Ultrasonic Phased Array Detection of Internal Defects in Composite Insulators

Chao Yuan, Congzhen Xie, Licheng Li School of electric power, South China University of Technology

Guangzhou 510640, PR China

Fuzeng Zhang China Southern Power Grid Co. Ltd

Guangzhou, 510000, PR China

and Stanislaw M. Gubanski Department of Materials and Manufacturing Technology

Chalmers University of Technology, SE 412 96 Gothenburg, Sweden

ABSTRACT To reduce the risk imposed by use of defected composite insulators on the operation

of power grids, this paper introduces a nondestructive ultrasonic phased array (UPA) technique that allows effectively test such insulators. The method offers a great potential by reducing inspection time as well as allowing for analyzing components characterized by a complex geometry. The UPA inspection system utilizes an open-ended rectangular waveguide sensor, operating at frequency of 2.5 MHz. The system is simple, safe and relatively inexpensive. In this work, samples of silicone rubber composite insulators with various types of detects are studied and the obtained results show that void defects in the bulk of the insulator housing are easiest to be detected. Holes under insulator sheds can also be detected by the edge of scanning range. For the defects near the core-shed interface, the detection becomes possible by comparisons with sample without defects.

Index Terms - Ultrasonic phased array, nondestructive testing, internal defects, composite insulator.

1 INTRODUCTION

COMPOSITE insulators are nowadays widely applied in transmission overhead lines due to their limited volume, light weight, superior mechanical properties, excellent anti-pollution properties and high-dielectric strength In order to secure a reliable manufacturing procedures of composite insulators, an effective elimination of units containing internal defects, appearing in form of voids, delaminations and interfacial debonding, is required already at the production stage. Otherwise, serious electric field distortions [1] and long-term effects of high intensity erosive discharges may cause insulator failures under service conditions [2-6]. An efficient nondestructive test technique could therefore be useful for finding and localizing such imperfections in composite insulators.

Various nondestructive test methods have so far been described and proposed in literature. Infrared (IR) thermography allows detecting the temperature rise of the insulator resulting from partial discharge activity or leakage

current concentration in parts of the insulators. The technique appears suitable for detecting voids, cracks and tracking damages [7]. Nevertheless, the temperature increase induced is easily influenced by effects of sunlight, wind, moisture, ambient temperature and other factors that can cause large temperature changes on insulator surface. On the other hand, measurements of electric field distribution along and around insulators also provide an attractive possibility Examples are provided in [8], where the electric field method allows to detect artificial and real defects, including an inserted wire (8 cm long) or a visible splits (42 cm long). However, under humid or polluted condition, the diagnoses of faulty insulator became problematic. X-ray can be used offline for getting an integral view of interested parts or a whole insulator [9]. As the degree of absorption of X-rays is determined by the density, consistency, and the thickness of the detected objects, a comparably low resolution and bad sharpness of defect edges due to scattering are important disadvantages, not allowing finding smaller defects. In contrast, computer tomography (CT) appears as an effective method for detection of air bubbles (1 mm) in GRP cores of hollow-core composite insulator [9]. The method is however too expensive and complicated for use in

Manuscript received on 11 February 2015, in final form 15 August 2015, accepted 15 August 2015. 

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DOI 10.1109/TDEI.2015.005225

the manufacturing process. Shearography is an optical method based on high sensitive to deformations under mechanical or thermal loads [10, 11]. It is indicated that the method is useful to detect flaws, delaminations and debonded areas in composite systems.

The common feature of all the above listed methods is that they are nondestructive, which eventually allows for using them for quality assurance and fault diagnostics during manufacturing process or even in the field. However, since most of them are time consuming and expensive in use, none is suitable for a routine daily testing.

Ultrasonic techniques have initially been used in two ways to detect defects in electrical insulation [12, 13]. The first method is based on measuring acoustic emission from a faulty insulator produced by corona discharges originated at the defects. However, practical experiences have demonstrated a high sensitivity of the technique to the background noise, which makes the determination of the defect location difficult. The second method, known as active or nondestructive ultrasound technique, relies on listening to echoes of sound signals coupled into the tested objects. The location and nature of defects can be analyzed based on parameters like amplitude, time, and phase shift of the returning signal. McGrail et al [14] indicated that voids less than 1mm in diameter could be found this way at the interface between XLPE insulation and the semiconductor sheath in cables. Xie et al [15] proved that ultrasonic inspections of composite insulator could identify voids (0.5mm) on the interfaces between the glass fiber reinforced core and silicone rubber housing in composite insulators. Scanning laser acoustic microscope (SLAM), utilizing high frequency ultrasonic waves in the range from MHz to GHz, was also found useful for detecting delaminations and misorientation of fibers in fiber reinforced plastics [16]. Unfortunately, as the ultrasonic techniques have fixed focus and single angle beam interrogation of the test component, it becomes burdensome to inspect defects at different depths. Especially for defects located under complex structures, like for example defects under insulator sheds, the detection appears very difficult.

Yet another way of ultrasonic nondestructive testing can be based on ultrasonic phased array (UPA) technique, which has continuously been developed over the recent 30 years. It was initially applied in medicine [17] and thereafter, following the fast development of computer and electronic control technology, it has widely been used in various industrial fields [18, 19]. This technique is especially efficient in inspections of objects having complex shapes, location of defects at different depths and identifying different types of defects.

This paper presents an application of UPA technique for detecting internal defects in composite insulators. Different composite insulator samples containing different artificially introduced defect types, like air voids, paper strips, drill holes under sheds, interfacial air gaps were tested. In addition eroded insulator samples taken from the field were

analyzed. The results of the tests and their detection efficiency are discussed.

2 BACKGROUND The UPA technique applied in this paper uses a custom

probe composed of 24 piezoelectric wafers, connected linearly and forming array elements. By controlling time delay of excitation pulse waveforms in each element, an ultrasonic beam can be created and send into the tested object. In turn, reflected signals are also received. The ultrasonic beam can be focused at different depths and be deflected to any azimuthal angle, is the desired features when inspecting geometrically complex objects.

UPA test technique has the following advantages [20, 21]:

• the technique is fast, reliable and relatively inexpensive,

• ultrasonic beam deflection allows the angles of incidence to be varied with only one probe,

• ultrasonic beam focusing allows the use of a single probe for working at desired depth,

• no special operator skills are needed for successful defect detection,

• the technique is environmentally friendly and safe,

• the testing system can be battery operated and portable,

• the technique provides real-time imaging capability.

In order to maintain good directivity and depress energy

leakage of the ultrasonic beam, the shape of the acoustic field must be focused and its location well controlled. Therefore, precise control of the delay for each element is demanded. A schematic view of the ultrasonic beam deflection by controlling the delay of array elements is illustrated in Figure 1. Based on the theory of wave superposition, the delay time of randomly adjacent elements ∆τ can be calculated as

∆τ sin 1

Figure 1. Schematic view of the ultrasonic beam deflection.

526 C. Yuan et al.: Ultrasonic Phased Array Detection of Internal Defects in Composite Insulators

where θ is the deflection angle of the ultrasonic beam, d the distance between elements, and c is the wave speed in the medium.

A schematic view of the focusing of ultrasonic beam is on the other hand presented in Figure 2. The focusing is in this case accomplished by combining the spherical and linear timing relationships of the elements for focusing the beam at a given range and propagating at a specific azimuth angle. The focusing delays can be calculated by the following formula [22]:

1 1 2 2

where is the delay for the nth element (n=…, -2, -1, 0, 1, 2, …), F is the focal length, and is a constant to keep the delays positive.

For a single element, the pressure distribution can be expressed as [23]

, ,/ sin

sin2

sin2

[j( -kr)] 3

where r is the radial distance from the simple source, the angular frequency, j a unit imaginary number, and p0 is a function of the wave number k. According to Huygens’ principle [24], the pressure distribution of the phased arrays for focusing and steering is the superposition of the pressure of single elements,

, ,/ sin

sin2

sin2

sin2

exp 4

and

12 tan

∆ ∆ sin ,

∆2 tan

where is the steering angle from the center of array, ∆ the time delay for pure steering.

3 EXPERIMENTAL In this work, the used UPA imaging system utilized a

custom waveguide operating at frequency of 2.5 MHz and consisting of 12 array elements spaced apart by 0.8 mm. Each of the elements was excited by a pulse produced by digitally controlled transmitter. In order to ensure an optimum of total transmission and acoustic pressure transmission coefficient, thickness of sound transparent layer was selected to be a quarter of the ultrasonic wave. The resulting footprint of the waveguide was larger than its size (9.6 mm x 7 mm).

A block diagram of the experimental setup is shown in Figure 3. Contact between the waveguide and the investigated object was during the measurements secured by a coupling agent. The ultrasonic transmission beam traveled through the coupling agent and penetrated into the insulator sample. The reflected signal was picked up using the same waveguide and was transmitted to a PC. After the collation and analysis of the data, the reflected signal was converted into an image and displayed in the PC. A picture showing the contact linear UPA probe during inspection of a composite insulator sample is shown in Figure 4. The UPA test system was provided by Zhongke Innovation Co, Ltd (China).

Composite insulator samples consisting of a GRP core and silicone rubber sheath and sheds were used in the measurements. To mimic the presence of these defects in composite insulator, three kinds of artificial defects were introduced, as shown in Figure 5a-c. These included an air filled void of 2 mm in diameter placed in the middle of a silicone rubber plate (a), paper strips of 0.3 mm thickness and 10 mm length located 5 mm above the sheath-core interface (b), and a 1mm hole drilled under insulator sheds (c) .

Figure 3. Block diagram of the experimental setup.

 

Digital controller

Waveguide

Insulator

PC & Display System

DC Power supply

Figure 2. Schematic view of ultrasonic beam focusing.

 

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In addition, in June 2012 a routine inspection performed on

a 500 kV DC line (6 years of service) identified by means of an IR detector obvious local overheating regions in several composite insulators, close to the high voltage ends, as indicated in Figure 6a. The high voltage end in one of these insulators was cut into several segments, exhibiting in their cross sections and erosion defects of different degrees. One of the identified defects is shown in Figure 6b and its scanning electron microscope (SEM) picture in Figure 6c. The defect penetrates into the GRP core and its depth is 1.2mm below the sheath-core interface. Formation of such erosion defects is attributed to a combined action of temperature, corona activity and moisture, most probably being an initial stage of the brittle fracture process.

The width of UPA probe is smaller than the space between sheds, so it could be placed on the sheaths with or without the sheds. All the sheds were removed to make it easier to transport those composite insulators. The plate sample has a thickness of 8mm and all other silicone rubber samples have a thickness of 6.5 mm. The effective thickness range for the used UPA test system is 4 mm to 50 mm. A summary of the all investigated types of insulator defects, indicating their position and shape, is presented in Figure 7.

4 RESULTS AND DISCUSSION Ultrasonic transmission parameters of the media involved

in the test are shown in Table 1. As ultrasonic signals were both reflected and transmitted when crossing heterogeneous structures, acoustic pressure reflection coefficient was calculated as

k 5

where pk represents reflected acoustic pressure amplitude and pd incident acoustic pressure amplitude. Z1 and Z2 stand for acoustic impedances of the involved interface materials. When ultrasonic beam was reflected by the sheath-core interface, its acoustic pressure reflection coefficient was equal to k = 0.52. In cases when ultrasonic beam was reflected by the defects containing air, as the air

acoustic impedance value is almost zero, its acoustic pressure reflection coefficient was k = -1. The reflected ultrasonic signals were used to produce a color coded map depending on the amplitude of reflected acoustic pressure and its depth. The closer to the red, the higher the amplitude of reflected ultrasonic signal was.

Figure 8a illustrates an air void in the middle of silicone rubber plate on a GRP core material under the inspection by UPA probe. In Figures 8b, the color map obtained on the object without air void is shown. The red highlighted area at the top of the (b) image is a blind zone. Aftershocks of the probe are the main cause of its existence and defects located near the surface of the material sample cannot be thus identified. A wedge can be placed on the surface of the tested material sample to offset or reduce the blind zone. In addition, there is some noise, displayed as light blue stripes under the blind zone. One possible reason for its appearance is a quantified phase error of the signals emitted by the array

Table 1. Ultrasonic transmission parameters.

Medium Acoustic velocity

(m/s)

Density (g/cm3)

acoustic impedance (g•cm-2•s-1)

SIR 1060 1.56 0.17*106

GRP 2570 2.1 0.54*106

Air 340 1.29*10-3 0.00004*106

Figure 7. General presentation of the position and the shape of the defects.

Figure 6. Erosion defect near the sheath-core interface. (a) IR image, (b)erosion defect and (c) SEM image.

  (a) (b) (c)

Figure 5. Test sample with artificial defects. (a) Air void, (b) drill hole and (c) paper strips.

 (b)(a) (c)

Figure 4. Inspection with contact linear UPA probe

528 C. Yuan et al.: Ultrasonic Phased Array Detection of Internal Defects in Composite Insulators

elements. The red highlighted area in the middle of the image is the echo produced by reflection of the beam at the sheath-core interface, characterized by a large difference in acoustic impedances of the involved media (sheath material and GRP core). There are also some blue stripes below the interface echo signals caused by the back wall echo and superposition of multiple reflected signals. Figure 8c shows the map of the

object containing a void in the silicone rubber plate. A teal block pattern can clearly be seen in the middle of the image. As the acoustic impedance of air is much different from that of silicone rubber, most of ultrasonic signals are reflected by the air void. The teal block pattern demonstrates the size and location of the air void. In addition, there are also a few light blue stripes caused by cracks surrounding the void.

Figure 11. UPA detection of erosion defect located at sheath-core interface of composite insulator (a) schematic view of test object, (b) color map of UPA signal in insulator without erosion defect, (c) color map of UPA signal in insulator with erosion defect.

Paper strips

Blind Zone

Interface

(a) (b) (c)

SIR sheath

GRP core

UPA probe

Figure 10. UPA detection of paper strips located at sheath-core interface of composite insulator; (a) schematic view of test object, (b) colormap of UPA signal in insulator without paper strip, (c) color map of UPA signal in insulator with paper strip.

Figure 9. UPA detection of hole drilled under shed of composite insulator; (a) schematic view of test object, (b) color map of UPA signalin insulator without hole, (c) color map of UPA signal in plate with hole.

Figure 8. UPA detection of air void located inside of silicone rubber plate; (a) schematic view of test object, (b) color map of UPA signalin plate without void defect, (c) color map of UPA signal in plate with void defect.

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In the conventional acoustic testing, the ultrasound signal can only be transmitted along the direction perpendicular to the surface of the probe. It therefore has a narrow detection range and cannot be used in objects of complex geometry. By contrast, UPA probe consists of a number of elements, which results not only in a wider scanning range but also provides a possibility for changing the direction of observation. This feature, allows for example to detect defects under the sheds of composite insulator. UPA detection patterns of artificially and naturally created defects in composite insulators are presented in Figures 9-11. Figure 9 illustrated the consequence of drilling a hole through GRP rod into the insulator sheath. In Figure 9b, presenting the color map of UPA signal in an insulator without drilled hole, long red block pattern appears in the middle of the image. Such a strong and wide reflection represents the sheath-core interface echo. Figure 9c shows the map of insulator with the hole under its shed. An additional red block pattern appears in the upper right side of the map, which indicates location of the defect. By adjusting focus direction and focus depth of the ultrasonic beam, more ultrasonic signals propagate in the direction of the hole and the defected pattern becomes clearer. In addition, as the ultrasonic signals reflected by the hole are located above the echo of sheath-core interface, this means that the drill holes penetrate to the silicone rubber sheath.

Similarly as described above, Figure 10 illustrates the consequence of placing paper strips at the sheath-core interface of a composite insulator. Because of jamming of the interface echo signal, it becomes more difficult to identify precisely the thin defect. However there still appear a difference in the image of the objects with and without the strips, which allows estimating defect’s position and size. The red block pattern in the middle of the scan of Figure 10b indicates, similarly as in Figure 9b, the sheath-core interface echo. In addition yellow stripe patterns can be observed above the red interface echo in Figure 10c, which results from reflections of the strips and appear earlier than the interface pattern. The yellow patterns are very thin, which appear consistent when taking into account the thickness of the strips. One may thus conclude that even presence of paper strips above the sheath-core interface can be detected by UPA technique.

Figure 11 illustrates the result of detection of erosion defect located near the sheath-core interface. This type of defect is the most difficult to detect because most of the ultrasonic signals are reflected by the sheath-core interface. Figure 11b shows the color map of UPA signal in the area without the defect. Different from previous detection, all the reflected signals including interface, blind zone and erosion defects are weakened to the same extent to make erosion defects are displayed more obvious. The sheath-core interface become blue-green strips pattern in the middle of this detection image. In figure 11c there appears in addition a red-yellow strip pattern within the sheath-core interface one, which indicates the ultrasonic reflected signals coming from the defect are stronger. A new interface is created of silicone rubber and air and because of the difference in acoustic impedances of both the media the

reflection is amplified. As compared with the traditional ultrasonic probe, the UPA probe can thus generate stronger signals, which facilitates detection of defects located near the sheath-core interface.

5 CONCLUSION The ability to detect internal defects in composite

insulators has been one of main concerns related to their successful implementation in power networks. In this respect the ultrasonic phased array (UPA) opens new range of possibilities. UPA technique provides many advantages and one among the most important is that UPA probe has a wider scanning range and focus direction and depth can be adjusted by controlling delays of signals sent by array elements. This feature also opens a possibility of technique application for inspecting geometrically complex objects.

The results of measurements presented here show that various types of defects and their location could successfully be detected by UPA technique. The investigations included artificially created and naturally appearing defects. Void defects located in the middle of one of the contacting media of the silicone rubber and GPR core interface were easiest to be detected. Drill holes under insulator sheds could also be detected by the edge of scanning range. For the defects near or at the interface, the best results were obtained by contrasting with scans of no defected regions. It is postulated that UPA testing has the potential to become effective, economic and robust mobile tool for inspecting the integrity of composite insulators in production and service conditions.

ACKNOWLEDGMENT Thanks for the financial and technical support of the

National Natural Science Foundation of China (project no. 51107043) and National Engineering Laboratory for Ultra High Voltage Engineering Technology (Kunming, Guangzhou) (2011440002070906). The author Yuan Chao thanks the China Scholarship Council (CSC File No. 201406150008) for the financial support.

REFERENCES [1] G. Vaillancourt, S. Carignan, and C. Jean, "Experience with the

detection of faulty composite insulators on high-voltage power lines by the electric field measurement method," IEEE Trans. Power Delivery, Vol. 13, pp. 661-666, 1998.

[2] M. Kumosa, L. Kumosa, and D. Armentrout, "Failure analyses of nonceramic insulators. Part 1: Brittle fracture characteristics," IEEE Electr. Insul. Mag., Vol. 21, No. 3, pp. 14-27, 2005.

[3] M. Kumosa, L. Kumosa, and D. Armentrout, "Failure analyses of nonceramic insulators: part II - the brittle fracture model and failure prevention," IEEE Electr. Insul. Mag., Vol. 21, no. 3, pp. 28-41, 2005.

[4] B. Lutz, L. Cheng, Z. Guan, L. Wang, and F. Zhang, "Analysis of a Fractured 500 kV Composite Insulator - Identification of Aging Mechanisms and their Causes," IEEE Trans. Dielectr. Electr. Insul., Vol. 19, pp. 1723-1731, 2012.

530 C. Yuan et al.: Ultrasonic Phased Array Detection of Internal Defects in Composite Insulators

[5] J. Andersson, S. M. Gubanski, and H. Hillborg, "Properties of interfaces in silicone rubber," IEEE Trans. Dielectr. Electr. Insul., Vol. 14, pp. 137-145, 2007.

[6] J. Andersson, S. M. Gubanski, and H. Hillborg, "Properties of interfaces between silicone rubber and epoxy," IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 1360-1367, 2008.

[7] E. Spangenberg and G. Riquel, "In service diagnostic of composite insulators EDF's test results," 10th Int'l. Sympos. HV Eng., Vol. 4, pp. 139-142, 1997.

[8] G. H. Vaillancourt, S. Carignan, and C. Jean, "Experience with the detection of faulty composite insulators on high-voltage power lines by the electric field measurement method," IEEE Trans. Power Delivery, Vol. 13, pp. 661-666, 1998.

[9] A. Merten, "Non-destructive test methods for hollow-core composite insulators," Int'l. Conf. High Voltage Eng. Application (ICHVE), pp. 536-539, 2010.

[10] Y. Hung, W. Luo, L. Lin, and H. Shang, "Evaluating the soundness of bonding using shearography," Composite Structures, Vol. 50, pp. 353-362, 2000.

[11] S. Gubanski, A. Dernfalk, J. Andersson, and H. Hillborg, "Diagnostic methods for outdoor polymeric insulators," IEEE Trans. Dielectr. Electr. Insul., Vol. 14, pp. 1065-1080, 2007.

[12] T. McGrail, "Ultrasonic NDT for defects/degradation in insulation," IEEE Colloquium on Characterisation of Dielectric Materials, , pp. 1/1-1/5, 1994.

[13] D. W. Auckland, C. D. Smith, and B. R. Varlow, "Ultrasound-a diagnostic tool for NDT of insulation degradation," 6th Int'l. Conf. Dielectric Materials, Measurements and Applications, pp. 115-118, 1992.

[14] T. McGrail and C. Smith, "Acoustic and ultrasonic techniques for condition monitoring," IEEE Colloquium, Monitoring Technologies for Plant Insulation, pp. 1-3, 1994.

[15] X. Cong-zhen, Y. Zhang, H. Yan-peng, Y. Xue-jun, and W. Qiang-hua, "Application of Ultrasonic Flaw Detector to Internal Defects in Composite Insulators," High Voltage Eng., Vol. 10, pp. 2464-2469, 2009 (in Chinese).

[16] M. Oishi, "Nondestructive evaluation of materials with the scanning laser acoustic microscope," IEEE Electr. Insul. Mag., Vol. 7, No. 3, pp. 25-30, 1991.

[17] B. O. Kenneth, J. Benkeser, and A. Leon, "An ultrasonic phased array applicator for hyperthermia," IEEE Trans. Sonics Ultrasonics, Vol. 31, p. 527, 1984.

[18] J. Habermehl, A. Lamarre, and O. NDT, "Ultrasonic phased array tools for composite inspection during maintenance and manufacturing," in 17th World Conf. Nondestructive Testing, Shanghai, China, pp. 1-5, 2008.

[19] C. Frederick, D. Zimmerman, and A. Porter, "High-density polyethylene piping butt-fusion joint examination using ultrasonic phased array," Journal of Pressure Vessel Technology, vol. 132, p. 051501, 2010.

[20] B. W. Drinkwater and P. D. Wilcox, "Ultrasonic arrays for non-destructive evaluation: A review," NDT and E Int'l. Colloquium, Vol. 39, pp. 525-541, 2006.

[21] J. Poguet, J. Marguet, F. Pichonnat, and L. Chupin, "Phased array technology: concepts, probes and applications," J. Nondestructive Testing & Ultrasonics (Germany), Vol. 7, pp. 1-6, 2002.

[22] L. Azar, Y. Shi, and S. C. Wooh, "Beam focusing behavior of linear phased arrays," NDT & E Int'l.l, Vol. 33, pp. 189-198, 2000.

[23] S. C. Wooh and Y. Shi, "Optimization of ultrasonic phased arrays," in Review of Progress in Quantitative Nondestructive Evaluation, ed US: Springer, 1998, pp. 883-890.

[24] A. McNab, A. Cochran, and M. Campbell, "The calculation of acoustic fields in solids for transient normal surface force sources of arbitrary geometry and apodization," J. Acoustical Soc. America, Vol. 87, pp. 1455-1465, 1990.

Chao Yuan was born in Hunan Province, China, in

1987. He received the B.Sc. (Electrical engineering)

from Hunan University, China, in 2010. He is

currently pursuing the Ph.D. degree at South China University of Technology, China. Since 2014 he has

been a visiting Ph.D. student in High Voltage Engineering at Chalmers

University of Technology, Sweden. His research interests include aging

mechanism of composite insulators, dielectric material characterization,

nondestructive testing.

Congzhen Xie (Corresponding author) was born in

Shaanxi Province, China, in 1973. She received a

B.S. degree in Wuhan University of Hydraulic and

Electrical Engineering, Wuhan, P.R. China, in 1994.

She received the Ph.D. degree at the South China

University of Technology in 2010. Presently she is

an associate professor of Electrical Engineering

College of South China University of Technology.

Her major research fields are high-voltage insulation, composite insulators

and detecting techniques.

Licheng Li was born in Jiangsu Province, China, in

1941. He received the B.S.E.E. degree from the

Tsinghua University, China, in 1967. From 1967 to

1980, he was with GanSu Electric Power

Construction Corporation as a chief engineer. From

1980 to 1982, he was the project manager and

Principal Engineer of China Extro-High Voltage

Tech. Company. He joined China Southern Power

Company (the predecessor of China Southern Power Grid Company, CSG)

in 1990 where he is at present a secretary-general of consulting group of

CSG. His major research fields include HVAC/HVDC transmission

network, paralleling system operation and stability, wide-area

measurement and control.

Fuzeng Zhang was born in Shandong Province,

China, in January 1979. He received the B.Sc. and

M.Sc. degrees from the School of Automation,

Northwestern Polytechnic University, China, in

2001 and 2004, respectively. He received the Ph.D.

degree from the Department of Electrical

Engineering, Tsinghua University, Beijing, China,

in 2008. He is currently working in Ultra-high

Voltage Laboratory of Technology Research Center of China Southern

Power Grid. His research interests are HV engineering and HV outdoor

insulation.

Stanislaw M. Gubanski (M'89-SM'90-F'01)

received the M.Sc. (high voltage engineering) and

Ph.D. degrees (material science) from the Technical

University of Wroclaw, Poland, in 1973 and 1976,

respectively. He was a Research Fellow at the

University College of North Wales Bangor, U.K

from 1976 to 1977, and a senior lecturer at the

Technical University of Wroclaw, Wroclaw, Poland,

from 1977to 1988. Afterwards he was associate professor (1989-1996) at

the Royal Institute of Technology, Stockholm, Sweden. Currently, he is

professor in High Voltage Engineering at the Department of Materials and

Manufacturing Technology, Chalmers University of Technology. He is a

Senior Associate Editor of the IEEE Transactions on Dielectrics and

Electrical Insulation.

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