6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
Weather Influence Analysis on the Lightning
Protection Current Estimation
Dragos Machidon, Marcel Istrate
Power Engineering Department
“Gheorghe Asachi” Technical University
Iaşi, Romania
machidon.dragos@tuiasi.ro
where a, b and c are constants proposed by different
researchers.
Abstract— Lightning protection current is a key parameter
when estimating the lightning protection zones and also in other
analysis like those regarding the overhead line’s behavior at
direct lightning strokes. The insulators’ flashover is simulated
through some ATP MODELS module considering their voltagetime characteristic while considering the influence of different
weather parameters such as air pressure, temperature and
humidity. The entire analysis is conducted on a 400 kV overhead
power line.
An incorrect assessment of the orientation distance or the
attractive radius will be reflected into wrong sized protection
areas, thus increasing the risk of shield failures. As it can be
observed from (1) and (2) the values of the orientation
distance and attractive radius are higly dependent on the
lightning protection current intensity, which makes that a
proper estimation of this parameter is more than necessary.
Keywords— lightning protection current, weather influence.
I.
The lightning current intensity depends on a lot of factors
and it’s unique for each lightning strike. On the other hand the
lightning protection current is a concept related to the analyzed
equipment being unique for each power equipment. Basically,
the lightning protection current represents the minimum
intensity of the lightning current capable to generate a voltage
surge which will cause a breakdown or a flashover of the
power equipment’s insulation. Thus is obvious that not all the
lightning strikes are dangerous for the power installations but
only those having a current intensity equal of greater than the
installation’s specific lightning protection current.
INTRODUCTION
By their nature the power substations and overhead power
lines are well exposed to lightning strikes. Their protection
against such phenomena represents a significant preoccupation
since the early design stages and will continue later on through
their entire life span. The goal of reducing the number of
lightning strokes in the power installation’s active parts is
achieved by installing protective systems consisting in a mix of
vertical rods and ground wires, which should attract most of
the lightning strokes. A proper design of lightning protection
system is vital for the integrity and safety of the world’s wide
power systems. On the other hand there is also a great concern
on the overhead line’s behavior in case of direct strikes.
The specific normative propose some values for the
lightning protection currents, but the use of these prescribed
values may lead to incorrect results in engineering calculations,
as they were established through some gross generalizations
and massive simplification of the lightning orientation process.
When designing a lightning protective system the
dimensions of the obtained protected zones must be known. In
order to estimate the dimensions of the protected zones over
the time several methods were developed, such as the
laboratory models method, electrogeometrical theory’s
methods like the rolling sphere method [1] and elliptic model
[2] and the new attractive radius method [3] developed on the
basis of the latest theory regarding the lightning’s orientation
process, known as leader progression model.
A more appropriate way to estimate the lightning protection
currents is to simulate the entire transient regime caused by
lightning strikes in specific power installation using the ATPEMTP software. In this procedure an important aspect is
related to the manner in which the insulators’ flashover process
is simulated. For this study the authors will use the insulator’s
voltage-time characteristic approach, according to which the
lightning protection current will be determined as the minimum
intensity of the lightning current that generates a voltage surge
which will intersect the insulator’s voltage-time characteristic.
The bond between the lightning protection zones
dimensions’ and the lightning orientation process is possible
by considering the influence of the lightning protection current
intensity, as described in the expressions of the orientation
distance, R, and the attractive radius, Ra, presented below [4]:
R  aIb
(1)
Ra  a  h b  I c
(2)
The entire analysis will be conducted for a Romanian
typical 400 kV overhead power line from the eastern part of the
country, considering the influence of different weather
parameters like air pressure, temperature and humidity. The
goal is to determine how the atmospheric condition variations
over an entire year will reflect on the value of the lightning
protection current, knowing the fact that the atmospheric
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
conditions can have a significant impact on the insulator’s
flashover voltage.
II.
The modules which simulate the flashover were realized in
MODELS component of ATP program, on basis on following
observations [5]:
ATP MODEL OF THE POWER LINE
- When phase conductors or ground wires are stroked the
shape of the overvoltage is quite similar to the lightning
current’s one, so the analysis involves normalized pulse
voltage 1.2/50 μs.
The aim of ATP simulation of transient due to direct
lightning strokes in power lines is to analyze time evolution of
voltages and to determine the protection currents. The ATP
model of the 400 kV analyzed line contains the following
components [5]:




- When the top of a tower is stroked, the overvoltage waves
have a short back, half-amplitude time being of 6÷8 μs. The
behavior of insulators at short back pulses is not entirely
known, some test on 110 kV lines’ insulators showing an up to
50 % increase of flashover voltages, by report with that
corresponding to the normalized pulse. An average increase of
38 % can be considered, both for positive and for negative
pulses.
two overhead line’s spans and three towers;
two lines, at the sending end of the two spans, having
such a length that the reflected waves from theirs ends
do not affect the simulations` results;
the insulators of the three towers and modules that
simulate the line’s insulators flashover;
lightning pulse current generator.
- At a lower slop of the lightning current, the overvoltage
has lower amplitude and the flashover time increases till 25 μs.
However, the flashovers produced at bigger times than 6 μs are
rare [6], so the protection current can be determined only for
those flashovers that take place in maximum 6 μs from the
beginning of the transient regime on the insulator (in
concordance with J.G. Anderson’s method).
The line between adjacent towers is modeled by four multiphased nominal Π-equivalent circuits with distributed
parameters. The Π circuits’ are frequency independent and are
calculated for 0,5 MHz, they are not transposed and the
influence of corona discharge is neglected.
The extreme lines are modeled in the same way but they are
long enough to avoid the influences of reflected waves at their
receiving ends.
- For power lines having nominal voltage higher than 220
kV, the operating voltage favors insulators’ flashover. This is
the reason why the potential of phase conductors is considered
to be equal with the average value of operating voltage, for a
half-period, and in opposite polarity with tower’s potential.
The propagation phenomena along the towers and their
earthing grids involve equivalent circuits having distributed
parameters. The authors’ choice was to model the PAS type
towers by mean of distributed parameters Π equivalent circuits.
Each of such a circuit models the different parts of the metallic
construction, as Fig. 1 shows [5]:
Gnd
- MODELS modules must initiate to generate voltage-time
characteristic only when the insulators start to be stressed by
the pulse voltage. So, these modules must considerate the
propagation time from stroked point to tower;
Gnd wire
2
III.
WEATHER INFLUENCE ON INSULATOR’S VOLTAGE-TIME
CHARACTERISTIC
Phase A
Phase B
As presented in the previous sections of the paper the
power line’s insulators flashover will be simulated using their
voltage-time characteristic. If this parameter is not available
then it can be estimated using the following expression [7]:
Phase C
Distributed
parameters ∏
equivalent circuit
V (t )  V50%  1  kiz / t
(3)
where: V50% represents the critical flashover voltage (CFO) and
kiz is a constant depending on the nominal voltage of the line
(kiz = 3,41–for 400 kV).
Zg
Fig. 1. The ATP model of a PAS tower
The insulator’s CFO is determined for standard
atmospheric conditions, while the power lines are operating in
non-standard atmospheric conditions, which for our country are
varying on large ranges throughout a year, as passing through
different seasons. As an expected consequence it is obvious
that the insulator’s flashover will occur at different voltages
than for the case of standard atmospheric conditions.
Tower’s equivalent scheme is earthed through an R-L
lumped parameters circuit, in this stage being not considered
the earthing grid as a distributed one. Line’s insulators chains
are modeled through their pulse capacity in association with
modules that simulate the flashover. The pulse capacity of 400
kV insulators is about 140 pF [5].
Experimentally determining the insulator’s CFO for
different atmospheric condition in a laboratory controlled
environment is almost an impossible task, thus for practical
purposes is more useful to use the standard CFO correlated
with different correction factors that considers the influence of
The flashover arc is modeled by an R-L lumped parameters
circuit, in this stage the dependence of the arc parameters by
the current being neglected, because the aim of analysis is to
determine protection currents.
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
insulator’s CFO even if the humidity influence is neglected
(V50%A – 2).
air pressure, temperature and humidity. This procedure is
already widely used, being recommended by IEEE and IEC
standards [8], [9]. Thus for estimating the critical flashover
voltage in varying atmospheric condition, when considering the
lightning impulse voltage, the following mathematical
expression can be used [10]:
V50% A    H c  V50% S
TABLE I.
Month
(4)
January
where: δ is the relative air density, Hc is humidity correction
factor, and V50%S is the critical flashover voltage determined in
standard atmospheric condition.
The relative air density is defined as:
P  T0

P0  T
WEATHER CONDITIONS FOR THE ANALYZED TIME INTERVAL
t
[ºC]
-8,00
h
[%]
87,87
P
[kPa]
100,41
February
-5,25
90,18
100,53
March
11,75
72,71
100,47
April
13,54
75,70
100,00
May
16,52
69,79
100,00
June
18,67
71,70
99,38
100,05
July
21,14
74,75
August
21,14
70,72
99,28
where: P0 and T0 are the standard pressure and temperature
with the temperature in degrees Kelvin and P and T are the
ambient pressure and temperature.
September
17,00
67,17
100,25
October
10,50
83,29
100,54
November
-2,83
89,23
100,42
In design or selection of the insulation level, wet or rain
conditions are assumed, and therefore Hc = 1.0 [10]. However
the humidity correction factor can be determined with (6):
December
-3,18
89,06
100,49
(5)
H

H c  1  0.0096    11


TABLE II.
3
where: H is the absolute humidity in g/m and δ is the relative
air density.
IV.
STUDY CASE
The analyzed line is a Romanian’s typical 400 kV overhead
power line realized on PAS towers and equipped with CTS
160-1 glass insulators. The length of the insulators is about
3230 mm, and the flashover voltage for negative lightning
strokes, is considered to be of 1792 kV. The phase conductors
and ground wires sag is of 10 m and respectively of 8 m. The
length between two adjacent towers is of 375 m.
The overhead power line is located in the eastern part of the
country. The meteorological information where collected from
a weather station disposed on the same area with the power line
and for simplification are assumed to be the same over the
entire power line’s span. The monthly mean values of air
temperature, relative humidity and air pressure were calculated
and are presented in the Table I.
V50%A - 1
[kV]
1794,17
V50%A - 2
[kV]
1963,96
January
δ
[kg/m3]
1,096
0,914
V50%S
[kV]
1792
February
1,086
0,919
1792
1788,60
1946,09
March
1,021
0,958
1792
1752,88
1828,81
Month
(6)
INSULATOR’S CFO DEPENDING ON WEATHER CONDITIONS
Hc
April
1,009
0,970
1792
1753,89
1808,89
May
0,999
0,977
1792
1749,45
1790,28
June
0,986
0,992
1792
1751,70
1766,07
July
0,984
1,011
1792
1782,72
1763,01
August
0,976
1,006
1792
1759,21
1749,58
September
1,000
0,976
1792
1748,87
1791,77
1838,29
October
1,026
0,963
1792
1769,84
November
1,075
0,924
1792
1779,96
1926,75
December
1,077
0,923
1792
1781,68
1930,36
If the humidity’s influence is considered, the value of the
insulator’s CFO (V50%A – 1) is a little lower than the standard
one and is recorded over the entire time interval, as graphically
represented in Fig.2.
Using the procedure described in the previous section,
corroborated with the data presented in Table I, the relative air
density was determined and then the insulator’s critical
flashover voltage was calculated first considering and then
neglecting the influence of humidity through the correction
factor Hc. The obtained values are presented in Table II, where
V50%A – 1 is the critical flashover voltage calculated considering
the correction imposed by the humidity factor Hc, while the
V50%A – 2 is the insulator’s CFO depending only on the relative
air density.
When analyzing the data presented in Table II, one may
notice that the sub unitary values of the relative air density
recorded from April to August lead to a smaller value of the
Fig. 2. Comparison between insulator’s standard CFO and the CFO values
determined when considering the weather influence
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6TH INTERNATIONAL CONFERENCE ON MODERN POWER SYSTEMS MPS2015, 18-21 MAY 2015, CLUJ-NAPOCA, ROMANIA
Another important observation is that the values of
insulator’s CFO (especially the V50%A – 2 determined without
considering the humidity’s influence) are smaller than the
standard one for five months on a year, between April and
August, this being the exact time interval when the
thunderstorms activity is the most important. For the rest of the
year it proves that atmospheric conditions have a positive
influence as the determined insulator’s CFO is higher than that
for the standard conditions.
used in other applications regarding the overhead power line’s
behavior at lightning strikes.
These higher values don’t pose any interest as they will
reflect in a higher lightning protection current. A conservative
approach would suggest that power installations’ lightning
protective system must be designed according to the smallest
value of lightning protection current.
In this paper the authors analyzed the influence of
atmospheric conditions on the values of the lightning
protection currents for the case of a 400 kV overhead power
line. The ATP software package was used to simulate lightning
strikes in the overhead power line’s elements along with
insulator’s flashover process.
V.
The lightning protection current is a key parameter in
different applications like designing a lightning protective
system, or analyzing the overhead line’s behavior in case of
direct strikes.
Thus the smallest value of the insulator’s critical flashover
voltage when neglecting the humidity influence is recorded in
August as 1749.58 kV, only 42.42 kV lower than the standard
value. With this value and using (3) a new voltage-time
characteristic was determined for the power line’s insulator and
then simulations were conducted in ATP to determine the
values of the lightning protection current.
The influence of atmospheric conditions determined a
lower value of the insulators’ critical flashover voltage
especially between spring and summer seasons. As
consequence the values lightning protection currents are
slightly lower than those obtained for standard atmospheric
conditions.
Further analysis must be conducted in order to evaluate the
impact of the results obtained in this paper when actually
design the lightning protection zones of different power
installations, or analyzing the overhead line’s behavior in case
of direct strikes.
Different scenarios were considered as the lightning strikes
were simulated in phase conductors, line’s towers and ground
wires also. The obtained results are presented in Table III in a
comparative manner.
TABLE III.
CONCLUSIONS
LIGHTNING PROTECTION CURRENT VALUES
REFERENCES
Standard
Extreme atmospheric
atmospheric conditions
conditions
Ip
[kA]
Uflash
tflash
Uflash
tflash
[kV]
[μs]
[kV]
[μs]
Lightning strikes in phase conductors
2.50
2249
4.90
2.00
2211
5.02
Lightning strikes in line’s towers
2871
1.98
239
237
2819
2.34
Lightning strikes in ground wires
2120
6.7
191
2055
7.03
188
Note: Uflash and tflash are the voltage and time at which
insulator’s flashover occurs.
[1]
R.H. Lee, “Protection zone for buildings against lightning strokes using
transmission line protection practice”, IEEE Trans. Ind. Appl., 1977,
no.14, pp. 465-470.
[2] D. Machidon, M. Istrate, “A New Model Based On Electro-Geometrical
Theory For Estimating The Lightning Protection Zones”, Proceedings of
the 8th International Symposium on Advanced Topics in Electrical
Engineering, ATEE 2013, ISBN 978-1-4673-5978-8, 23-25 May 2013,
Bucharest, Romania.
[3] D.L. Machidon, M. Istrate, I.V. Banu, “Algorithm Based on Attractive
Radius for Estimating the Lightning Protection Efficiency”,
Proceedings of the 14th International Conference On Optimization Of
Electrical And Electronic Equipment, OPTIM 2014, pp. 27-32, 22-24
May, 2014, Brasov, Romania.
[4] V. Cooray, Lightning Protection, London, The Institution of Engineering
and Technology, 2010, p.212-239.
[5] M. Istrate, M. Guşă, “Analysis of lightning’s strokes on HV Lines in an
ATP approach”, Acta Electrotehnică, Proceedings of the 2nd
International Conference on Modern Power Systems MPS 2008, 2008,
p. 162-165.
[6] CIGRE Guide to Procedures for Estimating the Lightning Performance
of Transmission Lines, CIGRE Working Group 01 of Study Committee
33, October 1991.
[7] G. Drăgan, Tehnica Tensiunilor Înalte, Vol. II, Editura Academiei
Române, 2001.
[8] IEEE 1313.1, “IEEE Standard for Insulations Coordination, Principles
and Rules”, 1996.
[9] IEC Publication 71.1., “Insulation Coordination Part. I, Definitions,
Principles and Rules”, 1993-12.
[10] A.R. Hileman, Insulation Coordination for Power Systems, CRC Press,
Taylor
&
Francis
Group,
1999,
p.630-631.
As it can be observed from the data presented in Table III
the atmospheric condition have a small influence on the
lightning protection current values, as they are a little bit
smaller than those determined for the standard conditions, no
matter if the lightning strikes the phase conductor, ground wire
or line’s towers.
It is worth notice that this analysis was conducted by using
the mean values of the weather parameters in the considered
month. For the most severe weather conditions (air temperature
– t = 35 ºC, relative humidity – h = 31 % and atmospheric
pressure – p = 99.40 kPa on August the 4th) is expected to
obtain even lower values of the lightning protection currents
results, but is questionable if such specific values should be
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