NOTE Le présent Rapport technique ne contient pas de résultats de mesure détaillés relatifs aux surtensions. Par conséquent, la probabilité d'occurrence ne peut pas être évaluée.
Mots clés: surtensions

General Information

• Status: Published
• Publication Date: 12-Dec-2006
• ICS: 29.240.01 - Power transmission and distribution networks in general
• ICS: 33.100.01 - Electromagnetic compatibility in general
• Technical Committee: SC 77A - EMC - Low frequency phenomena
• Drafting Committee: WG 8 - TC 77/SC 77A/WG 8
• Current Stage: PPUB - Publication issued
• Start Date: 13-Dec-2006
• Completion Date: 31-Dec-2006

Standards Content (Sample)

TECHNICAL IEC
REPORT TR 61000-2-14
First edition
2006-12
Electromagnetic compatibility (EMC) –
Part 2-14:
Environment – Overvoltages on public electricity
distribution networks
Reference number
IEC/TR 61000-2-14:2006(E)
Publication numbering
As from 1 January 1997 all IEC publications are issued with a designation in the
60000 series. For example, IEC 34-1 is now referred to as IEC 60034-1.
Consolidated editions
The IEC is now publishing consolidated versions of its publications. For example,
edition numbers 1.0, 1.1 and 1.2 refer, respectively, to the base publication, the
base publication incorporating amendment 1 and the base publication incorporating
amendments 1 and 2.
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TECHNICAL IEC
REPORT TR 61000-2-14
First edition
2006-12
Electromagnetic compatibility (EMC) –
Part 2-14:
Environment – Overvoltages on public electricity
distribution networks
© IEC 2006 ⎯ Copyright - all rights reserved
No part of this publication may be reproduced or utilized in any form or by any means, electronic or
mechanical, including photocopying and microfilm, without permission in writing from the publisher.
International Electrotechnical Commission, 3, rue de Varembé, PO Box 131, CH-1211 Geneva 20, Switzerland
Telephone: [phone] Telefax: [phone] E-mail: [e-mail] Web: www.iec.ch
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International Electrotechnical Commission
МеждународнаяЭлектротехническаяКомиссия
For price, see current catalogue

– 2 – TR 61000-2-14 © IEC:2006(E)
CONTENTS
FOREWORD.4
INTRODUCTION.6

1 Scope.7
2 Normative references .7
3 Terms and definitions .7
4 Description of overvoltages .10
4.1 General .10
4.2 External overvoltages.11
4.3 Internal overvoltages.11
4.4 Overvoltage waveshape .11
5 Long duration overvoltages .12
5.1 Sustained earth faults.12
5.2 Broken neutral on LV network.12
5.3 Maloperation of voltage regulating equipment.13
5.4 Overvoltages due to voltage unbalances .13
5.5 Dispersed generation .14
6 Short duration overvoltages.15
6.1 Earth faults.15
6.2 Load rejection (sudden load loss) .16
6.3 Self-excitation .16
6.4 Resonance and ferroresonance.16
7 Very short duration overvoltages (transients).18
7.1 General description .18
7.2 Lightning .19
7.3 Switching .20
7.4 Summary of surge duration and cause.26
8 Effects of overvoltages on equipment .27
8.1 General considerations.27
8.2 Reduction in life of filament lamps .28
8.3 Effect of overvoltages on IT equipment.28
9 Case studies .29
9.1 General .29
9.2 Switching of LV power factor correction capacitor.29
9.3 Metal fusion furnace .30
9.4 Switching of MV power factor correction capacitor.31
9.5 DC traction system .32
9.6 Load switching .34
10 Protection against the effects of overvoltages.36
10.1 General considerations.36
10.2 Point on wave switching .36
10.3 Arcing horns and spark gaps .37
10.4 Overvoltage protection relays .38
10.5 Snubbers (high frequency RC filter).38
10.6 Uninterruptible power supply (UPS) systems .39

TR 61000-2-14 © IEC:2006(E) – 3 –
10.7 Surge protection device (SPD) .39
11 Conclusions.41
12 Recommendations.42

Bibliography.43

Figure 1 – Lightning impulse test voltage characteristic .11
Figure 2 – Broken neutral on LV network .13
Figure 3 – The effect of distributed generation on network voltage.14
Figure 4 – Line – Neutral temporary overvoltage on healthy phase for single phase line
– earth fault .15
Figure 5 – Typical transient overvoltage when energizing a capacitor bank.22
Figure 6 – Notching caused by power electronics switching .24
Figure 7 – ITI (CBEMA) curve for equipment connected to 120 V 60 Hz systems .29
Figure 8 – Voltage waveform distorted by the energization of a PFC capacitor .30
Figure 9 – Phase to ground overvoltage in case of a single (a) or multiple (b) faults .30
Figure 10 – Equivalent circuit.31
Figure 11 – Extruder connection – single line diagram .32
Figure 12 – Current waveforms (phases A and C) taken at the main LV circuit breaker.32
Figure 13 – Single line diagram of public transportation system .33
Figure 14 – Voltage waveforms associated with overvoltages on public transportation
system .34
Figure 15 – 20 kV line-to-earth voltages during breaking transformer current.35
Figure 16 – Spark gap .37
Figure 17 – Two-stage surge protection scheme .40

Table 1 – Surges on the low voltage network .26
Table 2 – Surges on the medium voltage network .27
Table 3 – Reduction of filament lamp life .28
Table 4 – Protective levels for typical MV surge arresters (effectively earthed neutral
systems) .41

– 4 – TR 61000-2-14 © IEC:2006(E)
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 2-14: Environment – Overvoltages on public electricity
distribution networks
FOREWORD
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9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of
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The main task of IEC technical committees is to prepare International Standards. However, a
technical committee may propose the publication of a technical report when it has collected
data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 61000-2-14, which is a technical report, has been prepared by subcommittee 77A: Low
frequency phenomena, of IEC technical committee 77: Electromagnetic compatibility.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
77A/540/DTR 77A/547/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.

TR 61000-2-14 © IEC:2006(E) – 5 –
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
A bilingual version of this publication may be issued at a later date.

– 6 – TR 61000-2-14 © IEC:2006(E)
INTRODUCTION
IEC 61000 is published in separate parts according to the following structure:
Part 1: General
General considerations (introduction, fundamental principles)
Definitions, terminology
Part 2: Environment
Description of the environment
Classification of the environment
Compatibility levels
Part 3: Limits
Emission limits
Immunity limits (in so far as they do not fall under the responsibility of the product
committees)
Part 4: Testing and measurement techniques
Measurement techniques
Testing techniques
Part 5: Installation and mitigation guidelines
Installation guidelines
Mitigation methods and devices
Part 6: Generic standards
Part 9: Miscellaneous
Each part is further subdivided into several parts, published either as International Standards
or as technical specifications or technical reports, some of which have already been published
as sections. Others will be published with the part number followed by a dash and a second
number identifying the subdivision (example: 61000-6-1).

TR 61000-2-14 © IEC:2006(E) – 7 –
ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 2-14: Environment – Overvoltages on public electricity
distribution networks
1 Scope
This part of IEC 61000 describes the electromagnetic environment with respect to the
voltages in excess of normal that are found on electricity supply networks operating at low
and medium nominal voltages and that can be impressed on equipment connected to those
networks, without considering further effects (e.g. amplification or attenuation) within an
installation. Since these overvoltages have the potential to hinder the functioning of electrical
and electronic equipment, they fall within the definition of electromagnetic disturbance in the
field of EMC. Various categories of overvoltage are described, based on relative magnitude,
duration and energy content.
This Technical Report describes the phenomena of overvoltages, it does not specify
compatibility levels and does not directly specify emission and immunity levels.
The report describes the various phenomena and processes that cause overvoltages,
including the transfer into the networks concerned of overvoltages that originate in or traverse
other networks and installations, including higher voltage networks and the installations of
electricity users. The effects of overvoltages on equipment are outlined. Some case studies of
overvoltage events are presented.
Recommendations are made regarding the general technical approach to mitigating the risk of
equipment being hindered from operating as intended by the effects of overvoltages. (It is not
the function of IEC publications to assign responsibility for mitigating measures to any of the
parties involved.)
The purpose of this report is to ensure that this important category of electromagnetic
disturbance is included in the description of the environment in Part 2 of IEC 61000. For that
purpose, only a brief description is provided of the various overvoltages and their causes and
effects. A much more detailed treatment can be found in IEC 62066. A UIE publication –
Guide to quality of electrical supply for industrial installations, Part VI: Transient and
temporary overvoltages and currents – has a similar content. Measurement methods are
specified in IEC 61000-4-30.
NOTE This Technical Report does not include detailed measurement results for overvoltages, therefore it is not
possible to provide an assessment of the probability of occurrence.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60050-161, International Electrotechnical Vocabulary (IEV) – Chapter 161: Electro-
magnetic compatibility
3 Terms and definitions
For the purposes of this document, the terms and definitions contained in IEC 60050-161 as
well as the following terms and definitions apply.

– 8 – TR 61000-2-14 © IEC:2006(E)
3.1
back flashover
flashover of phase-to-earth insulation resulting from a lightning stroke to that part of the
system which is normally at earth potential
3.2
breakdown
dielectric failure of an insulation under the effect of a strong electric field and/or by physico-
chemical deterioration of the insulating material
3.3
direct lightning stroke
lightning striking a component of the network, e.g.: conductor, tower, substation equipment,
etc.
3.4
declared supply voltage
U
c
normally the nominal voltage of the system. If by agreement between the electricity supplier
and the consumer a voltage different from the nominal voltage is applied to the supply
terminals, then this voltage is the declared voltage
3.5
disruptive discharge/flashover/sparkover
passage of an arc following dielectric breakdown
NOTE 1 The term “sparkover” (in French: “amorçage”) is used when a disruptive discharge occurs in a gaseous or
liquid dielectric.
NOTE 2 The term “flashover” (in French: “contournement”) is used when a disruptive discharge occurs over the
surface of a solid dielectric surrounded by a gaseous or liquid medium.
NOTE 3 The term “puncture” (in French: “perforation”) is used when a disruptive discharge occurs through a solid
dielectric.
3.6
indirect lightning stroke
lightning stroke that does not strike directly any part of the network but that induces an
overvoltage in that network
3.7
insulation coordination
selection of the dielectric strength of equipment in relation to the operating voltages and
overvoltages which can appear on the system for which the equipment is intended to operate,
taking into account the service environment and the characteristics of the available prevention
and protective devices
[IEV 604-03-08, modified]
NOTE In this instance, the term "dielectric strength of the equipment" means its rated or its standard insulation
level as defined in IEC 60071-1.
3.8
lightning arrester
surge diverter
/surge arrester/
surge protective device (SPD)
device designed to protect the electrical apparatus from high transient overvoltages and to
limit the duration and frequently the amplitude of the follow-on current

TR 61000-2-14 © IEC:2006(E) – 9 –
3.9
lightning impulse
voltage impulse of a specified shape applied during dielectric tests with a virtual front duration
of the order of 1 μs and a time to half value of the order of 50 μs
NOTE The lightning impulse is defined by the two figures giving these durations in microseconds; in particular the
standard lightning impulse is: 1,2/50 μs.
3.10
long duration overvoltages
overvoltage with a duration in excess of 10 min
NOTE The magnitude of a long duration overvoltage is typically given as a r.m.s. value.
3.11
nominal voltage
U
N
the voltage by which a system is designated or identified
3.12
overvoltage
any voltage having a value, either peak or r.m.s., exceeding the maximum value of the
corresponding declared voltage
3.13
per unit (p.u.)
methodology used to simplify equations and the presentation of electrical parameters by
expressing them as a fraction of a reference parameter:
⎛ Actual⎞
p.u. value =
⎜ ⎟
Base
⎝ ⎠
where the Actual and Base values are of the same quantity, e.g. voltage, current, impedance
etc.
NOTE Typically the Base value for voltage is the nominal voltage for fundamental frequency phenomena and the
peak line to ground voltage for transients.
3.14
power frequency withstand voltage
r.m.s. value of sinusoidal power frequency voltage that the equipment can withstand during
tests made under specified conditions and for a specified time
3.15
rise time (of a pulse)
the interval of time between the instants at which the instantaneous value of a pulse first
reaches a specific lower value and then a specific upper value
NOTE Unless otherwise specified, the lower and upper values are fixed at 10 % and 90 % of the pulse magnitude.
3.16
short duration overvoltage
voltage swell
power frequency overvoltage with a duration lasting greater than one period (one cycle) and
up to 10 min
NOTE The magnitude of a short duration overvoltage is typically given as a r.m.s. value.

– 10 – TR 61000-2-14 © IEC:2006(E)
3.17
surge
transient voltage wave propagating along a line or a circuit and characterized by a rapid
increase followed by a slower decrease of the voltage
[IEV 161-8-11]
NOTE In some parts of the world the term “Impulse” is used to describe a short duration overvoltage
characterised by a very rapid change in magnitude with a duration less than 200 μs.
3.18
temporary overvoltage
oscillatory overvoltage (at power frequency) at a given location, of relatively long duration and
which is undamped or weakly damped
NOTE Temporary overvoltages usually originate from switching operations or faults (e.g. sudden load rejection,
single-phase faults) and/or from non-linearities (ferroresonance effects, harmonics).
3.19
transient
pertaining to or designating a phenomenon or a quantity which varies between two
consecutive steady states during a time interval short when compared with the time-scale of
interest
[IEV 161-02-01]
3.20
very short duration overvoltage (transient)
overvoltage with a duration from less than a microsecond to several periods at fundamental
frequency
NOTE The magnitude of a very short duration overvoltage is typically given as a peak value.
3.21
voltage impulse
transient voltage wave applied to a line or equipment, characterized by a rapid increase,
followed generally by a slower non-oscillatory decrease of the voltage
3.22
front time
T
for a lightning impulse voltage T is a virtual parameter defined as 1,67 times the interval T
between the instants when the impulse is 30 % and 90 % of the peak value on the test voltage
curve (points A and B, Figure 1)
3.23
time to half-value
T
for a lightning impulse voltage T is a virtual parameter defined as the time interval between
the virtual origin, O , and the instant when the test voltage curve has decreased to half the
peak value
4 Description of overvoltages
4.1 General
Overvoltages are an intrinsic phenomena present on all networks. Overvoltage events can be
created in the public network or in the electricity user's installation. The dynamic response of
a network to load switching, both planned and unplanned (faults) will result in the storage and
release of energy. This transfer of energy will cause an overvoltage to be propagated within
the network.
TR 61000-2-14 © IEC:2006(E) – 11 –
4.2 External overvoltages
Overvoltages that are caused by events that are external to an installation, for example:
lightning strokes and faults on adjacent higher voltage networks, are generally very short term
overvoltage travelling waves. They attenuate with distance and the wave front becomes less
steep. In addition there are longer term overvoltages caused by load rejection, open circuit
neutrals, faulty voltage control equipment and the effect of distributed generation.
4.3 Internal overvoltages
Events within an installation can give rise to overvoltages, for example: the switching of non-
linear load, switch arcing, and fuse operation.
4.4 Overvoltage waveshape
A common method of representing the waveshape of a very short term overvoltage is shown
in Figure 1. The important values are the front time, T and the time to half-value, T . For
1 2
example, typical values for a transient overvoltage caused by lightning are 1,2 μs for the front
time and 50 μs for the time to half-value (a 1,2/50 μs waveform).

U
1,0
0,9
0,5
0,3
t
T′
T
T = 1,67 T
T ′ = 0,3 T = 0,5 T
T
T
O O
IEC  2258/06
Figure 1 – Lightning impulse test voltage characteristic
NOTE Figure 1 is only meant to represent an example of one type of overvoltage. Other types of overvoltage are
described in IEC 60071-2.
Other very short duration overvoltages having the shape of a damped high frequency
oscillation can be caused by events such as energizing capacitor banks, although their
amplitude is often much lower than an overvoltage caused by a lightning stroke, and the rate
of occurrence can often be higher. This type of very short duration overvoltage can propagate
over long distances and across voltage levels, hence adverse effects can often be seen some
distance from the point of initiation. This is particularly true when the overvoltages are
transferred to the lower voltage networks where the resilience of equipment is at its lowest.
The situation at all voltage levels can be further exacerbated if a resonance condition is
created, i.e. when the frequency of the transient overvoltage is close to the natural frequency
of the network and or equipment connected to the network.

– 12 – TR 61000-2-14 © IEC:2006(E)
When more than one type of overvoltage event occurs simultaneously, it can lead to
overvoltages in excess of the values quoted for a single event.
5 Long duration overvoltages
The overvoltages presented in this clause are typically described as being of long duration,
however it should be noted that there will be instances where for a particular event the
overvoltage could last for less than 10 min.
The overvoltages presented in this section are 50/60 Hz overvoltages.
5.1 Sustained earth faults
In MV networks with isolated or high-impedance grounded neutral, this kind of fault will
produce line to earth temporary overvoltages on the healthy phases. The overvoltage will last
for the duration of the fault, this can be anything from parts of a second for conventionally
earthed systems up to some hours for systems earthed via a tuned reactance (Petersen coil
earthing). Generally the magnitude of the overvoltage will not exceed twice the nominal phase
to earth voltage, i.e. √3 × U, where U can be up to 1,1 × U if the voltage is at the maximum
N
of the acceptable MV range. The overvoltages last until the faulted section of network is
disconnected.
Earth faults on the MV network can result in temporary power frequency overvoltages
between live conductors and earth on the LV network. The duration and magnitude of these
overvoltages will be dependent on the fault conditions and the MV earth impedance, as
described above.
The majority of public LV distribution systems are operated with a solidly earthed (grounded)
neutral. Therefore when earth faults occur on the MV network that raise the ground potential
in the vicinity of the LV network it is possible for an overvoltage to exist between the phase
and earth conductors of the LV network. The duration is limited by the time taken for the MV
protection and circuit breaker to clear the fault, typically no more than 5 s. The magnitude of
the overvoltage will generally not exceed 1,5 kV r.m.s., however this is dependent upon the
impedance of the LV ground connection and the magnitude of the MV earth fault current.
IEC 62066 contains a comprehensive description of this type of overvoltage.
5.2 Broken neutral on LV network
For a three-phase LV network supplied from a star (wye) transformer winding or for a two
phase network supplied from a transformer with a centre-tapped neutral at LV (sometimes
referred to as a three-wire network), if the neutral becomes disconnected (e.g. broken due to
a fault), single-phase loads beyond the break could experience an overvoltage up to the line
voltage of the network. The exact magnitude of the voltage will be dependent upon the ratio of
the impedance (loads) connected across each phase of the network – see Figure 2 below.
This type of overvoltage can persist for several hours or, in rare cases, days until the neutral
has been reconnected or the faulty network has been disconnected in readiness for repair.
This disconnection is often by manual intervention following complaints of severe voltage
fluctuations which occur as a result of changes in load.

TR 61000-2-14 © IEC:2006(E) – 13 –
Transformer LV
winding
L
L
L
N
Broken
neutral
Load A Load B
IEC  2259/06
Figure 2 – Broken neutral on LV network
In the event of a broken neutral as shown in Figure 2 above, the voltage that appears across
load A and load B is determined by the relative magnitude of these two loads, i.e.:
⎛ Z ⎞
A
⎜ ⎟
Voltage across load A (U ) = U ⋅ ; and
L
A L
⎜ ⎟
1 2
Z + Z
⎝ A B⎠
Voltage across load B (U ) =
U − U
B L A
L
1 2
Hence, depending upon the values of Z and Z it is possible for U to vary between near
A B A
zero and full line voltage ( U ).
L
L
1 2
NOTE Depending on the impedances and their phase shift, the voltage on the unloaded phase, phase L (L -N),
3 3
could theoretically be higher than the full line voltage.
5.3 Maloperation of voltage regulating equipment
Maloperation of automatic voltage regulation systems can sometimes lead to long duration
overvoltages between 1,1 and 1,2 p.u. at most. For instance, this could be due to a loss of
regulator voltage reference causing the tap changer to boost the voltage at its maximum, or
inadequate line drop compensation settings following unplanned load transfer on a regulating
transformer. Appropriate voltage regulator blocking relays can minimize risks of such
situations.
5.4 Overvoltages due to voltage unbalances
The combined effect of voltage unbalances and steady state voltage close to the maximum
agreed voltage tolerance can result in long duration overvoltages. This is the case in
particular for effectively earthed distribution systems supplying single-phase loads connected
line-to-neutral through an equivalent Y-y earthed MV/LV transformer connection (typical in
North America). In such cases, not only negative-sequence voltages can be transferred due to
load unbalance, but also zero-sequence voltages as well. The latter also depends on the
zero-sequence system impedance. In some cases, the combination of steady state voltages
near the upper limit, and the negative-sequence plus the zero-sequence voltage unbalances
can lead to permanent line-to-neutral voltages on some phases in the range of 1,1 p.u. at MV
and LV. Voltage regulators whose voltage reference is connected line-to-neutral can however
compensate the effect of zero sequence voltage unbalance thus reducing risks of this kind of
overvoltage.
– 14 – TR 61000-2-14 © IEC:2006(E)
5.5 Dispersed generation
In the absence of distributed generation it is typical for public distribution networks to have
been designed on the basis that energy flows in one direction i.e. from the source (substation)
to the point of utilization. Therefore it is typical for the voltage to be a maximum at the source
and to decrease with distance away from the source.
In some areas it is typical for MV/LV transformers to have an adjustable transformation ratio
(tap setting). The tap setting can only be adjusted off load and is selected with a view to
offsetting the voltage drop in the MV network.
MV and LV networks are designed such that under conditions of no load the voltage at
source, be it MV or LV, is as close as possible to the maximum agreed voltage tolerance. This
should ensure that the supply delivered to loads at the remote end of the network will remain
within agreed tolerances at times of peak demand.
The presence of distributed generation within the network can have the effect of increasing
the voltage level at the point of connection and therefore modifying the voltage distribution
profile. In Figure 3, the lower curve shows how the network voltage decreases with distance
from source, while the upper curve shows how the voltage profile can be raised if distributed
generation (DG) is connected between the source and the receiving end. This effect is
exacerbated if generation export coincides with periods of low demand (load) on the network.
NOTE For the purposes of describing Figure 3, only the terms U and U are introduced.
R RDG
In the absence of distributed generation the voltage at the receiving end is U and as
R
mentioned previously, MV/LV transformers with off-load tap settings have been adjusted to
compensate for the line voltage drop ∆V. If distributed generation is connected to the network,
the receiving end voltage is raised to U and the LV will also rise. In the absence of
RDG
suitable voltage regulation or overvoltage protection it is possible that the distributed
generation could cause the network voltage to exceed agreed tolerance levels.
For networks with high source impedance (sometimes referred to as “weak networks”) the risk
of voltage rise could be the limiting factor in determining the amount of distributed generation
that can be connected.
Voltage
Circuit without DG
Circuit with DG
under full load
U
RDG
U
R
Distance
Sending Receive
DG
end end
IEC  2260/06
Figure 3 – The effect of distributed generation on network voltage

TR 61000-2-14 © IEC:2006(E) – 15 –
6 Short duration overvoltages
6.1 Earth faults
As explained in section 4 the method of earthing and the value of the neutral to earth
impedance will determine the magnitude of overvoltage that will occur on the healthy phases
during earth faults. Various types of neutral earthing are used from solid or effective to the
high impedance earthing.
The term effectively earthed neutral applies to a system, or portion of the system, where the
ratio of zero-sequence reactance to positive-sequence reactance is positive and not greater
than 3 and the ratio of zero-sequence resistance to positive-sequence reactance is positive
and not greater than 1, as viewed from a considered location for any condition of operation:
0 ≤ X /X ≤ 3 and 0 ≤ R /X ≤ 1
0 1 0 1
where
X is the positive-sequence reactance;
X is the zero-sequence reactance;
R is the zero-sequence resistance.
For effectively earthed neutral systems, the overvoltage on the healthy phases is limited to
less than 1,4 p.u.
Figure 4 below illustrates the maximum line to neutral (L-N) overvoltage on healthy phases for
different values of the impedance factors X /X and R /X used for defining the effectiveness
0 1 0 1
of the neutral earthing
1,7 p.u.
1,6 p.u.
1,5 p.u.
1,4 p.u.
1,25 p.u.
0 1 2 3 4 5 6
X /X
0 1
IEC  2261/06
Figure 4 – Line–neutral temporary overvoltage on healthy phase
for single phase line – earth fault
R /X
0 1
– 16 – TR 61000-2-14 © IEC:2006(E)
In some countries, in order to reduce earth fault currents, networks are earthed via a high
impedance component (resistor/reactor), this requires that all network components have to be
rated for full line voltage.
6.2 Load rejection (sudden load loss)
Sudden loss of load on the MV or HV networks can result in a temporary overvoltage before
the automatic voltage control can correct the situation and bring the voltage back within limits.
The magnitude of the overvoltage depends on the magnitude of the source impedance (lower
impedance systems will see less change in voltage) and the size and characteristics of the
load. Typically the overvoltage will be in the range of U + 3 %, but in rare cases, such as
N
faults, it could be up to 6 %. Typically it can take up to three minutes before the tap changer
can stabilise the situation and bring the voltage back within limits.
Higher overvoltages are possible in the case of weak supply systems (high source
impedance) or isolated power plants where the dynamic response of machines adds to over-
voltages, and in particular when a relatively long line or cable is left connected to the
generator following load rejection at the receiving end. This condition can even lead to the
Ferranti effect.
The Ferranti effect is a condition where the voltage at the receiving end of the line can rise to
a value in excess of that at the sending end of the line or cable. This phenomenon is due to
the voltage gain across the capacitive elements of the line or cable. The Ferranti effect can
result in significant overvoltages appearing at the receiving end of very long lines, therefore it
is more often a associated with HV and EHV networks and rarely a problem for MV networks.
The Ferranti effect also poses a risk of resonance and ferroresonance when the long lines are
terminated by unloaded transformer, leading to transformer saturation and an overvoltage
with a distorted waveshape. Where it is identified that overvoltages due to load rejection are
likely to reach an unacceptable level, protective systems need to be implemented to ensure
that such overvoltages will be of short duration.
6.3 Self-excitation
Self-excitation can occur where the load on a generator becomes capacitive or where a motor
is left disconnected with capacitors in parallel.
In the case of synchronous machines, the armature reaction can cause excessive voltage rise
as a result of an increase of the exciting flux. To maintain the voltage within the acceptable
range of values, a negative field may be needed, but it is not always sufficient.
Additionally, if the generator suddenly becomes islanded on a capacitive load following load
rejection, the generator will accelerate. The machine reactance increases with frequency
while the capacitive reactance decreases. Therefore it is important to make sure that self
excitation does not occur, not only at nominal frequency, but also as a consequence of
frequency variations resulting from load rejection. When self-excitation is possible during
overspeed conditions, it is important to ensure that the generator is disconnected quickly
before reaching the critical frequency in order to avoid losing control of the voltage.
Self excitation is also possible for induction generators and motors. As a countermeasure, it is
often recommended to limit the amount of reactive power compensation to less than about
30 %, of the motor or generator rating, to reduce the risk of self excitation in the case of
islanding or load rejection.
6.4 Resonance and ferroresonance
6.4.1 Resonance
As a result of the interaction between the reactive and inductive components that are part of
every power system, each system will be inherently resonant at a certain frequency or
frequencies. As such resonance itself is not an exceptional phenomenon, however resonance

TR 61000-2-14 © IEC:2006(E) – 17 –
as a cause of overvoltage may be exceptional, but should be recognised. There are two
general conditions that need to be satisfied before the normal voltage can be amplified by
resonance:
– there needs to be sources of harmonics at the right frequency or frequencies in order to
excite resonances;
– system damping (e.g. resistive loads) must be relatively small.
Therefore, resonance is more likely to cause overvoltages when combined with other sources
of overvoltages that create favourable conditions. For instance, overvoltages due to load
rejection may cause transformer saturation leading to harmonics which may be amplified by
resonance and add to overvoltages. Resonance is also possible during switching transients
such as switching-in an unloaded transformer with a harmonic rich content of inrush current.
This is a particular problem when switching a transformer that only has a capacitive load e.g.
energizing capacitors or filters or a long section of cable.
6.4.2 Ferroresonance
6.4.2.1 General
Ferroresonance is a rare phenomenon compared to single line earth faults. It is associated
with the saturation of magnetic cores in conjunction with relati
...

IEC TR 61000-2-14 ®
Edition 1.0 2006-12
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
colour
inside
Electromagnetic compatibility (EMC) –
Part 2-14: Environment – Overvoltages on public electricity distribution
networks
Compatibilité électromagnétique (CEM) –
Partie 2-14: Environnement – Surtensions sur les réseaux de distribution
publics
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IEC TR 61000-2-14 ®
Edition 1.0 2006-12
TECHNICAL
REPORT
RAPPORT
TECHNIQUE
colour
inside
Electromagnetic compatibility (EMC) –

Part 2-14: Environment – Overvoltages on public electricity distribution

networks
Compatibilité électromagnétique (CEM) –

Partie 2-14: Environnement – Surtensions sur les réseaux de distribution

publics
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 29.240.01; 33.100.01 ISBN 978-2-8322-6681-6

– 2 – IEC TR 61000-2-14:2006  IEC 2006
CONTENTS
FOREWORD . 4
INTRODUCTION . 6

1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Description of overvoltages . 10
4.1 General . 10
4.2 External overvoltages . 11
4.3 Internal overvoltages . 11
4.4 Overvoltage waveshape . 11
5 Long duration overvoltages . 12
5.1 Sustained earth faults. 12
5.2 Broken neutral on LV network. 12
5.3 Maloperation of voltage regulating equipment. 13
5.4 Overvoltages due to voltage unbalances . 13
5.5 Dispersed generation . 14
6 Short duration overvoltages . 15
6.1 Earth faults . 15
6.2 Load rejection (sudden load loss) . 16
6.3 Self-excitation . 16
6.4 Resonance and ferroresonance . 16
7 Very short duration overvoltages (transients) . 18
7.1 General description . 18
7.2 Lightning . 19
7.3 Switching . 20
7.4 Summary of surge duration and cause. 26
8 Effects of overvoltages on equipment . 27
8.1 General considerations . 27
8.2 Reduction in life of filament lamps . 28
8.3 Effect of overvoltages on IT equipment . 28
9 Case studies . 29
9.1 General . 29
9.2 Switching of LV power factor correction capacitor . 29
9.3 Metal fusion furnace . 30
9.4 Switching of MV power factor correction capacitor . 31
9.5 DC traction system . 32
9.6 Load switching . 34
10 Protection against the effects of overvoltages . 36
10.1 General considerations . 36
10.2 Point on wave switching . 36
10.3 Arcing horns and spark gaps . 37
10.4 Overvoltage protection relays . 38
10.5 Snubbers (high frequency RC filter) . 39
10.6 Uninterruptible power supply (UPS) systems . 39
10.7 Surge protection device (SPD) . 39

11 Conclusions . 41
12 Recommendations . 42
Bibliography . 43

Figure 1 – Lightning impulse test voltage characteristic . 11
Figure 2 – Broken neutral on LV network . 13
Figure 3 – The effect of distributed generation on network voltage . 14
Figure 4 – Line – Neutral temporary overvoltage on healthy phase for single phase line
– earth fault . 15
Figure 5 – Typical transient overvoltage when energizing a capacitor bank . 22
Figure 6 – Notching caused by power electronics switching . 24
Figure 7 – ITI (CBEMA) curve for equipment connected to 120 V 60 Hz systems . 29
Figure 8 – Voltage waveform distorted by the energization of a PFC capacitor . 30
Figure 9 – Phase to ground overvoltage in case of a single (a) or multiple (b) faults . 30
Figure 10 – Equivalent circuit . 31
Figure 11 – Extruder connection – single line diagram . 32
Figure 12 – Current waveforms (phases A and C) taken at the main LV circuit breaker . 32
Figure 13 – Single line diagram of public transportation system . 33
Figure 14 – Voltage waveforms associated with overvoltages on public transportation
system . 34
Figure 15 – 20 kV line-to-earth voltages during breaking transformer current . 35
Figure 16 – Spark gap . 37
Figure 17 – Two-stage surge protection scheme . 40

Table 1 – Surges on the low voltage network . 26
Table 2 – Surges on the medium voltage network . 27
Table 3 – Reduction of filament lamp life . 28
Table 4 – Protective levels for typical MV surge arresters (effectively earthed neutral
systems) . 41

– 4 – IEC TR 61000-2-14:2006  IEC 2006
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 2-14: Environment – Overvoltages on public electricity
distribution networks
FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote
international co-operation on all questions concerning standardization in the electrical and electronic fields. To
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with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
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indispensable for the correct application of this publication.
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The main task of IEC technical committees is to prepare International Standards. However, a
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data of a different kind from that which is normally published as an International Standard, for
example "state of the art".
IEC 61000-2-14, which is a technical report, has been prepared by subcommittee 77A: Low
frequency phenomena, of IEC technical committee 77: Electromagnetic compatibility.
This bilingual version (2019-03) corresponds to the monolingual English version, published in
2006-12.
The text of this technical report is based on the following documents:
Enquiry draft Report on voting
77A/540/DTR 77A/547/RVC
Full information on the voting for the approval of this technical report can be found in the
report on voting indicated in the above table.
The French version of this standard has not been voted upon.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
The committee has decided that the contents of this publication will remain unchanged until
the maintenance result date indicated on the IEC web site under "http://webstore.iec.ch" in
the data related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates
that it contains colours which are considered to be useful for the correct
understanding of its contents. Users should therefore print this document using a
colour printer.
– 6 – IEC TR 61000-2-14:2006  IEC 2006
INTRODUCTION
IEC 61000 is published in separate parts according to the following structure:
Part 1: General
General considerations (introduction, fundamental principles)
Definitions, terminology
Part 2: Environment
Description of the environment
Classification of the environment
Compatibility levels
Part 3: Limits
Emission limits
Immunity limits (in so far as they do not fall under the responsibility of the product
committees)
Part 4: Testing and measurement techniques
Measurement techniques
Testing techniques
Part 5: Installation and mitigation guidelines
Installation guidelines
Mitigation methods and devices
Part 6: Generic standards
Part 9: Miscellaneous
Each part is further subdivided into several parts, published either as International Standards
or as technical specifications or technical reports, some of which have already been published
as sections. Others will be published with the part number followed by a dash and a second
number identifying the subdivision (example: 61000-6-1).

ELECTROMAGNETIC COMPATIBILITY (EMC) –

Part 2-14: Environment – Overvoltages on public electricity
distribution networks
1 Scope
This part of IEC 61000 describes the electromagnetic environment with respect to the
voltages in excess of normal that are found on electricity supply networks operating at low
and medium nominal voltages and that can be impressed on equipment connected to those
networks, without considering further effects (e.g. amplification or attenuation) within an
installation. Since these overvoltages have the potential to hinder the functioning of electrical
and electronic equipment, they fall within the definition of electromagnetic disturbance in the
field of EMC. Various categories of overvoltage are described, based on relative magnitude,
duration and energy content.
This Technical Report describes the phenomena of overvoltages, it does not specify
compatibility levels and does not directly specify emission and immunity levels.
The report describes the various phenomena and processes that cause overvoltages,
including the transfer into the networks concerned of overvoltages that originate in or traverse
other networks and installations, including higher voltage networks and the installations of
electricity users. The effects of overvoltages on equipment are outlined. Some case studies of
overvoltage events are presented.
Recommendations are made regarding the general technical approach to mitigating the risk of
equipment being hindered from operating as intended by the effects of overvoltages. (It is not
the function of IEC publications to assign responsibility for mitigating measures to any of the
parties involved.)
The purpose of this report is to ensure that this important category of electromagnetic
disturbance is included in the description of the environment in Part 2 of IEC 61000. For that
purpose, only a brief description is provided of the various overvoltages and their causes and
effects. A much more detailed treatment can be found in IEC 62066. A UIE publication –
Guide to quality of electrical supply for industrial installations, Part VI: Transient and
temporary overvoltages and currents – has a similar content. Measurement methods are
specified in IEC 61000-4-30.
NOTE This Technical Report does not include detailed measurement results for overvoltages, therefore it is not
possible to provide an assessment of the probability of occurrence.
2 Normative references
The following referenced documents are indispensable for the application of this document.
For dated references, only the edition cited applies. For undated references, the latest edition
of the referenced document (including any amendments) applies.
IEC 60050-161, International Electrotechnical Vocabulary (IEV) – Chapter 161: Electro-
magnetic compatibility
3 Terms and definitions
For the purposes of this document, the terms and definitions contained in IEC 60050-161 as
well as the following terms and definitions apply.

– 8 – IEC TR 61000-2-14:2006  IEC 2006
3.1
back flashover
flashover of phase-to-earth insulation resulting from a lightning stroke to that part of the
system which is normally at earth potential
3.2
breakdown
dielectric failure of an insulation under the effect of a strong electric field and/or by physico-
chemical deterioration of the insulating material
3.3
direct lightning stroke
lightning striking a component of the network, e.g.: conductor, tower, substation equipment,
etc.
3.4
declared supply voltage
U
c
normally the nominal voltage of the system. If by agreement between the electricity supplier
and the consumer a voltage different from the nominal voltage is applied to the supply
terminals, then this voltage is the declared voltage
3.5
disruptive discharge/flashover/sparkover
passage of an arc following dielectric breakdown
NOTE 1 The term “sparkover” (in French: “amorçage”) is used when a disruptive discharge occurs in a gaseous or
liquid dielectric.
NOTE 2 The term “flashover” (in French: “contournement”) is used when a disruptive discharge occurs over the
surface of a solid dielectric surrounded by a gaseous or liquid medium.
NOTE 3 The term “puncture” (in French: “perforation”) is used when a disruptive discharge occurs through a solid
dielectric.
3.6
indirect lightning stroke
lightning stroke that does not strike directly any part of the network but that induces an
overvoltage in that network
3.7
insulation coordination
selection of the dielectric strength of equipment in relation to the operating voltages and
overvoltages which can appear on the system for which the equipment is intended to operate,
taking into account the service environment and the characteristics of the available prevention
and protective devices
[IEV 604-03-08, modified]
NOTE In this instance, the term "dielectric strength of the equipment" means its rated or its standard insulation
level as defined in IEC 60071-1.
3.8
lightning arrester
surge diverter
/surge arrester/
surge protective device (SPD)
device designed to protect the electrical apparatus from high transient overvoltages and to
limit the duration and frequently the amplitude of the follow-on current

3.9
lightning impulse
voltage impulse of a specified shape applied during dielectric tests with a virtual front duration
of the order of 1 μs and a time to half value of the order of 50 µs
NOTE The lightning impulse is defined by the two figures giving these durations in microseconds; in particular the
standard lightning impulse is: 1,2/50 μs.
3.10
long duration overvoltages
overvoltage with a duration in excess of 10 min
NOTE The magnitude of a long duration overvoltage is typically given as a r.m.s. value.
3.11
nominal voltage
U
N
the voltage by which a system is designated or identified
3.12
overvoltage
any voltage having a value, either peak or r.m.s., exceeding the maximum value of the
corresponding declared voltage
3.13
per unit (p.u.)
methodology used to simplify equations and the presentation of electrical parameters by
expressing them as a fraction of a reference parameter:
 Actual
p.u. value =
 
Base
 
where the Actual and Base values are of the same quantity, e.g. voltage, current, impedance
etc.
NOTE Typically the Base value for voltage is the nominal voltage for fundamental frequency phenomena and the
peak line to ground voltage for transients.
3.14
power frequency withstand voltage
r.m.s. value of sinusoidal power frequency voltage that the equipment can withstand during
tests made under specified conditions and for a specified time
3.15
rise time (of a pulse)
the interval of time between the instants at which the instantaneous value of a pulse first
reaches a specific lower value and then a specific upper value
NOTE Unless otherwise specified, the lower and upper values are fixed at 10 % and 90 % of the pulse magnitude.
3.16
short duration overvoltage
voltage swell
power frequency overvoltage with a duration lasting greater than one period (one cycle) and
up to 10 min
NOTE The magnitude of a short duration overvoltage is typically given as a r.m.s. value.

– 10 – IEC TR 61000-2-14:2006  IEC 2006
3.17
surge
transient voltage wave propagating along a line or a circuit and characterized by a rapid
increase followed by a slower decrease of the voltage
[IEV 161-8-11]
NOTE In some parts of the world the term “Impulse” is used to describe a short duration overvoltage
characterised by a very rapid change in magnitude with a duration less than 200 µs.
3.18
temporary overvoltage
oscillatory overvoltage (at power frequency) at a given location, of relatively long duration and
which is undamped or weakly damped
NOTE Temporary overvoltages usually originate from switching operations or faults (e.g. sudden load rejection,
single-phase faults) and/or from non-linearities (ferroresonance effects, harmonics).
3.19
transient
pertaining to or designating a phenomenon or a quantity which varies between two
consecutive steady states during a time interval short when compared with the time-scale of
interest
[IEV 161-02-01]
3.20
very short duration overvoltage (transient)
overvoltage with a duration from less than a microsecond to several periods at fundamental
frequency
NOTE The magnitude of a very short duration overvoltage is typically given as a peak value.
3.21
voltage impulse
transient voltage wave applied to a line or equipment, characterized by a rapid increase,
followed generally by a slower non-oscillatory decrease of the voltage
3.22
front time
T
for a lightning impulse voltage T is a virtual parameter defined as 1,67 times the interval T
between the instants when the impulse is 30 % and 90 % of the peak value on the test voltage
curve (points A and B, Figure 1)
3.23
time to half-value
T
for a lightning impulse voltage T is a virtual parameter defined as the time interval between
the virtual origin, O , and the instant when the test voltage curve has decreased to half the
peak value
4 Description of overvoltages
4.1 General
Overvoltages are an intrinsic phenomena present on all networks. Overvoltage events can be
created in the public network or in the electricity user's installation. The dynamic response of
a network to load switching, both planned and unplanned (faults) will result in the storage and
release of energy. This transfer of energy will cause an overvoltage to be propagated within
the network.
4.2 External overvoltages
Overvoltages that are caused by events that are external to an installation, for example:
lightning strokes and faults on adjacent higher voltage networks, are generally very short term
overvoltage travelling waves. They attenuate with distance and the wave front becomes less
steep. In addition there are longer term overvoltages caused by load rejection, open circuit
neutrals, faulty voltage control equipment and the effect of distributed generation.
4.3 Internal overvoltages
Events within an installation can give rise to overvoltages, for example: the switching of non-
linear load, switch arcing, and fuse operation.
4.4 Overvoltage waveshape
A common method of representing the waveshape of a very short term overvoltage is shown
and the time to half-value, T . For
in Figure 1. The important values are the front time, T
1 2
example, typical values for a transient overvoltage caused by lightning are 1,2 µs for the front
time and 50 µs for the time to half-value (a 1,2/50 µs waveform).

Figure 1 – Lightning impulse test voltage characteristic
NOTE Figure 1 is only meant to represent an example of one type of overvoltage. Other types of overvoltage are
described in IEC 60071-2.
Other very short duration overvoltages having the shape of a damped high frequency
oscillation can be caused by events such as energizing capacitor banks, although their
amplitude is often much lower than an overvoltage caused by a lightning stroke, and the rate
of occurrence can often be higher. This type of very short duration overvoltage can propagate
over long distances and across voltage levels, hence adverse effects can often be seen some
distance from the point of initiation. This is particularly true when the overvoltages are
transferred to the lower voltage networks where the resilience of equipment is at its lowest.
The situation at all voltage levels can be further exacerbated if a resonance condition is
created, i.e. when the frequency of the transient overvoltage is close to the natural frequency
of the network and or equipment connected to the network.
When more than one type of overvoltage event occurs simultaneously, it can lead to
overvoltages in excess of the values quoted for a single event.

– 12 – IEC TR 61000-2-14:2006  IEC 2006
5 Long duration overvoltages
The overvoltages presented in this clause are typically described as being of long duration,
however it should be noted that there will be instances where for a particular event the
overvoltage could last for less than 10 min.
The overvoltages presented in this section are 50/60 Hz overvoltages.
5.1 Sustained earth faults
In MV networks with isolated or high-impedance grounded neutral, this kind of fault will
produce line to earth temporary overvoltages on the healthy phases. The overvoltage will last
for the duration of the fault, this can be anything from parts of a second for conventionally
earthed systems up to some hours for systems earthed via a tuned reactance (Petersen coil
earthing). Generally the magnitude of the overvoltage will not exceed twice the nominal phase
to earth voltage, i.e. √3 × U, where U can be up to 1,1 × U if the voltage is at the maximum
N
of the acceptable MV range. The overvoltages last until the faulted section of network is
disconnected.
Earth faults on the MV network can result in temporary power frequency overvoltages
between live conductors and earth on the LV network. The duration and magnitude of these
overvoltages will be dependent on the fault conditions and the MV earth impedance, as
described above.
The majority of public LV distribution systems are operated with a solidly earthed (grounded)
neutral. Therefore when earth faults occur on the MV network that raise the ground potential
in the vicinity of the LV network it is possible for an overvoltage to exist between the phase
and earth conductors of the LV network. The duration is limited by the time taken for the MV
protection and circuit breaker to clear the fault, typically no more than 5 s. The magnitude of
the overvoltage will generally not exceed 1,5 kV r.m.s., however this is dependent upon the
impedance of the LV ground connection and the magnitude of the MV earth fault current.
IEC 62066 contains a comprehensive description of this type of overvoltage.
5.2 Broken neutral on LV network
For a three-phase LV network supplied from a star (wye) transformer winding or for a two
phase network supplied from a transformer with a centre-tapped neutral at LV (sometimes
referred to as a three-wire network), if the neutral becomes disconnected (e.g. broken due to
a fault), single-phase loads beyond the break could experience an overvoltage up to the line
voltage of the network. The exact magnitude of the voltage will be dependent upon the ratio of
the impedance (loads) connected across each phase of the network – see Figure 2 below.
This type of overvoltage can persist for several hours or, in rare cases, days until the neutral
has been reconnected or the faulty network has been disconnected in readiness for repair.
This disconnection is often by manual intervention following complaints of severe voltage
fluctuations which occur as a result of changes in load.

Figure 2 – Broken neutral on LV network
In the event of a broken neutral as shown in Figure 2 above, the voltage that appears across
load A and load B is determined by the relative magnitude of these two loads, i.e.:
 
Z
A
Voltage across load A (U ) =   ; and
U ⋅
A L
L
 
1 2
Z + Z
 A B
Voltage across load B (U ) = U − U
B L A
L
1 2
Hence, depending upon the values of Z and Z it is possible for U to vary between near
A B A
zero and full line voltage ( U ).
L
L
1 2
NOTE Depending on the impedances and their phase shift, the voltage on the unloaded phase, phase L (L -N),
3 3
could theoretically be higher than the full line voltage.
5.3 Maloperation of voltage regulating equipment
Maloperation of automatic voltage regulation systems can sometimes lead to long duration
overvoltages between 1,1 and 1,2 p.u. at most. For instance, this could be due to a loss of
regulator voltage reference causing the tap changer to boost the voltage at its maximum, or
inadequate line drop compensation settings following unplanned load transfer on a regulating
transformer. Appropriate voltage regulator blocking relays can minimize risks of such
situations.
5.4 Overvoltages due to voltage unbalances
The combined effect of voltage unbalances and steady state voltage close to the maximum
agreed voltage tolerance can result in long duration overvoltages. This is the case in
particular for effectively earthed distribution systems supplying single-phase loads connected
line-to-neutral through an equivalent Y-y earthed MV/LV transformer connection (typical in
North America). In such cases, not only negative-sequence voltages can be transferred due to
load unbalance, but also zero-sequence voltages as well. The latter also depends on the
zero-sequence system impedance. In some cases, the combination of steady state voltages
near the upper limit, and the negative-sequence plus the zero-sequence voltage unbalances
can lead to permanent line-to-neutral voltages on some phases in the range of 1,1 p.u. at MV
and LV. Voltage regulators whose voltage reference is connected line-to-neutral can however
compensate the effect of zero sequence voltage unbalance thus reducing risks of this kind of
overvoltage.
– 14 – IEC TR 61000-2-14:2006  IEC 2006
5.5 Dispersed generation
In the absence of distributed generation it is typical for public distribution networks to have
been designed on the basis that energy flows in one direction i.e. from the source (substation)
to the point of utilization. Therefore it is typical for the voltage to be a maximum at the source
and to decrease with distance away from the source.
In some areas it is typical for MV/LV transformers to have an adjustable transformation ratio
(tap setting). The tap setting can only be adjusted off load and is selected with a view to
offsetting the voltage drop in the MV network.
MV and LV networks are designed such that under conditions of no load the voltage at
source, be it MV or LV, is as close as possible to the maximum agreed voltage tolerance. This
should ensure that the supply delivered to loads at the remote end of the network will remain
within agreed tolerances at times of peak demand.
The presence of distributed generation within the network can have the effect of increasing
the voltage level at the point of connection and therefore modifying the voltage distribution
profile. In Figure 3, the lower curve shows how the network voltage decreases with distance
from source, while the upper curve shows how the voltage profile can be raised if distributed
generation (DG) is connected between the source and the receiving end. This effect is
exacerbated if generation export coincides with periods of low demand (load) on the network.
NOTE For the purposes of describing Figure 3, only the terms U and U are introduced.
R RDG
In the absence of distributed generation the voltage at the receiving end is U and as
R
mentioned previously, MV/LV transformers with off-load tap settings have been adjusted to
compensate for the line voltage drop ∆V. If distributed generation is connected to the network,
the receiving end voltage is raised to U and the LV will also rise. In the absence of
RDG
suitable voltage regulation or overvoltage protection it is possible that the distributed
generation could cause the network voltage to exceed agreed tolerance levels.
For networks with high source impedance (sometimes referred to as “weak networks”) the risk
of voltage rise could be the limiting factor in determining the amount of distributed generation
that can be connected.
Figure 3 – The effect of distributed generation on network voltage

6 Short duration overvoltages
6.1 Earth faults
As explained in section 4 the method of earthing and the value of the neutral to earth
impedance will determine the magnitude of overvoltage that will occur on the healthy phases
during earth faults. Various types of neutral earthing are used from solid or effective to the
high impedance earthing.
The term effectively earthed neutral applies to a system, or portion of the system, where the
ratio of zero-sequence reactance to positive-sequence reactance is positive and not greater
than 3 and the ratio of zero-sequence resistance to positive-sequence reactance is positive
and not greater than 1, as viewed from a considered location for any condition of operation:
0 ≤ X /X ≤ 3 and 0 ≤ R /X ≤ 1
0 1 0 1
where
X is the positive-sequence reactance;
X is the zero-sequence reactance;
R is the zero-sequence resistance.
For effectively earthed neutral systems, the overvoltage on the healthy phases is limited to
less than 1,4 p.u.
Figure 4 below illustrates the maximum line to neutral (L-N) overvoltage on healthy phases for
different values of the impedance factors X /X and R /X used for defining the effectiveness
0 1 0 1
of the neutral earthing
Figure 4 – Line–neutral temporary overvoltage on healthy phase
for single phase line – earth fault

– 16 – IEC TR 61000-2-14:2006  IEC 2006
In some countries, in order to reduce earth fault currents, networks are earthed via a high
impedance component (resistor/reactor), this requires that all network components have to be
rated for full line voltage.
6.2 Load rejection (sudden load loss)
Sudden loss of load on the MV or HV networks can result in a temporary overvoltage before
the automatic voltage control can correct the situation and bring the voltage back within limits.
The magnitude of the overvoltage depends on the magnitude of the source impedance (lower
impedance systems will see less change in voltage) and the size and characteristics of the
load. Typically the overvoltage will be in the range of U + 3 %, but in rare cases, such as
N
faults, it could be up to 6 %. Typically it can take up to three minutes before the tap changer
can stabilise the situation and bring the voltage back within limits.
Higher overvoltages are possible in the case of weak supply systems (high source
impedance) or isolated power plants where the dynamic response of machines adds to over-
voltages, and in particular when a relatively long line or cable is left connected to the
generator following load rejection at the receiving end. This condition can even lead to the
Ferranti effect.
The Ferranti effect is a condition where the voltage at the receiving end of the line can rise to
a value in excess of that at the sending end of the line or cable. This phenomenon is due to
the voltage gain across the capacitive elements of the line or cable. The Ferranti effect can
result in significant overvoltages appearing at the receiving end of very long lines, therefore it
is more often a associated with HV and EHV networks and rarely a problem for MV networks.
The Ferranti effect also poses a risk of resonance and ferroresonance when the long lines are
terminated by unloaded transformer, leading to transformer
...