The Effect of Surface Roughness on Corona-generated ... - IEEE Xplore

IEEE Transactions on Dielectrics and Electrical Insulation

Vol. 22, No. 2; April 2015

879

The Effect of Surface Roughness on Corona-generated Electromagnetic Interference for Long-term Operating Conductors Xingming Bian State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, North China Electric Power University, Beijing 102206, China Beijing Key Laboratory of High Voltage and EMC, North China Electric Power University, Beijing, 102206, China

Yuanjiu Wang, Liming Wang, Zhicheng Guan Graduate School at Shenzhen, Tsinghua University, 518055 Shenzhen, Guangdong Province, P.R.China

Shuwei Wan State Grid Chongqing Nan’an Power Supply Company, 400060 Chongqing, P.R.China

Lan Chen, Fangdong Chen and Xuesong Zhao Jibei Electric Power Company Limited, 100053 Beiijing, P.R.China ABSTRACT This paper presents a research of the surface roughness effects on corona-generated electromagnetic interference of long-term operating conductors. The detailed surface morphologies and roughness of typical long-term operating conductors were measured and analyzed. The radio noise measurement system based on the corona cage was discussed and the actual radio noise generated by the conductor in the measured cage was obtained, based on the parameters of the corona cage and the measured data. The radio noise tests were carried out under both dry and wet conditions. Under two types of conditions, the corona intensity was higher on the long-term operating conductors contrasted to the new conductors as a result of the anomalistic surfaces of the long-term operating conductors. The rate of decrease of corona inception voltages between the long-term operating conductors and new conductors was discovered to increase with the raise of surface roughness degree Ra in linear relationship. Approximate linear relation was also discovered between the averaged radio noise deviation and Ra. The rate of decrease of corona inception voltages under wet conditions was slightly higher (about 1%) than that under dry conditions. The difference between the radio noise of long-term operating conductors and that of new conductors was larger (1.3 dB) under wet conditions compared to dry conditions. Index Terms - AC corona discharge, electromagnetic interference, long-term operating conductor, surface condition, roughness, corona cage.

1 INTRODUCTION THE extent of the high voltage power grids has been progressively expanding with the rapid increase in energy demand in the recent several years. Corona discharge and its associated electric field, audible noise, electromagnetic interference may bring about severe electromagnetic problems near high voltage power transmission lines and substations. Therefore it has been a continuing and essential Manuscript received on 23 July 2013, in final form 11 September 2014, accepted 11 September 2014.

subject of research work by both electrical engineers and physicists [1-6]. Transient current may be induced as a result of the movement of electrons during the procedure of corona discharge in the conductor. The propagating corona current pulses will lead to electromagnetic interference (EMI) to electronic communication systems in the neighborhood of the high voltage power transmission lines [7]. Restricting the corona-generated EMI, also known as radio noise, to a receivable degree has become a significant cerebration in the arrangement and operation of both HVAC and HVAC power transmission lines [8].

DOI 10.1109/TDEI.2014.004212

880

X. Bian et al.: The Effect of Surface Roughness on Corona-generated Electromagnetic Interference

Up to now, previous research was chiefly concentrated on the theoretical predetermination model [9-14], the test and statistical analysis of the radio noise [15-20] generated from HVAC and HVDC power transmission lines. However, very little attention has been paid into the long-term operating effects of the conductor surface status on corona-generated radio noise as the research period could hardly be long enough (usually more than 10 years) in the test laboratory [21]. The radio noise basically depends on the corona discharge on the surface of the conductor; consequently, the clarification of the relationship between the conductor surface and the corona-generated radio noise is of evident significance. The surface roughness factor m was employed to evaluate the degree of roughness of the electrodes by Peek in 1929 [22], and it has been widely used in the research of corona discharge of the electrodes over the past several decades. It is reported in [23] that conductors with different surface conditions have different values of m. In most cases, m is in the range of 0.82 to 0.92 for new conductors while 0.4 to 0.8 for aged conductors in high voltage transmission lines. However, that conclusion is so subjective that different investigators or institutions may present different roughness factor values for the same conductors. It is discovered in the previous work that conductor aging could be advantageous to its surface as the amount and intensity of corona discharge on the conductor was reduced at working voltage with time [21]. It is found by Laforest and Whepley that the conductor surface condition greatly improved as time went on therefore the electromagnetic interference level turned out to be better. However, the laboratorial aging time of the small aluminum conductors was no more than 4 months, consequently; the conclusion could not be well universalized in evaluating the variation in the radio noise of long-term operating conductors in ac power transmission lines [24]. Booker carried out the research work on atmospheric aging effects on the corona performances of non-energized aluminum conductors in northern Indiana, USA. The aging periods of the conductors varied from 4 to 19 months in his tests. After the atmospheric aging period for 4 months, it was observed that the corona onset voltages of the conductors raised to a certain extent, at the same time, the audible noise level in 8 kHz expressed a decrease tendency. Unfortunately Booker did not keen on the investigation after the initial aging period [25]. According to the research of Juette, wet snowflakes or raindrops may accumulate on the surfaces of the conductors in bad weather, forming corona discharging sources, consequently, the radio noise and audible noise from an ac transmission line may become higher [26]. However, the effects of other contaminations adhering to the conductors on the radio noise were not studied. Currently there are a large number of transmission lines that have been in service for more than ten years, the surface of them will have exasperated. Atmospheric industrial pollution, acid rain, oxidation of the aluminum will have polluted the surface and coarsened such conductors [27-28].

The surface conditions, corona discharge processes and audible noise of some long-term operating conductors were reported in our previous work. The corona discharge properties of such conductors were extremely different from those of the new conductors as the surfaces of long-term operating conductors had been contaminated and roughened [29-31]. What’s more, the surface roughness degree was discovered to be a determinant element of the corona discharge intensity of the conductors when the external conditions were kept constant [31-32]. The measurement of the corona-generated electromagnetic interference, the interrelationship between the surface roughness and radio noise of the conductors is reported in greater detail in this paper.

2 SURFACE ROUGHNESSES AND TEXTURE OF THE CONDUCTORS The corona discharge of the conductors is closely related to their surface textures. In order to get the realistic materials, the long-term operating conductors used in this work were acquired from the practical high voltage ac transmission lines in North, Central and South China; two new conductors were also obtained for comparison. The information (location, age, annual average temperature and relative humidity of the city where the long-term operating conductors had been in service, surface roughness degree) of the tested conductors in this work is presented in Table 1. Table 1. The information of the conductors in this work. Annual Number

Location of the conductors

Age (years)

Average

Surface

temperature

roughness

and relative

Ra (μm)

humidity 1

2

Beijing (North China, above the farmland ) Anding (North China, near a cement factory)

10

11.6℃, 60%

3.53

12

11.9℃, 64%

5.08

Wuhan (Central China, 3

near a thermal power station and the Yangtze

30

16.4℃, 78%

7.46

River) 4

5

Huangshi(Central China, near a steel factory) Tianshengqiao(South China, near a coalmine)

20

15

Shenzhen (South China, 6

near a thermal power

10

station and the sea) 7

Guangzhou(SouthChina, near the sea)

20

16.6℃, 79% 16.8℃, 79% 21℃, 80% 22℃, 80%

8.93

13.88

18.17

20.62

8

Not put into service

0

0.73

9

Not put into service

0

0.81


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The accurate measurement of the conductors’ surfaces seems to be a fundamental aspect of this research. In this research, the non-contact three dimension (3D) phase shift MicroXAM surface profiler with a vertical resolution of 0.01nm shown in Figure 1 was used to measure the surface morphologies of the conductors. Figure 2 and Figure 3 are the measured surface morphologies of the long-term operating conductors (conductors 1-7 in Table 1) and two new conductors (conductors 8-9 in Table 1). It is clearly noticed from the nine pictures that the surfaces of the longterm operating strands are much rougher and more anomalistic than the new strands.

(a)

(b)

(c)

Figure 1. The non-contact 3D phase shift MicroXam surface profiler.

Figure 4 presents the schematic diagram of the measured values by the 3D phase shift MicroXam. The individual height value of each measurement point could be defined as zi,j, (i,j) is a measurement point in the surface of XY plane; M,N is the number of measurement points in X ,Y axis respectively. The average surface roughness (Ra) could be calculated as follows: 1 1 M N Ra   zi , j  zav M N j 1 i 1

(d)

(e)

(1)

where zav is the average value of all the height data points from several measurements. The average surface roughness Ra of the conductors in Figure 2 and 3 could be calculated according to Equation (1), the results are also presented in Table 1. It should be noticed that the average surface roughness of long-term operating conductors is much higher than that of new conductor, and the average surface roughness displays an increasing tendency with the decrease of latitude in China.

(f)

3 CORONA DISCHARGE AND RADIO NOISE TESTS ARRANGEMENT The corona discharge together with the radio noise of the conductors in Figure 2 and Figure 3 was studied in a corona cage. The corona cage could generally be regarded as

(g) Figure 2. The detailed surface morphologies of the strands of long-term operating conductors.

882


(a)

The transverse section of the corona cage kept square in all the tests, its size may be varied between 1.2 m and 2 m continuously. In this research, it was maintained at 1.7 m × 1.7 m. The outer shield cage was well linked to the earth so that the electromagnetic interference outside the corona cage could be effectively excluded. The experimental conductors or conductor bundles were installed centrally in the corona cage, and they were linked to an ac power source whose peak voltage was 300 kV.

(b) Figure 3. The detailed surface morphologies of the strands of new conductors.

Figure 5. The framework of the corona cage used in the experiment.

Figure 4. The schematic diagram of the measured values by the three dimension phase shift MicroXam surface profiler.

monophase test equipment, where the single conductor or bundled conductors are installed in the center of an earthed, mesh cage. The high surface electric field of the conductors can be well simulated at comparatively lower applied voltages as the test bundles are close to the ground cage. Compared to the experiments in the full-size test lines, the experiments of the various bundle geometries in the corona cage will be much more time-saving as well as more economical in financial costs [21]. For the reasons outlined above, corona cages have been generally applied in measuring and forecasting corona performances (radio noise, audible noise, corona power loss and so on) of high voltage power transmission lines [26, 33]. The corona performances of high voltage transmission lines are affected by many environmental factors and conductor surface features. Therefore, the systematic classifying and departing of these factors is quite important for aging effect investigations. Laboratory corona cages provide superiority in this respect as the external environmental conditions or particular surface features can be altered while other factors remain unchanged [29-33]. Figure 5 shows the framework of the corona cage used in the experiments and Figure 6 gives a full view. The whole length of the corona cage is 4 m, which comprises three sections, the 0.5m-long sections in both ends are earthed in order to overcome the border effects, the 3m-long central section is designed for corona measurement.

Figure 6. Full view of the corona cage utilized in the experiments.

The processes of corona discharge were recorded as videos by an ultra violet (UV) imager (as in Figure 7a), which developed our expertise in exploring such phenomena. The ‘photon count’ could be defined as the amount of photons every second in average during the passing minute. The ‘photon count’ could well represent the intension of the corona discharge (as in Figure 7b). The upper limit of the UV imager was not experienced as the peak photon count number exceeded 10 times the highest number inspected [2, 31].

(a) UV imager

(b) the photon counting model

Figure 7. The UV imaging detector and its photon counting model.


Vol. 22, No. 2; April 2015

Many elements, such as air pressure, temperature, humidity, test distance and the gain will have significant effects on the photon counting rate. The air pressure in the experiments was steady between 101.0 and 101.4 kPa, the temperature was kept within the range 17 to 18 oC and the relative humidity was maintained between 65 to 70%. The effect of wind could be neglected as the experiments were carried out indoors. The gain and distance were kept constant during all the experiments to avoid the deviations. The ac high voltages power supply were increased step by step in 5 kV as far as the corona discharge became violent, and after that the applied voltage was lower down by degree till the corona discharge vanished completely. The radio noise, caused by the interference current pulses, covers the frequency in the range of 100 kHz to 1 GHz; hence it is important to measure the interference current accurately in this research. The radio noise was measured by the interference current coupling circuit shown in Figure 8. The parameters of the current circuit in Figure 8b are determined as follows: R0 (the internal impedance of the EMI receiver) is 50 Ω, Z0 (the impedance of the cable, matches with R0) is 50 Ω， C (the capacitor filters the low-frequency signals) is 1 μF, R1 (the non inductive sampling resistance) is 50 Ω.

883

interference current induced from the conductor per meter, ε0 is the permittivity in the air. The interference current I0 measured by the EMI receiver is not equal to the interference current induced from the conductor in the measured cage It, however, their relationship could be obtained based on the circuit in Figure 9 under different frequencies. I It  0 (3) K

K

R1C  C  C30    R1  R0   jCC30 R1R0

(4)

where K is the proportion of I0 in It, ω is the angular frequency of the excitation, C30 could be measured by the impedance analyzer before the tests. Therefore, i can be calculated by I I (5) i= t  0 l lK where l is the length of the measured corona cage (m) [21]. Hence, the excitation function  can be written as follows: 2 0 I 0 (6) = KC0 l UEMI , the measured value of the EMI receiver, is the voltage across the current I0 in the equivalent impedance of the system( Zeq ) can be expressed as: U EMI =20lg(I 0  Zeq ) (7) According to equations (2) to (7) above, the radio noise generated by the conductor in the measured cage could be expressed as follows:  =U EMI  20lg(Z eq  l  K )+20lg( 2 0 C0 ) (8)

(a) the interference current coupling circuit in the corona cage

The parameters of the test conductors in this investigation are displayed in Table 2. Table 2. Zeq , K and C0 of the test conductors.

(b) the parameters of the interference current coupling circuit Figure 8. Schematic diagram of radio noise measurement system.

The equivalent circuit of the measurement system is described in Figure 9. C12, C13 is the equivalent capacitance between the conductor and the measured cage, guard cage respectively; C30 is the equivalent capacitance between the measured cage and the outer shield cage. The interference current induced from the conductor in the measured cage is It , I0 is the interference current measured by the EMI receiver. The quasi-peak data was utilized during the experiments, and the measuring frequency was selected to be 500 kHz uniformly. The electromagnetic interference level of the conductor can be evaluated by the excitation function brought forward by Gary [11]. 2 0i = (2) C0 where  is the excitation function, C0 is the capacitance between the corona cage and the conductor (F/m), i is the

Conductor

Diameter of the

type

conductor(mm)

Zeq()

K

C0 (pF/m)

LGJ240/40

21.66

24.99

0.50

12.53

LGJ300/40

24.00

24.98

0.50

12.82

LGJ400/65

28.00

24.96

0.50

13.30

LGJ500/35

30.00

24.95

0.50

13.53

Figure 9. The equivalent circuit of the radio noise measurement system .

884


4 RESULTS OF THE RADIO NOISE UNDER DRY CONDITIONS Figure 10 and Figure 11 present the intensity of stable corona discharge points on the surface of two typical longterm operating conductors (conductors 1 and 7 in Table 1) and new conductors (conductors 8 and 9 in Table 1) at the corona inception voltages. The graphs images were observed by the UV imaging detector under dry conditions in the corona cage.

The relationship betweenγand Ra are displayed in Figure 12, and can be described by the linear equation (10) below (with a maximum discrepancy of 1%).  =  0.856Ra  5.14  100% (10) The actual radio noise generated by the conductor may be obtained by equation (8), and the radio noise of the typical long-term operating conductors and new conductors under dry conditions is compared in Figure 13. It is obvious that under the same applied voltage, the radio noise of long-term operating conductors is higher than that of the new conductors as the intensity of corona discharge turns to be stronger. Long-term operating conductor Ra=3.53μm New conductor Ra=0.73μm 32 LGJ240/40 diameter 21.66mm

(a) conductor 8 (b) conductor 1 U0=97.6 kV, Ra=3.53 μm U0=104.8 kV, Ra=0.73 μm Figure 10. Contrast of the state of corona discharge points close to the surface of the conductors (both have the same diameter 21.66 mm) at corona inception voltage.

Radio Noise /dB

36

28 24 20 95

100

105

110 115 120 125 Applied Voltage /kV

130

135

(a) LGJ240/40

It is apparent that the corona intensity is higher on the long-term operating conductors contrasted to the new ones due to the rough and irregular surfaces of the long-term operating conductors. The relative deviation between the corona onset voltages of the new and long-term operating conductors, γ, is defined in equation (9). γ can well reflect the declining degree of the corona inception voltages for long-term operating conductors.

  (U 0new  U 0long ) U 0new  100%

(9)

24

35 30 25 20 15 10

100 105 110 115 120 125 130 135 140 145 Applied Voltage /kV

(b) LGJ400/65 Figure 13. Contrast of the radio noise between long-term operating conductor and new conductor under dry conditions.

The deviation of radio noise between the long-term operating conductor and new conductor under same applied voltage is defined as ᇞRN. The relationship between ᇞRN and Ra is shown in Figure 14. ᇞRN, as given in equation (11), turns to be higher in linear with the raise of Ra.

20 γ/%

Long-term operating conductor Ra=20.62μm 50 New conductor Ra=0.81μm 45 LGJ400/65 40 diameter 28.00mm

55

Radio Noise /dB

(a) conductor 9 (b) conductor 7 U0=118.6 kV, Ra=0.81 μm U0=92.1 kV, Ra=20.62 μm Figure 11. Contrast of the state of corona discharge points close to the surface of the conductors (both have the same diameter 28.00 mm) at corona inception voltage.

16

RN=0.747Ra +2.1

12 8 4

8

12

Ra/μm

16

20

Figure 12. The relationship between γ and Ra under dry conditions.

(11)

Therefore, when the designers or managers of power grid would like to predict the actual radio noise of long-term operating conductors in good weather based on the analytical methods in [23], the increment (ᇞRN) should be added to the calculated value as the actual radio noise will rise.


Vol. 22, No. 2; April 2015

20

12 8 4 4

8

12 Ra/μm

16

20

Figure 14. The relationship between ᇞRN and Ra under dry conditions.

5 RESULTS OF THE RADIO NOISE UNDER WET CONDITIONS The wet conditions of the long-term operating conductors could be formed by spraying some water on the surface of each conductor equally, and the amount of water was selected to be 500 mL in each test. This mean could simulate the longterm operating conductors in light rain or foggy weather well. 45

wet condition dry condition

Radio Noise /dB

40 LGJ240/40 diameter 21.66mm

35 30 25 20

95 100 105 110 115 120 125 130 135 Applied Voltage /kV (a) LGJ240/40, Ra=3.53 μm

wet condition dry condition LGJ400/65 50 diameter 28.00mm

Radio Noise /dB

60

All of the tests could be completed within 30 minutes, additionally; the corona current was not strong enough to generate enough energy to heat the conductor.For the reasons mentioned above, the conductor could remain moist during the test. According to the monitoring data from a non-contact infrared temperature measurement instrument, the deviation of the temperature of the long-term operating conductor was in the range of 0.5 to 2 oC during the whole process of the corona tests. The water was absorbed by the polluting impurities in the long-term operating conductor; this would contribute to the emission of the electrons, therefore the corona discharge points close to the conductors might appear at lower applied voltage. In the meantime, for the same type of conductors, the intensity of corona discharge and corona generated audible noise under wet conditions became more serious than that under dry conditions, and the increment of the audible noise was in the range of 3 to 5 dB as we found in our previous work [30]. Under the same applied voltage, the photon count of the corona discharge point of conductor 1 in Figure 10b increased from 4680 to 5590 as its surface became wet; similar case could also be detected for conductor 7 in Figure 11b, the photon count increased from 12370 to 14710. The radio noise of two typical long-term operating conductors under wet and dry conditions is contrasted in Figure 15. Similar to the results of audible noise in [30], for the same type of conductors, the radio noise under wet conditions is higher than that under dry conditions. The difference of the radio noise in the wet and dry cases is 4.0 to 8.9 dB in Figure 15a, 7.1 to 10.3 dB in Figure 15b, respectively. The tests of the new conductors under wet conditions were also carried out in this work. The preparation of the new conductors under wet condition was the same as that of longterm operating conductors. The rate of decrease of the corona inception voltages (γ’), the deviation of radio noise between the long-term operating conductor and new conductor (ᇞRN’) under wet conditions was also obtained, and they are presented in Figure 16 and Figure 17 respectively. The relationship betweenγ’, ᇞRN’ and Ra can be described by linear equations (12) and (13) below (with a maximum discrepancy of 1%).

 ' =  0.887Ra  6.07   100%

(12)

RN =0.771Ra +3.4

(13)

'

24

20

40 30 20 95 100 105 110 115 120 125 130 135 140 145 Applied Voltage /kV

(b) LGJ400/65, Ra=20.62 μm Figure 15. Contrasts of the radio noise between long-term operating conductors under wet and dry conditions.

γ'/%

ΔRN/ dB

16

885

16

12

8

4

8

12

Ra/μm

16

20

Figure 16. The relationship between γ and Ra under wet conditions.

886


the radio noise of the long-term operating conductors may still have great possibilities to become deteriorative than that of new conductors. And such phenomenon will be more apparent in light rain or foggy weather condition. For those reasons, the designers or managers of power grid in such areas should pay close attention to the variations of surface states and radio noise of long-term operating conductors.

24

ΔRN'/ dB

20 16 12

ACKNOWLEDGMENTS

8 4 4

8

12

Ra/μm

16

20

The authors acknowledge the financial support from National Natural Science Foundation of China 51377096, 51037001 and the Fundamental Research Funds for the Central Universities 2014QN39.

Figure 17. The relationship between ᇞRN’ and Ra under wet conditions.

By comparing equations (10) and (12), it can be found that the rate of decrease of corona inception voltages under wet conditions is slightly higher (about 1%) than that under dry conditions when Ra is constant. And by comparing equations (11) and (13), it can be observed that the difference between the radio noise of long-term operating conductors and that of new conductors was larger (1.3 dB) under wet conditions compared to dry conditions when Ra is constant. Therefore, when the designers or managers of power grid would like to predict the actual radio noise of long-term operating conductors in slight rain or heavy foggy weather based on the analytical methods in [23], the increment (ᇞRN’) should be added to the calculated value as the actual radio noise will rise.

6 CONCLUSION The radio noise measurement system based on the corona cage was discussed and the actual radio noise generated by the conductor in the measured cage was obtained, based on the parameters of the corona cage and the measured data. The radio noise tests were carried out under both dry and wet conditions. Under two types of conditions, the corona intensity was higher on the long-term operating conductors contrasted to the new conductors due to the irregular surfaces of the aged conductors. The rate of decrease of the corona inception voltages between the long-term operating conductors and new conductors was discovered to increase with the raise of surface roughness degree Ra in linear relationship. Approximate linear relation was also discovered between the averaged radio noise deviation and Ra. The rate of decrease of corona inception voltages under wet conditions was slightly higher (about 1%) than that under dry conditions. The difference between the radio noise of longterm operating conductors and that of new conductors was larger (1.3 dB) under wet conditions compared to dry conditions. The significance of the present investigation could be concluded as: in several air polluted or coastal areas in China, the surface conditions of long-term operating high voltage transmission conductors may be very rough, this may lead to the drop of the corona onset voltages and the increasement of the corona discharge intensity. Although it is good weather,

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Xingming Bian was born in Jiangsu province, China, in 1985. He received the B.S. degree in electrical engineering from Huazhong University of Science and Technology, Wuhan, China, in 2006. He received the Ph.D. degree from the Department of Electrical Engineering at Tsinghua University, Beijing, China, in 2012. He carried out his postdoctoral research at Graduate School at Shenzhen, Tsinghua University, from 2012 to 2014. Now he is an associate professor at North China Electric Power University. His research interests are electromagnetic environment, corona and streamer discharge, conductor aging, anti-pollution materials, high voltage insulation and the optimization of insulator grading rings in power systems.

887 Lan Chen was born in Beijing, China, on 1 December 1986. He received the B.S. degree in electrical engineering from Tongji University, Shanghai, China in 2009. He received the Ph.D. degree from the Department of Electrical Engineering at Tsinghua University, Beijing, China, in 2014. Now he works at State Grid Jibei Electric Power Company Limited. His research interests are electromagnetic environment and corona discharges. Yuanjiu Wang was born in Jiangxi Province,China, on 30 November 1990, and received the B.S degree from the Department of Electrical Engineering, Tsinghua University, Beijing, China, in 2012. He is a M.S. candidate in the Electrical Engineering at Tsinghua University, Beijing, China. His research interests are electromagnetic environment and corona discharges.

Shuwei Wan was born in Guizhou Province, China, in 1988. He received the B.S and M.S. degrees from the Department of Electrical Engineering, Tsinghua University, Beijing, China, in 2011 and 2013, respectively. Now he works at the State Grid Chongqing electric power company in China. His research interests are electric field calculation, the application of helicopter and corona discharges. Fangdong Chen was born in Hubei province, China, in 1970. He received the B.S. degree from the Department of Electrical Engineering, North China Electric power University Beijing, China, in 1993. He received the M.S. degree from the Department of Electrical Engineering, Tsinghua University Beijing, China, in 2010. He is a senior engineer and works as a vice chief engineer in power apparatus maintenance company, jibei electric power company limited. His research interests are the application of helicopter, electric field calculation and electromagnetic environment in power systems. Xuesong Zhao was born in Beijng, China, in 1975. He received the B.S. degree from the Department of Electrical Engineering, Beijing Jiaotong University, Beijing, China in 1996. He is a senior engineer in power apparatus maintenance company, jibei Electric Power Company Limited. His research interests are the application of helicopter, the condition monitoring and diagnosis of extra high voltage power transmission liness. Liming Wang was born in Zhejiang Province, China, on 30 November 1963, and received the B.S., M.S., and Ph.D. degrees in high voltage engineering from the Department of Electrical Engineering, Tsinghua University, Beijing, P.R. China, in 1987, 1990, and 1993, respectively. He has worked at Tsinghua University since 1993. His major research fields are high voltage insulation, electrical discharges, flashover mechanisms on contaminated insulators, electromagnetic environment and applications of pulsed electric fields. Zhicheng Guan (M’06) was born in Jilin Province, China, on 10 November 1944 and received the B.S., M.S. and Ph.D. degrees in high voltage engineering from the Department of Electrical Engineering, Tsinghua University, Beijing, P.R. China, in 1970, 1981, and 1984, respectively. His major research fields are high voltage insulation and electrical discharges, flashover mechanisms on contaminated insulators, electromagnetic environment technologies, and applications of plasma and high voltage technologies in biological and environment engineering.