Download Lightning Stroke Against Overhead Electric Lines: A Comparison

Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
ISSN 1843-6188
SOME ASPECTS CONCERNING THE INFLUENCES OF SURGE
ARRESTERS UPON LIGHTNING BEHAVIOR OF OVERHEAD LINES
Iuliana HRISCU, M. GUSA, M. ISTRATE
E.ON Moldova Distributie Iasi, [email protected]
Technical University Iasi [email protected], [email protected]
handling, less sealing problems, better overload
performance, reduced cost and market prices – are very
important when thousands of arresters shall be widely
distributed over a utility's distribution system.
Furthermore, distribution systems have the highest
need for line arresters, as they are usually not protected
by shield wires and their pole footing impedances are
high. The situation is different for transmission lines.
In the most cases they are reasonably shielded and
have acceptably low footing impedances. Even at
locations, where this is not the case, a decision for
applying line arresters is not as simple as for
distribution lines due to technical and/or economical
constraints. Today, all over the world (Fig. 1), line
surge arresters are installed on overhead lines of
transmission systems with voltages up to 800kV.
However, comparing with distribution systems and
despite their many advantages, the installation of surge
arresters in transmission systems represents a
phenomena with a low speed evolution.
Abstract: Main external cause for overhead lines outage
that may bring negative consequences from technical and
economical point of view is represented by lightning strokes.
Researches upon this phenomenon has been focused upon
various aspects, starting with the lightning strike itself and
its consequences and following with finding solutions for
better protection of overhead lines. Many countermeasures
for improving the lightning performance of the lines have
been investigated. Regarding the lightning behavior of
overhead lines, the influence of surge arresters, used in
various configurations, on the insulation stress is of much
interest. The aim of this paper is to study the behavior of an
110kV single-circuit overhead line when surge arresters in
different configuration are used. The analysis underlines
the effect of using different values of tower foot resistance
and lightning current amplitude.
Keywords: surge arresters, lightning current, overhead lines
1. INTRODUCTION
In order to prevent flashover effects, metal oxide surge
arresters, installed in parallel with overhead line
insulators, were used in distribution systems since 1975
(first time in Japan, in a 33 kV system). In the ‘80,
again in Japan were installed for the first time overhead
line surge arresters in 66 kV, 77 kV and 138 kV
systems. Many other utility companies – not only the
Japanese ones – have followed this example, therefore
today in power distribution systems are installed and
operate several tens of million of arresters.
Apart from general well-known advantages of gapless
surge arresters, by comparison with the old SiC
arresters and protective gaps, this tendency was
sustained by developing in the 1980`s of composite
housing for distribution systems surge arresters. The
benefits of these technologies like less weight, easy of
Figure 2 Arresters installed on 77 kV overhead lines
2. FIELD EXPERIENCE
Several studies were made so far, in order to analyze
the efficiency of arrester installation methods, for
example only on the upper phase, on all three phases or
other combinations.
In Japan are frequently used three main equipping
modes, depending on the number of circuits of the line
(Table 1): # 1 – all three phases of one circuit on a two
circuits line; # 2 – all three phases of both circuits of
the line; # 3 – one or two phases on a circuit of two
circuits line [1] .
Another important aspect is the connecting mode of the
arrester: directly at the ends of protected insulator
string or in series with an external gap.
Figure 1 Arresters used on overhead lines
all over the globe
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
From Japan experience gained in this matter by the
year 1998, some conclusions could be drawn,
regarding the insulation coordination. Firstly, in order to
obtain an efficient operation of the arrester, it is essential
a correct grading of spaces between gap and lower end
of the insulator string. Secondly, in order the arrester to
efficiently interrupt the impulse current in a very short
time even in polluted environment and bad weather, it is
necessary to determine correctly the gap distances and to
use an arrester housing specially created for these
conditions. From the switching voltages and tripping
point of view, it was noticed that arrester performances
rise directly proportional with gap length. But, from
insulation coordination point of view, it was noticed that
performances are better when the gap distance are
shorter. In conclusion, it can be said that before using
arresters on overhead lines, detailed studies on
efficiency of different methods of installing and
Figure 3 Compact line model using line arresters
In 2008, CIGRE Conference in Croaţia, focused the
gained experience in line arresters utilisation field from
different parts of the world.
One of the many cases of line arresters application
presented at this Conference is one of the 123 kV Ston –
Komolac line, from southern Croatia, an one-circuit line
44 km length, protected with ground wire and that crosses
an area with intense lightning activity and high keraunic
coefficient up to 70 days/year. Furthermore, due to very
high soil resistivity in that area, the tower foot resistance
is very difficult to be maintained below an accepted value.
Therefore this line was considered to have very low
performances regarding lightning behavior.
In the summer of 2007, 110 arresters were installed
hanging below the phase conductor (as in Fig.4), on
some of the 144 towers of Ston – Kolomac line, in
order to improve line performances with 50 – 60%.
Between the installation moment and when that article
was presented, 8 months have passed so that, as the
authors point out, the period was too short to generalize
the resulted conclusions. But in this period, only 4
outages occurred, equivalent to 6 outages per year rate,
and that can be seen as an improvement of line
behavior by 52%. If this outage rate maintains itself at
a low value, it will be considered to extend this pilot
project to other important transport lines. [3]
Along with the utilization of arresters on above
mentioned line, 61 of the arresters installed on the most
exposed towers, were equipped with Excount-II
monitoring sensors, (Fig 5) with main goal to
determine arresters behavior on line in moments when
surges occur. This real time scanning intelligent system
allow remote control and wireless data acquisition, as
Table 1 Application methods on OHL circuits
coordination of all component elements are needed. [1]
In Nordic states, appeared, starting with 1990, an
increasing demand of 420kV compact overhead lines,
especially in the residential areas, in order to reduce
electromagnetic field intensity and visual impact.
Because this area is characterized by a relatively low
keraunic coefficient (< 20 days/year), the absence of
grounding wires and application of line arresters was
considered as a possible solution. In this matter, a
feasibility study, that considered the application of line
arresters on upper phase at each tower in the
compacted line area (Fig 3) in order to protect the line
to lightning strikes was developed. In this way, the upper
phase behaves like a grounding wire, assuring a 30º
shielding angle for lower phases. Compacting the line and
implicit reducing the magnetic field were more important
than reducing the number of outages due to lightning
strikes, so the results were considered satisfying when a
reduction with 50%, of magnetic field and a reduction
of tower pattern was obtained as well.
ELECTRA Magazine published in October 1999
number, an article with the main scope to classify the
requirements that arresters must accomplish in order to
operate properly, a description of functioning
mechanism, an arresters classification and the standard
tests that they must pass.[2]
Figure 5 Line arresters with monitoring sensors incorporated
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
ISSN 1843-6188
after installation of 21 arresters. In the next years, the
outages rate maintained at a very low level.
In Brazil, Companhia Energética de Minas Gerais
(CEMIG), the main utility company in Minas Gerais
area manages 30 substations and approximately 5000
km of overhead lines, outspreaded in south-east Brazil,
on an area equivalent to France. [5]
The main problems of CEMIG regarding overhead line
service are generated by two factors: the keraunic
coefficient of the area and soil resistivity of some
regions. This environmental conditions determined
CEMIG to adopt some measures to avoid energy
losses, financial penalties and clients complains.
Figure 4. Line arresters hanging on phases of an OHL
well as monitoring the arresters activity revealing the
number and location of surge levels and current
flowing. [4]
China is one of the countries with most widely
applications of line surge arresters. Since ‘90, there
were installed several thousand arresters on lines
operating at voltages between 35 kV and 220 kV,
encountering no problems in operation.
As in many other countries, also in China lightning
strike are the main cause for 40 – 70% of outages
number, mostly in areas with high lightning frequency,
high soil resistivity and rough terrain. For example, in
near 20 years of operation, 19 of 33 outages registered
on 9 of the most important transmissions lines
operating at 500kV in Hubei area, resulted from
lightning strikes. Among all protection methods used
on international level as: installing of shield wires,
reducing the protection angle, reducing tower foot
resistance, increase the insulation level, the most
efficient was installing line arresters in parall with line
insulator strings (Fig 6).
Using line arresters, the outage number due to
lightning strikes decreased significantly. For example,
on 110 kV Xi-bai from Guangdong area, the outage
rate decreased from 15,5 outages per 100 km per year
Figure 7 Line arrester mounted on a 400kV
sustaining tower
One of the lines with low performances due to
lightning strikes is OHL 230kV Guilman Amorim –
Ipatinga 1, which in period 2000-2001 registered 6,
respective 5 outages. First actions in analyzing and
improving the performances were related to measuring
of tower foot resistance, which proved to be at very
high levels. In these conditions, the solution was the
installation of line arresters on overhead lines. So,
through analysis and modeling, it was obtained the
optimum method to equip the towers, according to the
tower foot resistance values. All these actions had
satisfactory results that reflected upon energy quality
and clients satisfaction. [5]
In Romania, national company SC Transelectrica SA
who manages the transmission system with voltages
above 220kV, also confronted with the necessity of
improving both the reliability of transmission lines and
power quality offered to the customer. So, the company
focused on 220kV and 400kV overhead lines,
especially on the segments that cross areas with high
keraunic coefficient and high soil resistivity, where
shielding wires are missing. In a first stage, installing
the line arresters parallel with every insulator string on
OHL 400kV Braşov–Gutinaş sector 130-145 was
decided. (Fig. 7) [6]
Results show a decisive decrease of flashover rate on
this OHL after installing the line arresters, meaning
that this solution is justified both economical and
technical. There are still some operating problems due
Figure 6 Line arrester mounted on a sustaining tower
in 1999, to 4 outages per 100km per year in 2000,
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
ISSN 1843-6188
The comparative results between the cases in which
two types of arresters were used, presented in Fig. 9,
shows that the arrester type has no significant influence
upon the outages rate on the unprotected circuit 2,
when protected circuit 1 is equipped on the external
phases. [7]
As mentioned before, the arresters application on
overhead lines was studied also as a solution of
reducing the visual impact of the lines. Two
researchers from Denmark have made an analysis on a
400 kV line with the configuration presented in Fig.
10, in order to find if ground wires can be eliminated
and arresters can be used to assure line protection
instead. So, using PSCAD/EMTDC they have modeled
a line with frequency depending distributed
parameters. To the obtained model (which includes
also shield wires) were applied lightning current
impulses of 100kA – 1.2/50µs, considering that this
value can be applied to the system without producing
the insulation flashover at a certain tower.
The conclusions show that
in a this kind of
configuration of the 400kV
OHL, from the protection
level point of view, only
the
protection
with
arresters on upper phase at
all towers approaches to
Figure 10
the
protection
level
400kV line
configuration
obtained
with ground
in Denmark
wires, considering that in
this study only lightning
strikes on upper phase
were taken into analysis.
So, in order to renounce to
the shield wires in favor of line arresters application on
the upper phase, this analysis must be extended and
must be overviewed the lightning strikes on the other
two phases.
Another conclusion shows that renouncing to the shield
wires, installing of line arresters might demand a
resizing of the geometrical distances in tower
configuration, in the direction of increasing them. In
this case, must be reconsidered the visual impact
created by these modifications, and compared both
from the technical-economical point of view and safety
in operation.
As it can be seen in those presented above, line metaloxide arresters application on overhead lines in order to
protect them, remains a modern solution that can prove
to be both reliable and viable from the technicaleconomical point of view. But, the complexity of the
phenomena that occur when lightning surges appear
and arrester function leave enough space for questions
and opportunity studies.
Figure 8 Line arrester separated by phase conductor by
connection assembly breakage
to some errors in assessment of area environmental
stresses. These leaded to wrong dimensioning of the
arrester assemble and appearance of a weak point. Due
to extremely strong wind, the connection device broke
and three of the arresters were disconnected from the
phase conductor. Because of the same strong wind, the
arresters turned and the suspension shackle was
unscrewed from the arresters upper fixture, leading to
arresters falling down and be damaged. It is shown in
Fig 8 a part of the connecting wire hanging (red) and
another on the phase wire (yellow). So, these problems
were solved, in order to eliminate the damages related
to mechanical behavior. But, using the line arresters on
OHL is still both a modern one and viable one, in order
to improve the stability of power system.
3. RESEARCH RESULTS
From the theoretical point of view, the operation of line
arresters was modeled for different types of lines and
different installing methods, in different programming
environments, most frequent being used EMTP.
French researchers, in collaboration with HydroQuebec Canada, have modeled in a reviewed version
of EMTP (EMTP-RV), a 400 kV double-circuit line
and the results were presented in 2008 in Croatia. The
line is protected with two ground wires, but a
significant number of towers are situated in mountain
areas where an optimal value for the tower foot
resistance is hard to obtain. Therefore it was suggested
to use line arresters. Firstly it was decided to make a
detailed study of the technology to use was carried out,
concerning the type of arrester best fitted (gapped or
gapless), the arresters class, the towers where the
arresters will be installed and the phases that will be
protected. [7]
4. THEORETICAL BASIS
Figure 9. Flashover rate on circuit 2, when both external
phases of circuit 1 are protected by arresters
gapless
When lightning strikes the tower top, the impulse
current flows through the tower and its foot resistance
and gapped
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
to earth. If the ground wires are missing, the voltage
applied to the insulators strings can be considered
equal to tower potential at the console level. When
ground wires are present, a contribution is added by the
coupling with the phase conductors. The stress of line
insulation appears only at the stroked tower, and when
ground wire are present, to the next towers too. If a
flashover appears on an insulation string, a part of the
lightning current will be taken over by the phase
conductor. The size of this component depends on
ration between surge impedances of conductor and
tower. Therefore, current waves propagate in both
directions apart from stroken tower creating insulation
stresses at the adjacent towers.
Insulation stresses are mainly influenced by three
factors: lightning current amplitude, lightning current
front duration and tower foot resistance. Another
parameter, tower surge impedance can be considered
constant because it is identical for all towers of a line.
A problem that can be solved by assisted simulation is
the rate of voltage drop along the tower, which
influences the stress on insulation of different phases
and complicates in this way the determination of
critical current level. Because the tower model is a
RLC circuit, the front time of the lightning current
wave influences mainly the voltage drop on
inductance, so it is expected that while the front of
current impulse gets shorter, this voltage drop become
more important.
When grounding wires are missing, lightning can strike
the phase conductor and the lightning current
propagates in both directions, inducing the appearance
of a potential wave equal to ItZca/2. If an insulation
flashover does not appear, the voltage wave propagates
and attenuates due to losses on conductor resistance
and corona discharge. On the un-stroked phases, lower
voltages are induced by the coupling with stroked
phase. When the lightning strikes the ground wire
between the towers, the lightning current divides in
two equal components which flow in opposite
directions and arriving to the towers, they will draw a
part of it, so the amplitude of current flowing forward
on the ground wire decreases gradually.
ISSN 1843-6188
penetration so that the lightning might strike a phase
conductor. The tower is considered as a circuit with
uniform distributed parameters, without losses, having
three segments:
 the first, between tower top and R phase console,
 the second, between R phase console and S/T phases
console
 the last, between S/T phases console and footing
resistance.
In this way, the voltage distribution along the tower
can be observed. The tower surge impedance can be
computed with rel. (1)
2h 

ZT  60 ln 2
 1
r


(1)
Figure 11 Lightning strike in an OHL with ground wire in
absence/presence of arrester
where r is the radius of the tower considered as
cylindrical.[8]
The behavior of the overhead line was studied
regarding the insulation stress and current amplitude
through arresters, in some cases: with and without
grounding wire and with and without arresters
simultaneously. For all above mentioned cases a
parametrical study was accomplished, considering
credible ranges of variation for main factors: the front
time of the current impulse and tower foot resistance,
keeping the same amplitude of lightning current.
In the first step they were used both ramp and standard
double-exponential shaped current impulse, in order to
verify the results obtained analytically and to confirm
the accuracy of simulations. A first observation is that
in case of lightning strike to the top of tower,
homologue values of voltages on phase insulation
strings and tower foot resistance are considerably
higher when standard shaped current impulse was used.
The explanation is that standard shape impulse has
variable front slope and the maximum one is higher
than that in case of ramp shape impulse with the same
front time. In case of lightning strike in the span, the
simulations conducted to similar results for both shapes
of the lightning current impulse.
So, in order to be closer to the reality, in the foregoing
simulations only standard double-exponential shaped
current impulse was used. Also in the foregoing
5. CASE STUDY
It will be considered a one circuit 110 kV overhead
line, build on concrete towers. The ATP model of line
is made by 10 spans of 200 m ended by two segments
of 25 km with length, in order to avoid the influence
of the reflected waves from the opposite end. The line
is modeled as a traveling waves circuit. In order to
observe easier the phenomena that will appear, the
symmetry of ATP model will be preserved relating to
the striking point. If the lightning current strikes at the
span middle, the symmetry of the model will be
maintained by adding a new span, equally divided by
the impact point. When grounding wire is present it is
considered implicitly that it will capture the lightning
strike, excluding the possibility of shielding
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
observations, for easy comparing the results, the
current impulse front time will be 1.5 s.
Considering the lightning strike at the tower top when
ground wire is present (Fig. 11) and comparing the line
behavior in absence of arresters (Fig. 12), respectively
of arrester presence on R phase (Fig. 13), the following
conclusions may be drawn:
- in case of lightning strike at tower, the presence of the
arrester limits the voltage on protected phase insulation to
15% up against the case in which the arrester is missing;
- when the arrester is missing, the voltage top value on
R phase at the adjacent tower decreases with 50%;
- the presence of the arrester on a phase (R in this case)
limits the voltage top value only on the protected
insulation strings, while the stresses of same phase
insulation on adjacent towers is at least equal to that in
case the arrester is missing (if not higher);
- for the other two unprotected phases (S and T), the
top voltage value on insulation strings at hited tower
drops with aproximately 30% when arrester is present
on R phase.
From the current through arresters point of view,
considering the case when the lightning hits the tower,
and the arrester is present only on R phase, the current
through arresters represents 20% of lightning current if
the line is not protected with grounding wire and 10%
when the grounding wire is present. If the insulation on
S phase is also protected by an arrester, the value of the
current through this arresters drops at 10%, respectively
7% of the lightning current. The presence of arrester also
on the T phase does not change very much the current
distribution, unless maybe in a small rate the current
through S phase arrester (t predictible aspect).
Coming back to the more conservative cases, when the
current impulse front time varies beteen 0,5 s and 3
s, in order to see its influence and the influence of
tower foot resistance, following conclusions can be
drawn, valid in all studied cases:
- increasing the time of lightning current slope reduces
the voltage levels on phase line insulation. The
slightest dependence appears for voltages on tower foot
resistance, which is expected to happen because they
are modeled only by their dispersion resistance. The
tower potential and stresses on insulation strings
decrease when the time of current slope increases,
because the tower inductance component Ldi/dt,
reduces accordingly.
- the tendency of voltages decreasing when of lightning
current front time increases depends on the tower foot
resistance too, becoming smaller while the foot
resistances become higher.
1
UR
2
3
4
5
1
US
2
3
4
5
Figure 12. Lightning strike at tower top (GW present)
Stresses on hit tower insulation and the following 4 towers
2
3
UR
4
5
1
1
US
2
3
4
5
Figure 13. Lightning strike at tower top (GW present –
arrester on R phase) . Stresses on hit tower insulation and
the following 4 towers
 Before install arresters on overhead lines, detailed
studies are necessary concerning their behavior, the
efficiency of different methods of installation and
coordination of component elements. ;
 The presence of arresters assures the insulation
protection of the respective tower, but might increase
the stresses on insulation of the adjacent towers;
 The increase of front time of the lightning current
impulse reduces the voltages on line phases insulation;
 The decrease of stresses when time front of current
impulse increases, is influenced by the tower foot
resistance, becoming smaller while the foot resistances
become higher.
6. CONCLUSIONS
 In the last years the application of line surge arresters
on overhead lines is developing exponentially,
transforming this solution in one of higher and higher
performance and viable from technical-economical
point of view.
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Scientific Bulletin of the Electrical Engineering Faculty – Year 10 No. 3 (14)
7. REFERENCES
[1]
33-301 Experience and Effectiveness of
Application
of
Arresters
to
Overhead
Transmission Lines, T. KAWAMURA Shibaura
Institute of Technology, a.o. CIGRE SESSION
1998
[2] 33.11.03 Application of Metal Oxide Surge
Arresters to Overhead Lines – Working Group
33.11. Task Force 03 A. Schei, Conventor of WG
33.11 – ELECTRA No 186, OCTOBER 1999
[3] Lightning Performance Improvement Of 123 kV
Line Ston – Komolac By Use Of Line Surge
Arresters M. PUHARI, a.o.,
CIGRE
COLLOQUIM Cavtat 2008
[4] First Experience in Monitoring of Line Surge
Arresters Installed on 110 kV Transmission Line
Ston – Komolac in Croatia, S. BOJIĆ, a.o. Croatia
[5] Application of line surge arrester on a 230 kV
transmission
line
CEMIG’s
Experience,
A.C.O.ROCHA, a.o. BRAZIL
[6] Romanian Field Experience in Mounting and
Exploitation of Line Arresters On High Voltage
Overhead Electric Lines, Stelian Alexandru Gal –
CN Transelectrica SA, a.o. Romania
[7] Reduction of the double-circuit flashovers on a 400
kV overhead line, A. Xemard, a.o. Canada
[8] Hileman A., Insulation Coordination for Power
Systems, CRC Press, 1999
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