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Reducing Ice Accretion Using ‘Super-Hydrophobic’Coatings on Conductors & Insulators


TESTING

Reducing Ice Accretion Using ‘Super-Hydrophobic’Coatings on Conductors & Insulators CIGRE Brochure 438 (2010) reviews systems to predict and monitor ice accretion on power networks and also discusses the different anti-icing and deicing methods currently available. These countermeasures are divided into both ‘active’ methods (such as mechanical, thermal or vibration) and ‘passive’ methods, which include specialized coatings for conductors that may be applicable for insulators as well. Current international work on these anti-icing coatings is presently concentrated within CIGRE WG B2.44 whose Convener is Masoud Farzaneh of Canada. Since power companies in Scandinavia often experience ice & snow issues affecting lines, they have undertaken their own research efforts relating to these types of coatings. This article, contributed by Igor Gutman of STRI, Miroslav Radojcic of Norway’s Statnett and Lillemor Carlshem of Sweden’s Svenska Kraftnät reports on recent investigations in this field.

Photos: INMR ©

Winter storm in eastern Canada in 1998 brought down hundreds of transmission and distribution structures, with ice accretions on conductors over 75 mm thick.

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at a significant icing rate. Moreover, this pattern continued even when the separated ‘islands’ of ice grew together. Chinese researchers apparently obtained similar results using their own super-hydrophobic conductors. Abnormal appearance of ice accretion on super-hydrophobic conductors: (at left) results from STRI test; (at right) taken from paper by Jiang Xingliang at IWAIS 2013 Plenary Session.

Based on this encouraging finding, a test program was organized to assess several desirable factors versus traditional conductors, including: • Lower ice accumulation • No significant increase in corona effects in the form of radio interference and audible noise • Low ageing • Comparable visually • Offering hydrophobicity recovery

Schematic of ice screening test.

Photos courtesy of STRI

Six different conductor & coating combinations were selected for the laboratory tests and the ‘original’ conductor samples were all Parrottype with 38.25 mm outer diameter. The six conductors tested were as follows:

Ice test performed in laboratory (left) and outdoors (right). In principle, preventing ice accretion on overhead lines is feasible using coatings that have low adhesion against ice. Indeed, experiments years ago in Canada demonstrated reduced ice adhesion compared with fully hydrophilic surfaces. For example, silicone-based materials could theoretically have 3 to 5 times lower adhesion to ice than do the bare metallic conductors. To test this, a BLX conductor prototype impregnated with 5% silicone was manufactured and tested at STRI about 10 years ago. Results, however, showed that there was no evidence that ‘standard’ hydrophobic materials result in lower ice adhesion than do those that are hydrophilic. At IWAIS (International Workshop on Atmospheric Icing of Structures) Conferences between 2009 and 2011, research groups presented results using super-hydrophobic materials (i.e. those characterized by a contact angle of about 150° and developed using nano-technology). Such materials apparently prevent

water drops from collecting on surfaces and thus impede ice forming on conductors and insulators. According to some reports, superhydrophobic coatings with small contact angle hysteresis were found to have from 5 to 6 times lower adhesion compared to reference samples made of polished aluminum. With the support of power companies in Scandinavia, STRI carried out an ice-screening test of a superhydrophobic coating during which very specific behavior of the coating was obtained. On hydrophilic conductors tested as reference, (i.e. new, blasted, painted and acid treated), exposure to water spray resulted in radial growth of ice that within a short time led to a smooth surface and circular cross-section. Consequently, ice growth rates were almost identical on these conductors. Behavior of the super-hydrophobic conductor, however, proved to be different in as much as ice grew from frozen droplets on the surfaces and these remained separate even

1. New, never used conductor taken from a drum 2. Conductor with hydrophobic RTV silicone coating 3. Conductor with super-hydrophobic coating 4. Blasted conductor whose surface had been treated to make it less shiny 5. New conductor with standard paint for metal 6. Conductor treated with acid to reduce visual impact Experiments on insulators with superhydrophobic coatings also took place. A. Ice Accumulation To perform a simple screening test to compare the ability of different conductors to withstand ice accretion, samples were fixed in a rotating drum. This principle was first developed in the laboratory (for glazed type of ice) and then outdoors (primarily for rime ice). Laboratory ice accretion tests were conducted inside a climate chamber where temperature was kept between -7°C and -4°C. Before testing, conductor samples were precooled to -7°C for a period of 24 h. Outdoor tests were then performed at about -10°C.

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In order to evaluate adhesion between ice and conductor under field conditions, a specialized tool was developed by STRI. This tool proved effective and was even later offered to CIGRE WG B2.44 to assist standardization of future field measurements. Indeed, it is felt important to standardize measurement of the impact of different remedial measures on ice coverage and to do this under outdoor conditions.

Utilization of hydrophobic nanoparticles would allow for very fast recovery of hydrophobicity but probably not allow penetration of hydrophobicity through pollution layers (as is usually the case with low molecular weight components in silicone rubber). One of the most important findings from this test was that the isolated frozen drops on the super-hydrophobic coating could easily be removed just by touching, i.e. ice adhesion was very low at this early stage. On hydrophilic conductors, by contrast, ice builds

Tool developed by STRI for field comparison of ice adhesion.

Photos: INMR ©

Ice testing in the laboratory using a representation of glazed ice (i.e. transparent ice of high density) did not reveal any significant improvement in ice accretion performance as a result of the super-hydrophobic nano-coating. Similar amounts of ice, by weight, were accreted on all six conductors. However, the appearance of this ice on the two hydrophobic conductors (i.e. the RTV and the nano-coated) differed from the smooth ice found on other specimens. This difference was most pronounced on the superhydrophobic coating that continued to feature a ‘bumpy’ surface, even after 3 h of ice accretion. The reason for this could be strong polar properties of molecules at the surface of the coating that result in orientation of the water molecules, thereby affecting formation of ice, even at some distance.

Photo courtesy of STRI

A 1.4 kg weight was used, sliding on a rod, and this rod was placed on the conductor so that it rested under its own weight. This weight was then lowered onto the conductor, first from the first half of the rod’s length (15 cm) and then from its full length of 30 cm – respectively referred to as ‘half punch’ and ‘full punch’.

Facilitated ice shedding from wind would prove beneficial to reducing accretions on conductors. (Photo INMR) 141

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Set-up for AN test. up simultaneously on large portions of the wetted surface, resulting in strong adhesion. This fundamental difference might prove important for real applications where stresses due to wind and mechanical movement

of the conductor could play a major role in ice shedding. From the outdoor test, it was confirmed that, in the case of the super-hydrophobic conductor, a

Results of AN test. Acid treated conductor (lowest curve) is much shorter and should not be compared with others.

thinner layer of rime type of ice was observed compared to the others. No such difference, however, was seen from laboratory tests in which glaze ice was created and a possible explanation for this may be that, in the case of the super-hydrophobic conductor, initial ice accretion has a different character. B. Corona & Audible Noise Corona effects and in particular audible noise (AN) are important parameters when investigating superhydrophobic conductors. Common knowledge is that traditional hydrophobic conductors (i.e. newly greased or RTV-coated) usually have higher AN levels than service-aged or intentionally sand-blasted ones. In addition, proposals for special hydrophilic paints to reduce AN have been put forward and verified. Therefore, it was seen as especially important to also demonstrate this in the case of super-hydrophobic conductors.

Photos courtesy of STRI

AN measurements were performed under dry and wet (rain) conditions for different conductors, i.e. new, sandblasted, painted, acid-treated, RTV-coated & super-hydrophobic. The conductors were tensioned horizontally, one by one, 2 m above the floor inside STRI’s climate test hall.

Change of form of voltage of water drops on super-hydrophobic surface at different voltage gradients (observed during AN test under rain).

Noise was measured using a Norsonic Sound Analyzer equipped with a microphone placed 4 m away from the conductor and 1 m above the floor. Equipment was configured to record the spectrum from 10 Hz to 20 kHz at 1 min intervals. Rain was simulated by 10 nozzles mounted on a horizontal beam, hung parallel to the conductor and placed 2 m away horizontally and 5 m

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above the floor. This arrangement corresponded to an average rain intensity of 3.5 mm/h upon the conductor, i.e. corresponding to light summer rain but higher than IEC standard rain. Tests were performed under both dry and rain conditions at representative maximum voltage gradients of 12, 14, 16, 18 and 20 kVrms/cm, by adjusting test voltage. The test involved: measurement of dry background noise (reference spectrum), application and stepwise increase of voltage with noise measurement at each voltage level, application of rain and measurement of noise at each voltage level, and finally, switch off and measurement of wet (rain) background noise level.

Set-up for 1000 h salt fog tracking and erosion test.

For tests under rain, the performance of all types of conductors was found similar (excluding the short acid treated conductor, because of its different length). Influence of different background levels is taken out through subtraction. Excluding the short acid treated conductor, it is clear that the super-hydrophobic conductor performs similarly to the average conductor.

C. Low Ageing The goal here was to study long-term performance of conductors with a super-hydrophobic coating using the 1000 h salt fog test (IEC 62217), prescribed as a typical tracking and erosion test for composite insulators.

Photograph to rank visual impact of different conductors.

Photos courtesy of STRI

One possible explanation is the appearance of water drops on the energized conductor. Photos were taken in conjunction with AN measurements and it is clear that water was present only on the top surface (in the case of hydrophilic conductors water drops normally collect on the bottom surface). Moreover, the influence of electric field can be seen as drops extend in a radial direction (and even ‘explode’) as voltage increases from level E1 to E2. Of course, it would be desirable to repeat similar tests on a test span in the field under natural wetting conditions; however early indications are that, due to its super-hydrophobic properties, the coating might behave similarly to standard conductors (which is basically different to the comparison of normal hydrophobic / hydrophilic conductors referred to earlier).

Typical image of tower with three different insulators (coatings) and three different backgrounds used for automatic image analysis (taken at test site). A super-hydrophobic and an RTV coated conductor were tested in a moisture-sealed corrosion-proof chamber with 45 m³ volume. The two conductors were mounted using composite line insulators hanging from the roof and tested at elevated

voltage to ensure corona activity on the surfaces. From a degradation point of view, the two coatings passed the 1000 h test and remained properly adhered to conductors. The super-hydrophobic 143

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Photo courtesy of STRI

Test set-up for resistance measurements over time.

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.

Resistance measurement of polluted plates over time (averaged values).

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 Due to this large number of example, it could prove promising parameters, it was decided to initially to also use this coating to increase compare visibility through a simple pollution flashover performance, as comparison whereby samples of is the case for RTV silicone. A test coated conductors were mounted was therefore carried out to compare outdoors on a rig. Differences in standard HTV silicone rubber and visibilities for the different conductor super-hydrophobic material, as types was documented and compared described above for conductors but through photographs taken under applied now to porcelain plates. different conditions and then The samples were tested over a period averaged base on a ranking supplied of 10 days and the program consisted by 8 employees. of resistance measurements to check speed of hydrophobicity recovery. The RTV-coated, new and blasted Rate of recovery was also checked conductors were evaluated as least after water immersion. The target visible. Based on photos, the dark pollution level was an ESDD of about conductors (i.e. the painted and of 0.15 mg/cm2. After initial trials the acid-treated) were assessed as during pre-testing, it was decided most visible. The super-hydrophobic 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. ď ¸

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Reducing Ice Accretion Using ‘Super-Hydrophobic’Coatings on Conductors & Insulators