Transformer Oil

a petroleum or synthetic oil used as an insulator and coolant in transformers and other oil-filled electrical equipment. Petroleum transformer oils are also used in oil circuit breakers for extinguishing an electric arc when the current is switched off.

Most transformer oils are petroleum oils. They are obtained by refining the appropriate petroleum distillates with the aid of selective solvents (phenol or furfural), sulfuric acid, or adsorbents or by hydrogenation. The process of extracting oils from raw material containing paraffin hydrocarbons includes a dewaxing stage.

Transformer oils should have a high dielectric strength, a high

Figure 1. Impedance transformers: (a) coaxial and (b) wave-guide quarter-wave types with fixed resistance, (c) adjustable coaxial double-stub, (d) adjustable coaxial with dielectric bushings, (e) wave-guide type with double tee junction; (1) and (2) adjustable plungers, (3) and (4) movable dielectric bushings, (5) H-arm, (6) transformer input, (7) E-arm, (8) transformer output, (D) diameter of outer conductor of coaxial line, (d₁), (d₂), and (d) diameters of inner conductor of coaxial line from the generator side, from the load side, and in the transformer portion, respectively, (b₁), (b₂), and (b) dimensions of smaller side of cross section of rectangular wave guide from the generator side, from the load side, and in the transformer portion, respectively, (l) distance between centers of the dielectric bushings, (λ) effective wavelength in the line, (ε) dielectric constant. Dotted rectangles denote the location of movable plungers in the E- and H-arms of the tee junction.

resistivity, a small tangent of the dielectric loss angle, good resistance to oxidation, low viscosity, and low volatility. Petroleum transformer oils have a viscosity of 6–10 × 10^–6 m²/sec at 50°C, a pour point not higher than –45°C, a flash point not lower than 135°C, a tangent of the dielectric loss angle not greater than 0.026–0.005 at 90°C, and a dielectric constant of 2.2–2.3; they should not contain water or any impurities. All grades of transformer oils produced in the USSR contain not less than 0.2 percent antioxidant additives, such as Ionol (2,6-di-tert-butyl-4-methylphenol).

The most widely used synthetic transformer oils are liquids based on chlorinated biphenyls and trichlorobenzene, for example, Geksol and Sovtols. Hydrocarbon, organosilicon, and organophosphorus synthetic oils are also used in certain types of special transformers.