(JP10306766-A)      The system includes upper and lower pond by-pass paths that are branched near a pump turbine end. A predetermined junction is shared with other pump turbines. A rotation speed increase sensor is provided to at least one of the pump turbine unit group.    When the rotation speed increase sensor determines that the rotation speed of the specific pump turbine unit exceeds a predetermined value and is higher than a rated speed, a predetermined command is sent by the specific pump turbine unit to other pump turbine units. The closing velocity of a guide blade is limited to a predetermined value that is lower than the pump turbine migration velocity.    ADVANTAGE -   Restrains water hammer due to abnormal raise of hydraulic pressure, in shared pipe line between pump turbine unit group. Does not need to test time-difference cut-off of predetermined load of pump turbine units. Attains reduction of cost since only one rotation speed increase sensor is used.    (US6042331-A)      At least a first pump-turbine out of the pump-turbines has a rotational speed increase detector, and when it is detected that the rotational speed of the first pump-turbine has exceeded a second predetermined value higher than a rated speed. A command is issued also to at least one of the pump-turbines other than the first pump-turbine to limit the guide vane closing speed to a speed approximately equal to or lower than the speed after the shift to the slow closing in the pump-turbine.    A second predetermined value is set above the range of rotational speed variations which can occur during the normal power generating operation in a connected state of the pump-turbines to an electric power system.    ADVANTAGE -   Remedies an abnormal water hammer interference related to the S-characteristic and occurring between number pump-turbines sharing conduits, especially an abnormal increase of water pressure which occurs when the load of a pump-turbine is shut shown with a time difference.    (US6210100-B1)      At least a first pump-turbine out of the pump-turbines has a rotational speed increase detector, and when it is detected that the rotational speed of the first pump-turbine has exceeded a second predetermined value higher than a rated speed. A command is issued also to at least one of the pump-turbines other than the first pump-turbine to limit the guide vane closing speed to a speed approximately equal to or lower than the speed after the shift to the slow closing in the pump-turbine.    A second predetermined value is set above the range of rotational speed variations which can occur during the normal power generating operation in a connected state of the pump-turbines to an electric power system.    ADVANTAGE -   Remedies an abnormal water hammer interference related to the S-characteristic and occurring between number pump-turbines sharing conduits, especially an abnormal increase of water pressure which occurs when the load of a pump-turbine is shut shown with a time difference.