Dr David J Grieve (email@example.com
) developed the interactive Java pages for this website.
A number of uncoupling failures of insulator strings occurred on high voltage transmission lines crossing bushveldt in Namibia. These uncoupling failures were triggered by rifle fire damage to several of the toughened glass insulators, which then allowed flashover to occur across insulators in a string (a typical string is illustrated in Figure 2
). The insulators in question are U70BL type and are shown in Figure 1
. These are ball and socket 11 kV disc insulators with galvanised cast iron caps. The glass insulator is glued into the cap using Portland cement. The mode of failure is illustrated in Figure 3
and involves fracture of the galvanised cast iron cap during a arc flashover, allowing the connecting pin to pull out of the cement and cracked glass in the insulator cap. This results in dropping live high tension power cables on the ground with associated danger to life.
The electrical energy available in an arc flashover can be estimated, and is much less than the energy required to burst undamaged cast iron caps. The question posed by the power supply company related to the reasons for this failure, i.e. what had happened to the caps to allow them to suffer fracture in this manner under relatively low electrical energies.
This case study answers this question by linking metallography with fracture toughness knowledge, supported by fractography of failed caps.
Figure 1 U70BL Insulator
Figure 2 Typical insulator string
Figure 3 Fractured cap
Proceed to second part
of case study.