How Transmission Systems Deal with Failures

Failures in transmission systems are dealt with by re-routing current to bypass the faulty section of line. The ability of the network to do this depends on its degree of interconnectedness and therefore the availability of conductors in the right place to provide an alternative route. In addition, these conductors need to have the ability to carry additional load above their normal capacity without overheating - which, in the case of overhead lines, causes dangerous sagging and irreversible changes in the metal cable and in the case of underground cables, thermal damage to the insulation.

Unlike overhead lines that have only a limited time before they overheat, underground cables have a significant ability to carry overload currents. This is because underground cables are in intimate contact with soil, which gives them a greater thermal mass than an overhead cable sitting in air.

Overhead line operators can increase their rated capacity by around 30% on average by allowing for the weather conditions that affect how the cables are cooled. On warm, still days, the line will not be able to carry an overload for any length of time, as the normal maximum summer operating temperatures may be close to the annealing temperature for the wire structure.

Underground cables with internal real time monitoring of temperature can increase their capacity by up to 100% and carry up to twice their rated load for at least 90 minutes. This means that strategically placed cable links can perform a valuable role in increasing overall system reliability.