10
2006 by Taylor & Francis Group, LLC.
Insulators and
Accessories
George G. Karady
Arizona State University
Richard G. Farmer
Arizona State University
10.1 Electrical Stresses on External Insulation...................... 10-1
Transmission Lines and Substations . Electrical Stresses .
Environmental Stresses . Mechanical Stresses
10.2 Ceramic (Porcelain and Glass) Insulators..................... 10-7
Materials . Insulator Strings . Post-Type Insulators .
Long Rod Insulators
10.3 Nonceramic (Composite) Insulators ............................. 10-9
Composite Suspension Insulators . Composite Post Insulators
10.4
Insulator Failure Mechanism ....................................... 10-13
Porcelain Insulators . Insulator Pollution . Effects of
Pollution . Composite Insulators . Aging of Composite
Insulators
10.5 Methods for Improving Insulator Performance ......... 10-18
Electric insulation is a vital part of an electrical power system. Although the cost of insulation is only a
small fraction of the apparatus or line cost, line performance is highly dependent on insulation integrity.
Insulation failure may cause permanent equipment damage and long-term outages. As an example, a
short circuit in a 500-kV system may result in a loss of power to a large area for several hours. The
potential financial losses emphasize the importance of a reliable design of the insulation.
The insulation of an electric system is divided into two broad categories:
1. Internal insulation
2. External insulation
Apparatus or equipment has mostly internal insulation. The insulation is enclosed in a grounded
housing which protects it from the environment. External insulation is exposed to the environment. A
typical example of internal insulation is the insulation for a large transformer where insulation between
turns and between coils consists of solid (paper) and liquid (oil) insulation protected by a steel tank. An
overvoltage can produce internal insulation breakdown and a permanent fault.
External insulation is exposed to the environment.