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Experimental Study on Lighting Shielding Performance of ±500 kV
HVDC Transmission Lines Hengxin HE, Junjia HE, Dandan Zhang, Li
DING
College of Electrical & Electronics Engineering Huazhong
University of Science & Technology
Wuhan, China Email: [email protected]
Zhenglong JIANG, Cheng WANG, Huisheng YE Hunan Electric Power
Test and Research Institute
Changsha, China
Abstract—This paper is to investigate the lightning shielding
performance of the HVDC transmission line when taking the dc
operation voltage into account. Firstly, the equivalency of the
scale experiment is discussed from the following aspects: (a) the
determination of test scale factor; (b) the electric field
simulation of downward leader; (c) nominal field simulation of the
HVDC transmission line. Secondly, a test circuit is established
taking into account of the isolation of the dc charge circuit and
surge generator. The exposure space distribution of the HVDC
transmission line is obtained from a large amount discharge
experiments. The total number of recorded discharge is over 7500
times. The competition between upward streamers originating from
the grounded wire and polar conductor is observed by using the
high-speed digital camera. Finally, the effectiveness mechanism of
the dc operation voltage on the lightning shielding performance of
HVDC transmission line is analyzed according to the recorded
phenomenon.
Keywords-±500 kV HVDC transmission line; lightning stroke
shielding; scaled experiment; test equivalency; exposure space;
upward streamer
I. INTRODUCTION Lightning strokes to overhead transmission lines
is a usual
reason for unscheduled supply interruptions in the modern power
systems. Field study indicates that more than 90% lightning stroke
outage accidents for extra high voltage (EHV) and ultra high
voltage (UHV) transmission lines are caused by shielding failure
flashover. To improve the shielding effect of transmission line
against lightning strokes is one of the key problems of the
transmission line design, which aims at both maintaining
reliability and reducing costs.
The lightning shielding failure flashover rate of the HVDC
transmission lines is higher than that of the EHVAC transmission
lines in China. For the ±500 kV Jiang-Cheng HVDC transmission line
which is a part of the Three-Gorges power energy transmission
project and has been put into operation in 2004. The average
lighting shielding failure flashover rate is 0.38 flashes/100km·a.
A total of thirteen lighting shielding failure trip-off accidents
occurred. Eleven trip-off accidents of them occurred on the
positive polar of the bipolar HVDC system[1]. The similar results
are reportd from the operation experience of ±500 kV Tian-Guang and
±500 kV Gui-Guang HVDC project in China [2].
It is believed that the simulation experiment is an effective
way to determine the probability of lightning strokes attaching to
the power facilities and to estimate the shielding effect of
lightning rod or shielding wire [3-7]. The credibility of the
lightning simulation experiment depends on how exactly it
reconstructs the actual lightning attachment process. Over the past
three decades, many experiments have been carried out in
high-voltage laboratories to develop a more valid way to simulate
the lightning attachment process by using long sparkle discharge.
Most of these experiments were carried out by using two typical
configurations. One is a rod-plane gap that the rod electrode is
energized with positive impulse voltage and the plane is grounded
potential [8]. It results in a positive leader incepting from the
rod. Another typical configuration is an inverted rod-plane gap
[5]. In this configuration, the overhead plane electrode is
energized with negative impulse voltage and the rod is located on
the ground at zero potential. It results a positive upward leader
inception from the rod. In this paper, a traditional rod-plane gap
is adopted. The rod electrode is energized with negative impulse
voltage. The test object is replaced by a scaled model. Therefore,
the encounter process of the downward leader and upward leader can
be simulated by the negative streamer inception from the rod
electrode and the upward streamer inception from the scaled model,
respectively.
The purpose of this paper is to investigate the lightning
shielding performance of the HVDC transmission line concerning the
effectiveness of dc operation voltage by employing a simulation
experiment approach. Firstly, the equivalency of the scaled
experiment is discussed from the following aspects: (a) the
determination of test scale factor; (b) the electric field
simulation of downward leader; (c) nominal field simulation of the
HVDC transmission line. Secondly, a test circuit is established
concerning the isolation of the dc charge circuit and surge
generator. The exposure space distribution of the HVDC transmission
line obtained from a large amount discharge experiments and the
typical discharge phenomenon are described in section 3. Finally,
the effect mechanism of the dc voltage on the lightning shielding
performance of HVDC transmission line is preliminary analyzed
according to the recorded phenomenon.
978-1-4244-2487-0/09/$25.00 ©2009 IEEE
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II. TEST EQUIVALENCY
A. Test scale factor The lightning simulation experiment is
generally carried
out by using a configuration commonly known as a rod-plane gap.
The negative downward leader channel is simulated by a rod
electrode applying surge impulse to it and the electric power
facilities under test are replaced by their miniature model. As
shown in Fig.1, Hc is the height of the cloud, D is the horizontal
distance between return stroke channel and ground object, DM is the
distance between discharge strike point to the ground and the test
object in laboratory experiment. It is impossible to simulate the
whole process of lightning discharges by laboratory experiments
with conventional lightning or switching impulses and only the
final stage of lightning strokes to the grounded objects can be
simulated by laboratory discharge in long air gaps under impulse
voltages [3].Consequently, the test scale factor can be defined as
the ratio of the streamer length of the negative downward leader
zFL and the final jump distance of the laboratory long air gap
discharge zFD:
FLFD
zkz
= (1)
The return stroke incepts when the streamer ahead the negative
downward leader tip reaches the ground according to the propagation
mechanism of natural cloud-to-ground lightning flashes. Hence the
final jump distance is approximately equal to the streamer length
of the negative downward leader zFL, which can be also roughly
calculated by the empirical formula of strike distance. In this
study, the final jump distance to the ground is calculated by using
the downward leader charge distribution model proposed in [9] and
[10], which is based on electrostatic considerations of measured
waveforms of the return-stroke current. The downward leader core is
assumed as the region where the electric field exceeds 3.0MV/m
which is the critical ionization field of air. The length of the
streamer ahead of the downward leader is calculated by the ambient
potential profile ahead of the downward leader and by the constant
electric field maintained inside the established negative corona
region which is equal to 750.0kV/m for a negative streamer [11].
Therefore, the calculated results of the streamer length at the
final jump of the negative downward leader as a function of the
magnitude of lightning current is shown in Fig. 2. The maximum
lightning current magnitude simulated here is approximately -40.0kA
and the corresponding final jump length zFL is 50.22 m.
Meanwhile, the maximum rod-plane gap length of the scaled
experiment is 1.5m and corresponding breakdown voltage is
approximate -1200.0kV under negative 250/2500 sswitch impulse
voltage. It is in the acceptable error range that the discharge
final jump distance zFD of scaled experiment is equal to the length
of rod-plane gap HL considering the average electric field of
negative streamer is 750.0kV/m. Thus the scale factor k is equal to
40.0 considering certain surplus margin. And the height of the
scaled model is given by
FDMFL
zhh hk z
= = ⋅ (2)
Where h is the height of the actual grounded object under test
and k is the test scale factor.
Rod electrode
Thunder cloud
zFL
HC
HL Streamer
Downward leader
zFD hM
Scale model
Grounded object
h
DDM
Fig. 1 Illustration of the scale factor of lightning shielding
simulation experiment
Fig. 2 the final jump length to the ground as a function of the
magnitude of lightning current of downward leader
B. The electric field simulation produced by the downward leader
This section is devoted to discuss one of the most important
aspects to ensure the credibility of the scale experiment.
Assuming E1 (t) is the electric field in the vicinity of an actual
grounded object during the final stage of the negative downward
leader attachment to the grounded object, and E2(t)is the electric
field nearby the test model during the discharge process in
laboratory experiments. The equivalency of the scale experiment can
be guaranteed, if the variation of E2(t) is consistent with the
variation of E1 (t).
According to the superposition principle, the spatial electric
field E1(t) during the period of the negative downward leader
attachment to the HVDC transmission line has two components Es(t)
and Ei(t). Ei(t) is the total electric field produced by the HVDC
transmission line which will be discussed in the following section.
Es(t)is the electric field produced by the charge in the downward
leader channel which can be calculated by the following
methods.
The average velocity of the negative downward leader vdcan be
calculated by the empirical formula proposed in [14]:
4 1/ 3d 8 10v I= × (3) Where I is the magnitude of lightning
current. The average progress velocity of the downward stepped
leader was found to be in the range of (1.8-2.2) ×105m/s from some
observations of lightning strikes by Berger et al. From equation
(10), using a lightning current I is equal to 30.0 kA, the velocity
is 2.49×105m/s, which is consistent with Berger’s observations.
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Combining with the leader channel model mentioned in section .A,
the electric field at ground level is calculated when the magnitude
of lightning current I is respective 10.0, 20.0 and 30.0 kA, and
the corresponding horizontal distance Dis 50.0, 100.0 and 150.0m
from the lightning channel, which is shown in Fig. 3. The
calculation results confirm that the electric field increases
significantly when the downward leader attaches to the ground in
the final stage, and the total duration time is approximately in
the range of 200.0~1000.0 s. It is consisted with the results of
artificially triggered lightning experiments [15] and the close
lightning electromagnetic measurements of the nature
cloud-to-ground flashes [16].In order to simulate the electric
field variation excited by the descending leader in the vicinity of
the grounded object during the final stage, the standard 250/2500 s
negative switch impulse voltage is applied to the rod electrode in
the scaled experiments.
Fig. 3 Electric field waveforms in the vicinity of lightning
stroke target at ground level
C. Nominal field simulation of the HVDC transmission line
Another component of the spatial electric field E1(t) during
the negative downward leader attachment to the HVDC transmission
line is Ei(t), which is the total electric field produced by the
HVDC transmission line. However, only the nominal field of the HVDC
transmission line is concerned and the ion current field is
neglected in this study for a simplification consideration.
In order to select the valid magnitude of dc voltage applied to
the scaled model line, the surface electric field Ec on the polar
conductors of the actual HVDC transmission line and the scaled
model line under monopole operation mode are calculated
respectively by using the charge simulation method (CSM). The
configuration position of the grounded wires and polar conductors
are located according to the geometrical size of G4-40 which is a
type of straight tower widely used in ±500kV HVDC transmission
project in China. The calculation results are shown in Tab. 1.
where rw is the radius of grounded wire, rc the radius of
sub-conductor and Udc the dc voltage applied to the polar
conductors, respectively. It indicates that the surface electric
field of scaled model line coincides with the surface electric
field of the actual HVDC transmission line if the applied dc
voltage is in the range of 15.0~16.0kV. The four-bundle polar
conductors of the actual transmission line are simulated by a
single conductor in the scaled model line.
Moreover, the electric field distribution at ground level of the
scaled model line is measured by using the EFA–3 electric
field sensor when the dc voltage applied to the positive polar
of the model line is 16.0kV. The measuring result coincides with
the calculation result of the electric field distribution at ground
level of the actual ±500 kV HVDC transmission line, as shown in
Fig.4. The comparison implies that the nominal electric field
distribution in the vicinity space of the scaled model line can be
excellently agree with that of the actual ±500 kV HVDC transmission
line, if the dc voltage applied to the positive polar of the scaled
model line is in the range of 15.0~16.0kV. Tab. 1 the surface
electric field strength of a HVDC transmission line and
the model line Transmission Lines rw/cm rc/cm Udc/kV Ec/
kV/cm
Actual HVDC transmission lines
0.550 1.186 550.0 22.347
Scaled model line 0.055 0.100 15.00 22.328 Scaled model line
0.055 0.100 16.00 23.816
Fig. 4 Measured and calculated electric field distribution at
ground level of model transmission line under monopole operation
mode contrasting
with that of the actual HVDC transmission line
III. EXPERIMENTAL CONFIGURATIONS
A. Test Objects & Electrode Configurations The purpose of
this paper is to study on the lightning
shielding performance of the ±500 kV HVDC transmission line by
using the experimental approach. The test object is the 1:40 scaled
model line of the actual ±500 kV HVDC transmission line. It
includes two 1:40 scaled model towers of G4-40 which is a type of
straight tower widely used in ±500 kV HVDC transmission project in
China. And it also contains the scaled model line segment connected
to the scaled model towers. The four-bundle polar conductors and
the grounded wire of the actual transmission line are replaced by a
single conductor in the scaled model line with the radius of 1.0mm
and 0.55mm, respectively. The length of the insulator string scaled
model is 14.0cm which is made by epoxy rod with a diameter of
16.0mm. The shielding angle of the scaled model line is 11.3°.
The negative downward stepped leader before the final jump stage
is simulated by a rod electrode. The length of the negative
downward leader can account for approximately over 90.0% of the
whole gap length between the thunder cloud and the ground in
natural cloud-to-ground flashes. Therefore the rod electrode should
be as long as possible to reproduce the spatial electric field
distribution beneath the downward stepped leader. A copper rod
electrode with length of 2.0m and diameter of 18.0mm is adopted in
the experiments. The rod electrode whose lower tip is fabricated
into a cone with an apex angle of 30° is set aslant with a degree
of 30° to ensure that every discharge incepts from the lower tip of
the rod electrode.
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Finally, the upper tip of the rod electrode is connected with
the surge generator and hoisted by the driving crane located on the
roof of the high-voltage test hall.
B. DC charge and isolation circuit As discussed in section (c),
in order to simulate the
nominal electric field distribution of the HVDC transmission
line, a dc voltage in the range of 15.0kV~16.0kV is applied to the
polar conductor of the scaled model line. In the experiments, the
maximum magnitude of applied surge voltage to the rod electrode is
up to -1500.0kV. Therefore, the isolation of the dc charge circuit
and the surge generator circuit should be considered when polar
conductor of the scaled model line is attached by the primary
discharge. The dc charge and isolation circuit proposed in the
paper is shown in Fig.5: The ac resistance of water-resistance R1
is 400.0k , the capacitance value and surge withstand voltage of C2
is 0.65 F and 75.0kV, respectively. As illustrated in Fig. 6, a
250/2500 s negative switch impulse voltage with a magnitude of
286.0kV is directly applied at point A, and the over-voltage
waveform of point B indicates that the maximum voltage magnitude
and the voltage rising rate at point B can be significantly
restricted by using the energy absorption branch R1-C2. In
addition, the over-voltage of the test transformer outlet can be
effectively restricted by using water-resistance R2. The ac
resistance of R2 is 160.0 k .
The charge volume accumulated on the surface of the polar
conductor of the scaled model line relies heavily on the grounding
status of the polar conductor, which has significant influence on
the upward streamer inception condition of the polar conductor
[17]. As shown in Fig.6, the capacitor C1provides a charge
accumulation path from the ground to the polar conductor of the
scaled model line. The capacitance value and surge withstand
voltage of C1 is 300.0pF and 400.0kV, respectively. In order to
avoid the external insulation flashover of capacitor C1, a 2.0cm
needle-needle protection air gap is employed to parallel connection
to the scaled insulator string. The surge breakdown voltage of this
protection air gap is approximately 48.0kV. The protection air gap
breaks down if the polar conductor is attached by the primary
discharge. The voltage rising on the capacitor C1 can be
effectively restricted. Moreover, the over-voltage of the test
transformer outlet can be limited in a security range combining
with the dc charge and isolation circuit.
1-rod electrode 2-grounded wire 3-polar conductor4-scaled model
of insulator string 5-protection air gap 6-scaled tower model7-post
insulator 8-silicon-diode 9-test transformer 10-voltage regulator
11-
isolation transformer Fig. 5 Illustration of the test
circuit
Fig. 6 Impulse response of DC charge test circuit (1-Applied
switch impulse voltage at point A; 2-Response waveform at point
B)
C. Observation equipment and Test procedure The discharge
process is recorded by the PCO.1200hs high-
speed digital camera. It is produced by the PCO company in
Germany. And the photographic speed in the experiments is 1000
frames/s.
The probability of polar conductor attached by the primary
discharge at a certain position Pc(x, y) is tested by the following
procedure. Firstly, the rod electrode tip is placed at (x, y). A
total number of 50 impulses are applied to the rod electrode for
each rod electrode tip location (x, y). Then the attachment times
to the grounded wire ng and polar conductor nc by the primary
discharge are recorded. Finally, the shielding failure probability
Pc(x, y) of polar conductor at position (x, y) can be yielded by
dividing nc by the total number of applied impulses. Following
that, if we change the position of the electrode tip (x, y), the
exposure space distribution of the polar conductor can be obtained
by repeating the procedure mentioned above which is shown in the
Fig. 7.
Fig. 7 Illustration of test arrangement for investigating the
distribution of the probability of polar conductor attached by the
primary discharge at a
certain position Pc(x, y)
IV. TEST RESULTS AND DISCUSSIONS This experiment is carried out
in the high-voltage test hall
of Hunan Electric Power Test and Research Institute (EPTRI) in
China from June to August in 2007. The total number of the recorded
discharge is over 7500 times.
A. The Pc(x, y) distribution of the scaled model line The Pc(x,
y) distribution of the positive polar conductor is
obtained respectively when the dc voltage applied to the
positive polar is 0.0kV and 16.0kV by repeating the test procedure
proposed in section . C. The equal probability
-
curves Pi of the positive polar attached by the primary
discharge can be calculated by employing the curve fitting tools
Datafit 7.02.
All the tested spatial locations of the rod electrode tip are
categorized into five groups according to the probability of the
positive polar conductor attached by primary discharge when the
applied dc voltage is equal to 0.0kV. The corresponding probability
intervals are: [0.0%, 20.0%], [20.0%, 40.0%], [40.0%, 60.0%],
[60.0%, 80.0%] and [80.0%, 100.0%]. All the spatial points
mentioned above are illustrated in Fig.8. The calculated equal
probability curves Pi show a good agreement with the test
results.
The equal probability curves of the positive polar attached by
the primary discharge are obtained by adopting the same procedure
mentioned above when the applied dc voltage is equal to 16.0kV. As
shown in Fig. 9 the exposure space of the polar conductor expands
significantly when the applied dc voltage to the positive polar is
equal to 16.0kV.
Pi=20.0% Pi=40.0% Pi=60.0% Pi=80.0% Pc>0.0% Pc>20.0%
Pc>40.0% Pc>60.0% Pc>80.0%
1- 2- 3- 4-
Fig. 8 the exposure space distribution of G4–40 (Udc=0.0 kV)
1-Udc=16.0 kV Pc=10.0% 2-Udc=16.0 kV Pc=50.0% 3-Udc=16.0 kV
Pc=90.0%
4-Udc=0.0 kV Pc=10.0% 5-Udc=0.0 kV Pc=50.0% 6-Udc=0.0 kV
Pc=90.0% Fig. 9 the comparison of exposure space distribution of
G4-40 taking into
account of the applied dc voltage
B. Effect of dc operation voltage magnitude and polarity on the
Pc(x, y) distribution The rod electrode tip is located at x=1400.0
mm. The Pc(x,
y) distribution is obtained by changing the y coordinate of the
rod tip when the dc voltage applied to the polar conductor is -
30.0, −16.0, 0.0, 16.0, 30.0 kV, respectively. The test results
are shown in Fig.10. It indicates that the probability of the polar
conductor attached by the primary discharge increases significantly
as the applied positive dc voltage increases. And the exposure
space reduces because of the effect of the negative dc voltage.
Fig. 10 Distribution of exposure space variation with the
polarity and magnitude of DC voltage at x=1 400.00 mm
C. Effect Mechanism of dc operation voltage The effect mechanism
of the dc voltage on the Pc(x, y)
distribution is preliminary analyzed according to the discharge
phenomenon recorded by the high-speed digital camera.
The typical discharge phenomena are illustrated in Fig. 11. Fig.
11 (a) and Fig. 11 (b) show the typical phenomena of the grounded
wire and polar conductor attached by the primary discharge
respectively when the applied dc voltage is equal to 0.0kV.
As shown in Fig. 11 (a) and Fig. 11 (b), although the upward
streamer incepts from the grounded wire, the polar conductor is
attached by the primary discharge when the applied dc voltage is
equal to 16.0kV. It can be concluded that the competition exists
between the upward streamers incepted from the polar conductor and
grounded wire. The incepted upward streamer from the polar
conductor takes the lead in encountering the downward streamer
incepting from rod electrode. Therefore, the primary discharge
attaches to the polar conductor even if the upward streamer incepts
from the grounded wire.
The applied positive dc voltage increases the surface electric
field of the polar conductor. It is easier to satisfy the upward
streamer inception criterion for the polar conductor than the
situation which the applied positive dc voltage is zero. Meanwhile,
this competition phenomenon is also shown in Fig. 11 (e) and Fig.
11 (f). Although the upward streamer incepts from polar conductor,
the grounded wire is attached by the primary discharge when the
applied dc voltage is equal to 16.0kV. It confirms the conclusions
discussed above.
-
(a) (b) (c)
(d) (e) (f)
Fig. 11 Photograph of discharge phenomenon recorded by high
speed camera. (a) and (b) is the typical phenomena of the grounded
wire and polar conductor attached by the primary discharge
respectively when the applied dc voltage is equal to 0.0kV; (c) and
(d) is the typical phenomena of the polar
conductor attached by the primary discharge even if the upward
streamer incepts from the grounded wire; (e) and (f) is the typical
phenomena of the grounded wire attached by the primary discharge
though the upward streamer incepts from the polar conductor;
V. CONCLUSIONSThis paper has described the investigation method
and
results of the lightning shielding performance of the HVDC
transmission line. The scale experiments were carried out to study
the exposure space distribution of the HVDC transmission line
taking into account the effect of the dc voltage. The main
contributions of present work are summarized as follows:
1) The credibility of the lightning simulation experiment
depends on how exactly it reproduces the electric field E(t)
variation in the vicinity of the grounded object in the final stage
of the lightning attaching to the grounded object. E(t) consists of
two components Es(t) and Ei(t) which is produced by the downward
leader channel and the HVDC transmission line, respectively. Es(t)
can be simulated by using a rod electrode energized with negative
switch impulse. It is recommended the rise time of the applied
impulse voltage is in the range of 200.0~1000.0 s taking into
account of the average velocity of the natural negative downward
step leader. Ei(t) can be simulate by applying a dc voltage in the
range of 15.0~16.0 kV to the 1:40 scaled model line. The encounter
process of the downward leader and upward leader can be simulated
under this configuration.
2) A test circuit is established taking the isolation of the dc
charge circuit and surge generator loop into account.
3) The competition between upward streamers originating from the
grounded wire and polar conductor is observed by using the
high-speed digital camera. The applied dc voltage makes it easier
to satisfy the upward streamer inception criterion for the polar
conductor. Although the upward streamer incepts from the grounded
wire, the polar conductor
can still be attached by the primary discharge taking into
account the effect of dc voltage.
ACKNOWLEDGMENTThe authors wish to thank all of the anonymous
reviewers,
whose comments assisted in improving the clarity of the
paper.
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Experimental Study on Lighting Shielding Performance of ±500 kV
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