lightning protection of overhead power distribution lines
TRANSCRIPT
Lightning Protection of Overhead Power Distribution
Lines---Outage Aspects, Mitigation Methods and
Future Projects---
Shigeru Yokoyama
Shizuoka University
Abstract—This document shows the lightning outage aspects
on overhead power distribution lines (OPDLs) mainly in Japan
and a designing method of the insulation level of OPDLs. Based upon detailed analysis of them the author proposed the
mitigation methods, which is thought to be meet risk
management of OPDLs. Moreover the author indicates the future
subjects related to lightning protection of OPDLs.
Keywords—lighting, lightning protection, distribution line,
middle voltage line,outage, surge arrester, overhead ground wire
1. INTRODUCTION
Lightning damage countermeasures in overhead power
distribution lines (OPDLs) are quite important for stable
supply of electricity. However, actual policies and the specific
lightning countermeasures vary according to a country and an
electric power company without clear and definite policies.
Globally speaking, study of lightning damage
countermeasures in OPDLs that we often see is about
“theoretical review regarding occurring possibility of
flashover due to indirect lightning hit and direct lightning hit”
[1]. It is of course one of the important items of lightning
damage countermeasures in OPDLs, but it is necessary to
consider many other issues for reducing the degree of damage
in actual OPDLs.
In this paper, the author evaluates the result of conventional
studies that have been conducted in Japan in wider view.
Based on this, the author elucidates the best lightning damage
countermeasures in OPDLs and proposes necessary studies
needed in the future.
Followings are the main themes:
(1) Summary of studies of lightning damage and its
countermeasures in the past
(2) Clarifying the goal for lightning damage countermeasure
(3) Challenges for future technology development
2. WHAT ARE THE COUNTERMEASURES FOR?
Lightning damage countermeasures mean protecting OPDLs
from the influence of lightning without a doubt, but it is not
very clear what damage to reduce specifically. Followings are
important possible damages and items related to their
countermeasures.
2.1 Countermeasure for long-time electrical service
interruption
It is important to consider not only protecting flashover
at insulating points but also follow current
countermeasure for early recovery.
Structure of electric distribution system / efficient use of
automatic distribution system
2.2 Countermeasure for short-time electrical service
interruption
Reduction of flashover is a main target.
2.3 Countermeasure for momentary voltage drop
Balancing with the countermeasures at consumers
2.4 Countermeasure for facility damage
Insulation coordination ---- enforcing insulation for
important facility
Countermeasure against power frequency follow current
3. LIGHTNING DAMAGE ASPECTS
In Japan, there used to be quite a large gap between the actual
lightning damage rate and theoretical one. In fact, there was a
difference of around ten times between the analysis result of
flashover percentage and the actual damage in Japan [2], but
reviewing various research results in the past 20 years has
made this gap significantly smaller.
The author would like to show the aspects of lightning damage
in Japan to clarify the target of lightning protection.
3.1 The result of photo observations
In 1994 Yokoyama has urged Japanese electric power
companies and conducted camera observations of the lightning
damage caused to OPDLs nation-wide [3-5]. Five electric
power companies out of 9 in Japan conducted camera
observation with the initial urge and many of them made
collaborative study with Central Research Institute of Electric
Power Industry using around 130 cameras totally. The manner
of installation of still cameras is shown in Figure 1. The
observation of these companies is called No. 1 Group
Observation.
Later on, Tokyo Electric Power Company (TEPCO)
conducted the same observation using 100 cameras since
1997 [6]. With these two groups of observation, many
lightning strike conditions to OPDLs and the surroundings
were observed.
3.2 Damage due to induced effect of nearby lightning
stroke
In average, lightning surge arresters are usually installed with
200-meter intervals in Japan, and an event of flashover in the
insulation of OPDLs, caused by induction of lightning
2013 International Symposium on Lightning Protection (XII SIPDA), Belo Horizonte, Brazil, October 7-11, 2013.
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Fig.1 Installation of still cameras on an electric pole
Fig.2 Direct lightning hit to an overhead power distribution
line
in the neighborhood of an OPDL, was almost none.
Yokoyama already indicated the occurrence of damage to be
almost none by indirect lighting stroke in his research paper[7],
This fact confirms his statement.
3.3 Direct lightning stroke
When lightning directly stroke an OPDL of No. 1 Group of
electric power companies, power frequency follow current
continued in almost half of the cases ( See Figure 2) and it has
become clear that the rest of the half does not produce follow
current. The similar result has also been shown in the
observation by TEPCO. The occurrence of flashover by direct
lightning strikes is influenced by many factors such as the
peak value and wavefront length of lightning current, the
interval of surge arresters, grounding resistance value,
existence and the number of overhead ground wires, the
number of low-voltage distribution lines, and the number of
metal communication wires, etc. In any case, it was
demonstrated that a certain protection can be obtained from
direct lightning strike depending on the conditions, even
though the insulation strength of Japanese distribution line is
between 60 – 200 kV [8, 9].
0% 20% 40% 60% 80% 100%
in Summer
in Winter
Mountainous
Area
Ratio
Surge arresters Conductor Transformer insulator Switch Others
51
24
8
5
21
51
24 3 2 15
34 8 8 5
25 6 7 3
Fig.3 Outage ratio of overhead power distribution lines in
summer and in winter
Ar Ar
Pole mounted Air Switch
Ground relay
Telephone
Watt hour meter
ZnO×3Paper valve×1
Outages of surge arresters
6kv / 3kV
Tr
3kV / 100/200V
Office
Building Limiter operates
Telephone, Fax, PC
Warehouse
Distributionbord
Antenna
Direct lightning
Wireless communications facilities
Communication cable
Drop wire
Watt hour meter
Ar
Paper valve×1
Outages of surge arresters
50m
30m
80m
70m
Fig. 4 Surge arrester breaks due to backflow current of winter
lightning
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3.4 Burnt damage of surge arresters
・・・Effect of winter lightning and backflow current・・・
Figure 3 shows the outage rate of each facility of OPDLs in
summer season as well as in winter season. This data shows
that the outage rate of surge arrester break is larger in winter
season than in summer one. It indicates that there are many
lightning flashes, which has large amount of charges, in winter
[10]. Surge arrester break often occurs on the last pole of a
distribution line (closest pole to a consumer) [11, 12]. This indicates that the surge arrester break occurs due to the
backflow current from a high structure or a tower, which is hit
by a lightning stroke. This estimation was clarified by camera
observation later (Fig.4) .The authors clarified that installation
of overhead ground wires is more effective than the upgrading
of surge arrester capacity against surge arrester break [13,14].
3.5 Effect of nearby trees on lightning protection of OPDLs Still camera observation revealed that lightning discharge
progressed in the direction of trees and finally attached to the
overhead ground wire of a nearby OPDL (Fig.5). An
experiment for corroboration, which was done at the Shiobara
Testing Yard of CRIEPI, confirmed the same phenomena
(Fig.6) [15-17]. This result shows that the existence of nearby
trees does not give good effect upon lightning protection of an
OPDL.
Figure 7 showed the case that lightning discharge first
attached to a tree and then lightning discharge jumped into an
OPDL [18]. This phenomenon was also corroborated by the
long-gap discharge experiment, which was done at the
Shiobara Testing Yard of CRIEPI. This flashover may result in
wire-cut outage in the middle part of two poles. Usually wire-
cut outage occurs in the close area of a pole, because
insulation strength is stronger between two wires than in an
insulator on the pole.
4. CONDITIONS FOR DESIGNING LIGHTNING
DAMAGE COUNTERMEASURES
Necessary conditions for designing lightning damage
countermeasures in specific OPDLs are shown as follows ;
4.1 Lightning phenomena in a target region
・Number of lightning flashes, peak value and front steepness
of lightning current and their relationship
・Winter lightning
4.2 Soil conductivity
4.3 Surrounding conditions
・Forest and woods, buildings, transmission lines
4.4 Horizontal configuration and vertical configuration of
phase conductors
・ There is basically no much difference between two
configurations concerning lightning protection, but it has
something to do with the judgment of omitting a lightning
arrester, which is possible for Japanese insulation system [19].
4.5 Existence of low-voltage distribution lines and
telecommunication wires
They have almost the same effect as an overhead ground wire
[20, 21].
・Effective against direct lightning hit to OPDLs
Fig. 5 Lightning discharge to an overhead power distribution
line close to trees
Fig. 6 Effect of a tree on the progressing direction of a
discharge
・ Very effective for preventing burnt damage of surge
arresters
4.6 Use of insulated wire for a phase conductor
・ Enforcement of insulation [22]
・ Location of lightning attachment[23]
・Fast melting of a wire due to power frequency follow
current[22]
5. HOW TO DECIDE THE BASIC INSULATION
STRENGTH OF OPDLs
5.1 Background for deciding insulation strength
Basic insulation level is low in distribution voltage class, so it
is impossible to fully prevent from lightning. Therefore, it is
common to tolerate with systematic insulation for inner
abnormal voltage (switching overvoltage, temporary power
frequency overvoltage) normally generated in the system and
to take measures of anti-lightning and anti-salt damage
considering atmospheric conditions of the target area.
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(a) Picture taken by a close camera
(b) picture taken by a distant camera
Fig. 8 Lightning stroke on a tree and side flashes to an
overhead power distribution line
5.2 In case lightning surge arrester is not used
It is hard to think distribution lines without a lightning surge
arrester, but suppose there is distribution line without it, the
basic insulation strength is decided by the following
considerations:
(1) Tolerable to frequent switching surge
The frequency of switching surge is not decided with
uniformity. It refers to the degree where the local power
company cannot secure reliability.
(2) Tolerable to frequent temporary overvoltage
(3) Considering the decrease of insulation strength due to
contamination
5.3 in case lightning surge arrester is used
(1) Selecting the lightning surge arrester with V-1
characteristic (clamping voltage) or discharge starting voltage
of gap, so that it will not be frequently damaged by switching
surge or temporary overvoltage.
(2) In such a case, consider the influence of degradation of a
lighting surge arrester by repeated impression of switching
surge.
(3) For a lightning surge arrester with a gap lightning
protection design must be done by gap discharge starting
voltage. On the other hand, clamping voltage of a lighting
surge arrester without a gap is used for protection design.
5.4 Conclusion
(1) Since insulation distance of insulators in a distribution line
is relatively short with 10 to 30 cm, insulation strength may
greatly change depending on the method of installation of
conductors.. Additionally we should consider workability and
tolerability to mechanical strength for securing insulation
distance. It is almost impossible to consider short insulation
distance of 9 cm or less. 50% flashover voltage of a high-
voltage pin insulator with least insulation used for 6.6kV
OPDLs in Japan is 80 – 90 kV.
(2) In this case, insulation of an insulator is decided without
considering switching surge and temporary overvoltage and
lighting protection design is done based on such insulation.
In Japanese 20 kV class distribution lines, there was an
example of lowering insulation strength to the limit by
reviewing the amount of generated switching surge in detail.
Even in such a case, the insulation strength is not less than 100
kV [9].
(3) Since lightning surge arrester, overhead ground wire,
grounding resistance value and insulation strength are
mutually related, all such factors must be considered and
outage rate must be calculated.
6. POSSIBLE LIGHTNING DAMAGE
COUNTERMEASURES
--Why do we use lightning surge arresters for lightning
protection of OPDLs ?---
6.1 Is it realistic to take measures only with overhead
ground wire and low-resistance grounding?
Without considering the transient property of grounding
resistance, it looks like possible to take lightning protection
measures by making grounding resistance low. For example,
if we consider grounding resistance to be 5 ohms and the peak
value of lightning current to be relatively large 50 kA,
insulation is possible with 40 cm tall insulator since rise in
grounding potential is 250 kV.
6.2 Cost of low resistance grounding
But in reality, a large scale grounding work is required to
secure grounding value of a few ohms, so it is hard to believe
that the cost of such measure will be smaller than the measure
using lightning surge arresters as a commons sense.
Furthermore, if the front steepness of lightning current wave
shape is considered, the effect of distant grounding is
significantly decreased.
6.3 Selecting method of high insulation level and insulated
arm
It is one possible measure to set the ground resistance to a
realistic value of 10 to 30 ohms and to make insulation level
of insulators high. But making insulation of the insulator high
means heavy load in a power pole, leading to the increased
construction cost. Therefore, it is not common to use this
method.
Additionally, compared with the method of a lightning surge
arrester explained below, this method requires enforced
lighting protection capability at the transformer on a power
pole. Therefore it is necessary to install a large capacity
lightning surge arresters at the transformer pole.
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6.4 Lightning damage countermeasure using lightning
surge arresters
Considering the above, it is common to take measures of
lighting protection using lightning surge arresters in OPDLs.
Based on the research made by the author [7], if a lightning
arrester is installed at intervals of 300 meters, there will hardly
be flashover caused by indirect lightning strokes in OPDLs
even with a very low insulation level. Therefore it is only
necessary to take measure for direct lightning. The effect of
using only lightning surge arresters is greatly influenced by
the value of grounding resistance, so it is necessary to strictly
analyze the sensitivity in order to know the degree of effect
using the parameters such as lightning nature, characteristics
of lightning surge arrester and ground resistance values, etc.
6.6 Application of surge arresters and overhead ground
wire
It becomes apparent that the combined use of lighting arresters
and an overhead ground wire will give better effect compared
with individual use respectively [24]. For this reason, in case
full-scale direct lightning countermeasures are pursued, this is
one of the main countermeasures.
In this method, it becomes apparent that ground resistance
should not necessarily be low to protect OPDLs [25].
6.7 High grounding resistance and consumer (low-voltage
distribution line) overvoltage invasion
In 6.6, it states that “it is not always necessary to decrease
grounding resistance to low value.” This means preventing
flashover at an insulator by suppressing voltage in an insulator.
If grounding resistance is kept higher with this method,
flashover may not happen at high voltage side of OPDLs, but
grounding potential increases significantly. For this reason,
grounding potential greatly increases at a power pole where a
transformer is equipped and current increase at low
distribution lines becomes large. This leads to the increase of
lightning overvoltage invading to the consumers’ equipment
and may possibly expand the lightning damage of the
consumers.
7. POWER FREQUENCY FOLLOW CURRENT
COUNTERMEASURES
The destruction of equipment may occur only with flashover,
but the damage of insulators and the meltdown of electric wire
can be often caused by the thermal function of current due to
power frequency voltage following the flashover. This means
if we take the measure of reducing the influence of follow
current caused by power frequency voltage, we may reduce
the damage. In Japan, countermeasures against breaking wires
and insulator damage have been traditionally taken.
7.1 High-speed current interrupt
If a transformer station can detect fault current in an early
stage, it can open up a current breaker and reduce the damage
due to follow current. This method has a certain effect for
preventing the breaking of wire.
Since distribution lines spread like a web from a substation, it
may be difficult to detect short-circuit current at a distant
location from the substation.
7.2 Application of arcing horn[26]
Arcing horns at insulators are used to prevent damage at
insulators, so that arc of flashover or power follow current
keeps the distance from insulator’s surface. There are
various types of arcing horns.
7.3 Use of melting-resistant wire [9]
Insulated wire has shorter meltdown time than naked wire [22].
In Japan improved melting-resistant type insulated wires were
developed and some electric power companies have used them.
The number of twisted wires in an insulated wire is reduced
and the wire diameter is made larger so that the following
effects can occur and prevent the wire to break easily.
(1) Increasing the heat capacity of wire
(2) Improving heat conductivity towards longitudinal direction
(3) As surface contact with each wire improves thermal
conduction, resulting in suppressing heat increase in a wire
8. RELATION BETWEEN THE RATE OF DAMAGES
DUE TO INSULATION FLASHOVER AND THAT DUE
TO SURGE ARRESTER BREAKAGE[27]
Damage of Surge arresters rarely occurs due to current of
summer lightning. On the other hand, damages of surge
arresters occurs frequently in OPDLs at the west coast of
Japan in winter., because winter lightning sometimes include
long continuing current. In Japan simple outages due to
flashover in an insulator or a pole transformer have been
greatly reduced, resulting in increase of the rate of outages of
surge arresters in electric power companies facing to the west
coast of Japan. For example the rate of surge arrester outages
out of total outages on OPDLs reaches to 50% for Hokuriku
electric power company .
9. DESIGNING PROCEDURE OF LIGHTNING
PROTECTION MEASURES
From the above consideration, procedure of lightning
protection measures for OPDLs are proposed as follows;
9.1 Setting up insulation strength
・Evaluation of switching surges and temporary overvoltage
of power frequency
・Insulation strength may be used as one of parameters for
AFOR (Analysis on Flashover Rate of Specific Overhead
Power Distribution Lines)
・ Weight of insulators, ease of transportation and
construction
9.2 Setting up the level of reliability for specific region
9.3 Setting up environmental conditions
9.4 Setting up configuration of OPDLs, the number and
position of low-voltage lines
9.5 Lightning protection without surge arresters in OPDLs
・ Determination of insulation strength and the value of
grounding resistance by means of AFOR
9.6 Lightning protection with surge arresters and without
an OGW(overhead ground wire)
・Calculation of FOR (Flashover Rate of Specific Overhead
Power Distribution Lines ) by means of AFOR
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・Large effect of grounding resistance
9.7 Lightning protection with surge arresters and an OGW
・Calculation of FOR by means of AFOR
・Small effect of grounding resistance
9.8 Lightning protection for the area of frequent winter
lightning
・Calculation of the rate of surge arrester damage by means of
AESA (Analysis of Consumed Energy in Surge Arrester)
9.9 Detection method of damaged facilities and
maintenance of them
9.10 Cost performance
Fig.8 shows one example of the procedure of lightning
protection design for OPDLs.
10. CHECK POINTS ON LIGHTNING SURGE
ANALYSIS FOR OPDLS
10.1 Power frequency voltage
・As the maximum voltage of power frequency is not so
large compared with insulation strength, usually the effect of it
may be ignored .
・Closely simultaneous flashovers in two or three phases do not occur because of the difference of phase voltages.
10.2 Insulation characteristics of insulators, pole
transformers and other facilities
・ Short-wavetail lightning overvoltages which occurs
frequently in OPDLs because of short interval of surge
arresters [28]
・Special insulation characteristics of insulated wires
10.3 Lightning attachment
・ Important effect of trees ・・・ attraction of discharge,
side flash
・Lightning attachment characteristics of insulated wires[23]
・Lightning attachment characteristic of an electric pole
10.4 Transient phenomena
・Impedance of an electric pole・・・around 200Ω for surge
impedance
・Fast surge phenomena around a pole transformer [29,30]
10.5 Capacity of surge arresters
・ Japanese experience・・・ Surge arresters with energy
capacity of 15kJ is thought to be adequate for usual summer
lighting. But 15kJ may be not sufficient capacity for winter
lightning.
・Damage of surge arresters occurs frequently due to back
flow current from a high tower hit by lightning.
11. RESEARCH TARGETS OF LIGHTNING
PROTECTION FOR OPDLs
(1) Setting up a reliability target of lightning protection for
OPDLs of specific country and region
(2) Technology transfer of a well experienced electric power
companies to other ones
・Grasp of lighting characteristics of target regions
No
Yes
Yes
No
Yes
No
No
・Enviromental condition
・Soil conductivity
･Lightning flash density
･Number and
configuration of
low-voltage lines
･etc...
Yes
START
Minimum insulation strength
･Switching surge, Temorary overvoltage
･Ease of construction, transportation and maintenance
Setting up of reliability of a
target region for lightning
surge
Setting up
Repead procedure A
Calculation of cost
performance
END
Applying Lightning
surge arresters
Applying an
OGW
Calculation of RFO By
means of AFOR
Calculation RFO By
means of AFOR
considering
grounding resistance
Calculation RFO By means
of AFOR
Simultaneous calculation
･Surge arrester damage by means of AESA
･RFO by means of AFOR
Determination of
・grounding resistance
･number of OGW
･grounding interval
･configuration of OGWs
Region of frequent
winter lightning
Necessity of recalculation of
Mitigation factor due to
surrounding buildings and low-
voltage lines
Selection of the best lightning
protection measures
A
Fig. 8 Flow chart of lighting protection design for OPDLs
OGW : Overhead Ground Wire
RFO : Rate of Insulation Flashover
AFOR : Analysis on Flashover Rate of Specific Overhead Power
Distribution Lines
AESA : Analysis of Consumed Energy in Surge Arrester
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Fig. 9 Damage on a concrete pole due to lightning hit
(3) Clarification of burnout mechanism of a surge arrester and
development of detection method of its deterioration
・The mechanism of burnout of surge arresters have not been
fully studied .
(4)Installation of V-t characteristics and effect of short
wavetail overvoltage on flashover into AFOR simulation
(5) Installation of transient characteristics and soil discharge
characteristics related to grounding characteristics into AFOR
simulation
(6)Development of effective detection of lightning damages
(7) Lightning flashover characteristics of contaminated
insulators
(8) Effect of high grounding resistance on invading surge into
consumers
(9) Establishment of field investigation method of lightning
outages
・Ablation of concrete from an electric pole hit by a lightning
stroke (Fig.9)
12. CONCLUSIONS
(1)The author summarized the lightning outage aspects.
Observation by means of still cameras clarified the following
items.
・Lightning outages due to indirect lightning is very rare.
・Surge arrester burnout often occurs in OPDLs ,which are
located along the west coast of Japan.
・Outages due to side flash from a tree hit by lightning occurs
occasionally. Nearby trees do not play a shielding role.
(2) There was large difference between the rate of flashover
calculated by a surge simulation program and actual lightning
outage rate. In order to adjust the difference, parameters of
lightning phenomena and the correlation between parameters
should be used precisely. The existence of low–voltage lines
and telecommunication wires should be taken into
consideration. Moreover flashover characteristics of short
wavetail is also important to adjust the difference.
(3) Mitigation of damage due to power frequency follow
currents is one of important lightning protection measures. As
examples, application of arcing horns, high-speed current
interrupt and use of melting-resistant wire are taken.
(4)Insulation strength of insulators are decided taking
switching surge, temporary overvoltage and reduction of
insulation due to contamination. Insulation strength is one of
parameters in addition to surge arresters, an overhead ground
wire and grounding resistance.
(5) Simple outages due to flashover in an insulator or a pole
transformer have been greatly reduced, resulting in increase of
the rate of outages of surge arresters in electric power
companies facing to the west coast of Japan. Surge arrester
damage is influenced by energy capacity and manufacturing
method of it.
(6) Field investigation of actual damage aspects is quite
important in order to design effective protection measures.
Still camera observation is also important for clarifying the
effectiveness of lightning protection measures in a specific
region.
(7) According to the above results the author extracts the
future research projects, which include surge analysis,
lightning observation, high-voltage experiments and field
investigations on damage in OPDLs.
In order to solve these problems, cooperation of engineers of
power companies and manufacturers and university
researchers is very important matters.
Most appropriate protection measures for OPDLs should be
established every electric power companies and specific
regions.
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