Transient overvoltages and currents: lightning surges

APPLICATION NOTE
TRANSIENTS & OVERVOLTAGES: LIGHTNING SURGES
UIE
July 2015
ECI Publication No Cu0135
Available from www.leonardo-energy.org

Publication No Cu0135
Issue Date: July 2015
Page i
Document Issue Control Sheet
Document Title: Application Note – Transients & Overvoltages: Lighting Surges
Publication No: Cu0135
Issue: 03
Release: July 2015
Author(s): UIE
Reviewer(s): Roman Targosz, Stefan Fassbinder
Document History
Issue Date Purpose
1 June 2007 Original publication
2 January
2012
Adoption into the Good Practice Guide
3 July 2015 Review
Disclaimer
While this publication has been prepared with care, European Copper Institute and other contributors provide
no warranty with regards to the content and shall not be liable for any direct, incidental or consequential
damages that may result from the use of the information or the data contained.
Copyright© European Copper Institute.
Reproduction is authorised providing the material is unabridged and the source is acknowledged.

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CONTENTS
SUMMARY ........................................................................................................................................................ 1
INTRODUCTION .................................................................................................................................................. 2
ORIGIN OF LIGHTNING SURGES ............................................................................................................................. 4
DIRECT FLASHES TO OVERHEAD LINES............................................................................................................................4
INDUCED OVERVOLTAGES ON OVERHEAD LINES ..............................................................................................................4
OVERVOLTAGES CAUSED BY COUPLING FROM OTHER SYSTEMS .........................................................................................4
LIGHTNING SURGES TRANSFERRED FROM MV SYSTEMS ...................................................................................................5
SURGE MAGNITUDE AND PROPAGATION IN MV SYSTEMS..................................................................................5
SURGE TRANSFER TO THE LV SYSTEM..............................................................................................................6
SURGES CAUSED BY DIRECT FLASH TO LV LINES ..............................................................................................................7
LIGHTNING SURGES INDUCED INTO LV SYSTEMS .............................................................................................................7
OVERVOLTAGES CAUSED BY FLASHES TO THE STRUCTURES OR NEARBY ..............................................................................7
CONCLUSIONS.................................................................................................................................................... 9

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SUMMARY
This is a second publication in a series of Application Notes on transient overvoltages and transient currents in
AC power systems and customer installations.
For a general introduction to the subject, read Cu0134 – Transients and Overvoltages: Introduction.
This Application Note provides a more detailed description of lightning surges and their origins. It explains the
differences and consequences of:
 Direct lightning flashes
 Lightning flashes on MV and LV power lines – both nearby and at a distance
 Overvoltages caused by resistive, inductive, and capacitive coupling with a system carrying a lightning
current
The closer the lightning flash is, the higher the stress on the concerned structure. However, the likelihood of
such an event is lower than that of a remote lightning flash. Statistical considerations are an essential part of
the decisions to be made.
The structure of the power system that suffers from a lightning flash also has a significant influence on the
severity of the consequences. Although this structure is under human control, its parameters are generally
determined by considerations other than lightning protection.

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INTRODUCTION
Lightning is an unavoidable event that affects power systems through several mechanisms. The obvious
interaction is a flash on the power system, but other coupling mechanisms can also produce overvoltages, as
shown in Figure 2. To aid in establishing a better understanding of the diversity of mechanisms, this subclause
first presents a summary of the basic parameters of a lightning bolt between a cloud and any object at the
earth level.
Figure 1 – Possible lightning stroke terminations.
Significant lightning parameters include waveforms, amplitudes, and frequency of occurrence. The literature
contains data obtained by measurements as well as data produced by computations. Three types of coupling
mechanisms are reviewed that can produce overvoltages in low-voltage systems. While this discussion makes
reference to overvoltages, consideration of the current asociated with the overvoltage or the current initially
causing the overvoltage is an important aspect of the subject.
In the event of a direct strike on an electrical system, the immediate threat is the flow of lightning current
through the earthing impedances resulting in overvoltages. The effective impedance of the lightning channel is
high (a few thousand ohms). As a practical matter, the lightning current can be considered an ideal current
source. In the event of a nearby flash, the immediate threat is the voltage induced in circuit loops, which in
turn can produce surge currents. In the case of a distant flash, the threat is limited to induced voltages.
Therefore, the response of an electrical system to the lightning event is an important consideration when
assessing the threat.
The severity of the overvoltage appearing at the end-user facility reflects the characteristics of the coupling
path. Factors include distance and nature of the system between the point of flash and the end-user facility,
earthing practices and earth connection impedance, presence of SPDs along the path, and branching out of the
distribution system. All of these factors vary over a wide range according to the general practices put into
effect by the utility as well as local configurations.
The annual frequency of thunderstorm days worldwide is shown in Figure 2. Long-used for risk assessment,
this information is now being superseded by maps of flash density for regions where a lightning detection

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system is in operation. Flash density maps provide more accurate information than the traditional
thunderstorm days, and it is expected that they will supersede thunderstorm maps as they become more
widely available.
Figure 2 – Map of annual thunderstorm days.

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ORIGIN OF LIGHTNING SURGES
Lightning surges in electrical systems can generally be classified according to their origin:
1. Current surges due to direct lightning flashes on overhead lines, including back-over events
2. Induced overvoltages on overhead lines due to flashes at some distance, and the resulting surge
currents
3. Overvoltages caused by resistive, inductive, and capacitive coupling from systems carrying lightning
currents, and the resulting surge currents
These classes of overvoltages are briefly described in the following paragraphs.
DIRECT FLASHES TO OVERHEAD LINES
As mentioned earlier, the effective impedance of the lightning channel is high, and the lightning current can, as
a practical matter, be considered as an ideal current source.
The resulting overvoltages are therefore determined by the effective impedance that is seen by the lightning
current. For a flash to an overhead line conductor, the impedance in the first moments is determined by the
characteristic impedance (surge impedance) of the line. Given the typical values of characteristic impedances,
ranging from tens of ohms to 400 ohms, very high overvoltages occur that can be expected to cause flashover
to earth long before the service entrance of a building becomes involved. Therefore, the lightning surge
appearing at service entrances, while reflecting the severity of the lightning flash and its distance, bears no
resemblance to the actual lightning current. One should not attempt to relate published statistics on total
lightning current to the lightning surges impinging on the service entrance.
INDUCED OVERVOLTAGES ON OVERHEAD LINES
Due to the changes in electromagnetic field caused by a lightning flash, surges are induced in all kinds of
overhead lines, even at a considerable distance from the flash. The voltages have essentially the same value
for all conductors because the phase separation is small compared to the distance to the flash.
For instance, in a high-voltage line with a 10 metre conductor height for a lightning current of 30 kA, the
induced voltage is in the order of 100 kV for a flash at a 100 metre distance. For a low-voltage line with a
height of 5 metres, a current of 100 kA (the 5% percentile level according to IEC 61312 (E.1.3)) will induce a
voltage of about 2 kV even at a distance of 10 kilometres. However, here again, the induced voltage does not
necessarily appear at the service entrance: the high levels will provoke flashover to earth or operation of a
surge arrester, so that the surges appearing at the service entrances are more likely to be in the range of only a
few kilovolts. As noted before, these surges involve significant overvoltages but relatively small current levels.
This is in contrast with surges resulting from direct flashes where the current source aspect of the
phenomenon results in current surges that reflect the dispersion of the original stroke current among the
paths offered to this earth-seeking current.
OVERVOLTAGES CAUSED BY COUPLING FROM OTHER SYSTEMS
A lightning flash to earth or to a part of a system normally at ground potential can result in an earth potential
of high value at the point of flash (and in the vicinity). This phenomenon will cause overvoltages in electrical
systems using this point of earth as reference for their earthing system.

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Figure 3 – Example of resistive coupling from lightning protection system.
Figure 3 gives an example of such a case. The potential rise of the earthing system is determined by the
lightning current and the effective earthing impedance. In the first moment, the potential of the earth
electrode is determined by the local impedance that might be, for instance, 10 ohms. This means that a high
voltage is produced between the earthing system and electrical installations inside the building, with a high
probability of causing either insulation breakdown or the operation of SPDs. Following such events, current
impulses can flow into the various systems, determined mainly by their impedance to earth. In this way,
overvoltages are produced in the power supply system as well as in other connected services (tele-
communication, data and signalling systems, et cetera). Furthermore, overvoltages are transferred to other
buildings, structures, and installations. For instance, all power installations supplied from the same distribution
transformer as the one struck by lightning can be affected. Due to the high electromagnetic fields caused by
the lightning current, inductive and capacitive coupling to electrical systems that are close to a lightning path
can also cause overvoltages of concern (especially on electronic and data systems) causing failures and/or
malfunctions.
LIGHTNING SURGES TRANSFERRED FROM MV SYSTEMS
Because their structures are longer and higher than other structures located in their vicinity (houses, trees),
MV overhead lines are in general more exposed to lightning than LV lines. The number of lightning flashes
affecting the line depends on the keraunic level of the local area. The propagation of the surge through the MV
system and the transfer rate to the LV system depend on the physical construction of the system. Some
important differences can exist between the designs used in different countries.
The lightning surges in MV systems are caused by flashes or are induced by nearby flashes. In addition, back-
flashovers can occur from flashes striking earth wires or extraneous metal parts of structures or equipment, or
striking the earth close to a line structure.
SURGE MAGNITUDE AND PROPAGATION IN MV SYSTEMS
The surge propagation depends on the MV system structure and, in particular, on the surge-protective devices
installed. High-level lightning surges are generally attenuated quickly during their propagation on the line. This

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is due to losses and flashover across the line insulators. In practice, after a few line spans, the magnitude of an
overvoltage is reduced to the insulation levels of the line isolators. With the exception of direct strokes to the
MV/LV transformer or its vicinity, it can be assumed that overvoltages in an MV system are limited by the
insulation level of the line isolators. In a 20 kV system; this is about 150 kV to 180 kV. For wooden pole lines
without earthed cross-arms, however, much higher surges can occur.
A second limitation of the surge level is provided by the surge-protective devices. These are usually located at
the primary side of the C MV/LV transformer or at the entrance of an underground network. These protection
devices might be ZnO or SiC surge arresters or air gaps. The residual overvoltage (in the range of 70 kV for a
20 kV system, for instance) depends on the rated value and earthing impedance of the protection devices.
When air gaps are used, one can expect the lightning surge to be followed by a power frequency follow
current that can generate a temporary overvoltage.
SURGE TRANSFER TO THE LV SYSTEM
The overvoltage surges generated in the MV system by lightning are transferred to the LV distribution system
in two different ways:
1. By capacitive and magnetic coupling through the MV/LV transformer
2. By earth coupling
The transferred surge magnitude depends on many parameters including:
 LV earthing system (TT; TN, IT)*
 LV load
 LV surge-protective devices
 Coupling conditions between MV and LV earthing
 Transformer design
In case of a direct lightning flash to the MV line, the surge arrester operation or an insulator spark-over diverts
the surge current through the earthing system and can produce a resistive earth coupling between the MV and
LV systems. An overvoltage is transferred to the LV system as shown in the typical case of Figure 4. Depending
on the earthing impedance values, this earth coupling overvoltage can be much higher than the capacitive
coupling through the transformer.
Figure 4 – Typical earth-coupling mechanism

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In a TN system, if the neutral is also earthed at the customer installation, smaller overvoltages will occur. It
should also be noted that this kind of resistive coupling can be avoided by using a separate earthing system for
the LV portion of the transformer.
A typical value of the overvoltage transmitted by capacitive and electromagnetic coupling to the secondary of
the MV/LV transformer side is 2% of the MV phase-to-earth voltage between phase and neutral conductors
and 8% between phase conductor and earth. In some situations, this MV/LV transfer rate can be higher.
Induced lightning surges on the MV system produce much less surge current than direct flashes (usually less
than 1 kA). In practice, the overvoltages are transferred to the LV system only by capacitive coupling and do
not exceed a few kilovolts. In such cases, the overvoltage induced directly in the LV system (at least in the
portion that is near the lightning impact point) is in general higher than the one transferred from the MV side.
If an SPD operates or a sparkover occurs, the current will be small (usually less than 1 kA), and accordingly the
resistive coupling is negligible.
SURGES CAUSED BY DIRECT FLASH TO LV LINES
Extremely high potential overvoltages are produced when direct lightning flashes occur on overhead lines.
Flashover will result between all line conductors, and in most cases will earth somewhere in the vicinity of the
flash (primarily at the poles). Flashover can also occur in non-protected installations supplied by that line.
In a combined overhead line/cable system, the overvoltages will be somewhat reduced due to the lower surge
impedance of cables compared to overhead lines. The amount of reduction depends on the current duration
and on the total capacitance to earth of the system. However, this reduction is usually not sufficient to avoid
overvoltages exceeding normal insulation levels in LV installations. Therefore, direct flashes to LV lines should
generally be expected to cause damage.
As a practical matter, the overvoltages can be limited by SPDs that might be installed at the distribution
transformer and at the consumer's premises. However, such devices will be highly stressed, and a high risk of
damage to SPD elements can be expected in the case of direct flashes unless the SPDs are specifically designed
for that purpose.
LIGHTNING SURGES INDUCED INTO LV SYSTEMS
Estimates of prospective induced overvoltages in LV systems due to lightning at some distance from an
overhead line can be derived from considerations discussed in the lightning protection literature. For instance,
induced overvoltages in excess of what normal LV insulation can withstand might even occur for lightning
flashes up to 10 km distance from the line. This kind of surge is therefore a main concern in LV distribution
systems that include overhead lines, even though the end-user is more likely to be exposed to the residual
overvoltages associated with the flashover.
Lightning-induced overvoltages occur mainly between conductors and earth. The voltage difference between
the conductors is initially small, especially when twisted conductors are used. However, considerable line-to-
line stresses can also occur due to different loads on phase conductors (depending on the LV system),
interactions of surge protective devices, possible flashovers, et cetera.
OVERVOLTAGES CAUSED BY FLASHES TO THE STRUCTURES OR NEARBY
The preceding subclauses have addressed the situation prevailing when surges conducted by the power supply
impinge on the facility. A different situation occurs when lightning strikes a structure that is one of several
structures supplied in parallel by an LV power system. In that case, the earth-seeking current divides among

Page 8
the various paths available. These include local earth – the building earthing system – as well as distant earth
points through any and all metallic paths, primarily the power supply cable.
The current dispersion among the available paths will produce overvoltages primarily between the conductors
and local earth. Depending on the configuration of the LV installation and the presence or absence of SPDs,
these overvoltages can be large or can be moderate.
Overvoltages between conductors and local earth stress the insulation of connected equipment. Such
equipment usually has sufficient robustness to withstand these levels per IEC 60664 [E.1.3] recommendations.
In contrast to the equipment insulation, the working components of power equipment are stressed by
overvoltages appearing between conductors. At first glance, it might be rationalized that the most threatening
situation would be the overvoltages applied to the working components of the power equipment. However,
overvoltages to earth can become a problem, not so much for the power equipment insulation but as a result
of shifts in reference potentials between the power system and the communications system that may be
connected to the equipment.

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CONCLUSIONS
Lightning surges originate beyond human control. Their severity at the point of utilization of electric power
depends on many parameters determined by the point of impact of the lightning flash and by the structure of
the power system. Although this power system structure is under human control, its parameters are generally
determined by considerations other than lightning protection.
Lightning surges may be classified according to their point of impact: direct flashes, nearby flashes, and flashes
occurring at some distance. For direct flashes, the surges result from the flow of lightning current in the
structure of interest and the associated earthing system. For nearby flashes, the surges result from induction
of voltages in the conductor loops, and to some extent, a rise in earth potential associated with the lightning
current. For distant flashes, the surges are limited to those induced in circuit loops. Precise time and location
of individual lightning flashes available from lightning detection networks is useful for trouble-shooting specific
problems as well as improving the accuracy of risk assessment.
The closer the point of impact of the lightning flash, the higher the stress on the concerned structure. But the
likelihood of a direct or nearby flash is lower than that of a more remote lightning flash. In any case, statistical
considerations involving specific risk analysis, are an essential part of the decisions to be made concerning
protection against lightning surges (see IEC 62305, particularly Clause 2).

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