High-voltage bushings for use on transformers and breakers are made in several principal types, as follows:
A. Composite Bushing:
A bushing in which insulation consists of two or more coaxial layers of different insulating materials.
B. Compound-Filled Bushing:
A bushing in which the space between the major insulation (or conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with a compound having insulating properties.
C. Condenser Bushing:
A bushing in which cylindrical conducting layers are arranged coaxially with the conductor within the insulating material. The length and diameter of the cylinders are designed to control the distribution of the electric field in and over the outer surface of the bushing. Condenser bushings may be one of several types: 1 . Resin-bonded paper insulation; 2 . Oil-impregnated paper insulation; or 3. Other.
D. Dry or Unfilled Type Bushing:
Consists of porcelain tube with no filler in the space between the shell and conductor. These are usually rated 25 kV and below.
E. Oil-Filled Bushing:
A bushing in which the space between the major insulation (or the conductor where no major insulation is used) and the inside surface of a protective weather casing (usually porcelain) is filled with insulating oil.
F. Oil Immersed Bushing:
A bushing composed of a system of major insulations totally immersed in a bath of insulating oil.
G. Oil-Impregnated Paper-Insulated Bushing:
A bushing in which the internal structure is made of cellulose material impregnated with oil.
H. Resin-Bonded, Paper-Insulated Bushing:
A bushing in which the major insulation is provided by cellulose material bonded with resin.
I. Solid (Ceramic) Bushing:
A bushing in which the major insulation Is provided by a ceramic or analogous material.
