Insulation coordination from the probabilistic point of view

The problem of the design of the insulation characteristics of an electric power system was solved, up to very recent days, and is still solved in a large majority of cases, in a deterministic way, i.e. in choosing the highest expected dielectric stress from one side and the lowest expected dielectric strength of the equipment from the other side, and by providing a suitable gap between the two, in order to cover errors, unknown factors, and aging of the equipment. In this paper, the principles of an optimal insulation design is recalled and the cost of failure risk is introduced into the economical optimum design of transmission systems. A mathematical expression is given for two simple cases of optimum design to which the insulation coordination of a transmission system can be reduced in general. The importance of knowing the breakdown probability of an insulating component, as a function of the crest value of a given overvoltage, in these optimum designs is also pointed out and the methods to calculate such probability are exposed. A description of the experimental method is also given, which can be applied to any component samples, and of the calculation methods which enable one to derive the breakdown probability of a complex component from knowledge of the breakdown probability of its elements. Also examined is how the withstand test selections modify the breakdown probability distribution of a component. Finally, it is shown how on the basis of the above mentioned methods, one can have only estimations of breakdown probability distributions and how the uncertainties of such determinations can be taken into account in intelligent designing of insulation