Infrared detection and discrimination method for high voltage line defects
Presented in the article [1] with hair hot relative to the rise of temperature of the ambient temperature to determine thermal defects, and gives the different load power flow under the different wire connector overheating police temperature table, when the testing point of the rise of temperature of the ambient temperature is greater than the specified in the table of the temperature rise of the police thought that there are defects, and according to the police in the table determine the temperature rise of defect types, this method is simple, intuitive, practical strong, but in the line of infrared testing the following deficiencies:
1) for overhead transmission lines, due to the constraints, may not accurately measuring circuit around the environment temperature, humidity, wind speed and detection distance, generally USES the ground environment temperature, humidity, wind speed as a line of environmental parameters and estimate the detection distance, measured by such hot relative to the rise of temperature of the ambient temperature exists error, will bring hot defect judgment error;
2) For HVDC and AC lines, even if the same material, the same environmental conditions, due to skin effect and proximity effect, under the same load current, the heating of AC lines should be more serious than that of DC lines. However, it is limited to stipulate the temperature rise of the police boundary only according to the wire type and load current in the article [1].
3) heating characteristics of different equipment, different materials vary, under different conditions of temperature rise should be allowed each are not identical, such as solar radiation, the tested object attached to a certain temperature, then the police should clearly and without solar radiation temperature is different, obviously, simply use this method to analyze the thermal defects is not convenient and accurate.
"Electric equipment guidelines for the application of infrared diagnosis technology for all the current type equipment of hot fault with relative temperature rise judgment method is put forward, the method through the analysis of the relation of relative temperature difference and contact resistance, relying on the electric power industry standard" power equipment preventive test procedures "(DL/T596) of the provisions of the contact resistance, determine the analysis of the current criterion of relative temperature rise to equipment, hot defects.
This method determines the judgment method from the internal cause of heating, overcomes the influence of some environmental factors and load current on the measurement results, and provides guidance for the infrared diagnosis of power equipment. However, for the infrared detection of lines, this criterion has deficiencies:
1) At present, there is no national standard for the quantitative contact resistance of wire fittings in operation in China, and the power industry standard "Power Equipment Preventive Test Procedure" on which this guideline is based has no provisions, so it is difficult to determine the judgment standard of relative temperature difference;
2) The criterion for judging the relative temperature rise of all other diversion equipment other than circuit breakers and switches in this guide and article [2] is the same as that of disconnectors, and the contact resistance requirements of disconnectors are different from those of line fittings;
3) The relative temperature rise method cannot consider the additional temperature rise caused by solar radiation;
4) The guidelines also point out that when the actual temperature rise of the generating hotspot is less than 10K, the relative temperature difference and the relative resistance deviation will have a large dispersion, which will affect the accurate judgment of equipment defects, and this type of thermal defects account for a large proportion on the line;
5) the guide that if the load rate is close to 100%, you can refer to the "AC high voltage electrical appliances in the long-term work of the heat" (GB763-90) to implement.
For the heat of high-voltage overhead transmission wires, the "heat of AC high-voltage appliances in long-term work" (GB763-90) requires that the maximum allowable working temperature of steel-core aluminum strand is +70℃. According to the "General Technical Conditions of Power Metallurgics" (GB2314-85), the electrical contact performance of power metallurgics should meet the following requirements:
1) The resistance between the two ends of the wire junction should not be greater than the resistance of the same length wire;
2) The temperature rise at the wire junction should not be greater than the temperature rise of the connected wire;
3) The carrying capacity of all fittings bearing electrical loads shall not be less than that of the installed conductor.
According to the rules above, can be identified in the normal load case, running in the tube, strain clamp, adjustable plate, the temperature of the place such as second-line league board should be the same as the conductor of the transmission line or smaller than it, therefore, can get tested object near the normal operation of the temperature of the conductor as the reference temperature, namely for a place with thermal defects, can send hot 5 m away from the wires or the temperature of the line hardware reference temperature.
At this point can be absolute temperature difference method is used to determine: take the 5 m far near object normal operation of the highest temperature of the conductor or line hardware for reference temperature Ta, the temperature of the measured object for T, Δ T = T - Ta, judging by the Δ T thermal defects, this method can eliminate the influence of solar radiation caused by the additional temperature rise.
At the same time, due to the homogeneity, the error caused by the inaccuracy of detection distance, ambient temperature, humidity, wind speed and other parameters is also reduced.
In the judgment criteria of defects, the following considerations are taken into account:
1) for the infrared thermal imager, there is a certain measurement error, such as the very advanced THV570 and the other side of the PM280, their accuracy of ±2%(reading range) or ±2℃;
2) for line has been running for a long time, due to the pressure has damage to the conductor, in long run there is corrosion, wear and tear for the running line hardware such as in the tube, we think there should be a allowed temperature range, and don't have to be strictly in accordance with GB2314-85 standard, namely the line hardware temperature don't have to be lower than the normal operation of the temperature of the wire around or equivalent;
3) Due to the measurement error and the randomness of reference point selection, the selected reference point temperature has a certain range of variation. According to the theoretical analysis and practical observation results, this temperature range is less than 2℃;
4) As the actual environmental conditions vary greatly, any infrared thermal imager can not provide a universal correction scheme consistent with the actual situation of the detection site. In addition, the infrared thermal imager requires the input of environmental parameters to correct the measurement temperature, so the measurement results have a certain error.
In analysis, and refer to the relevant national standards and experience both at home and abroad, at full capacity (not rated load, average infrared thermal imager can be converted to the rated load), for high voltage line discriminant take Δ T for fever under 10 ℃ when can think there's a slight contact with hidden dangers (general thermal defects), Δ T more than 10 ℃ but not more than 30 ℃ for major defects, namely Δ T more than 30 ℃ is the defect of emergency.
The error of measurement and so on various aspects, Δ T judge defect boundary value is not absolute, according to the measurement case is particular analysis.
According to the above criteria, it is better to judge thermal defects in actual testing.
Conclusions and Suggestions
The absolute temperature difference method is a suitable and reasonable method to identify the thermal defects of overhead transmission lines.
reference
Wu Cunheng et al. Early diagnosis and prevention of conductor joint overheating.
East China Electric Power, 1995,23 (11) : 24
2 Hu Shizheng. Relative temperature difference judgment method and criteria for infrared diagnosis of electrical equipment.
Power Grid Technology, 1998,22 (10) : 10
Preventive test procedure for electrical equipment.
Power Industry Standard of the People's Republic of China.DL/T596-1996
4 Application Guidelines of Infrared Diagnostic Technology for Live Equipment
5 Code for Preventive Test of Electric Equipment (DL/T596)
6 "AC high voltage electrical appliances in the long-term work of the fever" (GB763-90)
7 General Technical Specifications for Electric Power Metallurgy (GB2314-85)