TECHNICAL BROCHURE

550
WG
Lightning Protection of Low-Voltage C4.408
Networks Procedures

Members
A. Piantini, Convenor (BR), B. Hermoso Alameda (ES), A. Borghetti (IT),
A. C. Britten (ZA), A. Galván Diego (MX), T. Funabashi (JP), L. Grcev (MK),
A. M. Haddad (UK), J. Hoeffelman (BE), M. Ishii (JP), J. McDaniel (US),
J. Michaud (FR), C. A. Nucci (IT), R. G. Olsen (US), M. Paolone (IT),
F. Rachidi (CH), B. Richter (CH), P. E. Munhoz Rojas (BR),
A. Rousseau (FR), V. Shostak (UA), W. H. Siew (UK),
A. S. Telento (HR), S. Yokoyama (JP)

Corresponding Members
H. Geldenhuys, (ZA), R. Zeng, (CN)

Introduction provision of an up-to-date review of the status of

why and how LV networks are to be protected against
Improvement of the reliability and power quality levels lightning is the essential reason for the preparation of
of electric systems is widely acknowledged as a matter this TB.
of increasing concern and priority; this is primarily
so because today’s equipment in medium-voltage Previous CIGRE work on lightning protection of
(MV) and low-voltage (LV) networks is much more distribution networks was conducted by CIRED/
susceptible to deficiencies in power quality than in the CIGRE JWG 05 and JWG C4.402 - “Protection of MV
past. Distribution networks are often located in areas of and LV Networks against Lightning”, who produced the
high lightning ground flash densities, and are therefore following documents:
inherently subject to disruption by lightning. This is, - “Protection of MV and LV networks against
however, not a new phenomenon in MV systems - what lightning - Part 1: Common topics”;
are new factors are the higher density of connected MV - “Protection of MV and LV networks against
and LV customers than previously was the case, and the lightning - Part 2: Lightning protection of medium
consequent growth in exposed and vulnerable electrical voltage networks”.
equipment being used in LV networks in particular.
Research done during the last years into the MV aspects The CIGRE WG C4.408 “Lightning protection of
of this problem has concentrated on the need to gain low-voltage networks” was established in 2009 to
a better understanding of the characteristics of the accomplish additional activity focused on LV power
lightning overvoltages. This research has been expanded networks and to prepare an application guide for
to include LV lines. The main reason for doing this is distribution utilities on practical methods of lightning
that the much lower overvoltage withstand levels of LV protection.
lines and networks (compared with those of MV lines)
make such networks, and especially the large amounts Scope
of vulnerable electronic equipment connected to them,
inherently susceptible to lightning. It has therefore been The TB relates to the general characteristics of
found necessary to do further research and thereby lightning transients of different origins on LV systems
to build on the excellent work already done on MV (voltages less than 1 kV) as well as their dependence upon
systems. Accordingly, the research reviewed in this TB the network configuration, connected equipment, and
deals not only with the characteristics of transients in LV stroke and earth electrical parameters. Also within the
networks, but also the development and standardisation scope of the TB are the appraising of the effectiveness of
of surge protection for the connected equipment. With surge protective devices on the mitigation of lightning
the foregoing as background, it can be stated that the overvoltages and the provision of guidance on •••

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the protection of LV networks and relevant equipment,
including specific applications such as wind turbines,
Chapter 6 treats the main principles of lightning
protection of LV systems. The most important
C4.408
photovoltaic, and d.c. traction systems. definitions and concepts related to surge protective
devices (SPDs) are given and general guidelines for
The analysis comprises an extensive literature review typical applications are described.
and makes use of data obtained from measurements
and calculations carried out using accurate models. Chapter 7 discusses the impact of the application
The simulations consider typical LV line configurations of surge protective devices for LV lines of different
and simple but effective models to represent the configurations, including consumers’ installations and
frequency-dependent behaviour of distribution the secondary side of distribution transformers.
transformers and consumers’ installations. The TB
refers mainly to LV networks containing overhead Chapter 8 deals with lightning protection for specific
lines, as underground networks are much less prone to applications for which lightning strokes are among
lightning disturbances. the most dangerous threats, namely wind turbines,
photovoltaic, and d.c. traction systems. It gives a general
Description of the TB overview of these systems and specific lightning-related
problems and provides guidance on the selection of SPDs.
This TB is organized into nine chapters. The first sets
the context of the study. In Chapter 9, the main conclusions are presented.

Chapter 2 provides a summary of the topics covered Conclusions

in the brochures produced by JWG C4.402. Part 1
presents basic information and principles related to The following conclusions have been drawn from the
lightning protection of MV and LV networks, whereas findings discussed in the TB:
Part 2 presents an application of the information
provided in Part 1 to the lightning protection of MV • cloud discharges give rise to bipolar pulses with
networks. very fast rise times and should be taken into account
in the evaluation of the interference problems caused
Chapter 3 presents information on the most by LEMP. A typical lightning flash within a distance
important network parameters required for the of a few kilometers may induce voltage pulses of some
assessment of lightning transients on LV systems, hundreds of volts peak-to-peak;
including high frequency models of distribution • due mainly to the shielding provided by nearby
transformers for the analysis of transferred surges. trees and structures, LV networks are in general
not that prone to direct strikes, but in some rural
Chapter 4 deals with consumers’ installations and and semi-urban areas exposed lines longer than
provides information about the sensitivity of various 1000 m do exist. In case of direct strikes, the
LV apparatus to the effects of lightning. Simplified resulting overvoltages lead to multiple flashovers
circuits, suitable for representing the frequency and unprotected connected equipment can be
dependent behaviour of the input impedances of damaged;
typical installations, are also presented. • a direct strike to an end-user building causes an
earth potential rise that may lead to the operation
Chapter 5 is devoted to the analysis of the of SPDs or to flashovers between the structure and
main mechanisms which can give rise to lightning the line conductors. In both situations a portion
transients on LV networks. The basic characteristics of the stroke current is injected into the power
of the surges associated with cloud discharges, direct line, producing overvoltages that propagate along
strikes to the LV system – including those to end-user the network;
installations -, indirect strikes, and transference from • voltages induced by nearby lightning have a high
the MV system are described. Particular emphasis frequency of occurrence and often reach large
is given to the latter two, which are in general the magnitudes. The severity of the surges depends
most important on account of their amplitudes and on many parameters. Secondary systems are in
frequencies of occurrence. The influences of the general more susceptible to subsequent strokes,
network configuration, LV power installations, and of but severe surges can also be produced by the
parameters such as the stroke current magnitude and first stroke. Phase-to-ground induced voltage
waveshape, distance between the line and the strike magnitudes can reach some tens of kilovolts in
point, and soil resistivity on the lightning overvoltages various points along the network. Phase-to-neutral
are discussed in the chapter. voltages of some kilovolts are common if •••

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surge protective devices are not used. If the strike
point is close to the line, an appreciable fraction
even if surge arresters are placed close to the
primary terminals. The mechanism is explained in
C4.408
of the total current may enter the system from the the TB. Although the application of LV arresters
neutral earth connections; can significantly reduce the lightning damage
• in the case of direct strikes to the MV network, rates of exposed transformers, it does not prevent
short duration pulses of several tens of kilovolts overvoltages from arising at service entrances;
may be transferred to the secondary circuit. The • use and location of SPDs for buildings and
overvoltages are characterised by oscillations structures with and without lightning protection
caused by the various reflections that occur systems are illustrated and discussed in the TB.
throughout the LV network and are strongly The application of SPDs to a power installation
affected by the spacing between adjacent earthing can effectively reduce the local overvoltages to
points and flashovers across MV and LV insulators. acceptable limits, but in some circumstances
The presence of SPDs at various places of the LV this may result in higher voltage stresses at
line does not prevent high voltages from arising at unprotected premises. Therefore, unless they are
unprotected points; applied at every service entrance, exposed sensitive
• the estimation of voltages transferred from the equipment can be damaged. Additionally, voltage
MV to the LV side requires the use of an adequate oscillations caused by reflections at various points
high-frequency transformer model. As shown in within the installation can give rise to internal
the TB, the transferred voltages estimated using overvoltages with higher magnitudes than those
the p-capacitance model tend to be substantially limited by the SPDs placed at the service entrance;
higher than those obtained using more accurate • overvoltage protection in wind power,
models. Attenuation of the overvoltage along photovoltaic, and d.c. traction systems requires
the secondary network can be observed only for special consideration. Stresses from lightning
stroke locations distant from the LV portions of surges can be very different depending on the
the system; system studied and each case requires specific
• in regions of high lightning activity, surges considerations. The TB provides guidance
originating in the LV side can be responsible for a and examples showing possible solutions for
great number of transformer failures or damages, overvoltage protection of these systems. 