Abstract: the overhead high voltage transmission line is the artery of power system, and its operation state directly determines the safety and benefit of power system. In practice, the high voltage overhead line break accidents caused by overload and aging of equipment also occur. Infrared detection has the characteristics of long-distance, no power cut, no contact and no disintegration, which provides an advanced means for power system line status monitoring. However, there are few detection experience on the line and there is no corresponding standard. Therefore, this paper presents a method of temperature rise in the alarm range when infrared thermoimager is used to detect the line [ one ] And relative temperature rise method [ two ] The shortcomings of the analysis. Combined with the actual situation, the absolute temperature difference discrimination method is proposed.
Key words: the method of alarm boundary temperature rise, relative temperature rise method, absolute temperature difference discrimination method
1. Infrared detection and discrimination method for high voltage line defects
writing [ one ] The paper proposes that the temperature rise of the relative ambient temperature of the generating hot spot is used to judge the thermal defects, and the alarm limit temperature rise meter for the overheated connection of different wires under different load current is given. When the temperature rise of the detected point to the ambient temperature is greater than the alarm temperature rise specified in the table, it is considered to be defective, and the type of defect is determined according to the alarm temperature rise in the table. This method is simple, intuitive and practical, But in the infrared detection of the line, the following shortcomings exist:
one ) For the overhead high voltage transmission line, due to the limitation of conditions, it is impossible to accurately measure the ambient temperature, humidity, wind speed and detection distance around the line. Generally, the ground environment temperature, humidity and wind speed are used as the environmental parameters of the line to estimate the detection distance. Therefore, the temperature rise of the relative ambient temperature of the hot spot measured is incorrect, which will inevitably lead to the error of thermal defect judgment ;
two ) For high voltage DC and AC lines, even if the same materials and the same environmental conditions, due to skin effect and proximity effect, the heating of AC lines under the same load current condition should be more serious than that of DC [ one ] It is limited to only regulate the temperature rise of the alarm boundary according to the conductor type and load current ;
three ) The heating characteristics of different equipment and materials are different, and the allowable temperature rise under different conditions shall be different. For example, when there is solar radiation, a certain temperature rise will be added to the tested object. At this time, the temperature rise of the alarm boundary should be obviously different from that without solar radiation. Obviously, it is not convenient and accurate to simply use this method to analyze the thermal defects.
The application guide of infrared diagnosis technology for live equipment puts forward a relative temperature rise judgment method for thermal fault identification of current induced thermal equipment. This method analyzes the relationship between relative temperature difference and contact resistance, and relies on the standard of electric power industry "preventive test procedure for electric equipment" ( DL / T596 ) The relative temperature rise criterion for analyzing the thermal defects of current induced heat equipment is determined according to the provisions of contact resistance. This method determines the judgment method from the internal causes of heating, overcomes the influence of some environmental factors and load current on the measurement results, and has guiding effect on the infrared diagnosis of power equipment. However, for the infrared detection of the line, the criterion is insufficient
one ) At present, there is no national standard for the quantitative contact resistance of the line fittings in operation. The standard of electric power industry, the preventive test code for power equipment, which is the basis of the guideline, is not specified, so it is difficult to determine the relative temperature difference judgment standard ;
two ) This guide and document [ two ] The relative temperature rise judgment standard of all other diversion equipment other than circuit breaker and switch is the same as that of isolation switch, while the contact resistance requirements of disconnector are different from those of line fittings ;
three ) The relative temperature rise method can not consider the additional temperature rise caused by solar radiation ;
four ) The guide also points out the actual temperature rise of the hot spot < At 10K, the deviation between relative temperature difference and relative resistance will be more dispersed, which will affect the accurate judgment of equipment defects, and this type of thermal defects accounts for a large proportion of the line ;
five ) The guideline considers that if the load rate is close to 100 % , it can refer to the heating of AC high voltage electrical appliances in long-term operation ( GB763—90 ) To execute.
For the heating of high voltage overhead transmission wires, the heating of AC high voltage electrical appliances in long-term operation ( GB763—90 ) The maximum allowable working temperature of steel core aluminum strand is required to be + 70 ℃, according to general technical conditions for power fittings ( GB2314—85 ) The electrical contact performance of power fittings shall meet the following requirements:
one ) The resistance between two ends of the conductor joint shall not be greater than the resistance of the same length of conductor ;
two ) The temperature rise at the connection of conductor shall not be greater than that of the connected conductor ;
three ) The current carrying capacity of all fittings bearing electrical load shall not be less than that of the installed conductor.
According to the above rules, it can be recognized that under normal load operation, the temperature at the junction pipe, tension clamp, adjustment plate, and connecting plate of the second line shall be the same or smaller than the conductor of the transmission line. Therefore, the temperature of the normal running conductor near the tested object can be taken as the reference temperature, i.e. for the places with thermal defects, The temperature of wire or wire fittings can be taken as reference temperature 5m away from the hot spot.
At this time, absolute temperature difference method can be used to judge: take the maximum temperature of conductor or wire fittings running normally 5m away from the object under test as reference temperature Ta, and the temperature of the measured object is t, Δ T = T - TA, according to Δ T is used to judge the thermal defect, which can eliminate the effect of additional temperature rise caused by solar radiation. At the same time, due to the homogeneity, the error caused by the inaccuracy of the parameters such as the detection distance, ambient temperature, humidity and wind speed is reduced.
In terms of the criteria for the judgment of defects, it is considered that:
one ) For infrared thermoimagers, there are some measurement errors, such as the advanced thv570 of SATH and PM280 on the other side, and their accuracy is ± two %( Reading range ) or ± 2℃ ;
two ) For the lines that have been running for a long time, due to the damage to the conductor during crimping, corrosion and wear in the long-term operation, for the line fittings in operation, such as connecting pipes, we think that there should be a permitted temperature rise range, rather than strictly complying with the standard of gb2314-85, That is, the temperature at the wire fittings is not necessarily lower or equal than the temperature of the surrounding normal running conductor ;
three ) Because of the measurement error and the randomness of the reference point selection, the temperature of the selected reference point has a certain range of variation. From the theoretical analysis and the actual observation results, the temperature range is shown < 2℃ ;
four ) Because of the difference of the actual environment conditions, any infrared thermal imager can not give a general correction scheme which is consistent with the actual situation of the detection site. In addition, the environmental parameters of the temperature correction required by the infrared thermoimager are error, so the measurement results have some errors.
In summary, the analysis is made at full load with reference to relevant national standards and domestic and international experience ( When it is not rated load, the general infrared thermograph can be converted to rated load ) , for the high voltage line heating discrimination Δ It is considered that there is a slight contact hazard when t is below 10 ℃ ( General thermal defects ) ， Δ T is a major defect if it exceeds 10 ℃ but not more than 30 ℃, Δ T is an emergency defect when it exceeds 30 ℃. Due to the errors in measurement and other aspects, Δ T judge the limit value of defects is not absolute, and it should be analyzed according to the measurement. According to the above criteria, the thermal defects are judged in the actual test.
3 conclusions and suggestions
It is a more suitable and reasonable method to distinguish the thermal defects of overhead transmission lines by absolute temperature difference method.