Interconnected electrical networks do not have available any significant storage capacity of their own. Therefore, suitable measures must be taken for a continuous matching of generation and consumption of electrical energy within the network. Typically, each power plant unit must possess a certain control reserve of its design capacity which must be available within a few seconds. For the UCPTE network of Western Europe the requested control reserve is 5%. This paper gives a general description of the concept of a spinning reserve system for supplementary steam supply in conventional power plants with the steam generated from hydrogen/oxygen combustion. It is mainly focused on the steam generating component itself, the H-2/O-2 Steam generator. Special emphasis is put on the closed loop control system of the propellant supply for the steam generator which has to be exactly stoichiometric and the on-line quality control of the produced steam. The current status of development of such a H-2/O-2 Steam generator is demonstrated showing recent test results of pressure and temperature rise as well as combustion efficiency and steam quality which have been obtained with the experimental version of this steam generator.