7.3 Insulation Systems

The insulation used between turns and between windings in a transformer is chosen to satisfy the service conditions of that transformer. Insulation life is related to the operating temperature, and standard temperature classes of insulation have been established for this purpose (see Table 7.2). At least two temperatures must be considered in insulation systems. There is an overall average temperature that can be determined by resistance rise of the conductors, and there are hot spots caused by uneven heat dissipation in the coils. Ratings for some standard insulation classes are shown as defined in ANSI C57 standards. All are based on a 40 C maximum ambient and a 24-hour average ambient of 30 C for distribution and power transformers.

Class 105 insulation is almost universal for transformers under 1 kVA and for oil-filled units. Insulation materials include kraft paper, cotton tape, enameled or Formvar wire, wood, pressboard in oil filled units, and similar low-temperature organics. Small oil- filled transformers are sometimes used for high-voltage service or other applications where the insulating qualities of oil are required. Transformers with the higher temperature insulation classes are usually smaller and lower in cost but have higher copper losses. However, the smaller frame of...