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Article
Lightning flashover characteristics of a full-scale AC 500 kV transmission
tower with composite cross arms
Qian Wang, Xidong Liang, Yufeng Shen, Shuming Liu, Zhou Zuo, Yanfeng
Gao
PII: S2095-8099(22)00159-X
DOI: https://doi.org/10.1016/j.eng.2021.09.021
Reference: ENG 981
To appear in: Engineering
Received Date: 10 March 2021
Revised Date: 18 July 2021
Accepted Date: 29 September 2021
Please cite this article as: Q. Wang, X. Liang, Y. Shen, S. Liu, Z. Zuo, Y. Gao, Lightning flashover
characteristics of a full-scale AC 500 kV transmission tower with composite cross arms, Engineering (2022), doi:
https://doi.org/10.1016/j.eng.2021.09.021
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© 2022 THE AUTHORS. Published by Elsevier LTD on behalf of Chinese Academy of Engineering and Higher
Education Press Limited Company.
Research
Smart Grid and Energy Internet—Article
Lightning Flashover Characteristics of a Full-Scale AC 500 kV Transmission
Tower with Composite Cross Arms
Qian Wang a, Xidong Liang a,*, Yufeng Shen b, Shuming Liu a, Zhou Zuo a, Yanfeng Gao c
a State Lab of Power System, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China
b Shandong Electrical Engineering &Equipment Group Co., Ltd., Jinan 250024, China
c State Grid Jibei Electric Power Co., Ltd. Research Institute, North China Electric Power Research Institute Co., Ltd., Beijing 100045, China
* Corresponding author.
E-mail address: lxd-dea@mail.tsinghua.edu.cn (X. Liang).
ARTICLE INFO
Article history:
Received 10 March 2021
Revised 18 July 2021
Accepted 29 September 2021
Available online xxxx
Abstract: Overhead transmission lines (OTLs) have always been the major means of power delivery. With the
significant increase of transmission voltage and transmission capacity, the dimensions of transmission towers are
increasing accordingly, resulting in extensive occupation of land resources. Towers with composite cross arms are a
promising solution to this problem, considering the remarkable performance of composite line insulators. In this
research, a full-scale alternating current (AC) 500 kV model of a transmission tower with composite cross arms is
manufactured and applied under a lightning overvoltage of different polarities. The developing process of streamer-
leader discharge is recorded with a high-speed camera, and the major path of the flashover is identified. The flashover
voltages are measured and corrected to standard conditions while considering the air humidity and air density, and
clearly confirm the polarity effect. The tower’s lightning-withstand level is calculated based on the tower structure and
the flashover characteristics. Based on the results obtained from full-scale experiments, the feasibility of composite
cross arms is confirmed, and a structural optimization is proposed.
Keywords: Composite cross arms; Transmission tower; Lightning flashover; Streamer discharge; Lightning withstand
1. Introduction
In the past few decades, impressive development has occurred in large-capacity power transmission over long
distances, due to technological advances and an increased demand for electric power [1–2]. Overhead transmission lines
(OTLs) have always been the major means of power delivery and will continue to be the preferred alternative in the
following years [3]. However, the construction of regular transmission towers takes a great deal of steel. Moreover,
with the increase of voltage level, towers have grown significantly in dimension. Taking alternating current (AC) 500
kV transmission lines as an example, power transmission corridors of 45–60 m are needed. Even with compact towers,
corridors of 28–43 m are necessary, resulting in the large occupation of land resources [4].
To address this issue, cross arms were introduced to power systems in Japan in the 1960s and have now been used in
various countries under different voltage levels. In addition to saving land, the use of fixed cross arms eliminates the
negative effect of wind [5–6]. With the widespread application of silicone rubber (SR) insulators, composite cross arms
are becoming increasingly popular. Compared with steel-based material, polymer-based material is lighter and exhibits
better corrosion resistance [7–9]. However, for cross arms used under high voltages such as 500 kV, a single horizontal
composite insulator cannot meet the requirement of mechanical strength, considering the weight of the conductor [9–
11]. Therefore, a more complicated structure has been proposed: In addition to horizontal insulators to maintain spacing
for insulation, diagonal insulators are added in order to provide tensile strength [12]. This change has now been widely
accepted.
The external insulation characteristics of the composite cross arms consist of the pollution flashover characteristics
along the insulator surface under a working voltage and the air gap flashover characteristics between the conductor and
the tower under an impulse overvoltage. Since the flashover characteristics over long distances cannot be directly
extrapolated from the results over short distances, full-scale pollution flashover tests of AC 500 kV composite cross
arms have been conducted and the negative exponential relationship between the salt deposit density and flashover
voltage has been confirmed [5,13]. The insulation configuration at the power frequency is mainly determined by the
pollution flashover characteristics along the surface, while under an overvoltage such as a lightning overvoltage or a
switching overvoltage, the air gap between the conductor and the tower is the component most likely to flashover [14–
17]. Theoretical and experimental results have described the flashover characteristics of simple electrode systems such
as rod-rod and rod-plane electrodes [18–19]. Researchers have also focused on the impulse flashover characteristics of
2
towers without cross arms [20–21]. Nonetheless, the actual air gap of transmission towers cannot be well described by
ideal models or previous structures, which made it necessary to conduct overvoltage flashover tests using a full-scale
tower with composite cross arms. Unfortunately, the results are still very limited due to the rigorous requirements for
testing. As a result, the influence of the introduction of composite cross arms on a tower’s lightning protection still
remains uncertain.
A full-scale AC 500 kV model transmission tower with composite cross arms was manufactured for the investigations
described in this paper. Lightning overvoltages of different polarities were applied, and the flashover characteristics
were measured. The developing process of the streamer-leader discharge was recorded, and the major path of the
flashover was identified. The measured flashover results were corrected to standard conditions and clearly confirmed
the polarity effect of lightning flashover. The lightning-withstand level of the tower with composite cross arms was
derived based on the results. The feasibility of composite cross arms was thus confirmed, and a structural optimization
is proposed accordingly.
2. Experimental setup
2.1. Model transmission tower
A full-scale AC 500 kV asymmetric transmission tower with a total height of 53 m was manufactured, as shown in
Fig. 1(a), where R is 3.7 m and L1 and L2 are 6.4 and 6.9 m, respectively. The tower was designed based on a regular
OTL with suspension insulators, and its dimensions were validated by simulation [9]. Since the structures of the lower
two phases were symmetric, only two sets of composite cross arms on the same side of the tower were installed during
the test; these are referred to as the “upper phase” and “lower phase” in this paper. Based on previous research, the
influence of the omitted phase on the electric field distribution and the parasitic capacitance to the tower (or ground) of
the other two phases is negligible [9,12]. Compared with regular towers, transmission corridors of 2–3 m could be saved
by introducing composite cross arms. In order to better simulate the actual operating conditions, a four-conductor bundle
of at least 10 m was fixed to the cross arms. In addition, grading rings were installed at both ends of the cross arms and
shielding rings were installed near the conductors. Moreover, in order to reduce the tower height for the convenience of
testing, only the upper component of the tower was kept, while the lower part away from the cross arms was removed.
As a result, the actual height of the model tower was reduced to 17 m. Furthermore, considering that the distance
between the conductor and the grounding conductor must not be less than 1.5 times the flashover distance according to
test standards [22], the model tower was lifted by a jib when the lower phase was being tested, as shown in Fig. 1(b),
making sure that the distance between the conductor and the grounding conductor was more than 10 m. It is worth
noting that the tests for both the upper and lower phases were performed with the tower lifted to the same height.
(a) (b)
Fig. 1. The model transmission tower with two sets of composite cross arms on the same side for the experiment. (a) Schematic diagram;
(b) actual image.
3
Each set of composite cross arms consisted of four insulators, as shown in Fig. 2(a). A pair of regular line composite
insulators were applied for diagonal stretching, with a total length of 5032 mm. In addition, a pair of large-diameter
insulators with an insulating length of 4689 mm were utilized for horizontal support. The sheath of the horizontal
insulator, whose diameter was 320 mm, was made of SR composites with hydrophobic transfer ability. The selection
and dimensioning of the insulators were validated by a simulation of the electromagnetic environment and pollution
flashover tests [5,9]. However, the trunk of the horizontal insulators was filled with polyurethane (PU) foam for better
sealing and weight reduction, instead of the glass-fiber-reinforced plastics used in line insulators. A cross-section of the
horizontal insulator and the PU foam material are shown in Fig. 2(b).
(a) (b) (c)
Fig. 2. (a) Structure of a set of cross arms; (b) cross-section of the horizontal insulator filled with PU foam; (c) PU foam material used for
internal insulation.
2.2. Voltage waveform settings
The nominal voltage of the impulse voltage generator, which was used in the impulse overvoltage tests, was 7200
kV, and the actual waveform of the applied positive lightning overvoltage is shown in Fig. 3. According to the waveform
measured, the deviations of the time to crest and the half peak value were 29.2% and −17.4%, while the voltage
overshoot was 8%, all of which meet the requirements of the International Electrotechnical Commission (IEC) standard
[22]. In addition, the internal capacitance of the impulse voltage generator was 18.75 pF, while the external capacitance
was around 1400 pF and consisted of the capacitance of the voltage divider and the ground capacitance of the voltage
divider, the high-voltage (HV) leads, the equipment, and the voltage equalizer [24].
Fig. 3. Waveform of the positive lightning overvoltage generated with a time to crest of 1.50 μs and a half peak value time of 41.91 μs.
2.3. Photographing the flashover process
The process of lightning flashovers involved typical streamer-leader discharges. An observation system was set up,
as shown in Fig. 3. The closing coil was used to trigger the oscilloscope upon sensing the impulse voltage. Once the
oscilloscope was triggered, the high-speed camera was immediately triggered to shoot the discharge process. The
images recorded were then transmitted to a personal computer (PC).
4
Fig. 4. Schematic figure of the setup for the observation of the lightning flashover process.
3. Results and discussion
3.1. Flashover path under lightning overvoltage
Based on the tower structure with composite cross arms, there are three possible flashover paths for the breakdown
of the air gap. In Fig. 5, d1 represents an air gap flashover between the inner grading ring of the horizontal insulator and
the tower, while d2 and d3 represent an air flashover between the grading rings at both ends of the horizontal insulators
and diagonal insulators, respectively. The air gap distance of each flashover path is shown in Table 1.
(a) (b) (c)
Fig. 5. Possible flashover paths under lightning overvoltage. (a) Flashover between the tower and the inner grading ring of the horizontal
insulator, d1; (b) flashover between the grading rings at both ends of the horizontal insulators, d2; (c) flashover between the grading rings
at both ends of the diagonal insulators, d3.
Table 1. Air gap distances of different flashover paths.
Upper phase
Lower phase
d1 (m)
d2 (m)
d3 (m)
d1 (m)
d2 (m)
d3 (m)
3.98
4.67
4.23
4.10
4.43
4.56
In order to better photograph the flashover progress, the tests were all conducted in the evening. The air gap flashovers
along the three different paths were photographed and are shown in Fig. 6. For both the upper phase and the lower
phase, 20 positive and negative lightning overvoltages were applied until a flashover occurred [22]. The number of
flashovers along the various paths were counted and are shown in Table 2. The results clearly indicated that the air gap
between the tower and the grading ring on the inner side of the horizontal insulators (d1) was the major flashover path
(90% for the upper phase and 55% for the lower phase). This finding helped to confirm that lightning flashover would
mostly occur over the shortest air gap [25]. Moreover, the distinction between different polarities suggests that this
result is more evident under a positive voltage, whereas the results under negative voltage show more diversity.
(a) (b)
5
(c) (d)
Fig. 6. Typical air gap flashover images of different flashover paths. (a) d1m (flashover to the middle of the tower); (b) d1s (flashover to the
side of the tower); (c) d2; (d) d3.
Table 2. Number of flashovers over various paths.
Upper phase
Lower phase
d1 (m)
d2 (m)
d3 (m)
d1 (m)
d2 (m)
d3 (m)
36
4(neg × 3 + pos ×1)
0
22
9(neg × 5 + pos × 4)
9(neg × 7 + pos × 2)
neg: negative lightning overvoltages; pos: positive lightning overvoltages.
Further observation showed that there are convex nuts at the middle of tower. In addition, there are bolts at both sides
of the tower with lengths of about 10 cm, for the ease of climbing the tower. Therefore, due to these small units, a
structure with several tips is formed on the side of the tower. The tips further reduce the insulating distance, making the
streamer-leader discharge easier to form. The detail of the flashover caused by the tips is shown in Fig. 7.
(a) (b)
Fig. 7. Tip structures on the tower result in a decrease of flashover voltage. (a) Convex nuts at the middle of tower; (b) bolts at both sides
of the tower.
3.2. Flashover process under lightning overvoltage
The flashover processes of both the upper phase and lower phase under a negative overvoltage were shot, as shown
in Figs. 8 and 9. With the increase of voltage in Fig. 8(b), streamer discharge first occurred from the grading ring of the
horizontal insulators near the conductor, toward the grading ring near the tower. Next, as shown in Fig. 8(c), five
streamer channels occurred toward different directions. The luminescence of the streamer discharge was faint but
uniform, with a width of 1–1.3 m according to the picture. Then, dendritic leaders appeared at both ends of some of the
streamer channels, as shown in Fig. 8(d). Finally, as shown in Fig. 8(e), the leader extended along the direction of the
streamer channels and eventually formed the main discharge with an intense glow. Moreover, considering that the
formation of the streamer over the gap (which was around 4 m) took about 10 μs, the developing speed of the negative
steamer was on the order of 107–108 cm·s−1, based on the images. The process shown in Fig. 9 is similar to that shown
in Fig. 8. The three flashover paths mainly differed in terms of the location of the flashover; the processes of the air
breakdown were uniform.
6
Fig. 8. Flashover process of the upper phase (shooting speed: 180 064 fps; shooting interval: 5.55 μs; exposure time: 4.82 μs). (a) −3.05μs;
(b) 2.5 μs; (c) 8.05 μs; (d) 13.61 μs; (e) 19.16 μs.
Fig. 9. Flashover process of the lower phase (shooting speed: 130 140 fps; shooting interval: 7.68 μs; exposure time: 7.11 μs). (a) −3.75μs;
(b) 3.93 μs; (c) 11.61 μs; (d) 19.29 μs; (e) 25.96 μs.
3.3. Flashover characteristics under lightning overvoltage
Since a single discharge has great randomness, an up-and-down method was applied to obtain the 50% probability
flashover voltage, [26]. Each series of tests involved 20 lightning flashovers. The results of all 80 lightning
𝑈
50%
flashover tests are shown in Fig. 10 and belong to the same set as the results shown in Table 2. Since the condition of
the air plays an important role in determining the flashover characteristics, the air temperature T, relative humidity RH,
and pressure P were recorded at the beginning, middle, and end of each series of tests. The average value of three
measurements was taken as the condition of the air for the series, as shown in Table 3, where σ represents the variance
of the flashover voltages.
Fig. 10. Lightning flashover characteristics of composite cross arms conducted by the up-and-down method.
Table 3. Flashover voltages and the condition of the air for the lightning overvoltage tests.
Location
U50% (kV)
σ (%)
T (°C)
RH (%)
P (Pa)
Upper (+)
2090.0
2.4
25.7
62.8
79.24
Upper (−)
2268.6
2.0
28.2
40.9
79.32
Lower (+)
2038.7
1.9
28.4
38.9
79.19
Lower (−)
2372.3
2.8
26.3
47.6
79.25
3.4. Correction of test results for air humidity
7
Since the condition of the air significantly influences the flashover process, the test results needed to be corrected to
standard air conditions (i.e., temperature , pressure , and absolute humidity )
𝑡
0
=
20 ℃
𝑃
0
=
101.3
kPa
ℎ
0
=
11
g
·
m
―
3
in order to be comparable [28]. In general, the flashover voltage along the surface or over the air gap rises with an
increase in air humidity: The higher the humidity, the greater the number of electrons captured by water molecules and
the smaller the average free path of electrons. However, once the relative humidity reaches 80% or higher, the discharge
becomes destructive and irregular, especially for discharges on the surface. According to Ref. [22], the flashover voltage
under standard humidity based on can be derived as follows:
0
50%_h
U
𝑈
50%
0
50% 50%
50%_ =
1 + 0.01 11
h
U U
Uk
h
(1)
where , , and denote the absolute humidity, relative air density, and correction factor for humidity, respectively,
ℎ
𝛿
𝜔
while is an intermediate parameter. Among them,
𝑘
𝛿
=
𝑃
𝑃
0
×
273.15
+
𝑇
0
273.15
+
𝑇
(2)
where and represent the air pressure and temperature measured, respectively. Next, another intermediate parameter
𝑃
𝑇
is defined as follows:
𝑔
𝑔
=
𝑈
50%
500𝐿𝛿𝑘
(3)
Then, is determined by the value of according to Table 4 [22]. And denotes the length of the shortest flashover
𝜔
𝑔
𝐿
path. The flashover voltages after being corrected for air humidity are shown in Table 5. The results suggest that
humidity plays a minor role in the findings reported here.
Table 4. Correction factor for air humidity and correction factor for air density determined by .
𝜔
𝑚
𝑔
𝑔
𝜔
𝑚
< 0.2
0
0
0.2–1.0
g(g – 0.2)·0.8−1
g(g – 0.2)·0.8−1
1.0–1.2
1.0
1.0
1.2–2.0
(2.2 – g)(2 – g)·0.8−1
1.0
> 2.0
0
1.0
Table 5. Flashover voltages and condition of the air during the lightning overvoltage tests.
Location
U50% (kV)
(g·m−3)
ℎ
𝛿
k
g
𝜔
(kV)
0
50%_h
U
Upper (+)
2090.0
15.0
0.767
1.085
1.26
0.867
1947.8
Upper (−)
2268.6
11.2
0.762
1.037
1.44
0.527
2226.3
Lower (+)
2038.7
10.8
0.760
1.032
1.27
0.852
1985.1
Lower (−)
2372.3
11.7
0.766
1.043
1.45
0.519
2323.5
3.5. Correction of test results for air density
Like humidity, air density strongly influences the flashover characteristics. Previous research has shown that the
flashover voltage is higher in denser air [5]. The tests in this paper were conducted in Kunming, China, which has an
altitude of about 2100 m. The correction factor for altitude is specified in the IEC standard, which is valid in areas that
are no more than 2000 m high [22]. Thus far, there has been little test experience and no widely accepted method for
higher altitudes. In our case, the current correction formula for 2000 m or lower was extended and used, since the
altitude in this research was only slightly beyond the range. Therefore, the flashover voltage under standard air density
was derived as follows:
0
50%_
U
0
50%
50%_ m
U
U
(4)
where has the same definition as in the previous part and denotes the correction factor for air density, which can
𝛿
𝑚
be derived according to Table 4. The flashover voltages after being corrected for air density are shown in Table 6. The
results suggest that the influence of air density on flashover voltage was much more significant than that of air humidity
in this research. As a comparison, when cross arms are not used, the insulating length of a typical suspension composite
insulator is about 4152 mm [27]. Previous research indicates that the U50% of an insulator under positive lightning
impulse is around 2567 kV (corrected to standard conditions). This suggests that the application of composite cross
arms would not reduce the lightning protection performance in terms of flashover voltage.
8
Table 6. Flashover voltages after correction for air density.
Location
(kV)
0
50%_h
U
𝛿
g
𝜔
(kV)
0
50%_
U
Upper (+)
1947.8
0.767
1.26
1.0
2539.5
Upper (−)
2226.3
0.762
1.44
1.0
2921.7
Lower (+)
1985.1
0.760
1.27
1.0
2612.0
Lower (−)
2323.5
0.766
1.45
1.0
3033.3
Based on the results corrected to standard conditions, the flashover voltages of the lower phase were slightly higher
than those of the upper phase; this occurred because the air gap distance of the major flashover path (d1) for the upper
phase was shorter than that for the lower phase. Moreover, the flashover voltages under negative polarity were
significantly higher (about 15%) than those under positive polarity. These findings coincide with the test results from
other researchers using simplified electrodes or scaling electrodes [28].
It is well known that the breakdown of an air gap requires not only sufficient voltage, but also sufficient time under
lightning overvoltage. In this research, breakdown occurred at the wave tail of the lightning impulse, as shown in Fig.
11(a). The relationship between breakdown voltage and flashover time is shown in Fig. 12(b) and confirms that the
lower the voltage, the longer the time required for the gap breakdown. The findings also confirm the polarity effect,
considering the significant difference shown in the results. Furthermore, since the three flashover paths had different
volt-second characteristics, the test results showed an obvious variation, especially for the upper phase under positive
voltage.
(a) (b)
Fig. 11. (a) Waveform of lightning flashover for the upper phase under positive overvoltage; (b) relationship between breakdown voltage
and flashover time.
3.6. Lightning-withstand ability
Based on the test results and the structure of the transmission tower, the lightning-withstand level and the probability
of lightning trip-out were derived [29]. According to operation experience, most lightning strikes have negative polarity
and strikes on the tower are in the majority, which corresponds to applying a positive overvoltage to the conductor [28].
Therefore, the calculation was based on positive flashover voltages of the upper phase. In general, there are two kinds
of lightning strike locations: strikes on the tower and strikes on the conductor. For the complete tower with composite
cross arms used in this research, the lightning-withstand level for a lightning strike on tower was calculated as follows
𝐼
1
[28,30]:
𝐼
1
=
𝑈
50%
(
1
―
𝑘
)
𝛽
𝑅
𝑔
+
𝛽
𝐿
t
𝜏
𝑓
(
ℎ
𝑠
ℎ
𝑡
―
𝑘
)
+
ℎ
𝑐
𝜏
𝑓
(
1
―
𝑘
0
ℎ
𝑔
ℎ
𝑐
)
(5)
where denotes the coupling coefficient between the ground wire and the conductor. For the tower used in this research,
𝑘
𝑘
=
𝑘
0
𝑘
1
=
0.20
×
1.28
=
0.256 (6)
where and respectively denote the coupling coefficient of the double ground wire to the outer conductor, and the
𝑘
0
𝑘
1
corona correction factor for the lightning strike on the tower top, whose values are specified in Ref. [24]. In addition,
𝛽
denotes the current shunting coefficient of the lightning strike on the conductor, which refers to the ratio of current
9
flowing through the tower to the total lightning current, and represents the ground resistance of the tower. For the
𝑅
𝑔
AC 500 kV tower used in this research,
𝛽
=
0.88,
𝑅
𝑔
=
7 Ω (7)
Moreover, represents the equivalent inductance of the tower, which is related to the tower height. Here,
𝐿
𝑡
𝐿
t
=
53
×
0.5
=
26.5
μH
(8)
where represents the wavefront time, which is usually 2.6 μs when dealing with lightning protection; denotes the
𝜏
𝑓
ℎ
𝑠
height of the suspension point of the conductor, which was 38 m and was also the height of the composite cross arms;
denotes the height of the tower, which was 53 m; and and respectively denote the average height of the
ℎ
𝑡
ℎ
𝑐
ℎ
𝑔
conductor and the ground wire. In this research,
ℎ
𝑐
=
ℎ
2
―
2
3
𝑓
𝑎
=
38.0
―
2
3
×
12.0
=
30.0
m
(9)
ℎ
𝑔
=
ℎ
1
―
2
3
𝑓
𝑔
=
53.0
―
2
3
×
9.5
=
46.7
m
(10)
where and denote the sag of the conductor and the ground wire, respectively. Based on the data above,
𝑓
𝑎
𝑓
𝑔
𝐼
1
=
2539.5
[
(
1
―
0.256
)
×
0.88
×
7
+
26.5
×
0.88
2.6
(
38
53
―
0.256
)
+
30
2.6
(
1
―
0.20
×
46.7
30
)
]
=
152.4
kA
(
10
)
The lightning-withstand level for a strike on conductor was derived as follows [24]:
𝐼
2
𝐼
2
=
4𝑈
50%
𝑍
(11)
where represents the surge impedance of the OTL, which is determined by the conductor material, structure of the
𝑍
bundle, and so forth. In China, the respective OTL surge impedance of AC 220, 330, and 500 kV is typically 400, 303,
and 278 Ω [28]. Thus,
𝐼
2
=
𝑈
50%
𝑍
=
4
×
2539.5
278
=
36.54
kA
(12)
In China, the typical probability that the amplitude of lightning current will exceed can generally be derived as
𝐼
follows [28]:
lg
𝑅
=
―
𝐼
88
, 𝑜𝑟 𝑅
=
10
―
𝐼
88
(13)
Then, the probability of the lightning current exceeding the lightning-withstand level for a strike on the tower and
𝑅
1
a strike on the conductor are
𝑅
2
𝑅
1
=
10
―
152.4
88
=
1.85% (14)
𝑅
2
=
10
―
36.54
88
=
38.44% (15)
If the lightning current exceeds the lightning-withstand level, a flashover will occur over the insulation part of the
transmission lines. However, the duration of the lightning overvoltage is only a few tens of microseconds; it is too short
for the high-voltage switches to operate. As a result, tripping is only caused when the flashover channel that is broken
down by the impulse voltage develops into a stable power frequency arc. The processes involved above have a certain
randomness. Thus, in addition to the lightning-withstand level, the probability of lightning trip-out, , is also used as
𝑛
an integrated index to evaluate the lightning protection performance of transmission lines:
𝑛
=
𝜂𝑁𝑅
(16)
where denotes the arc over rate; that is, the probability of an air gap developing into a power frequency arc after
𝜂
flashover under impulse voltage. denotes the estimated total number of lightning strikes on transmission lines of 100
𝑁
km in one year. For the tower used in this research, which was in a typical plains area of China:
𝜂
=
0.87
, 𝑁
=
36.4
times
/(100
km
·𝑎) (17)
10
Moreover, assuming that the shielding angle , then the probability of a strike on the tower, , and the risk
𝛼
=
14°
𝑔
of shielding failure, , could be derived by the Monte–Carlo method, considering the tower structure:
𝑅
𝛼
𝑔
=
16.67 %, 𝑅
𝛼
=
0.19 %
(18)
As a result, the total trip-out rate is the sum of the lightning trip-outs caused by both strikes on the tower and strikes
on the conductor:
𝑛
=
𝜂𝑁𝑅
=
𝜂𝑁
(
𝑔𝑅
1
+
𝑅
𝛼
𝑅
2
)
=
0.87
×
36.4
×
(
0.1667
×
0.0185
+
0.0019
×
0.3844
)
=
0.098
time
/(100
km
·𝑎) (19)
For comparison, the lightning trip-out rate of a regular AC 500 kV transmission tower is ,
0
.081
time
/(100
km
·𝑎)
which means that, on average, there are 0.081 trip-outs per 100 km of transmission lines annually [28]. These findings
suggest that the introduction of composite cross arms would result in the degradation of lightning protection
performance. Considering the factors influencing , a smaller protection angle could be used to enhance the lightning-
𝑛
withstand ability.
4. Conclusions
In this paper, a full-scale AC 500 kV transmission tower with composite cross arms was manufactured and applied
under a lightning overvoltage of different polarities, which simulated the actual operation condition well. The discharge
process was recorded, and the flashover characteristics were measured. Based on the results, which were corrected to
standard conditions, the lightning-withstand level was calculated. Several conclusions were derived and are presented
below.
It has been confirmed that lightning flashover mostly occurs over the shortest air gap; the tip structures on the
tower make the formation of the streamer easier.
Air humidity and density both influence the lightning flashover characteristics. Air density, which is
determined by air temperature and pressure, played the major role in the lightning flashover characteristics
recorded here.
A significant polarity effect was observed for the full-scale transmission tower based on the measured results.
Under negative lightning overvoltage, the is about 15% higher than that under positive voltage.
𝑈
50%
The lightning-withstand ability of the transmission tower was degraded after the application of composite cross
arms. Thus, a protection angle smaller than 14° is recommended.
Acknowledgments
The authors would like to acknowledge the National Natural Science Foundation of China (Grant No.51977116) for the
support for this work.
Compliance with ethics guidelines
Qian Wang, Xidong Liang, Yufeng Shen, Shuming Liu, Zhou Zuo, and Yanfeng Gao declare that they have no conflict
of interest or financial conflicts to disclose.
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