Study of detecting defect porcelain insulators using infrared imaging technique

Study of detecting defect porcelain insulators using infrared imaging technique

Qiu Changtao, Sun Xintiang
NE China Institute of Electric Power Engineering
Jitin City 132012, China

Hou Shanjing
Xian Laboratory of Thermal Power Engineering
Xian 710032, China

Abstract
The detection of defect porcelain insulators on high voltage transmission lines is a very important problem for guaranteeing operating security. In order to investigate the thermal imaging characteristics of defect porcelain insulators, an AC simulation experiment under the laboratory condition and a field testing were carried out by using the infrared imaging technique. The insulators were energized at 110KV. The instrument used is an infrared detector, AGENA-870. Also, a DC simulation experiment at the laboratory for porcelain insulators of 500KV was performed by using the same infrared detector.

Results show that the infrared imaging technique is a new effective and simpler technique for detecting the AC defect porcelain insulators, but it is not feasible for DC defect porecelain insulators.

Introduction
The operating security of transmission lines is a very important problem concerning the reliable and economical operation of power systems. According to the statistical data in large power networks over the world, the contingency caused by defect insulators is about 85 percent of the whole transmission line contingency. At present, the insulators used in transmission systems in China are almost porcelain. So electrical engineers do still pay their attention to the detection of defect porcelain insulators. Traditional detecting methods, such as, sparking gap method, isolation resistance method, impulse corona current method etc., can not meet the need of site detection owing t their deficiency of tower crimbingm strong laboring, low efficiency or too much environmental influence etc. Since the infrared imaging technique was applied to power-to-power system, many organizations and authors in China have been engaging in experiment research work or on site testing. In this paper, by using the infrared imaging technique, an AC simulation experiment at the laboratory and a field-testing for porcelain insulators of 110KV were carried out. The instrument employed was an infrared detector, AGENA-870. Also, a simulation experiment at the laboratory for DC porcelain insulators of 500KV was performed using the same detector.

Results show that the infrared imaging technique is a new effective and simpler technique for detecting the AC defect porcelain insulators, but it is not feasible for defect DC porcelain insulators.

Infrared imaging detection methods
A body whose temperature is higher than 0°K must radiate infrared energy of 0.75-100mm. The relationship between the radiation energy and the temperature of the body must comply with stephen-Bolzmannian theorem, i.e.,

W = e^-dT⁴

Where
W - Radiation power per unit area;
d - Radiation Rate of the body;
T - Absolute temperature of the body;
e - Stephen-Boizmannian Constant.

According to the value of insulation resistance, the insulators on high voltage transmission lines can be divided into two kinds, normal insulator whose insulation resistance is higher than 300MW, and defect insulator whose insulation resistance is less than 300MW. The defect insulators can also be divided into two kinds, low resistance insulator whose insulation, resistance is between 10-300MW and zero resistance insulator whose insulation resistance is tower than 10MW.

For on-line operation insulators, there exists a voltage aross each of them some teak age. currents and dielectric polarization currents would be caused and flow through its body. this may produce dissipation of energy and makes their temperature higher than ambient temperature. however,. each insulator is under different environment, so it has different temperature. We can use the infrared imaging detector to identify the defect insulators from their thermal images. this is so called infrared imaging detection.

AC Simulation Experiment at laboratory
The simulation experiment was carried out under laboratory condition. We used the suspension insulators X-4.5 as test sample. The infrared detector, AGEmA-870 was used for detection. As shown in Fig. 1, the insulators along the string were coded in sequence 1,2, 8 from up to down. The string was energized at AC voltage 66 KV corresponding to phase voltage of 110KV transmission line, for 1-hour duration. When the temperature of insulators tended to thermal stable, we measured the iron-hat temperature of each nit. The ambient temperature as 22C and the indoor moisture 62. Table 1-2 list the position number and insulation resistance of the tested insulators corresponding to Test 1 and Test 2, respectively. The number marked with means defect insulator. The temperature distribution along the string are similar to the voltage distribution across each insulator, i.e. the insulators near by the line side have a little higher temperature than those near by the ground-side. But on the whole, the temperature distribution along the string is almost homogeneous, no obvious sudden

Table.1 AC Simutation Experiment at laboratory

No.	1	2	3	4	5	6	7	8
Resistance MW	10000	1000	7500	10000	15000	10000	120	1700
Temperature0° C	24.0	23.9	23.8	23.7	23.8	24.1	25.3	24.3

Change. When an insulator is defect as shown in Table 1, the insulator #7 has resistance of 120MW, and its temperature is higher than those of unit #6 and #8, where occurs-1 sudden change of temperature. From the thermal imaging as shown in Fig. 2, the image of insulator #7 appears especially bright. When an insulator is defect to zero resistance, as Test 2 shown in Table 2, the insulator #7 has resistance of 1MW, and its temperature is much lower than those of #6 and #8, where occurs a sudden change of temperature. From the thermal image, we can see that the image of insulator #7 becomes especially dark, as shown in Fig. 3.

Table. 2 AC Simulation experiment at laboratory

No.	1	2	3	4	5	6	7	8
Resistance MWM	10000	10000	7500	10000	15000	10000	1	170
Temperature0° C	24.1	23.8	23.8	23.6	23.9	24.3	22.8	25.2


Field test of detecting defect porcelain insulators

Field test of detecting defect porcelain insulators
The test was conducted in NW China Power System. Using, the infrared detector AGEMA-871, the thermal image of defect insulators of 330KV were obtained and shown in Fig. 4 and Fig. 5 Fig. 4 shows the infrared images of two tensile insulator strings. Along the upper string, there is a defect insulator, which has insulation resistance of 3MW and temperature at 28.1°C. The insulation resistances of the insulators on both sides are 1500MW and 500MW, and their temperatures are 33.4°C and 32.1°C, respectively. The lower string is a normal string. The insulation resistance of each unit is large than 300 MW.

Fig. 5 shows the infrared image of a suspension insulation string on a tower. From Fig. 5 we can see that the insulator of 200MW at position 3 has highest. Temperature, at 39_10C, and the other insulator of 600MW at position 2 has temperature at 34.30C.

From above field test and the previous laboratory experiments we can conclude that the insulator with insulation resistance lower than 300MW has higher temperature, and that with insulation resistance close to zero has lower temperature by comparing with the temperature of normal insulator.

Simulation experiment under laboratory condition for DC insulators
A simulation experiment at laboratory for detecting DC defect insulators of 500 KV was conducted by using infrared imaging technique. Using different infrared detectors to detect many defect insulators of different insulation resistances, there appears not obvious thermal effect, so that it is impossible to identify which insulator is defect along the string.

A part of thermal image of DC insulators is shown in Fig. 6. The insulation resistance and temperature of the string are listed in Table 3, from which we can see that the temperature distribution along the string is comparatively homogeneous. It is difficult to identify the defect insulator. This is because of very weak leakage current and no dielectric polarization current under the DC voltage condition. therefore, the infrared imaging detection is not feasible for detecting DC defect insulators.

Table. 3 DC Simulation test at laboratory

No.	--	21	22	23	24	25	26	27	28
Resistance MW	--	50	8000	6000	7500	1.5	9000	8000	10000
Temperature 0°C	--	32.3	32.5	32.3	32.4	32.3	32.4	32.3	32.4

Conclusion

The AC defect porcelain insulators can be exactly identified by using infrared imaging technique. This avoids the deficiency of lower crimbing detection. Insulators with insulation resistance of 10-300MW have higher temperature and appear brighter thermal image than normal insulators. Those AC insulators with resistance less than 10MW have lower temperature and no obvious image.
The infrared imaging technique is not feasible for detection DC defect insulators.

References

El-Arabaty, A., A. Nosseir (1979), "Measurements of Temperature of High - Voltage Insulators Using Infrared Technique," 3nd International Symposium on HVE, Milan, 1979.