The voltage control of the French EHV power system is organised in three levels (primary, secondary and tertiary) which concern distinct geographical areas and time constants. The secondary voltage control co-ordinates the action of the different generating sets at regional level. Proper operation of this control pre-supposes the possibility of dividing the EHV network into uncoupled zones and of choosing a descriptive node (pilot node) for each zone. This article presents the studies conducted to reach an automatic determination of zones and pilot nodes based on the examination of the structure of the network. The concept of electrical distance has been developed and its close relationship with the theory of information underlined. In addition, this concept allows the concepts of structural controlability and observability of proximity to be extended to the power system. Combined use of this electrical distance and typological analysis algorithms has proven an effective method for the identification of the secondary voltage control zones of the French grid.