Download designing concept on lightning protection of overhead power

IX International Symposium on
Lightning Protection
26th-30th November 2007 – Foz do Iguaçu, Brazil
DESIGNING CONCEPT ON LIGHTNING PROTECTION OF OVERHEAD
POWER DISTRIBUTION LINE
Shigeru Yokoyama
Central Research Institute of Electric Power Industry, Japan
[email protected]
2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196 JAPAN
Abstract - The principle is shown for lightning protection of power distribution lines taking the effects of surge arresters,
overhead ground wires and their combined use into consideration. Moreover an outline of a rational design method targeting
direct lightning hits, induced overvoltages and backflow currents from high structures.
1 INTRODUCTION
It is no exaggeration to say that changes in high-voltage distribution line equipment have had an impact on lightning
protection. Over the past 40 years or more, full-fledged studies and research have been conducted on lightning
protection for high-voltage distribution lines, and the fruits of such labors have made their way into lightning protection
design guidelines at different times. As research advances, the number of lightning outages has been steadily falling, but
lightning outages still account for a high percentage of failures on high-voltage distribution lines. In fact, they rank as
the number one or two leading cause, depending on the year in Japan.
Since high-voltage distribution lines have relatively weak insulation compared to power transmission lines, not only
direct lightning hits, but also induced overvoltages (indirect lightning hits) when lightning strikes nearby can cause
outages. The discharging of insulated locations due to overvoltages is not the only trigger for outages. Burnouts on
properly operating surge arresters caused by lightning currents with a long duration, which are common with winter
lightning, also cannot be ignored in some regions. In short, there are many causes of lightning outages on power
distribution lines, and in formulating a method of protection, a great many related matters must be considered.
This paper explains the results of the author’s research for 38 years on llightning protection of overhead power
distribution lines.
2 LIGHTNING INDUCED OVERVOLTAGES ON POWER DISTRIBUTION LINES
Lightning voltage induced by nearby strokes is one of the major causes of overvoltages which threaten insulation of
power distribution lines. Numerical calculations were carried out by dividing the length of the distribution line and time
into finite portions and solving the partial differential equation. The method for numerical calculation is described by the
author in an earlier paper [1]. The calculated values show close agreement with the measured values for both the peak
value and the waveform [2].
3 DISTRIBUTION SURGE ARRESTER BEHAVIOR DUE TO LIGHTNING INDUCED VOLTAGES
Using a calculation program for the analysis of lightning induced voltages on a multi-conductor system, the protection
effects of surge arresters on three- phase distribution lines were studied [3].
Figure1. shows the relationship between surge arrester installation intervals and the maximum line voltages. As 70kA for
the peak value of return stroke current, and 100 meters for the minimum distance between the lightning flash point and
the distribution line (YSL), installation of surge arresters every 200 meters provides the distribution line of lightningimpulse-withstand voltage of 100kV with good protection effects. Previous lightning observation studies have reported
that about 50% of lightning flashes have a peak current value of less than 30kA. Taking this into account, we can assume
that a surge arrester installation interval of 200 meters gives fairly good protection effects for a medium high voltage
distribution lines.
4 SHIELDING EFFECT OF OVERHEAD GROUND WIRE FOR LIGHTNING INDUCED OVERVOLTAGES
(FIGURE 2)
If an overhead ground wire has only one grounding point, the grounding wire current can be given by Eq. (1) using
induced voltage (U G (t ) ) which is considered to arise on the overhead ground wire with no grounding terminal [4,5]:
I G (t ) =
U G (t )
ZG / 2 + R
(1)
where ZG is the self surge impedance of an overhead ground wire G; and R is the grounding resistance of the overhead
ground wire.
Fig.1 Relationship between surge arrester installation intervalsand the maximum line voltages
,a: Lightning flash point is nearest to a surge arrester installation pole.
,b : Lightning flash point is nearest to the point halfway between two surge arrester installation poles.
Taking into account electromagnetic coupling between an overhead ground wire and a phase conductor, the current
flowing into the overhead ground wire reduces induced voltage. Suppressed induced voltage [ U ′ (t ) ] is expressed by
P
1
U P′ (t ) = U P (t ) − Z PG ⋅ I G (t ) (2)
2
where U P (t ) is the lightning induced voltage on a phase conductor with overhead ground wire not grounded; and Z PG
is the mutual surge impedance of an overhead ground wire and the phase conductor.
Lightning induced voltages are proportional to the conductor height as
U G ( X , t ) hG
=
U P ( X , t ) hP
(3)
At the grounding point of the overhead ground wire (X =X ), the shielding effect is expressed as follows:
Z PG
h
U ′ (X 2 , t )
η= P
= 1−
⋅ G
(4)
U P (X 2 , t )
2R + Z G hP
1
IG
2
1
IG
2
US =
1
IG • ZPG
2
IG
R
(U′ )
P
(U )
P
Fig.2 Shielding effect of overhead ground wire
where is the shielding factor (the ratio of the induced voltage with an overhead ground wire and that without an
overhead ground wire). Shielding factor is likely to be 55% to 70% for an actual distribution lines. [8,9] Equation (4)
shows that the shielding factor is determined only by geometrical array of conductors and earth resistance.
Induced overvoltage [kV]
No Protection
O.G.W. only
S.A. and O.G.W
S.A. only
!
Grounding of
O.G.W.
Distance from Left End (m)
S.A
Fig.3 Distribution of overvoltage peak along the line.
Distance between line and L.S.P. : 50m
L.S.P. : Lightning strike point
O.G.W. : Overhead ground wire
S.A. : Surge arrester
5 PROTECTION OF POWER DISTRIBUTION LINES AGAINST LIGHTNING INDUCED
OVERVOLTAGES BY MEANS OF SURGE ARRESTERS AND OVERHEAD GROUND WIRES
The combination of surge arresters and an overhead ground wire is frequently used to protect equipment and line
insulation against lightning on overhead power distribution lines in Japan. In recent years, many surge arresters have
been installed along with an overhead ground wire to provide better lightning protection, and their installation intervals
have gradually become shorter. The protective effects of surge arresters alone, an overhead ground wire alone, and the
combination of surge arresters and an overhead ground wire were therefore compared using a computer program to
analyze the overvoltages induced by nearby lightning strokes. The results were as follows [6]:
(1) If surge arresters are installed at every point where the overhead ground wire is grounded, the combination of
surge arresters and an overhead ground wire has almost the same effect as surge arresters alone.(Figure3)
6 EFFECT OF AN OVERHEAD GROUND WIRE AGAINST DIRECT LIGHTNING HITS
CRIEPI has shown the effectiveness of an overhead ground wire against direct lightning hits using a large impulse
generator in its Shiobara Testing Laboratory. From Figure 4 it is revealed that overhead ground wires are fairly effective
against direct lightning hits[7].
Fig.4 Comparison of the effect of protective measures of an overhead power distribution line against direct lightning hits
START
R ea d a n d stor e in p ut d a ta
C irc u it of p o w er d istr ibu tio n lin e
P ara m eter s of ligh tn in g
R a n d o m n u m be r g en er a tio n
a n d selection of p a r am eter s
K = k+ 1
G en e ra te r an d o m n um be rs P (K )
S ele ct p ea k ligh tn in g curr en t I ( K )
G en e ra te r an d o m n um be rs
Q (K )
S elec t X distan ce of ligh tn in g strik in g p o in t
G en erate r an d om n um bers
R (K )
S elec t Y distan ce of ligh tn in g strik in g p o in t
D e c id e n ece ssa r y ca lcu la tion ar ea
No
C a lcu late ligh tn in g str ikin g d istan c e D g
K = N?
Y es
P ossib ility o f fla sh ov er ?
Y es
D irect h it?
No
N u m er ica l ca lcu lation o f
ligh tn in g in du c ed o v er voltag e s
Y es
E M T P fo r
d ir ect h its
•C ou n t th e n um b er of sin g le-ph a se flash ov e r o u ta g es
•C ou n t th e n um b er of d ou ble -p h a se (or m or e)
fla sh over ou ta g es
No
K = N?
Y es
C a l c u late th e ra te of fla sh ov e r o u ta g e s
END
Fig.5 Flow chart of program to calculate both direct and indirect lightning hits power distribution lines
Fig.6 Relation between various lightning protection design and suppressing ratio of lightning outage rate against non-protection system. (two-phase
flashover failure, Grounding resistance : 200ohm, IKL:30 )
7 DEVELOPMENT OF ANALYTICAL METHOD FOR OUTAGE RATIO DUE TO LIGHTNING ON
POWER DISTRIBUTION LINES
CRIEPI developed a program which can calculate the lightning outage ratio due to both direct lightning hits and indirect
lightning hits. The flowchart of the developed program is shown in Figure 5 and some sample results of the program are
shown in Figure 6.
8 SELECTION OF PROTECTION MEASURES FOR POWER DISTRIBUTION LINES
8.1 Summary on Protection Effects of Surge Arresters and an Overhead Ground Wire
We described the principles behind the effects surge arresters, overhead ground wires, and their combined use in
Chapter 3,4,5 and 6. How they should be applied in order to establish a rational method is crucial.
This section will provide an outline of a rational design method targeting direct lightning hits and induced overvoltages
(indirect lightning hits).
As discussed in Chapter 3, almost all lightning overvoltages from induced overvoltages (indirect lightning hits) can be
suppressed by installing lightning arresters every 200m.Futhermore, the effect of surge arresters alone against an
induced overvoltage is nearly the same as the combined effect of surge arresters and an overhead ground wire. It was
also pointed out that an overhead ground wire always suppresses lightning overvoltages at a fixed rate regardless of the
maximum value of overvoltages. Figure 7 illustrates the tendency of the preventative effect against lighting outages due
to induced overvoltage based on this information.
The effect against direct lightning hits is generally represented as shown in Figure 10 based on the fact that the
combined effect of surge arresters and overhead ground wires is high. The rate of outages due to induced overvoltages
and direct lightning strikes varies by the insulation level. The lower the insulation level, the greater the increase in the
outage rate due to induced overvoltages. Ultimately, if lightning outages due to direct lightning strikes and induced
overvoltages are combined and plotted them on the y axis, and then plot the installation interval of lightning protection
equipment on the x axis, the total results are shown in Figure 9. If surge arresters are installed on all poles, the outage
rate is zero.
Fig.7: Effect of Surge Arresters and Overhead Ground wire Against Induced Overvoltoges
Fig.8 Effect of Surge Arresters and Overhead Ground wire Against Direct Lightning Hit
Fig.9: Reduction of Outages by Surge Arresters and Overhead Ground Wire
8.2 Lightning Protection Design Methods for Power
Distribution Lines with Low Insulation Levels
The following can be said based on Figure 9.
(1)
(2)
(3)
Shortening the grounding interval for overhead ground wires tends linearly reduce outages, but it is not possible to drastically
limit failures.
Increasing the number of surge arresters enables large reduction in failures due to induced overvoltages.
For the combined effect of suge arresters and overhead ground wires to arise, suppressing outages due to direct lightning hits is
crucial. This effect is rather prominent when surge arresters are installed every 200 m. When the interval is larger than around
200m, simple increase of the number of surge arresters installed is effective for reducing the failures overall.
The policy for applying lightning protection is shown in Figure 10. If the outage rate is reduced to 50% in Figure 10, it
would be possible to attain our goal by slightly increasing the surge arrester installation rate, compared with combining
surge arresters and overhead ground wires, and therefore, surge arresters alone would be considered effective. Further
reducing the lightning outage rate (about 20% in the figure) requires that the installation interval be considerably
shortened to achieve a reduction by surge arresters alone, and the possibility of combining surge arresters and overhead
ground wires emerges.
8.3 Lightning Protection Design Methods for Power Distribution Lines with Relatively High Insulation Levels
As shown in Figure 10, the outage rate due to induced overvoltages is high on power distribution lines with low
insulation strength. In this case, increasing the installation rate of surge arresters alone is often effective. As shown in
Figure11 on the other hand, when insulation strength is high, the effective range of combining the use of surge arresters
and an overhead ground wire rises because protection against direct lightning hits becomes the focus.
Fig.10 : Lightning Protection Design Methods for Power
Distribution Lines with Low Insulation Levels
Fig.11: Lightning Protection Design Methods for Power Distribution Lines with Relatively High Insulation Levels
9 PREVENTION OF LIGHTNING OUTAGES ON POWER DISTRIBUTION LINES LOCATED IN
MOUNTAINOUS AREAS
Mountainous Areas are prime locations for installing radio relay stations, mobile phone base stations, antennas for TV
and radio broadcasts, and Coast Guard facilities. Most of such equipment and facilities receive its electricity from power
companies, and lightning not only causes them to fail, but also results in outages on the power distribution lines that
supply electricity.
In the past, it was common to apply the same lightning protection policy were applied to mountainous areas. This
chapter discusses methods of protection against lightning outages on power distribution lines located in mountainous
areas.
9.1
Phenomena and Environment Involved in Lightning Outages on Power Distribution Lines Located In Mountainous
Áreas
(1)
High-resistance grounding in mountainous áreas
The most common cause of lightning outages on power distribution lines located in mountainous areas is backflow
current. The higher the grounding resistance of tall structures, the greater the current that flows back onto power
distribution lines when such structures are hit by lightning(Figure 12).
In mountainous areas, securing low grounding resistance is usually difficult.
(2)
Focus of lightning outages on tall structures (triggered lightning)
With winter lightning ,which occurs frequently along the Sea of Japan, lightning discharges progress from tall structures
built on the ground, and as a result, the discharge path is split in upward directions. Conversely, with summer lightning,
the discharge path is split in a downward direction, except for lightning that hits extremely tall structures.
(3)
Lightning with an extremely high charge
Fig.12 Backflow current into overhead power distribution line
Summer
Current Waveform
Winter
Fig. 13 Typical lightning current waveform
Fig.14 Surge arrester damages due to backflow current of a lightning stroke
The duration of stroke current is extremely long with winter lightning in Japan(Figure 13). The charge is represented by
an integrated value of the time of the instantaneous lightning current, and numerous examples of this value exceeding
300 coulombs have been observed with winter lightning, and there have even been cases where it exceeds the value for
summer lightning by a factor of 100 or more. This is why surge arrester and surge protection element burnouts (Figure
14)frequently occur.
9.2 Lightning Outage Performance of Power Distribution Lines Located in Mountainous Areas
Mountainous areas are extremely important to modern society because they serve as location of mobile phone base
stations, and this trend will continue. Maintenance in such areas is extremely difficult due to snow cover and road
condition, resulting in situations where a single failure may take several hours to recover. This makes lightning
protection for power distribution lines located in mountainous areas a crucial issue.
9.2.1 Lightning Outages on Power Distribution Lines Located in Mountainous Areas
Classified by Equipment Type
Figure 15 shows the results of our study on the rate of lightning outages occurring on power distribution lines located in
mountainous areas by equipment type. Among equipment the rate of high-voltage arrester outages exceeded 50%, far
higher than all other types of equipment, followed by 24% for pole transformers (including primary cutout switches), 7%
for electric meters etc. In winter lightning areas including flat areas the rate of lightning outages on surge arresters in
winter is higher than those in summer. The rate of lightning outages on surge arresters in winter, including flatlands, is
about 25% overall(Figure 15). On power distribution lines located in mountainous areas, the rate of lightning outages on
surge arresters further doubles, and severe failures due to the energy of lightning current is assumed.
It has been clearly shown that for lightning protection of power distribution lines located in mountainous areas, the
preferential study of preventive measures for surge arrester burnouts, or in other words, a lightning energy
countermeasure, is required.
9.3
A Proposal for Effective Lightning Protection for Power Distribution Lines Located in Mountainous
Areas
Even after standard lightning protection is applied to power distribution lines, excessive failures due to winter lightning
are being experienced in mountainous areas. Therefore, the application of new effective lightning protection is required,
but this is offset by the demand for raising efficiency, including the lowering of construction costs. Counterbalancing the
two sides requires ascertaining the lightning outage performance of lightning protection in the region, and then based on
an evaluation on the quantitative effects of each type or protection, performing rational lightning protection design.
Fig.15 Outage ratio of facilities on power distribution lines
In light of the matters that have been revealed through research thus far, this section will discuss effective methods for
lightning protection that target overhead power distribution lines located in mountainous areas.
9.3.1 Main Points in Lightning Protection Locations
The main points in lightning protection locations on power distribution lines located in mountainous areas are as
follows:
(i)
Since most lightning outages on power distribution lines located in mountainous areas occur due to backflow
current caused by lightning strikes on customers that have tall structures, lightning protection must be
preferentially applied to transformer poles to which lightning current flows back.
(ii) Since the outage rate of pole transformers for supplying power to customers is about 40%, protecting such
equipment is crucial.
(iii) Surge arresters are installed to protect the pole transformers mentioned in (ii) above, as a general rule. Since these
surge arresters often burn out due to the high energy of winter lightning, it is necessary to take preventative
measures against such accidents.
9.3.2 Methods of Lightning Protection on Power Distribution Lines Located in Mountainous Areas
Raising the discharge withstand capacity of surge arresters themselves has a major effect in preventing their burnout
when they are installed on power distribution lines that have overhead ground wires. It is therefore necessary to
coordinate their installation with the stringing of overhead ground wires. The coordination of overhead ground wires and
the increased discharge withstand capacity of surge arresters will be discussed on the following page.
(1)
Parallel installation of surge arresters
The parallel circuits of 10 kA surge arresters (gapless) used for raising discharge withstand capacity have different V-I
characteristics than the gapless 2.5 kA surge arresters that are generally used. Since the residual voltage of 10 kA surge
arresters is higher, almost all discharge current flows onto the 2.5 kA surge arresters, and the high discharge withstand
capacity of the 10 kA variety processes almost no energy. It is difficult to expect 10 kA surge arresters to operate
effectively to distribute and effectively reduce the processed energy, and therefore, it is necessary to remove low
discharge withstand capacity surge arresters that are prone to lightning outages. Equally distributing the processed
energy by installing surge arresters in parallel for backflow current requires surge arresters that are gapless and have
equivalent V-I characteristics.
Fig.16 Relation between Number of Overhead Ground Wires and Outage Rate
of Surge Arresters
(2)
Effect of increasing number of overhead ground wires
Figure16 shows the relationship between the number of overhead ground wires and the surge arrester burnout rate. This
figure is comparison using 100 for the surge arrester burnout rate on line where 2.5 kA surge arresters are installed
without any overhead ground wires. On power distribution lines where no overhead ground wires are strung, the
installation of such wires can reduce the surge arrester burnout rate to between 1/3 and 1/5, which means they are
effective in preventing surge arrester burnouts. Consequently, it is desirable to preferentially install overhead ground
wires over other types of lightning protection.
Two overhead ground wires enables the surge arrester outage rate to be reduced to about 1/3 of one overhead ground
wire , and three overhead ground wires enables an addition reduction of 1/3 compared with two overhead ground wires.
In short, the use of multiple overhead ground wires has a major effect, and based on this effect, it is desirable to select
the number of wires in line with the protection level.
(3) Method of applying multiple overhead ground wires
The use of multiple overhead ground wires increases the paths over which lightning current flows from the facilities,
reduces the current flowing onto high-voltage distribution lines via surge arresters, and lowers the rate of surge arrester
burnouts. Since spacing overhead ground wires too close together only yields the effect of a single wire, it is necessary
to maintain a certain distance between them. On the other hand, overhead ground wires and high-voltage lines should be
spaced close together because the mutual induction between them suppresses the lightning surge current intruding onto
the high-voltage line, or in other words, reduces the surge arrester discharge current. Broadly speaking, overhead ground
wires are spaced apart and positioned on the outside of the phase conductor as much as possible. When two overhead
ground wires are strung above a high-voltage line, the effect is lowered because they are close to each other. It is
acceptable for them to share the same grounding.
According to Figure 16, raising the discharge withstand capacity of surge arresters from 2.5 kA to 5 kA when there are
no overhead ground wires, can only lower surge arrester burnouts to between about 2/3 and 3/4. However, when an
overhead ground wire is strung, surge arrester burnouts can be lowered to between about 1/3 and 1/2, thereby pointing to
its high lightning protection effect. In short, raising the discharge withstand capacity of surge arresters is more effective
when applied to lines on which an overhead ground wire has been strung.
Figure 17 is the example of a distribution line with four overhead ground wires in Japan.
Fig 17 Power distribution line with four overhead Ground wires
10 CONCLUSIONS
We have shown that lightning outages on power distribution lines are caused by induced overvoltages (indirect lightning
hits), direct lightning hits, and backflow current. (Figure 18)
(1)Backflow current is a problem on power distribution lines, particularly those located in mountainous areas, supplying
power to tall structures, which are prone to being hit by lightning. Since the long duration of lightning current in
backflow current results in a great many surge arrester burnouts, it is a tremendous problem in winter lightning areas
(areas along the coastal region of the Sea of Japan and mountainous areas).
Induced
Overvoltages
Direct
Lightning Hits
Backflow
Currents
Effect of Overhead Ground Wire(s))
(in case of
lines with surge
arrester)
(Damage of
surge arresters
Can be reduced)
Fig.18 Effect of Overhead Ground Wire
(2) Installing surge arresters every 200 m on power distribution lines with insulation around 100kV extremely reduces
the rate of insulation failure due to induced overvoltages (indirect lightning hits). Only direct lightning hits need be
taken into account in this case.
Since distribution lines are lower in their insulation levels than transmission lines, they are relatively weak against
various kind of lightning surges. And materials used for distribution lines and methods of preventing lightning damage
are different by companies. In addition types of lightning damagse are different by locations of distribution lines. It is
therefore important to determine up to which types of lightning damage to prevent and then to implement most relevant
countermeasures based on the result of computer calculation. In future we will target further cost reduction by preparing
a lightning characterization map, which will show area-by-area frequencies and characteristics of lightning strokes.
11 REFERENCES
[1] S.Yokoyama, “Calculation of Lightning Induced Voltages on Overhead Multiconductor Systems”, IEEE. Trans.,
Vol.PAS-103, pp.100-108, 1984.
[2] S.Yokoyama, K.Miyake, H.Mitani and N.Yamazaki : “Advanced Observations of Lightning Induced Voltage on
Power Distribution Lines” , IEEE Trans, PWRD-1.No.2.pp.129-139, 1986
[3] S.Yokoyama : “Distribution Surge Arrester Behavior due to Lightning Induced Voltages”, IEEE Trans., Vol.PWRD-1, No.1,
pp.171-178, 1986.
[4] S.Yokoyama, K.Yamamoto and H.Kinoshita : “ Analogue Simulation of Lightning Induced Voltages and its Application for
Analysis of Overhead-Ground- Wire Effects”,IEE, Proc. Vol.132, Pt.C, No.4, pp.208-216, 1985.
[5] S.Yokoyama, H.Mitani, K.Miyake and N.Yamazaki : “Corroboration of Induced Lightning Voltage Suppressing
Effect of Overhead Ground Wire Using Natural Lightning”,Trans. IEE, Japan, Vol. 105-B, No.2, pp.125-132, 1985
[6] Y.Moro-oka, S.Yokoyama and A.Asakawa : “Protection of Power Distribution Lines against Lightning-induced
Overvoltages by Means of Surge Arresters and Overhead Ground Wires”, 8th ISH, Yokohama, Japan. 1993.
[7] S.Yokoyama and A.Asakawa : “Experimental Study of Response of Power Distribution Lines to Direct Lightning
Hits", IEEE Trans., Vol. PWRD-4, No.4, pp.2242-2248, 1989.