According to a DayCor® User
“Each has its advantages and disadvantages yet the multi-spectral method combines the 3 effectively to improve the detection accuracy”
A 660KV DC transmission line connects 2 provinces in China Yinchuan and Jiaodong spanning 1335 km, including 3.3 km over the Yellow River. Composite insulators were selected for the line due to their hydrophobicity, dimensions, weight, ease of installation and handling. With composite insulators, for DC in particular, it is necessary to perform inspections to ensure that: insulation material is not degraded; hydrophobicity is kept; both mechanical and electrical strengths are good; seal is intact and no abnormal discharges exist. Acknowledging the advantages of detecting damages in their early stages the State Grid Shandong electric power company with the Stat grid electric power research institute conducted a study that lead them to creating a maintenance operation guide.
Out of the existing detection technologies the researchers depicted UV imaging, IR and visible, claiming that when the three are combined they can provide a better view of the condition of the insulators. UV camera depicts surface discharges that indicate existing contamination and imperfections and lead to leakage current, tracking and degradation of the insulator’s sheath and brittle fracture of the rod; IR reveals dielectric loss, water ingress that lead to brittle fracture; visible imaging discloses the viewable failures .
To benefit from concurrent use of the 3 detection methods there is a need to perform the testing under conditions that match all three. As such, temperature, relative humidity, weather, visibility, electromagnetic interferences, and background radiation – were considered resulting in the decision to run the test under wind speed of 0.5m/s or less, humidity of 85% or less, no electromagnetic interference, background radiation balanced and 2 hours after sunset.
The combined inspection resulted in the decision to treat immediately composite insulators with serious corona discharge or temperatures that exceed those prescribed. It was further decided to replace all insulators with fractures, pitting deeper than 30% , creepage distance that exceed 10% of the insulator’s creepage distance, seal failure, carbonized sheath due to flashover.
Interestingly, all 3 detection methods indicated that the surface of a composite insulator has many discharge points. In this case, all 3 methods confirmed the same abnormal finding and the surface visible imaging of the spotted areas showed sheath pitting. In DC systems, versus AC, corona partial discharge emission is longer and stable and therefore the outcome damage is greater and heat is produced that can be detected by an IR imager. Another feature of DC systems is the higher rate of contamination and deposition of electrostatic dust, higher than in AC voltage, which is attributed to the unidirectional movement of charged particles on the surface of insulators.
Consequently it was concluded that visible inspection with binoculars, infrared and ultraviolet methods have each advantages and disadvantages but the multi-spectral method combines the three effectively and can verify findings in case where findings overlap, improve the detection accuracy of composite insulator, and provide a reliable comprehensive diagnostics of the actual state of the tested insulator.