Oil Insulation

Although, in electronic apparatus, there is a tendency toward the use of dry-type transformers, frequently voltages are so high that air clearances are impracticable and oil-filled containers must be used. In Fig. 42 the curves show rms breakdown voltage versus creepage distance under oil. An example will show the advantage of oil filling.

Fig. 42. Creepage curves of solid insulation under oil.

From Figs. 40 and 42 it will be seen that 10-in. creepage distance is required in air to withstand a 1-minute breakdown test of 60 kv on insulation 0.5 in. thick, whereas in oil only 2-in. creepage distance is required.

Curves of Fig. 42 are for pressboard or Micarta under oil. Some kinds of porcelain have less creepage strength than these materials. On the other hand, some grades of glass and polystyrene are much better and withstand 150 kv for 1 minute with 2 in. of creepage path.

In high-voltage low-current power supplies, these special materials are used to save weight and space. At 50 kv or more, sharp edges and points should be avoided by the use of round terminals, leads, and coils.

Only high grades of insulating oil are used for this purpose. Tests are run continually to check condition of the oil. Oil is stored in such a manner as to keep out moisture and dirt and avoid extremes of temperature. Where very high voltages are used, as in X-ray apparatus, oil filling is done under vacuum to remove air bubbles, and containers are sealed afterwards to prevent moisture from entering. Mica insulation is not used in oil because oil dissolves flexible bonds.

Often a high-voltage transformer can be integrated with some other component, such as a tube socket, capacitor, or another transformer. This is desirable from the standpoint of space conservation, provided that adequate clearances to the case are maintained. "Packaged" power supplies are sometimes made in this fashion to facilitate assembly and repair.