Electric Power Generation, Transmission, and Distribution

Results show that a majority of insulators (48%) are subjected to very light pollution and only 7%
operate in heavily polluted environments. Figure 10.15 shows the typical cause of composite insulator
failures. The majority of failures are caused by deterioration and aging. Most electrical failures are caused
by water penetration at the interface, which produces slow tracking in the fiberglass rod surface. This
tracking produces a conduction path along the fiberglass surface and leads to internal breakdown of the
insulator. Water penetration starts with corona or erosion-produced cuts, holes on the weather shed, or
mechanical load-caused separation of the end-fitting and weather shed interface.
Most of the mechanical failures are caused by breakage of the fiberglass rods in the end fitting. This
occurs because of local stresses caused by inappropriate crimping. Another cause of mechanical failures
is brittle fracture. Brittle fracture is initiated by the penetration of water containing slight acid from
pollution. The acid may be produced by electrical discharge and acts as a cathalizator, attacking the
bonds and the glass fibers to produce a smooth fracture. The brittle fractures start at high mechanical
stress points, many times in the end fitting.

10.4.5 Aging of Composite Insulators

Most technical work concentrates on the aging of nonceramic insulators and the development of
test methods that simulate the aging process. Transmission lines operate in a polluted atmosphere.

0.1 0.2

Equivalent Salt Deposit Density

(ESDD) in mg/cm^2

Electrical stress in

kV/cm

0.3

Silicone hydrophobic

Porcelain

Silicone hydrophibic

EPDM

0 0.4 0.5 0.6

30

40

50

60

70

FIGURE 10.14 Surface electrical stress vs. ESDD of fully wetted insulators (laboratory test results). (From
Application Guide for Composite Suspension Insulators. Sediver, Inc., York, SC, 1993. With permission.)

TABLE 10.6 Number of Standard Insulators for Contaminated Areas

System Voltage KV

Level of Contamination

Very light Light Moderate Heavy

138 6 = 68 = 79 = 711 = 8

230 11 = 10 14 = 12 16 = 13 19 = 15

345 16 = 15 21 = 17 24 = 19 29 = 22

500 25 = 22 32 = 27 37 = 29 44 = 33

765 36 = 32 47 = 39 53 = 42 64 = 48

Note: First number is for I-string; second number is for V-string.