In a concern for sustainable development, the rational use of electrical energy implies having transportation networks capable of transmitting large quantities of electrical energy over long distances as well as adequate storage plants. This double requirement is the consequence of the fact that electrical energy has to be produced and consumed simultaneously, as direct storage of large quantities is not economically feasible. Nowadays, as thermal power plants and run-off river plants are always working at their nominal power for economic reasons; it is not possible to cover the large space and time demand variations without having an efficient transportation network and adequate storage capacities. The recent start up of major windformer sites also implies the need for supplementary storage capacities. The inventory of large storage plants reveals the solution "pump-storage plant" as being one of the best in terms of storage potential, flexibility, reliability, response time, efficiency, risks and costs. Nearly all the pump-turbine groups operating today consist of synchronous motor-generators working at the network frequency and therefore at constant speed. The performance and efficiency of such groups can be significantly improved by using variable speed motor-generators, and more specifically doubly-fed asynchronous motor-generators. The few variable speed units in operation today are equipped with a cyclo-converter cascade. The purpose of the present thesis is to propose the replacement of the cyclo-converter solution by a back to back 3-level VSI cascade and investigate the complete regulation strategy. This solution compared to the former has the advantage of providing the possibility of regulating the reactive power in the rotor cascade as well as not polluting the network with sub-harmonics. The three steps followed in such a study, modelling, simulation and practical measurements have permitted the elaboration of a functional tool for the design of the control for such a system.