Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009
61
Measurement of Corona Characteristics and Electromagnetic
Environment of ±800 kV HVDC Transmission Lines under High
Altitude Condition
Zheng Zhang, Rong Zeng, and Zhanqing Yu
State Key Lab of Control and Simulation of Power Systems and Generation Equipments
Department of Electrical Engineering, Tsinghua University, Haidian District, Beijing 100084, China
Abstract— Measurement of corona characteristics was carried out on newly constructed ±800 kV
HVDC test lines located in Kunming, China, with an altitude of 2100 m. Radio interference, audible noise, resultant electric field and ion current density at ground level were measured and
compared with the calculated results. All these corona characteristics were also measured at
other voltage levels for the purpose of comparison. The measured result show some difference
with those acquired above sea level and is significant for further studies of corona mechanism
under high altitude condition.
1. INTRODUCTION
Corona of power transmission lines refers to the discharge phenomenon accompanied with light
emission, produced by the ionization of air around the conductors, when the conductor surface
potential gradient exceeds a critical value, namely, the corona onset gradient [1]. The corona
characteristics of DC transmission lines are different from those of AC lines mainly because of
the magnitude of environmental space charge arisen from ionization in the space between the two
conductors of the lines and between each conductor and ground [2]. The corona effect would not
only cause corona loss (CL) but also influence the environment, which includes electric field effect,
radio interference (RI) and audible noise (AN), etc. Due to the existence of space charge caused
by corona, the electric field under HVDC transmission lines is actually the resultant electric field
formed by electric charges on the conductors and in the space together. Parameters used to describe
the electric field effect include resultant electric field (Es) and ion current density (Js) at ground
level, etc. Corona performance is an important consideration in the design and operation of HVDC
transmission lines.
From the 1970s, many countries such as USA and Canada have carried out experimental researches on the electromagnetic environment of HVDC lines [3, 4] and brought forward many empirical formulae for the calculation of CL, RI, AN, etc. However, most of these researches were
carried out under lower voltage levels and altitudes.
In recent years, the rapid economic development in China has called for great needs in electricity.
Under such circumstances, the ultra HVDC power transmission technique, which could realize power
delivery in larger capacity and longer distance, is developing rapidly in accordance with the nation’s
energy policy. The construction of a brand new power test base and ±800 kV HVDC test lines has
recently been completed in Kunming, a famous tourist city in southwest China with an altitude
of 2100 m. The corona performance of the lines with the voltage level above 800 kV and under an
altitude of more than 2000 m has never been researched and is a brand new and scholarly worthy
field to study.
2. MEASUREMENT FIELD, EQUIPMENT AND METHOD
The test line has 4 towers and a length of 800 m (Figure 1). The length of the segment between the
two towers in the middle is 410 m and the other two segments at each side are 195 m respectively.
The distance between the two poles is 22 m and the minimum height of the lines at mid-span is
18 m. The type of the mounted bundle is 6 × LGJ-630/45, with a diameter of 3.36 cm of a single
conductor and a distance of 45 cm between nearby sub-conductors in the bundle.
RI is measured using an ETS-Lindgren6507 loop antenna and a Schaffner SCR3502 radio receiver. The pedestal of the antenna is 1.5 m high. The section of the antenna is rotated parallel to
the lines in order to get the maximum readings, which are represented as quasi-peak values. AN
is measured using a TES1353 sound level meter held at 1.5 m high and the result is represented
as equivalent A-level. This frequency weighed network can best simulate the average response of
human ear to pure sound. Es is measured using a rotating voltage meter which has been calibrated
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PIERS Proceedings, Moscow, Russia, August 18–21, 2009
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measured
calculated
75
70
RI (dBuV/m)
65
60
55
50
45
40
35
-50 -40 -30 -20 -10
0
10 20
30 40
50
60 70
80
Distance from bipolar center (m)
Figure 1: Picture of the test lines and measurement
field.
Figure 2: Lateral profile of RI at ±800 kV.
before measurement, while Js is acquired with a 0.6 m × 0.6 m metal plate for receiving the ion
current flowing down to the ground. Js data is read in voltage unit from a voltage meter and then
converted into current density by dividing the resistance inside the meter and the area of the plate.
All the corona characteristics are measured point by point at predetermined location perpendicular to the lines. The data at each point should be read several times and the measurement result
is expressed as average values as well as maximum and minimum errors.
3. CORONA CHARACERISTICS AT NOMINAL VOLTAGE LEVEL
3.1. RI
Both measured and calculated results of the lateral profile of RI at ±800 kV are shown in Figure 2.
The calculation is based on the Special International Committee on Radio Interference (CISPR)
empirical formula for RI, which is internationally recognized:
¡ ¢
RI = 38 + 1.6(gmax − 24) + 46 log(r) + 5 log(N ) + ∆Ef + 33 log 20
D + ∆Ew
h
i
(1)
∆Ef = 5 1 − 2 (log(10f ))2
In the formula, gmax represents the maximum bundle gradient (kV/cm), r is the radius of the subconductor (cm), N is the number of sub-conductors in a bundle, D is the distance of the calculated
point from the positive pole (m), ∆Ew is a meteorological correction term with an addition of 3.3 dB
per 1000 m with a benchmark of 500 m, ∆Ef is a frequency correction term, and f represents the
frequency needed.
In Figure 2, the zero point of the abscissa represents the center of the two lines and the positive
direction represents the direction towards the positive pole. The frequency under test is 0.5 MHz
and the corresponding environmental RI is 53.8 dB. According to the figure, the measured RI attains
maximum beneath the positive pole and declines to either side. The maximum value is 68.8 dB.
The RI at 20 m from the positive pole is 63.1 dB. The calculated maximum and RI at 20 m from
the positive pole are 59.3 dB and 52.9 dB respectively. The calculated RI profile is in accordance
with the measured profile under and beyond the positive pole, and is 9 to 10 dB lower. However,
calculated RI attenuates faster along the negative side than the measured. This is probably due
to the difference of corona mechanism between higher altitude and above sea level. The inner
causation needs further investigation.
3.2. Es
The measured result of Es at ±800 kV is shown in Figure 3. The calculated resultant electric field as
well as the nominal electric field without space charge using the Electric Power Research Institute
(EPRI) semi-empirical formula [6] is also illustrated in the figure. It shows that measurements
carried out at different time vary in a range, which is mainly due to the variation of weather
parameters such as temperature, humidity, wind velocity and air pressure. The average value
at the maximum Es position near the positive pole is about 35 kV/m, while the minimum value
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009
63
near the negative pole is about −40 kV/m. The calculated result is generally accordant with the
measured one and the resultant field is 2 to 3 times of the nominal field, which manifests that the
presence of the space charge increases the electric field at ground level.
3.3. Js
The measured result as well as the calculated result using EPRI semi-empirical formula [6] for Js
at ±800 kV is illustrated in Figure 4. Measured results at different time fluctuate intensely for
the reason that the space charge is very keen to weather conditions especially the wind, and the
wind velocity could reach a maximum of 10 m/s at the test base. Thus, the moments when the
instant wind velocity is less than 2 m/s are chosen to record the Js data. The calculated result
near the positive pole is well consistent with the measured, but the absolute value of calculated
result near the negative pole is significantly higher than the measured counterpart. It is generally
recognized that the mobility of negative ions is larger than that of the positive and thus leads
to higher Js in absolute value under the negative pole. The measured result doesn’t reflect the
difference possibly because the wind may influence the Js profile and high altitude condition may
affect the ion mobility. The internal mechanism has to been further investigated.
120
60
40
08.12.28, measured
08.12.29, measured
calculated
nominal electric
field, calculated
80
60
2
Js (nA/m )
20
Es (kV/m)
08.12.28, measured
08.12.29, measured
calculated
100
0
-20
40
20
0
-20
-40
-60
-80
-40
-100
-60
-60
-40
-20
0
20
40
60
-120
-60
80
-40
Distance from bipolar center (m)
-20
0
20
40
60
80
Distance from bipolar center (m)
Figure 3: Lateral profile of Es at ±800 kV.
Figure 4: Lateral profile of Js at ±800 kV.
60
bipolar, measured
bipolar, calculated
positive, measured
negative, measured
AN (dBA)
55
50
45
40
-45
-30
-15
0
15
30
45
60
75
Distance from bipoalr center (m)
Figure 5: Lateral profile of AN at 950 kV.
4. CORONA CHARACERISTICS AT DIFFERENT VOLTAGE LEVELS
Measurement of corona characteristics is also carried out under other three voltage levels: ±500 kV,
±600 kV and ±950 kV. The result of AN, Es and Js are illustrated below.
PIERS Proceedings, Moscow, Russia, August 18–21, 2009
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Since the construction of the test base has not been completed, the vehicles onsite and airplanes
frequently passing by has introduced irregular ambient noise comparable with AN produced by the
lines at intervals, which makes it difficult to obtain proper AN data at any time. Bipolar, positive
and negative AN profile at the voltage level of 950 kV are available and illustrated in Figure 5. At
that time the ambient noise with the poles unenergized is 42 dB and has kept quite steadily in the
whole measuring process. Corona produced AN can be easily distinguished. The calculated bipolar
AN using the Bonneville Power Agency (BPA) empirical formula [5] as below is also illustrated in
the figure.
AN = −133.4 + 86 · lg gmax + 40 · lg deq − 11.4 · lg D
deq = 6.6 · n0.64 · d
(2)
In the formula, d is the diameter of the sub-conductor (cm), n is the number of sub-conductors in
a bundle, and gmax and D have the same meaning as in Equation (1). Bipolar AN is obviously
higher than monopolar AN, so as the positive AN than the negative AN. The result supports that
the positive corona is the main source of AN in HVDC lines. The maximum bipolar AN is about
55 dB under the positive pole. The calculated AN profile is in accordance with the measured, but
is holistically 4 dB lower than the measured result, which is possibly due to the increase of altitude.
The altitude of the test base is 2100 m. The benchmark of the formula is 500 m. It can be deduced
from the comparison that the altitude correction factor for AN is 2.5 dB per 1000 m.
Figures 6 and 7 show the measured Es and Js profiles at different voltage levels respectively.
The profile of the other three voltages is consistent with the ±800 kV result. The lines almost do
not generate corona at 500 kV, so there is hardly any space charge and ion current under the lines,
and the Es contains the nominal component only. As the voltage increases, the corona becomes
more intense and the value of both Es and Js increases.
60
40
30
600 kV fog, cold
800 kV sunny, cool
950 kV cloudy, cold
80
60
40
20
Js (nA/m 2 )
Es (kV/m)
100
500 kV snow, cold
600 kV fog, cold
800 kV sunny, cool
950 kV cloudy, cold
50
10
0
-10
20
0
-20
-20
-40
-30
-60
-40
-80
-50
-100
-45
-30
-15
0
15
30
45
60
75
90
Distance from bipoalr center (m)
Figure 6: Lateral profile of Es at different voltage
levels.
-45
-30
-15
0
15
30
45
60
75
Distance from bipoalr center (m)
Figure 7: Lateral profile of Js at different voltage
levels.
5. CONCLUSION
A thorough measurement study of the corona characteristics of HVDC transmission lines with the
nominal voltage of ±800 kV was carried out. The result shows that positive corona is the main
source of RI and AN. Tests at the nominal voltage show that measured RI is 9 to 10 dB higher
than the calculated result under and beyond the positive pole along the profile vertical to the lines,
and attenuates more slowly than the calculated profile along the negative side. The measured
Es is well in accordance with the calculated result, whereas the measured Js is lower than the
calculated in absolute value under the negative pole. AN measured at ±950 kV is 4 dB higher than
the calculated result along the profile and an altitude correction factor of 2.5 dB per 1000 m was
deduced. Measurements at different voltage levels reveal that all the corona characteristics increase
with the voltage and the profiles are similar.
Differences between measured results and calculated results using empirical formulae can be
concluded into two major reasons: the first is that all the corona characteristics are sensitive to
Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009
65
weather conditions, therefore long-term measurement is needed to acquire profiles expressed in
cumulative distributions; and the second is probably due to the difference in corona mechanism
under high altitude and thin air condition, which requires further investigations.
REFERENCES
1. Maruvada, P. S., Corona Performance of High-voltage Transmission Lines, Research Studies
Press Ltd, Baldock, Herfordshire, England, April 2000.
2. HVDC Transmission Line Reference Book, Published by EPRI, Palo Alto, California, USA,
1993.
3. Transmission Line Reference Book HVDC to ±600 kV, Published by EPRI, Palo Alto, California, USA, 1977.
4. Maruvada, P. S., et al., EPRI Report EL-1170, Bipolar HVDC Transmission System Study
Between ±600 kV and ±1200 kV: Corona Studies, Phase I, Published by EPRI, Palo Alto,
California, USA, 1979.
5. Zhao, W., HVDC Engineering Technology, China Electric Power Press, Beijing, China, May
2005.
6. EPRI Report EL-2257, Conductor Development, Published by EPRI, Palo Alto, California,
USA, February 1982.