Lines and substations - Insulation selection and coordination

The word coordination refers to actions aimed at harmonizing two or more things. It's a very common concept, frequently encountered in everyday life. In the technical world, it generally covers the harmonization of a constraint and a capacity to resist this constraint. Even more specifically, when designing an electrical network, it is necessary to select materials so that they can withstand the various stresses to which they are subjected on this network. Conversely, if the equipment exists and, consequently, the level of resistance is defined, then we can also consider limiting the stresses to values below this level. Coordination can therefore be achieved either by increasing strength or reducing stress. As always in technology, there is a compromise to be found between, on the one hand, the comfort of use aspect, which requires that stress resistance be much higher than the stress itself, and, on the other hand, the economic point of view, which requires that stress resistance be reduced to the bare minimum in order to minimize equipment costs.

Insulation coordination of an electrical network is concerned with voltage stresses and their counterparts, the dielectric strength of equipment, which characterizes the ability of insulation to withstand certain voltage levels without dielectric breakdown, i.e., without triggering. Insulation coordination therefore involves adjusting the in-service dielectric strength of equipment (lines, substations, transformers, alternators) to the levels of overvoltages likely to occur on this equipment during operation, so that the probability of a short-circuit due to the failure of a given insulation is acceptable from both an operational and an economic point of view. This adjustment is achieved either by reducing stresses, increasing dielectric strength or, finally, by acting simultaneously on these two parameters. One of the main difficulties of the insulation coordination process lies in the fact that both the voltage stresses applied to equipment and the dielectric strength of this equipment are probabilistic in nature; this dual uncertainty makes it tricky to adjust the dielectric strength to be specified in relation to the stresses.

At every point on a network, insulation coordination therefore involves determining the optimum insulation, taking into account the consequences of damage or service interruption. With this in mind, a gradation of insulation can be envisaged, i.e. the ability of insulation to withstand electrical stresses. For example, the risk of failure must be practically nil for insulators whose failure prevents a nuclear unit from being used for several days (transformer insulation), and very low for substation busbar insulation, where the consequences of an ignition are serious; on the other hand, a higher...