Capacitor tanks are commonly used in electrical power distribution systems to help maintain consistent line voltage levels. A capacitor tank consists of a metal casing that houses one or more capacitor windings and is filled with a dielectric fluid. The capacitor windings, which include a pair of metal foil electrodes separated by a layer of polypropylene film, are connected via taps to leads that pass through capacitor bushings and terminal caps to exit the capacitor body. The non-metallic capacitor bushings isolate electrical leads from each other and from the metal housing of the capacitor. The bushings also isolate the internal contents of the capacitor from the external environment and prevent the dielectric fluid from seeping out.
Key Parameters and Requirements
To ensure that the bushings provide the appropriate isolation and protection, they must be properly joined and sealed to both the terminal caps (upper end of bushing) and the capacitor tank cover (lower end of bushing). Capacitor bushings are made of an insulating material such as ceramic, glass, glazed material, epoxy, or other polymer. Durable metals, such as stainless steel, are used to fabricate capacitor tank covers, while other metallic materials, such as brass plated with tin, are used to construct terminal caps. The joints between these non-metallic bushings and the metallic components (tank cover and terminal caps) must meet performance, manufacturing, and cost requirements.
Because capacitor tanks are commonly found at the top of utility poles and in electrical substations throughout the world, they are exposed to a myriad of environmental conditions. The sealed joints between the capacitor bushings and the metallic components must be able to withstand a variety of environmental conditions such as high temperatures, substantial temperature fluctuations, prolonged exposure to sunlight, and exposure to wind, rain, snow, and ice. Furthermore, these joints must be capable of maintaining such high performance over a period of approximately 30 years.
Researchers at Cooper Technologies Company in Houston, Texas, ran performance tests on over a dozen commercially available epoxy resin products in an effort to identify compositions that may be used to seal capacitor tanks.1 First, the lap shear strength was determined for each product, both at a high temperature (75°C–90°C) and at room temperature (25°C). With a lap shear strength of 3782 psi at high temperature and 3006.92 psi at room temperature, Master Bond Supreme 10HT epoxy was cited as one of only four products that exhibited sufficient strength for the application.
The researchers also assessed the compatibility of Supreme 10HT epoxy with dielectric fluid. The team exposed cured seals to the commercially available aromatic dielectric fluid, Edisol VI, via a test apparatus, heating the test apparatus to 75°C for two weeks. They measured several properties of the dielectric fluid, including breakdown voltage, dissipation factor, DC current leakage, surface tension, acidity, and visual appearance, both before and after the heating period. Results indicated that the Supreme 10HT seals did not adversely impact the dielectric. In fact, the dielectric properties measured before the two-week heating process met typical target values and changed very little after the heating process.
The researchers concluded that Master Bond Supreme 10HT is capable of providing a seal between metallic and non-metallic components of a capacitor tank that can withstand the extreme stresses and environmental conditions required.