(US4641040-A)      The appts. includes an automatic controller enabling rotation of the turbo-machine at its speed of synchronism with the frequency of the network. A device makes it possobke, when the head of water or pressure head is less than a value equal to a defined fraction less than one-half of the nominal head, to cause the turbo-machine to rotate at a speed lower than the synchronous speed, and to insert between the network and the electrical machine a frequency matching device (7) of maximum power at least less than one-half of the nominal power.    The defined fraction of the nominal head is in the order of a quarter of the nominal head. The automatic controller causes the turbo-machine to rotate, when the head of water or pressure head becomes less than the set value, at the speed giving the maximum efficiency for that turbo-machine.    ADVANTAGE -   Reduces pumping consumption. (5pp)a    (GB2154807-B)      A low-head hydroelectric installation, of the type comprising: - at least one hydraulic turbo-machine constructed to give optimum efficiency under a defined head of water or pressure head (Hn), called the nominal head, corresponding to a nominal power (Pn) of the installation and for a speed of rotation corresponding to the speed of synchronism (ns) with the frequency of an electrical network, - at least one rotary electrical machine drive-coupled to the said turbo-machine and electrically connected to the said network, - means for automatic control for enabling the turbo-machine to be rotated at its speed of synchronism with the frequency of the network, - and means for enabling, when the head of water or pressure head is less than a preset value (Hd) equal to a defined fraction of the said nominal head (Hn), in all cases less than half of the latter, the turbo-machine to rotate at a speed lower than the said synchronous speed, and consequently for inserting between the network and the electrical machine a frequency matching device of maximum power at least less than half the said nominal power.