Ultra-violet detection (UV)

Ultra-violet detection (UV) method was not able to detect the defects when fully covered by the housing, removing a small part of the housing and thus uncovering the conductive end of the conductive defects.

The possibility of localizing initial corona activity constitutes an interesting technical challenge, especially in daylight conditions, thus different techniques are available for day and night measurements. For daylight corona cameras, the diagnostic indicator considered is the emission generated by the defects in the UV-C range (i.e. with wavelength in the range 240-280 nm), a bandwidth in which the solar light is filtered by the atmosphere. Corona emission intensity is calculated using the number of pulses of light emission (named “blobs” - Fig. 6). A counter gives a number proportional to the quantity of “blobs” received by the sensor.

Fig. 6: Example of “blob” counting by a daylight corona camera (245 kV tension insulators)

Very sensitive portable cameras are now available, and some of them are combined “multi-cameras”, which can also provide IR and visual observations with one instrument. The method is particularly efficient in detecting conductive/semi-conductive defects only developing on a section of the insulator, with the remaining part of the insulator in good condition and characterized by a very high resistivity, especially in dry conditions and low humidity.

Under these conditions, the IR measurements indicated a very low temperature increase (in the order of 1-2 degrees) [19]. The sensitivity of the method was investigated by simulating conductive defects (using metal wires located on the insulator surface) at the live side, ground side and at a floating potential (in the middle of the insulator) [19]. The results are summarized in Fig. 7, where the minimum detectable defect length (in % of the insulator length) is presented for different insulator ratings. The UV camera was generally capable of detecting all conductive defects longer than about 20-30% of the insulator length.
Similar results were obtained by substituting the metallic wire with a semi-conductive tape to reproduce typical tracking values [19].

Fig. 7: Minimum conductive defect length detectable by UV for different defect positions and for insulators of different ratings Un

The question remained about the capability of the method to detect defects located under the insulator surface, possibly leading to insulator flashunder. While the UV method was not able to detect the defects when fully covered by the housing, removing a small part of the housing and thus uncovering the conductive end of the conductive defects (usual condition in case of severe defects, the sensitivity of the method was seen again and very similar to that obtained with defects occurring on the surface.

In general, IR thermography and UV measurements principally detect different physical properties (heat and enhancement of the electric field in the form of corona respectively), thus a combination of these two methods/cameras or use of a multi-camera would be the optimum solution for the remote inspection of composite insulators, especially when a certain failure tendency is known for the age or vintage of the insulators in question.

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