(EP435182-B1)      Control system for operating a plural number of generating units (5,6,7) in a hydroelectric power station with high efficiency, wherein said hydroelectric power station furthermore comprises a head tank for storing intake water supplied from upstream via a gate (2) at a water intake opening, characterised by: tank level detection means (27) for detecting the water level of the head tank, intake level detection means (31) for detecting the water level at the water intake opening and opening degree detection means (33) for detecting the degree of opening of the gate (2), flow amount calculation means (32) receiving the output signals of the intake level detection means (31) and the opening degree detection means (33) for calculating the amount of water flowing into said head tank (4) on the basis of the detected intake water level and the detected degree of opening of the gate and delivering an output signal indicating the result of said calculation, processing means (34) receiving the output of the flow calculation means (32) and of the tank level detection means (27) and outputting flow amount objective value signals, for determining flow amount objective values for each generating unit on the basis of the calculated water into the head tank and the detected water level in the head tank, said flow amount objective values representing a distribution of water flow on the individual generating units so that each generating unit can generate power most efficiently and the total power generation output is maximised, a plurality of power objective value calculation means (38,39,40) receiving the flow amount objective value signals, wherein each power objective value calculation means (38,39,40) is associated to a corresponding generating unit (5,6,7) is designed for calculating the power that is most efficiency generated by the respective generating unit (5,6,7) on the basis of the flow amount objective values calculated by the operation control means (34), and provides a power objective value signal as output, a plurality of deviation calculation means (41,42,43), each being associated with a corresponding generating unit (5,6,7), having as one input the power objective value signal of the corresponding power calculation means (38,39,40) and as second input an actual output signal from the corresponding generating unit (5,6,7) for calculating the deviation of the power objective value from the actual output value and outputting signals indicating the power deviation (S49,S50,S51) as control signals for the corresponding generating unit.