Galloping and ice shedding of overhead transmission lines is a serious problem that is receiving growing attention across the globe. Moreover, related insulator flashover makes icing an even great threat to power system reliability.
For example, during the winter of 2008/9, Southern China experienced the most serious snow and freezing weather in half a century. The series of severe winter events started on Jan. 10 when a large dome of cold air enveloped most of the country. The power system was severely affected, with 17 of 31 provinces and autonomous regions forced to endure reduced power supply. Hunan and Jiangxi were particularly hard hit, with about 37% of all 500 kV towers in these provinces toppled by icing or galloping. One city alone, Chenzhou, had no power or water for nearly two weeks while it was reported that Fuzhou lost power for about 3 weeks. According to statistics, the number of power lines that suffered outages reached 36,740, the number of transformer shutdowns was 2018 and the number of collapsed 110 kV to 500 kV towers was 8,381. The direct resulting economic loss was more than 20 billion RMB (circa US$ 3 billion).
This edited 2013 contribution by Prof. Jia Zhidong of the Graduate School at Shenzhen, Tsinghua University discussed various anti-icing and de-icing methods for insulators recently investigated in China. Particular reference is made to application of semi-conductive silicone rubber coatings on insulators.
Anti-Icing of Suspension Insulators
Among the mitigation methods used to improve icing performance of insulators in the field are increasing leakage distance using profiles with greater shed-to-shed distance or changing insulator orientation, which decreases risk of ice bridging. Both methods have been used with success not only for porcelain and glass insulators but also when designing lines with composite insulators. Ice accretion on insulators can drastically reduce the effectiveness of insulation, leading to flashovers and outages. The irregular shape of insulators makes it difficult to develop devices for automatic de-icing so efforts to eliminate ice on insulator strings have focused on passive methods such as modification of surface characteristics. For example, use of hydrophobic materials on insulators can reduce adhesion between ice and substrate surface (although it cannot completely prevent ice formation) and ice will tend to slide off due to gravity. An alternative approach is development of ‘super-hydrophobic’ coatings or surfaces, where the contact angle of water is greater than 150°. In this case, water drops will easily roll or slide off the surface and are unlikely to stay long enough to become frozen or to adhere to the surface if they do freeze. Much effort has therefore been expended on improving hydrophobicity of surfaces.
Conventionally, there are two approaches to produce such super-hydrophobic surfaces: one is to create a rough surface on top of one which is already hydrophobic (i.e. contact angle > 90°) and the other is to modify the surface of materials already having low values of surface free energy. Yet another approach is to develop semi-conductive coatings, as described in the following sections.
Anti-Icing on Insulators Using Semi-Conductive Coating
Thermal methods have been considered to be an effective measure for de-icing and the idea was firstly tested in order to generate de-icing on conductors or ground wires. Heating conductors by the Joule heat generated by electric current has proven to be a practical method for preventing ice accretion. Ice coverage will also be significantly reduced when the surface temperature of the insulator maintains above 0°C.
In 2007, a kind of semi-conducting silicone rubber coating, developed by stirring conductive fillers into normal RTV, was successfully applied to insulators. Hydrophobicity and hydrophobicity transfer of the ice-covered surface prevent the formation of glaze and decrease the adhesion force between the ice layer and the material surface (Fig. 2a). Joule heating generated by the semi-conductive coating delays formation of ice. Conductive filler particles such as carbon black and carbon fibers were mixed into the RTV silicone rubber to reduce the electrical resistance of the rubber and make the material semi-conductive.
The semi-conductive silicone rubber was applied to the bottom side of the insulator to configure the surface resistance of the insulator in order to eliminate power loss and accelerated aging due to continuous thermal consumption when there is no precipitation. In this case, a conductive path forms when the top-side of the insulator is wetted by moisture or precipitation under certain conditions. Leakage current and electric discharge occurring between water droplets can generate enough heat to delay ice accretion. With this method, good anti-icing performance of test insulators was obtained under certain laboratory conditions (see Fig. 2b). However, effective anti-icing behavior depends on the heating generated by leakage current across the insulator surface. Apart from atmospheric conditions, coating resistance and coated area covering the insulator surface are the two important factors and can both be adequately controlled. Therefore, optimization of the coating applied onto the insulator was carried out starting 2011 (see Fig. 2c) and a series of icing tests were carried out aimed at finding the optimal design for preventing ice accretion on insulators.
Numerical Design of Semi-Conductive Coating on Insulators
Basic Process & Simplified Model
There are three stages during the icing performance of insulator strings whose bottom has been coated with a semi-conductive silicone rubber coating. When the whole insulator maintained a surface temperature higher than freezing, the insulator basically reached thermal equilibrium. During this thermal equilibrium stage, a liquid layer covers the non-coated area, as shown in the schematic in Fig. 2(a). The function of the water film is like a heat source, which conducts the heat toward the water film and the porcelain part of the insulator, and heat transfer with the air. The droplets captured by the insulator are heated to be above the freezing temperature, and then most of the droplets join the original water film to be a new liquid layer. In the meantime, some parts of the original water film fall down from the edge of the insulator to maintain water film of some thickness.
Fig. 3: Anti-icing process on insulators and simplified model used in numerical design.
As a part of the circuit, the coating played an important role in raising the surface temperature. In essence, it was a thermodynamics problem and can be expressed by the energy conservation equation. The energy was mainly divided into two parts, the energy loss due to the heat conduction, convection, radiation, and the evaporation of the water, and the heating power generated by the applied voltage and the leakage current. Therefore, it is not hard to calculate the surface temperature by using energy equation. Normally, the temperature can be used to evaluate the icing status of the insulator surface. In particular, temperature of the upper surface is a direct symbol of whether there is or is not ice accretion.
From https://www.inmr.com/semi-conductive-coatings-limit-ice-accretion-insulators/
