Journal of China University of Science and Technology Vol.53-2012.10
Chie-Yeh Wang
1
, Shih-Wen Lee
2
, Der-Chin Chen
3
汪啟業 1 李世文 2 陳德請 3
1 逢甲大學資訊電機工程系碩士班學生
3 逢甲大學資訊電機工程系副教授
2 中華科技大學電機工程系助理教授
1.3
Department of Electrical Engineering, Feng Chia University
2
Department of Electrical Engineering, China University of Science and Technology
摘 要
本文研究目的在建立紫外線鐵路電車線電暈放電量測系統，提出了紫外成像
技術在鐵路電車線系統中的具體應用，量測項目包括電氣設備電暈放電、設備表
面的局部放電探測及外部污穢程度和絕緣缺陷檢測等。
研究方法是使用紫外
線
成像儀和紅外
線
成像儀，
分別針對高鐵電力系統來源端電壓 161KV ，及電車線系統的 25 KV 電力
系統，進行電暈放電的紫外線信號實施量測，並分析量測距離及儀器增益對紫外
線檢測的影響。本方法優點
快速、準確地檢測輸電線路故障和放電。
期望透過本研究收集的檢
測資料可做為電車線系統 (OCS, Overhead Catenary System) 維修的參考依據。
關鍵字 ：紫外線檢測、電暈放電、電車線系統
ABSTRACT
In this research, the inspection method of the corona discharge by ultraviolet imagers and signal processing is developed. The method is used to detect the corona and arcing emitted on high voltage equipment. This solar blind filters enable the detection of weak UV signals in daytime with the high signal-to-background ratio. In the experiment, the 161 KV of the high-speed rail power source system and the 25 KV of the Overhead Catenary System are measure, respectively. The ultraviolet and infrared imagers are used to detect the faults and electric discharge in the transmission lines. The paper presents the analyses of the influences on the inspection caused by the measurement distances and the instrument gains. The results of the experiment
29

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method show the failure of insulator to be corrected. Through this study, the measurement data are established and used to find the defects of the equipment in the Overhead
Catenary System to reduce the occurrence of blackouts.
Keywords : ultraviolet(UV), corona discharge, Overhead Catenary System(OCS)
The electric construction of high-speed and high-capacity railway has become the major trend in the railway transport worldwide. Although railway electrification has its advantages, but the power lines often cause flashover accidents and damage the dielectric strength because of the contamination, humidity, material aging, and these all affect the quality of power supply. Therefore, the insulator property diagnosis of electrical equipment and early warning system has become the most important issues for the preventive maintenance. This paper uses the ultraviolet imager to detect the surface discharge of Overhead Catenary System on-site, and to find out the adverse signals of equipment insulation in early stage. The ultimate aim is to find out the early indicator of insulation deterioration by using the ultraviolet imager in order to improve power quality and stability of the power line, and reduce the power failure accidents of power system.
In the natural environment, the wavelengths (200 nm ~ 280 nm) in the ultraviolet light of solar radiation are almost absorbed by the stratospheric ozone, it is known as the solar blind [1]. When the corona discharges, it will ionize the gases. The radiation wavelength of the light is decided according to the types of the ionized gases. The wavelengths of the ionized nitrogen molecule are about 220 nm ~ 270 nm which are located in the solar blind band [2]. Hence, using these wavelengths to detect the corona discharge will avoid the interference of the solar radiation. Figure 1 and Figure 2 are carried out both in the daytime and at night.
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Journal of China University of Science and Technology Vol.53-2012.10
The UV corona inspection system is shown as Figure 3. The light sources received by the inspection system include the visible light, the ultraviolet light and the infrared light. The input lights are separated into two parts by using beam splitter. One part forms the visible image by way of the visible CCD. And the other part, which is filtered by the solar blind filter (SBF), retains the ultraviolet part. Then, the UV part is enlarged by the signal amplifier and converted into the visible high-resolution light by using
Intenstified Charged Coupled Device (ICCD). Finally, through the technique of image
31

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method fusion, a special processing, the ultraviolet image and the visual image are combined into a composite image [3].
The COUNTER value displayed on the UV imager is the UV photons per minute.
It can be expressed as the intensity of the corona activity. The value is closely related to the distance, instrument gain, atmospheric pressure, temperature and humidity. Hence, the above factors will influence the results of the corona discharge.
Infrared (IR) thermography is based on the IR radiation emitted by the objects, which is directly proportional to the fourth power of the absolute temperature of the object. The IR camera converts the IR radiation, emitted by the objects that are being focused, into a color scale. Thus, in the thermal image, the temperature of the objects is identified by its color. In many situations, an abnormality might be indicated by the increase or decrease of the surface temperature of the equipment. The temperature is directly related to the severity of the electric discharges of insulator. High temperature leads to decomposition of the polymer, and may bring the insulator to an electrical and eventually mechanical failure [4]. The IR image is shown as Figure 4.
In the experiment, the IR imager and the UV imager are used to measure the terminal voltage of 161KV provided by Taiwan Power Company (TPC), and the 25KV power system of the Overhead Catenary System, respectively. The power system of
Taiwan High Speed Rail is shown in Figure 5. First, the IR imager is used to identify where the insulators are due to the abnormal temperature rise. Then, the UV imager is
32

Journal of China University of Science and Technology Vol.53-2012.10 used to measure UV photons of corona discharge of the insulator. The indicated value of the COUNTER is used to estimate the insulation condition of the equipment, and then it is provided as a reference for the future maintenance of the equipment. The flow chart of UV imaging system is shown as Figure 6.
33

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method
Under the ideal condition, the intensity of emitted light waves is inversely proportional to the square of the distance in the light source of the uniform medium.
However, the COUNTER value is inversely proportional to the square of the distance is
34

Journal of China University of Science and Technology Vol.53-2012.10 not conformed to practical application. In order to clarify the impact of distance on the
COUNTER detection value, the COUNTER values are measured and their average values based on different distances are obtained. Table 1 and Figure 7 are for 25 KV on
OCS, Table 2 and Figure 8 are for 161 KV on TPC, are shown as follows, respectively:
Distance
(m)
10
15
20
25
30
1 2
COUNTER Value
3 4 5 6
328 313 408 365 343 430
270 233 238 292 241 265
146 131 182 124 160 116
89 73 96 87 94 109
32 43 58 24 36
Voltage ： 25KV GAIN ： 170
29
Average
365
257
143
91
37
The relationship between the distance and COUNTER value
400
300
200
100 y = -82.017x + 424.55
0
100 225 400
Square of the distance
625 900
35

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method
Distance
(m)
COUNTER Value
1 2 3 4 5 6
Average
10 2590 2617 2748 2549 2583 2711 2633
15 2210 2315 2413 2194 2330 2247 2285
20 1958 2013 1978 1859 1895 1989 1949
25 1342 1408 1511 1379 1431 1429 1417
30 1092 1116 1073 1013 1089 810 1032
Voltage ： 161KV GAIN ： 170
3000
The relationship between the distance and COUNTER value y = -406.98x + 3084
2500
2000
1500
1000
500
0
100 225 400
Square of the distance
625 900
Obviously, the higher the voltage is, the more the Counter value is. Or, the stronger the electric field is, the more intensity of the corona discharge is. Figure 7 and Figure 8 present the number of UV photons and show a good linear relationship with the square of the distance.
4.2
Through the optical system, the transmission loss of UV light will be increased and the number of UV photons will be reduced dramatically when it reaches the CCD board.
To improve the sensitivity of the instrument, it is necessary to adjust the gain of the UV photon when the UV light enters the instrument’s optical system. The gain directly
36

Journal of China University of Science and Technology Vol.53-2012.10 changes the COUNTER values. In other words, it affects the corona strength [5]. The data shown in Table 3 and the plot in Figure 9 are for the 25 KV of OCS. The data shown in Table 4 and the plot in Figure 10 are for the 161 KV of TPC.
A.
GAIN COUNTER GAIN COUNTER GAIN COUNTER
40 14 100 116 160 233
50
60
29
43
110
120
124
146
170
180
365
408
70
80
90
58
73
130
140
160
189
190
200
87 150 233 210
Voltage ： 25KV Distance ： 30m
591
638
715
The relationship between the gain and COUNTER value
800
600
400
200 y = 37.637x - 122.99
0
40 60 80 100
GAIN
120 140 160 180 200
37

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method
GAIN COUNTER GAIN COUNTER GAIN COUNTER
40 412 100 1423 160 2451
50
60
625
834
110
120
1689
1732
170
180
2573
2694
70
80
90
963
1015
130
140
1847
2103
190
200
1241 150 2249 210
Voltage ： 161KV Distance ： 30m
2706
2869
3106
3500
3000
2500
2000
1500
The relationship between the gain and COUNTER value y = 155.64x + 328.76
1000
500
0
40 60 80 100 120
GAIN
140 160 180 200
In Figure 9, the number of UV photons has a good linear relationship with the instrument gain when the gain is less than 160. In Figure 10, the number of UV photons has a good linear relationship with the instrument gain. On the other hand, in order to improve the detection sensitivity of the imager, the gain of the instrument is adjusted.
Thus it will change the instrument sensitivity, and it also indirectly affects the actual value of the UV photons.
It will be helpful for the real application of the instrument to understand the relationship between the gain and the detection value of UV COUNTER. If the
COUNTER value is less than 200, a high gain, such as, more than 150, is generally selected, in order to detect the weak corona source. When the COUNTER value is
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Journal of China University of Science and Technology Vol.53-2012.10 between 200 and 5,000, the range of the gain is chosen between 90 and 150 for easy comparison. When the UV COUNTER value is greater than 5000, a small gain, such as, less than 80, is chosen in order to avoid the mutual overlapping of the UV image, and also locate the corona source accurately.
Changes in pressure and temperature can affect the air density, and also affect the detection value of the COUNTER. Due to the various ultraviolet filters, the transmission efficiency of the optical system and the CCD sensitivity conditions of the
UV imager are varied as well. The COUNTER values between different instruments are within the error of ± 20%, and the COUNTER values in the same instrument are within the error of ± 3%. The observation angle will have some bias in the field tests, so the effects of pressure and temperature in the field tests are not considered [6].
The influence of humidity on the UV COUNTER value is more complicated. In some situations, the increase of the humidity will reduce the intensity of corona. Such as, on the wet surface of a series of the insulators, the conductance is increased and the voltage distribution is relatively more uniform, so it may reduce the corona intensity of insulators.
In most situations, the higher humidity always causes the greater intensity of corona, such as, the dissoluble material of the dirty on the surface of insulator will be more easily dissolved in water. Finally, when the leakage current rises, the local discharge phenomenon is more easily occurred on the insulator surface.
It is worth noting that there is currently no way to amend the uncertainty of dirty ingredients and moist situation and it also brings certain difficulties to detect the discharge of corona. Therefore, we should focus on building up the data monitoring of the corona discharge.
The accumulation of a certain number of examples will help us understand more about the influence of humidity on the COUNTER value.
39

The Corona Inspection of Ceramic Insulator by Ultraviolet Imaging Method
In the experiment, the ultraviolet image of the corona discharge was affected by the observation distance and the gain of the instrument. When the observation distance is farther, the points of light on the image become smaller; and when the gain of the instrument increases, the points of light on the image increase
The atmospheric conditions also have some impact on the UV imaging
When the relative humidity is higher, the light reached to the UV imager is absorbed and scattered by the drops of water in the air, thus affects the pixel intensity in the observed image of the UV imager.
When the dust and water droplets are on the surface of insulator at the same time, there is greatest impact on corona discharge of the insulator.
The general calibration method of UV imaging detection is characterized by its activity of the corona intensity by measuring the UV COUNTER number per minute in a specified region.
It represents the level of the corona discharge of the electrical equipment. However, due to the precision of the instrument, it just can be measured the number of times of discharge within a region, it is hard to measure the number of times of discharge in a precise location. In addition, due to the different discharge patterns, there is no obvious correspondence between the frequency and the intensity of the discharge. It cannot analyze the discharge status of the equipment quantitatively by the frequency of UV discharge.
So, using this method can only provide an auxiliary reference for analysis and judgment [7].
[1] Wenjun Zhou, Han Li, Xiao Yi, Jun Tu, Jianhui Yu, “A Criterion for UV
Detection of AC Corona Inception in a Rod-plane Air Gap”, IEEE, Vol.
18, No.1,
Feb, 2011.
[2] David L. Chichester, and Scott M. Watson, “Multispectral UV-Visual Imaging as a
Tool for Locating and Assessing Ionizing Radiation in Air”, IEEE, Vol.
58, No.5,
Oct, 2011.
[3] Yin Limin, and Zhangyu, “Ultraviolet Image Processing Method in Corona
Detection”, IEEE, Vol. 1, pp. 327-331, Oct, 2009.
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Journal of China University of Science and Technology Vol.53-2012.10
[4] L. Meyer, S. Jayaram and E. Cherney. “Thermal Conductivity of Filled Silicone
Rubber and its Relationship to Erosion Resistance in the Inclined Plane Test”,
IEEE Trans. Dielectr. Electr. Insul., Vol. 11. pp. 620-630, Aug, 2004.
[5] E. G. da Costa, T. V. Ferreira, M. G. G. Neri, I. B. Queiroz, A. D. Germano,
“Characterization of polymeric Insulators using Thermal and UV Imaging under
Laboratory Conditions”, IEEE, Vol.
16, No.4, Aug, 2009.
[6] “The application of the UV corona inspector for Insulator inspection”, Member of
Earth Technolgies Group, p.3, 2009.
[7] JIANG Zhi-Wen, YE Hui-Sheng, LEI Hong-Cai,ZANG Chun-yan, “Application of UV imaging technology in detecting electric power equipment fault”, p.13,
2009.
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