Xiaoxing Wei

Anti-icing performance of RTV coatings on porcelain insulators by controlling the leakage current

Considerable work has been done on preventing ice formation on insulators and the modification of the surface characteristics by increasing the contact angle and decreasing the adherent force have been tried with some degree of success. Heating by electric current has however proved to be an effective and practical method for de-icing transmission lines, but difficult to apply to insulators. A room temperature vulcanized (RTV) silicone rubber coating containing sufficient carbon black to render it partially conducting but not enough carbon to lose its surface hydrophobicity, has been investigated to determine if the heat generated would suffice to inhibit ice growth on the insulators. The heat exchange progress was analyzed and the leakage current through it to prevent ice forming was estimated and coatings developed accordingly. The anti-icing performances of these RTV silicone rubber coatings with different leakage current magnitudes were compared in a climate chamber. The results showed that the surface heating effect, together with the hydrophobicity, could significantly reduce the formation of ice on insulators.

Mechanism of salt migration in icicles during phase transition and its impact on ice flashover

This paper addresses the salt migration during phase transition of cooling water in the icicle growth period. This paper contains three parts, first, application of Ficks law in salt migration process; second, salt concentration measurement of icicles in artificial climate chamber; third, ice flashover experiments during melting period. Based on freezing model analysis, cooling water started to freeze from the water-insulator interface. Ficks diffusion law was applied to deduce the change of salt content in the liquid water. It was investigated that the melting water conductivity of icicle tips was proportional to the applied water conductivity. Freezing velocity (in mm/h) played important roles on the salt migration process during the growth of icicles and the melting water conductivity of an icicle tip under 2.6 mm/h was 1.66 times higher than under 3.6 mm/h. Special attention was paid to the ice flashover under non-bridged and bridged icicles. Icing experiments under energized condition showed that the conductivity of icicle tips increased by 70 percent when icicles grew to bridge the gap between adjacent sheds, which was a critical reason for the decline of the corresponding ice flashover voltage.

Study of anti-icing performance of insulator strings bottom-coated with semiconductive silicone rubber coating

Prevention of icing flashover on transmission lines has become an important problem. A new method for mitigating effects of glaze ice on energized insulators has been developed. Semiconductive silicone rubber was painted on the bottom of suspension disc insulators, while the upper ceramic surface was untreated. An electrically conductive path was formed when supercooled water or droplets bridged the uncoated areas. The power loss was limited when there was no precipitation. The heat and partial discharge energy generated by the leakage current melted the original ice accretion on the insulator surface. The electrical performance of the insulator string with this kind of coating under freezing rain conditions was comparable with that of a clean insulator string. The impact of the ambient temperature on the anti-icing performance was also evaluated. In addition, the performance of the insulator with bottom surface semiconductive coating under fog conditions was compared with an uncoated string.

Development of anti-icing coatings applied to insulators in China

Icing of transmission lines due to extreme weather conditions has caused many failures in power networks worldwide [1]. As China has much of its land 2,000 m above sea level, intense snow and ice storms at this altitude have caused significant losses to the economy, and the harsh conditions have made life very difficult in these regions [2]. The winter of 2008/2009 was particularly harsh with freezing rain, which continued for more than three weeks in the central provinces, causing 218 ice flashovers. In particular, Hunan and Jiangxi Provinces experienced the severest storms resulting in damage to 37% of the 500-kV power transmission towers, collapsing many towers. Considerable work has been done in China on understanding the reasons for, and preventing, the icing flashover of insulators [3]-[10], but in this article the various anti-icing and de-icing methods for insulators currently being investigated in China are discussed with particular reference to the application of semiconductive silicone rubber coatings on insulators.

Effect of the Parameters of the Semiconductive Coating on the Anti-Icing Performance of the Insulators

Partial coating of insulators with semiconductive silicone rubber has proved to be a potential method for reducing ice accretion on insulators. The performance of the coated insulator strings is mainly determined by the insulator parameters, the coating resistance, and the coating area covering the insulator surface. This research is to investigate the effect of using semiconductive silicone rubber coating on the anti-icing performance of ceramic insulators, considering the coating resistance and the coating area. A series of icing tests was carried out in a climate room and the results obtained from tests on a single insulator unit were extended to short insulator strings. The experimental results showed that the magnitude of the appropriate coating resistance was below 0.3 MΩ in the freezing condition recommended, and that the suitable width of the band of the uncoated area was about 5 to 8 cm. Since the main heating power was consumed in the region of the uncoated band, the uncoated area should better be close to the edge of the top surface of the insulator, in which case the icicles could fall off more easily.

Thermal and voltage distribution along insulator strings with semiconducting RTV coatings

Icing problems led a great damage to the power network over the world recent years and a lot of ways dealing with this problem are proposed. Semiconducting RTV coating applied to insulators was first put forward in 2007 and it makes a significant role in defeating the ice accretion on insulator surface. Its main advantage in anti-icing are combination of hydrophobicity and hydrophobicity transfer based on the ordinary RTV and the Joule heating effect created by the leakage current. This paper focus on the distribution of voltage and the heat effect of the semiconducting RTV applied to suspension insulator during freezing condition. The thermal distribution along the insulator string with semiconducting RTV coating applied with service voltage in cold condition is investigated using an infrared FLIR. And the distribution of the 7-units 110kV insulator strings is calculated by using finite element analysis. Results showed that the range of the resistance of the semiconducting RTV applied on insulator as a coating is about 106∼107 Ω can get a suitable temperature rise during the cold condition and the voltage distribution is uniform with semiconducting RTV.

Performance of insulators treated with semiconductive silicone rubber coating during fog and polluted conditions

Ice accumulation on insulator surface imposes serious problems on power systems. One approach is to develop a hydrophobic coating with semiconducting characteristic resulting in heat generation, which is only coated on the bottom side. This paper focuses on the performance of the insulators treated with semiconductive coating during fog and pollution conditions. Three kinds of samples were tested, including insulator without any coating, insulator coated with normal RTV coatings and the one with semiconductive coatings. The artificial pollution tests were carried out in a fog chamber. The leakage current and the flashover voltage during the fog and polluted conditions were investigated. FEM calculation was also used to help understand the anti-contaminate performance. Test results indicate that the pollution flashover voltage of the semiconductive ones is almost twice of the pure ones, but a little lower than the RTV ones. The primary causes of high pollution flashover voltage are the joint action of the hydrophobicity of the coating and the drying effect generated by the leakage current.