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> Invited Paper for WOMEL 2016
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Abstract—Lightning surges represent, as a rule, a major cause
of momentary and sustained outages on overhead power lines.
They may result from direct hits or induction by nearby strokes.
Although the magnitudes of the transients associated with direct
strokes are usually much higher than those induced by nearby
strokes, the latter are much more frequent and of more concern to
overhead distribution systems with rated voltage 15 kV and less.
In this paper, discussions are provided on the basic characteristics
of the lightning-induced voltages, their dependence on various
parameters involved in the induction mechanism, and the
effectiveness of the main protective measures. Some priority
issues in lightning research are also indicated.
Index Terms— electromagnetic induction, lightning, lightning-
induced voltages, overvoltage protection, power distribution lines.
I. INTRODUCTION
IGHTNING transients have usually an important impact
on the power quality of distribution systems. The
understanding of the characteristics of the overvoltages
induced by nearby strokes and their dependence on the several
parameters involved in the induction mechanism is essential
for the decision-making process about the most suitable
protection measure that could be adopted for a particular
power distribution network.
In the next sections the subject of lightning-induced
overvoltages on overhead power distribution lines is discussed
considering the following aspects: coupling models, voltage
characteristics and their dependence on various parameters,
and effectivenesses of the main protective measures. Finally,
some priority issues in lightning research are indicated.
II. VALIDATION OF COUPLING MODELS
Various coupling models have been proposed for the
analysis of the response of transmission lines excited by
external electromagnetic fields, as e.g. the Agrawal et al. [1]
and the Rusck [2] ones. As demonstrated by Cooray [3] and
Michishita and Ishii [4], for the case of an electromagnetic
field radiated by a lightning channel perpendicular to the
ground plane, these models are equivalent and lead to identical
results. Since the validity of any model should be verified,
Manuscript received March 2, 2016.
A. Piantini is with the Institute of Energy and Environment, University of
São Paulo, São Paulo, Brazil (phone: + 55 11 3091-2580; email:
whenever possible, by means of comparisons with reliable
experimental results, a scale model system was developed and
implemented with this purpose [5]-[8]. This unique facility
enabled tests to be performed under controlled conditions; data
obtained from the simulation of a wide variety of situations
were used to confirm the validity of the Agrawal et al. model
[1] and the Extended Rusck Model (ERM) [8], [9], as well as
for the analysis of the lightning electromagnetic pulse response
of complex electric power networks [6], [10], [11].
For illustration, comparisons between measured and
calculated induced voltage waveforms on a three-phase line
with either a shield wire 1 m below the phase conductors
(h = 10 m; hg = 9 m) and grounded every 450 m (ground
resistance Rg equal to zero) or surge arresters installed on all
phases every 450 m (Rg = 200 Ω) are presented in Fig. 1. The
detailed simulation conditions can be found in [8].
(a)
(b)
Fig. 1. Measured (scale model) and calculated (ERM) phase-to-ground
induced voltages at the point closest to the stroke location on a line with a
shield wire or surge arresters. Perfectly conducting ground. (Voltage and time
scales referred to the full-scale system.) Simulation details can be found in
[8]. a) line with a shield wire b) line with surge arresters
Lightning-Induced Voltages on Overhead Power
Distribution Lines
Alexandre Piantini, Senior Member, IEEE
L
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As mentioned in [8], the ERM is an extension of the Rusck
model in the sense that it allows to take into account situations
of practical interest such as the case of a line with various
sections of different directions, the presences of a multi-
grounded neutral or shield wire, and equipment such as
insulators, transformers, and surge arresters. The incidence of
lightning flashes to nearby elevated objects and the occurrence
of upward leaders can also be considered, as well as the effect
of the soil parameters. All the calculation results presented in
the next sections were obtained using the ERM.
III. CHARACTERISTICS OF LIGHTNING-INDUCED VOLTAGES
The induced voltage magnitudes and waveshapes depend on
many lightning parameters and are substantially affected by the
soil characteristics, the relative position of the line and the
stroke location, the observation point, and the network
configuration.
Despite their large variations, lightning-induced voltages are
characterized by shorter durations in comparison with
overvoltages associated with direct strokes. Examples of
measured induced voltages on two parallel 2.7 km long non-
energized conductors – one with and the other without surge
arresters - can be found in [12]. The conductors are distant 6 m
from each other and their height above ground is about 10 m.
Fig. 2 shows the voltages induced simultaneously on the
conductors by a lightning flash which hit a tree located 61 m
from the closer one. The distance between the measuring point
and the nearest surge arrester was about 30 m.
Fig. 2. Phase-to-ground voltages induced simultaneously on two conductors
(measuring points in front of each other). Lightning strike point was a tree
61 m far from the conductor with surge arresters. Adapted from [12].
In comparison with a line located in a rural area, an urban
line is characterized not only by a higher density of
transformers and surge arresters but also by many laterals and
the presence of structures and buildings in its vicinity.
Regarding the line configuration, the induced voltages are
affected not only by the presence of the laterals but also by the
conditions of their terminations. If the laterals are long,
matched or ended by surge arresters, the induced voltages tend
to decrease. As shown in [11], the shorter the distance between
the observation point and the nearest lateral, the larger the
voltage reduction. On the other hand, if the laterals are
relatively short and terminated by an open circuit, the induced
voltages may increase as the distance between the observation
point and the nearest lateral diminishes [11].
Nearby buildings cause two opposite effects: firstly, they
attract lightning to closer distances in comparison to the case
of a line in open ground and therefore the overvoltages may
reach high magnitudes; secondly, they shield the line and
reduce the lightning electromagnetic field, and this leads to a
reduction on the induced voltages. For a fixed distance
between the line and the stroke location, the higher the
buildings, the lower the voltage magnitudes. For illustration
and considering the test configuration shown in Fig. 3, Fig. 4
compares two phase-to-ground induced voltages measured at
the transformer of the main feeder located 20 m from the
lightning strike point, for buildings' heights of 5 m and 15 m.
The experiments were performed on a scale model [13].
Because of the presence of laterals, shorter distances
between surge arresters, and shorter distances between line and
stroke location, lightning-induced voltages on urban lines tend
to present stronger oscillations in relation to those induced on
lines located in rural areas.
Fig. 3. Top view of the test configuration (scale model) relative to the
presence of buildings in the vicinity of the line. Distance of 20 m between
main feeder and stroke location. Triangles and circles represent transformers
and surge arresters, respectively, while squares and rectangles denote blocks
with buildings 5 m high. "M" corresponds to the measuring point. All
dimensions in meters. Adapted from [13].
Fig. 4. Measured induced voltages on an urban line configuration for
buildings' heights of 5 m (Fig. 3a) and 15 m. Stroke current of 50 kA with
front time of 2 μs. Distance of 20 m between line and stroke location. All
parameters values referred to the full-scale system. Adapted from [13].
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IV. PROTECTION MEASURES
One alternative to reduce the frequency of flashovers
associated with indirect strokes is to increase the critical
flashover overvoltage (CFO) of the line structures. However, a
consequence would be that surges with higher magnitudes
would then travel over long distances, thus increasing the
stresses on line equipment.
Other options involve the use of a shield wire or the
application of surge arresters. An evaluation of the
effectivenesses of these alternatives in mitigating overvoltages
associated with both direct and indirect strokes is done in [14].
The use of a shield wire in conjunction with arresters on every
pole and every phase is effective against direct strokes and
theoretically eliminates flashovers, but in any case a cost-
benefit analysis of the possible solutions should be performed.
In this section the effect of shield wires and surge arresters
in reducing lightning-induced voltages are discussed. In the
simulations, performed using the ERM, the line, with phase
conductors at the height h = 10 m, and the shield wire 1 m
above the middle phase (hg = 11 m), is assumed to be 2 km
long and lossless. The diameter of all the conductors is 1 cm,
and the inductance of the down conductor is 1 μH/m. In [15],
Rachidi et al. demonstrate that when the line length does not
exceed a certain critical value (typically 2 km), the surge
propagation along it is not appreciably affected by the finite
ground conductivity as long as the conductivity is not lower
than about 0.001 S/m.
A. Use of a Shield Wire
Due to its coupling with the phase conductors, a shield wire
reduces the amplitudes of voltages induced from external
electromagnetic fields regardless of its position with respect to
the phase conductors. However, the greater the coupling, the
more significant the voltage reduction.
Besides its height, the effectiveness of a shield wire depends
on the combination of several parameters such as the
grounding spacing (xg), the ground resistance (Rg), the soil
resistivity (ρ), the stroke current front time (tf), and the relative
position of the stroke channel with respect to the grounding
points. An analysis of the influence of these parameters, based
on computer simulations and test results obtained from scale
model experiments, was carried out in [16] considering typical
configurations of a rural distribution line and perfectly
conducting soil. In [17] the analysis has been extended to the
case of a finite ground conductivity.
In order to illustrate the effect of the shield wire, Fig. 5
presents phase-to-ground and phase-to-shield wire induced
voltages corresponding to different values of the ground
resistance. In the calculations, the stroke current was assumed
to have a triangular waveshape with amplitude I = 50 kA,
tf = 3 μs, time to half-value of 50 μs, and propagation velocity
(vf) equal to 30 % of that of light in free space (c). As shown in
Fig. 5a, in the absence of the shield wire the magnitude of the
induced voltage would be 500 kV.
It can readily be seen from Fig. 5 that the dependence of the
phase-to-shield wire voltages with the ground resistance is
more significant than that of the phase-to-ground voltages. In
addition, the phase-to-ground voltages increase with Rg, while
the phase-to-shield wire or phase-to-neutral voltages have the
opposite behavior. As shown in [17], the influence of the
ground resistance on both voltages is more pronounced when
the stroke location is in front of a grounding point.
The results presented in Fig. 5 are an indication that a shield
wire can substantially reduce the magnitudes of lightning-
induced voltages between the terminals of power equipment.
(a)
(b)
Fig. 5. Phase-to-ground and phase-to-shield wire lightning-induced voltages
at the point closest to the stroke location for different values of the ground
resistance. Stroke location equidistant from the closest grounding points.
I = 50 kA; tf = 3 μs; vf = 0.3 c; d = 50 m; h = 10 m; hg = 11 m; xg = 300 m;
ρ = 1000 Ωm. a) phase-to-ground b) phase-to-shield wire
B. Application of Surge Arresters
The installation of line arresters on all phases of MV
distribution systems can be effective in reducing the
magnitudes of the lightning-induced voltages provided that the
arresters' spacing is not too large.
An example that illustrates the influence of the distance
between adjacent arresters is depicted in Fig. 6, relevant to
experiments performed on a scale model simulating a 1.4 km
long 15 kV distribution line. The MO surge arrester protective
level was about 34 kV (for an impulse current of 5 kA, 8/20 μs
waveshape). Additional test details are described in [14]. In
Fig. 6, all the quantities are referred to the full-scale system by
applying the corresponding scale factors.
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Fig. 6. Measured induced voltages on lines with and without surge arresters,
at the point closest to the stroke location, for arresters' spacings of 300 m and
600 m. Lightning strike point 70 m from the line and equidistant from two
sets of arresters. I = 38 kA; tf = 3.2 μs; vf = 0.11 c; Rg = 50 Ω.
As shown in [14], the greater the value of the ground
resistance or the stroke current steepness, the smaller the effect
of the surge arresters. Therefore, depending on the line
configuration, surges induced by subsequent strokes may reach
higher magnitudes than those associated with the first one [8].
V. PRIORITY ISSUES IN LIGHTNING RESEARCH
Despite the existence of reliable models for the analysis of
lightning transients, the application of such models for
establishing the probability that a particular lightning event
detected by a location system could lead to a power outage is
still a challenging issue. This is due mainly to the fact that
overvoltages induced by nearby strokes depend on many
parameters and are particularly affected by the stroke location
and current magnitude and waveshape. Although important
progress has been made in the last years, it is essential to
continue efforts to develop more reliable power equipment
models, improve the performance of lightning location systems
(LLS) and diminish the uncertainties in the estimation of the
stroke location and current parameters. Therefore, priority
research topics should include:
- additional measurements of lightning current parameters
(first and subsequent strokes) in different locations;
- improvement of the accuracy in the estimation of lightning
locations and current parameters (peak values and front
times) from remote electromagnetic field measurements;
- development of models for positive lightning flashes;
- better characterization of lightning transients;
- development of more accurate models to represent the
behavior of power equipment subject to lightning surges.
VI. CONCLUSION
Lightning-induced voltages on overhead distribution lines
depend on many parameters, which may combine in an infinite
variety of ways. This paper presented some discussions on the
characteristics of such transients and their dependence on
some important parameters involved in the induction
mechanism. Examples illustrating the mitigation effects of a
multi-grounded shield wire and line arresters were presented.
The analyses involved simulations using a reliable model and
experiments performed on either a full or reduced-scale
system. Some priority issues in lightning research were
indicated.
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