Development of High Thermal Conductivity Epoxy Composite for Large Current Switchgear

Since SF6 gas used as an insulating medium for switchgear is a greenhouse gas, its use needs to be reduced. Toshiba Corporation has developed solid insulated switchgear (SIS) in which the main circuit is molded with an epoxy composite. Since developing the 24 kV class SIS in 2002, a 72/84 kV class SIS has been launched. The high dielectric strength of the epoxy composites enables these solid insulation systems to provide compact equipment. However, it is necessary to consider the problem of heat radiation in SIS, as epoxy composite has low thermal conductivity. Therefore, development of high thermal conductivity epoxy composites has been desired for large current class SIS. In this study, we focus on magnesium oxide (MgO), which has a thermal conductivity having low-cost. The developed epoxy composite was filled with MgO having high thermal conductivity and SiO2 having a low coefficient of thermal expansion. By using the estimation equations of thermal conductivity and coefficient of thermal expansion, we could efficiently determine the formulation of the high thermal conductivity epoxy composite. The thermal conductivity of the developed material was 1.39 W/(m·K), which is equivalent to 2.5 times that of conventional material. In addition, other characteristics also showed excellent values, such as 23×10-6/K of the coefficient of thermal expansion and 2,513 mPa.s of viscosity. These values satisfied the required ones for large current SIS. Thermal analysis revealed that applying the high thermal conductivity epoxy composite to 2500 A SIS can reduce the temperature rise by about 10 K. By applying the high thermal conductivity epoxy composite, we are convinced that a large current of SIS with merits such as low maintenance and compactness will be realized.