Long term performance of composite insulators: Ageing status determined by diagnostic methods

摘要

The electrical characteristics of composite insulators under operational service conditions can be reflected by the electrical tests shown in table 5. The surface characteristics of composite insulators may have an impact on the electrical characteristics. In this way the surface characteristics may give the background of an drop of electrical characteristics after service. In this way in figure 2 the surface characteristics are arranged around the electrical characteristics. Of course, there may be relations between the surface characteristics such as between pollutants and the properties of weather shed material. Excellent hydrophobicity may conceal effects of strong surface roughness and high pollution level. In turn, a weak hydrophobicity does not mean necessarily a high leakage current what was experienced with a 123 kV silicone composite insulator after 20 years of service. It shows class 5 to 7 after service. This insulator was installed in an outdoor test field for 1,5 years subsequently, however, the leakage current lay in the same level as found for much younger insulators. In another outdoor test field silicone composite insulators showed a hydrophobicity of class 7, however leakage currents of only one tenth of conventional insulators of comparable leakage distances. The electrical characteristics of silicone composite insulators after long term service have been checked for 25 years with results showing that such insulators can withstand service stresses for decades. In comparison to that the evaluation of surface characteristics as introduced in this report is a relatively new approach to determine ageing conditions of composite insulators. However, several data from pollution levels are available. For silicone composite insulators, generally the NSDD was found to be much greater than found for the SDD. Furthermore, the SDD showed much lower levels as expected from sites from where the ESDD is known . All found low SDD levels correspond with minor drops of the electrical characteristics found by laboratory tests after long term service. From this findings conclusion could be drawn that the ESDD known from distinct sites affects silicone composite insulators regarding their electrical characteristics less than known for conventional and other insulators. On the other hand the low SDD found on silicone composite insulators corresponds rather well with the only minor influenced electrical characteristics of those insulators after long term service exposure. For some of the other surface characteristics maxima and minima cannot be excluded. Thus, it is most likely that further evaluation will need decades in order to show which correlation exists between surface characteristics and electrical characteristics regarding long term performance of composite insulators after long term exposure under service stresses. Laboratory tests carried out on composite insulators after long term exposure can lead to a quantitative assessment of the ageing status of the insulators. Some of the test procedures described above are time consuming. Some need expensive test facilities. On the other hand, the test procedures are repeatable and reproducible and therefore suitable as tools to realise trends regarding the ageing status. Considering a life expectancy of composite insulators of several decades it would be most appropriate to carry out laboratory tests as described above on mature products from which life time expectancy is already known qualitatively. Trends regarding the ageing status can be followed only if test samples are available after decades of outdoor exposure.