To prevent an operation deviating from an upper or lower limit input/output range and an operation deviating from a proper rotational speed and a proper guide vane opening, by accurately calculating a simulated total pump head or simulated effective head for control corresponding to an actual total pump head or actual effective head respectively which varies every minute with fluctuation of a water level in a surge tank, in an adjustable speed pumped storage generator having surge tanks and a water passage system where a variable speed pump turbine shares some of conduits with other turbines.

The generator includes: calculating a simulated total pump head signal by generating a lifted water amount signal from a static head calculated from a water level difference signal H1 between an upper and a lower surge tanks and a power command or its equivalent signal, in a pumped storage mode; calculating a simulated effective head signal by generating a flow rate signal from a static head calculated from a water level difference signal H1 between an upper and a lower surge tanks and a power command or its equivalent signal, in a power generation mode; and calculating a proper rotational speed or proper guide vane opening of the variable speed pump turbine, based on the simulated total pump head signal or simulated effective head signal.