Progress In Electromagnetics Research Symposium Proceedings, Moscow, Russia, August 18–21, 2009 61

Measurement of Corona Characteristics and ElectromagneticEnvironment of ±800 kV HVDC Transmission Lines under High

Altitude Condition

Zheng Zhang, Rong Zeng, and Zhanqing YuState 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 kVHVDC test lines located in Kunming, China, with an altitude of 2100 m. Radio interference, au-dible noise, resultant electric field and ion current density at ground level were measured andcompared with the calculated results. All these corona characteristics were also measured atother voltage levels for the purpose of comparison. The measured result show some differencewith those acquired above sea level and is significant for further studies of corona mechanismunder high altitude condition.

1. INTRODUCTION

Corona of power transmission lines refers to the discharge phenomenon accompanied with lightemission, produced by the ionization of air around the conductors, when the conductor surfacepotential gradient exceeds a critical value, namely, the corona onset gradient [1]. The coronacharacteristics of DC transmission lines are different from those of AC lines mainly because ofthe magnitude of environmental space charge arisen from ionization in the space between the twoconductors of the lines and between each conductor and ground [2]. The corona effect would notonly 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 causedby corona, the electric field under HVDC transmission lines is actually the resultant electric fieldformed by electric charges on the conductors and in the space together. Parameters used to describethe electric field effect include resultant electric field (Es) and ion current density (Js) at groundlevel, etc. Corona performance is an important consideration in the design and operation of HVDCtransmission lines.

From the 1970s, many countries such as USA and Canada have carried out experimental re-searches on the electromagnetic environment of HVDC lines [3, 4] and brought forward many em-pirical formulae for the calculation of CL, RI, AN, etc. However, most of these researches werecarried 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 powerdelivery in larger capacity and longer distance, is developing rapidly in accordance with the nation’senergy policy. The construction of a brand new power test base and ±800 kV HVDC test lines hasrecently been completed in Kunming, a famous tourist city in southwest China with an altitudeof 2100 m. The corona performance of the lines with the voltage level above 800 kV and under analtitude of more than 2000m has never been researched and is a brand new and scholarly worthyfield 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 thetwo 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 is18m. The type of the mounted bundle is 6 × LGJ-630/45, with a diameter of 3.36 cm of a singleconductor 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 re-ceiver. The pedestal of the antenna is 1.5m high. The section of the antenna is rotated parallel tothe lines in order to get the maximum readings, which are represented as quasi-peak values. ANis measured using a TES1353 sound level meter held at 1.5 m high and the result is representedas equivalent A-level. This frequency weighed network can best simulate the average response ofhuman ear to pure sound. Es is measured using a rotating voltage meter which has been calibrated

62 PIERS Proceedings, Moscow, Russia, August 18–21, 2009

Figure 1: Picture of the test lines and measurementfield.

measured

calculated

80

75

70

65

60

55

50

45

40

35

RI (d

BuV

/m)

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80

Distance from bipolar center (m)

Figure 2: Lateral profile of RI at ±800 kV.

before measurement, while Js is acquired with a 0.6m × 0.6m metal plate for receiving the ioncurrent flowing down to the ground. Js data is read in voltage unit from a voltage meter and thenconverted 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 perpendic-ular to the lines. The data at each point should be read several times and the measurement resultis expressed as average values as well as maximum and minimum errors.

3. CORONA CHARACERISTICS AT NOMINAL VOLTAGE LEVEL

3.1. RIBoth 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(

20D

)+ ∆Ew

∆Ef = 5[1− 2 (log(10f))2

] (1)

In the formula, gmax represents the maximum bundle gradient (kV/cm), r is the radius of the sub-conductor (cm), N is the number of sub-conductors in a bundle, D is the distance of the calculatedpoint from the positive pole (m), ∆Ew is a meteorological correction term with an addition of 3.3 dBper 1000 m with a benchmark of 500 m, ∆Ef is a frequency correction term, and f represents thefrequency needed.

In Figure 2, the zero point of the abscissa represents the center of the two lines and the positivedirection represents the direction towards the positive pole. The frequency under test is 0.5 MHzand the corresponding environmental RI is 53.8 dB. According to the figure, the measured RI attainsmaximum 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 20m fromthe positive pole are 59.3 dB and 52.9 dB respectively. The calculated RI profile is in accordancewith 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 dueto the difference of corona mechanism between higher altitude and above sea level. The innercausation needs further investigation.

3.2. EsThe measured result of Es at ±800 kV is shown in Figure 3. The calculated resultant electric field aswell 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 measurementscarried out at different time vary in a range, which is mainly due to the variation of weatherparameters such as temperature, humidity, wind velocity and air pressure. The average valueat 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 themeasured one and the resultant field is 2 to 3 times of the nominal field, which manifests that thepresence 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 Jsat ±800 kV is illustrated in Figure 4. Measured results at different time fluctuate intensely forthe reason that the space charge is very keen to weather conditions especially the wind, and thewind velocity could reach a maximum of 10m/s at the test base. Thus, the moments when theinstant wind velocity is less than 2m/s are chosen to record the Js data. The calculated resultnear the positive pole is well consistent with the measured, but the absolute value of calculatedresult near the negative pole is significantly higher than the measured counterpart. It is generallyrecognized that the mobility of negative ions is larger than that of the positive and thus leadsto higher Js in absolute value under the negative pole. The measured result doesn’t reflect thedifference possibly because the wind may influence the Js profile and high altitude condition mayaffect the ion mobility. The internal mechanism has to been further investigated.

60

40

20

0

-20

-40

-60

Es (

kV

/m)

-60 -40 -20 0 20 40 60 80

Distance from bipolar center (m)

08.12.28, measured

08.12.29, measuredcalculatednominal electric

field, calculated

Figure 3: Lateral profile of Es at ±800 kV.

08.12.28, measured

08.12.29, measuredcalculated

120

100

80

60

40

20

0

-20

-40

-60

-80

-100

-120-60 -40 -20 0 20 40 60 80

Distance from bipolar center (m)

Js (

nA

/m )

2

Figure 4: Lateral profile of Js at ±800 kV.

bipolar, measured

bipolar, calculated

positive, measurednegative, measured

60

55

50

45

40

AN

(dB

A)

-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.

64 PIERS Proceedings, Moscow, Russia, August 18–21, 2009

Since the construction of the test base has not been completed, the vehicles onsite and airplanesfrequently passing by has introduced irregular ambient noise comparable with AN produced by thelines at intervals, which makes it difficult to obtain proper AN data at any time. Bipolar, positiveand negative AN profile at the voltage level of 950 kV are available and illustrated in Figure 5. Atthat time the ambient noise with the poles unenergized is 42 dB and has kept quite steadily in thewhole measuring process. Corona produced AN can be easily distinguished. The calculated bipolarAN using the Bonneville Power Agency (BPA) empirical formula [5] as below is also illustrated inthe 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 ina bundle, and gmax and D have the same meaning as in Equation (1). Bipolar AN is obviouslyhigher than monopolar AN, so as the positive AN than the negative AN. The result supports thatthe positive corona is the main source of AN in HVDC lines. The maximum bipolar AN is about55 dB under the positive pole. The calculated AN profile is in accordance with the measured, butis holistically 4 dB lower than the measured result, which is possibly due to the increase of altitude.The altitude of the test base is 2100m. The benchmark of the formula is 500 m. It can be deducedfrom 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 donot 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 becomesmore intense and the value of both Es and Js increases.

500 kV snow, cold

600 kV fog, cold

800 kV sunny, cool950 kV cloudy, cold

60

50

40

30

20

10

0

-10

-20

-30

-40

-50

-45 -30 -15 0 15 30 45 60 75

Distance from bipoalr center (m)

90

Es (

kV

/m)

Figure 6: Lateral profile of Es at different voltagelevels.

600 kV fog, cold800 kV sunny, cool950 kV cloudy, cold

100

80

60

40

20

0

-20

-40

-60

-80

-100

-45 -30 -15 0 15 30 45 60 75

Distance from bipoalr center (m)

Js (

nA

/m

)2

Figure 7: Lateral profile of Js at different voltagelevels.

5. CONCLUSION

A thorough measurement study of the corona characteristics of HVDC transmission lines with thenominal voltage of ±800 kV was carried out. The result shows that positive corona is the mainsource of RI and AN. Tests at the nominal voltage show that measured RI is 9 to 10 dB higherthan 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 measuredEs is well in accordance with the calculated result, whereas the measured Js is lower than thecalculated in absolute value under the negative pole. AN measured at ±950 kV is 4 dB higher thanthe calculated result along the profile and an altitude correction factor of 2.5 dB per 1000 m wasdeduced. Measurements at different voltage levels reveal that all the corona characteristics increasewith the voltage and the profiles are similar.

Differences between measured results and calculated results using empirical formulae can beconcluded 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 incumulative distributions; and the second is probably due to the difference in corona mechanismunder high altitude and thin air condition, which requires further investigations.

REFERENCES

1. Maruvada, P. S., Corona Performance of High-voltage Transmission Lines, Research StudiesPress 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, Cali-fornia, USA, 1977.

4. Maruvada, P. S., et al., EPRI Report EL-1170, Bipolar HVDC Transmission System StudyBetween ±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, May2005.

6. EPRI Report EL-2257, Conductor Development, Published by EPRI, Palo Alto, California,USA, February 1982.