Infrared thermography (IR)

With Infrared thermography IR, the temperature distribution along the insulator axis is measured by means of an infrared camera, searching for hot spots associated with possible local defects.

Infrared thermography (IR):
A thermal emission is associated with local heating caused by a current flowing along a defective part of the insulator, characterized by relatively high conductivity in comparison to the intact insulating material. An example would be in the presence of tracking or other semi-conductive paths as shown in Fig. 4.

Fig. 4: Examples of internal defects investigated (left: tracking mark) and in (right: channel with low conductive moisture)

With IR, the temperature distribution along the insulator axis is measured by means of an infrared camera, searching for hot spots associated with possible local defects. Compact 5 cameras with high sensitivities and excellent performance were developed which currently permit a fast and reliable inspection of the insulators. An example of defect detection by IR is shown in Fig. 5. Guidelines for IR inspection from a helicopter are available. The method is particularly sensitive to defects developing between the housing and the core, leading possibly to a flashunder. In this case the fault current is passing through the defective zone causing a significant temperature increase. The phenomenon is particularly evident when the tracking affects large parts of the insulator or when a sufficient conductivity is present all along the insulator. This could be because of humidity ingress in the interface or because of wetting of the part of the insulator not yet damaged. On the contrary, temperature measurements by means of an infrared camera are not suitable to detect conductive or semi-conductive defects developing on only a small section of the insulator with the remaining part sound and characterized by high resistivity, especially if the measurements are made in a low humidity condition (with the insulator dry). In this case, corona may instead occur on the tip of the defect, with a very low current associated with it, thus leading to very limited temperature increase, hardly detectable in service. Corona measurements are thus an important tool as well, as discussed in the following chapter.

Fig. 5: Examples of clear IR detection of internal conductive defects (from left-to-right: in laboratory; at test station; in service)


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