Photo courtesy of STRI
Test set-up for resistance measurements over time.
Resistance measurement of polluted plates over time( averaged values). and the RTV coating both completely lost hydrophobicity by the end of the test, with the super-hydrophobic coating losing it in only a few days whereas the RTV coating lost hydrophobicity after about 500 h. Some 4 weeks after the test ended, measurements were repeated and both coatings were found to have recovered hydrophobicity. However, whereas the RTV coating had an almost complete recovery( WC 2 to 3) over the full conductor length, the super-hydrophobic conductor had only recovered over part of its length( i. e. 20 % of this conductor had a WC of 2 to 3, while the rest was completely hydrophilic with WC 7).
D. Visual Impact For many power companies these days, visual impact has become an important issue due to public objection to new lines. Visual impact of conductors and insulators( as well as other line components) depends on colour, shininess and size, the background( i. e. sky, land, forest, etc.), illumination and atmospheric conditions.
Due to this large number of parameters, it was decided to initially compare visibility through a simple comparison whereby samples of coated conductors were mounted outdoors on a rig. Differences in visibilities for the different conductor types was documented and compared through photographs taken under different conditions and then averaged base on a ranking supplied by 8 employees.
The RTV-coated, new and blasted conductors were evaluated as least visible. Based on photos, the dark conductors( i. e. the painted and the acid-treated) were assessed as most visible. The super-hydrophobic conductor was somewhere in between( although under intense sunlight the almost white conductor was judged most visible).
STRI was later involved in a similar project aiming to quantify visibility of insulators having different coatings and based on an observer looking at a transmission line with typical local backgrounds( e. g. forest and sky). Low visibility was also preferred here in order to minimize net visual impact. This method, known as automatic image analysis, has since been tested involving a number of images of an existing line obtained using web cameras.
E. Hydrophobicity Recovery The driving force for this part of the investigation was that similar material had passed the test at Koeberg Insulator Pollution Test Station in South Africa( considered to be a natural accelerated ageing chamber). However, the mechanism of hydrophobicity loss and recovery is not yet known for the new superhydrophobic material to be applied to conductors and insulators. For example, it could prove promising to also use this coating to increase pollution flashover performance, as is the case for RTV silicone. A test was therefore carried out to compare standard HTV silicone rubber and super-hydrophobic material, as described above for conductors but applied now to porcelain plates. The samples were tested over a period of 10 days and the program consisted of resistance measurements to check speed of hydrophobicity recovery. Rate of recovery was also checked after water immersion. The target pollution level was an ESDD of about of 0.15 mg / cm2. After initial trials during pre-testing, it was decided to apply the pollution layers to the samples by spraying. This resulted in a more even distribution of pollution across the insulator than dipping. The repeatability of application was very good( deviation was less than 5 % whereas 15-20 % is recommended by most pollution standards) and testing was performed using the simple test set-up for resistance measurements over time.
It is evident that there is recovery of hydrophobicity( measured as resistance of the wetted pollution layer) for both the HTV silicone rubber and the nanocoated super-hydrophobic samples. However, recovery of hydrophobicity of the super-hydrophobic samples appears to be much faster, although the exact mechanism is still not clear. At the same time, the superhydrophobic samples were far more hydrophobic than HTV silicone rubber after water immersion. Even though their hydrophobicity decreased partially after water immersion, they recovered it quicker than in the case of HTV samples.
Summary A number of tests for an initial( screening-type) evaluation of new types of super-hydrophobic coatings for conductors and insulators were identified. These included: ice tests in the laboratory and outdoors; audible noise testing; ageing tests; visual impact tests; and hydrophobicity recovery tests.
The findings provided positive indications in terms of the potential benefits to be realized and that would justify further investigation of such coatings. Further work, for example, could explore nano-coatings in regard to ice formation and adhesion properties and also ageing performance under typical field conditions. �
144