Multi-stage hydraulic machine and control method for multi-stage hydraulic machine

A control method for a multi-stage hydraulic machine which comprises a rotatable shaft;

• a plurality of runners fixed to said shaft and arranged from a highest-pressure stage;
• runner chambers in which are respectively housed said runners;
• a return channel connecting adjacent ones of said runner chambers with each other;
• movable wicket gates provided in at least the said highest-pressure stage and capable of moving between a position in which said movable wicket gates block said channel with one said runner chamber and a position in which said movable wicket gates open said channel;
• and a casing connected to said highest-pressure stage, said method comprising:
• a first step of causing said movable wicket gates to block said channel;
• a second step of feeding air to said runner chamber of the highest-pressure stage to lower a water level in said hydraulic machine to a draft tube positioned below said runner chamber of the lowest-pressure stage;
• a third step of shifting a mode of idling operation of said shaft to one of a turbine generating and pumping operation,
• said third step including:
• a first process of exhausting air in said runner chamber of the highest-pressure stage, whereby said water level rises to said runner chamber of the lowest-pressure stage;
• a second process of using the rotary centrifugal force of the lowest-pressure stage runner for feeding water from said draft tube to the return channel connected to said runner chamber of the lowest-pressure stage;
• a third process of feeding a part of the water fed into said return channel back to said draft tube;
• a fourth process of sequentially exhausting air from said runner chambers of stages below said highest-pressure stage, beginning with a stage immediately below said highest-pressure stage, except said lowest-pressure stage, after said runner chamber of said highest-pressure stage reaches an underwater shut-off state;
• a fifth process of stopping said exhaustion of a part of the water from said return channel to said draft tube in said third process and, at the same time, exhausting air in said runner chamber of said lowest-pressure stage, when said runner chambers of every stage except said lowest-pressure stage reach an underwater shut-off state;
• and
• a sixth process of stopping exhaustion of the air from said runner chambers of each pressure stage when the pressure of said runner chamber of said highest-pressure stage reaches a predetermined value;
• and
• a fourth step of opening said movable wicket gates.