Method and Diagnostic Basis of Infrared Thermal Imager Applied to Transformer Bushing Detection

The transformer bushing shows abnormal temperature due to internal damage, abnormal joints, leakage of insulators, etc. The application of infrared thermal imaging camera to the detection of transformer bushing can timely find out whether the bushing is in the working state of wolf howling, find problems in time, improve work efficiency and avoid accidents.

The transformer bushing is abnormally hot, the main reasons are as follows:

1. The general cap of the casing is in poor contact with the external wiring board or the internal conductive rod

2. Lack of oil inside the oil-filled casing

3. Defects inside the casing lead to heat generation

Characterization of transformer bushing thermal defects:

1. The most common heating of transformer bushing is caused by poor contact between the general cap of the bushing and the external wiring board or the internal conductive rod. In this case, a faulty defect, like the general cap at the top of the casing, has a higher surface temperature than the other two. (As shown in Figure 3 of the casing in the following figure)

Casing Figure 3

Casing Figure 4

The thermal image similar to the above will also appear when the three-phase load of the load transformer is unbalanced in the infrared thermal imaging application. Therefore, the maintenance personnel should pay attention to analyzing the reasons for this result when measuring the infrared thermal imaging camera.

Casing Figure 5

Casing Figure 6

2. The lack of oil inside the oil-filled bushing of the transformer can also be observed through the application image of the infrared thermal imager. Due to the difference in specific heat capacity between transformer oil and air, the speed of heat absorption and heat dissipation of these two media is different. Using an infrared thermal imager to observe the outer wall of the oil-filled bushing, a temperature boundary line can be seen - This line is the oil level line in the casing (as shown in Figure 4 of the casing above)

3. If there is a defect inside the casing, it may also cause abnormal heating of the casing. In this case, the overall temperature of the faulty bushing is generally higher than the normal two-phase (the infrared thermal imager is applied to the bushing in Figure 5 below)

4. If the internal or external joints of the casing have poor contact, or the contacts are oxidized and corroded, it may also cause abnormal temperature at the contact point of the casing. In this case, the faulty bushing contact point will show that the temperature of the contact is significantly higher than other normal points or lines or bushings (see Figure 6 for bushings below)

Application and detection basis of infrared thermal imager for thermal defect of transformer bushing

According to the judgment of DL/T 664-2008 9.1 current heating type equipment: poor contact between the casing general cap and the external hard plate or the internal conductive rod is a current heating fault, and the judgment is based on the following;

The situation of defects inside the casing is more complicated: it may be caused by a voltage-induced thermal fault, or it may be caused by the casing machine. The following is the basis for the diagnosis of bushing voltage induced heating defects in thermal imaging camera applications:

However, the outer wall of the casing is a layer of insulating ceramics with crossed thermal conductivity. By measuring the surface temperature, it is difficult for us to obtain the true temperature inside the device. In general, we recommend using a three-phase comparison method to measure whether the casing is abnormally hot. For the case where the casing is short of oil, it is a voltage-induced thermal fault. This kind of failure is a big hidden danger for the safety production of the power sector. In the actual operation, the inspectors should pay attention to observation to avoid major losses. The following are the diagnostic criteria for casing oil shortage:

Suggestions for Treatment of Thermal Defects in Transformer Bushings

1. If the abnormal heat is caused by poor contact between the general cap and the external wiring board or the internal conductive rod. It is recommended to measure the DC resistance of the bushing in the event of a power outage. In general, the DC resistance of one phase that is abnormally hot is higher than the other two. This proves that the high-voltage bushing has poor contact. The part causing the failure should be replaced.

2. If it is an internal defect of the casing, it may be caused by the damping of the casing or the mechanical damage of the internal parts of the casing. It is recommended to test the transformer oil inside the bushing to analyze the cause of the defect.

3. If the bushing is short of oil, it is recommended to first determine the location of the oil leak, repair or replace the oil leaked location in the event of a power outage, and add transformer oil to the bushing.

How to use thermal imaging camera for transformer bushing inspection?

The casing is often directly installed outdoors (some are also installed indoors), and there are many factors that interfere with the external environment. We recommend:

(1) When the infrared thermal imager is applied and tested, it is necessary to avoid measuring direct sunlight as much as possible, especially to avoid shooting at noon.

(2) If the heat map is not clear in the automatic mode coefficient infrared thermal imager, you can use the automatic mode to measure the temperature range of the oil pillow; then manually set the level and span, set the temperature range to the minimum, and include the previously measured temperature range. Temperature range (minimum temperature range is different for each model).

(3) There are generally three-phase bushings on site, and the working conditions are similar. The temperature of the same position of different phases can be compared, so that the fault of the bushing heating can be quickly found.