Research On Anti-icing Dc Composite Heating Material

The unbalanced distribution of energy in China determines the necessity of high-voltage long-distance transmission.The transmission line inevitably passes through complex terrain,landform and extreme climate areas,which makes the transmission line suffer more serious ice and snow disasters.Composite insulators are widely used in HVDC transmission lines because of their light weight,high strength and good pollution resistance.However,in severe ice-covered areas,the electrical performance of DC composite insulators has declined significantly,so the anti-icing performance of DC composite insulators needs to be improved.At present,electrothermal,hydrophobic and ice-melting anti-icing coatings have been applied to composite insulators,but the corresponding disadvantages are also obvious.Whether it is a fully coated or semi-coated electric heating type of insulation,in the rainy days when the ambient temperature is higher than the freezing point,the rain will bridge the electric heating coating on the surface of the insulator to cause leakage current to the insulator.But the insulator is not covered with ice at this time,thus leading the waste of energy from the power system.The hydrophobic anti-icing coating has a certain anti-icing effect only at the initial stage of ice coating.When ice on the insulators gets heavy,there is almost no anti-icing effect.The ice-melting coating generates ions dissolved in water by self-decomposition to lower the freezing point,and also consumes materials in rainy days,so it needs frequent maintenance.In view of the above situation,this paper proposes a method for improving the thermal insulation performance of silicone rubber at low temperature by using phase change materials,thereby delaying the generation of ice coating.In view of the shortage of electrothermal materials,this paper proposes the idea of using positive temperature coefficient material to control the volume resistivity of the coating by temperature,and then control the leakage current of the insulator.When the temperature of the positive temperature coefficient material is higher than the freezing point,its volume resistivity is lower.When the temperature is lower than the freezing point,its volume resistivity is higher,and the temperature change affects the leakage current change.In this paper,two kinds of heating materials were studied,and then the two materials were respectively combined with silicone rubber.Electrical properties and anti-icing properties of the composite silicone rubber were analyzed.The feasibility of using two heating materials for anti-icing of composite insulators was theoretically demonstrated and practical test.By analyzing the characteristics of phase change materials suitable for anti-icing,phase change materials suitable for anti-icing were selected.The phase change materials were microencapsulated and then it was mixed with silicone rubber by different percentage to make silicone rubber sheets.Volume resistivity,water repellency and self-cleaning property of silicone rubber sheets were detected.In view of the shortage of electrothermal materials,this paper analyzes the characteristics of materials suitable for anti-icing positive temperature coefficient materials,selecting the series of positive temperature coefficient materials suitable for anti-icing,studying the preparation method of positive temperature coefficient materials,analyzing the material characteristics of positive temperature coefficient.The positive temperature coefficient material and silicone rubber composite,the volume resistivity and water repellency of the composite silicone rubber were measured,and the effect of the positive temperature coefficient material on the volume resistivity of the silicone rubber was studied.The main work and conclusions of the paper are as follows:（1）The method of composite phase change material with silicone rubber to improve the thermal insulation performance of silicone rubber at low temperature was put forward.Phase change microcapsules were prepared by in situ interfacial polymerization using n-tetradecane as core material and silica as shell.The phase transition point and latent calorific value of phase change microcapsule powder during cooling stage are 2.3℃and130.8 kJ/kg.（2）The phase change microcapsule was compounded with silicone rubber and the silicone rubber sheet was made of the composite material.At low temperature,the thermal insulation effect of silicone rubber sheet increases with the increase of the proportion of phase change microcapsules in composites.When the mass fraction of phase change material in silicone rubber sheet reaches 13.0%,the composite silicone rubber sheet will have a constant temperature range above 0℃during the process of cooling to the ambient temperature.When the mass fraction of phase change microcapsules reaches 16.7%,the composite silicone rubber sheet can last 6 minutes longer than the ordinary silicone rubber sheet at above 0℃.Generally speaking,phase change materials improve the thermal insulation performance of silicone rubber at low temperature,while the degree of improvement is limited.There is still a long distance from anti-icing.（3）The idea of compounding positive temperature coefficient material with silicone rubber is put forward.The volume resistivity of positive temperature coefficient material is affected by temperature,and the volume resistivity of silicone rubber mixed with positive temperature coefficient material is affected by temperature.The feasibility of using insulators coated with positive temperature coefficient material to prevent ice is tested and verified.Using BaTiO₃ and TiO₂ as raw materials,15.0mol%of Sn⁴⁺was added to replace Ti which function as Curie point shifting agent.Y³⁺and Mn²⁺were used as the electron donor and acceptor respectively.Pre-firing and secondary sintering were carried out at 1150℃and 1260℃respectively to prepare Ba(Ti_0.850Sn_0.150)O₃ positive temperature coefficient material with a Curie temperature of 0℃.Its volume resistivity in the low temperature region and high temperature region is 10⁵Ω?m and 10⁷Ω?m.It is worth mentioning that its volume resistivity changes U-shaped with temperature.Using BaTiO₃ and TiO₂ as raw materials,32.4mol%of Sr⁴⁺was added to replace Ba which function as Curie point shifting agent.Y³⁺and Mn²⁺were used as the electron donor and acceptor respectively.Pre-firing and secondary sintering were carried out at 1150℃and1350℃respectively to prepare(Ba_0.676Sr_0.324)TiO₃ with a Curie temperature of 0℃.Its volume resistivity in the low temperature region and high temperature region is about 10²Ω?m and 10⁴Ω?m.（4）In this paper,composite coatings were prepared by mixing self-made positive temperature coefficient materials with silicone rubber.The volume resistivity of composite coatings was almost not affected by temperature change,and the volume resistivity of silicone rubber was not affected by temperature change.The composite coatings were coated on the surface of composite insulators,and the anti-icing test of composite insulators was carried out.The test results show that the composite coatings have no anti-icing effect.