Hydraulic Characteristics Of Large Fluctuation Transient Process For Hydropower Station With Air-cushion Surge Tank

China's existing air-cushion surge chamber hydropower stations are all high head,small flow power station.Which kind of hydropower station the air cushion surge chamber matched is still in the exploratory stage.As the air-cushion surge tank set up a high-pressure air cushion to suppress water level by gas dynamic characteristics,there is a lot of diffences between air-cushion surge chamber and nomal surge chamber.In this paper,analysis of the air cushion surge chamber is based on the variation of surge and pressure to find its features.It is suggested that the pressure in the air cushion surge chamber should be the sum of the surge pressure and the gas pressure.Compared with the normal surge tank to analys the adaptability about the pressure and water level control requirements.The main contents of this paper include the following parts:(1)Starting from the simple operation condition of the upstream air cushion surge chamber setted by the one turbine single tube.The wave surge and gas pressure are both based on the gas state equation and basic equation of the air cushion surge chamber.Comparing the period of surge fluctuation of the normal surge tank with the air cushion surge chamber,we can find that under the load condition,the fluctuation period of the air cushion surge chamber is shorter than that of the normal surge tank and under the condition of increasing load,the fluctuation period of the air cushion surge chamber is longer than that of the normal surge tank.Comparing the maximum and minimum pressure,we can define the capability about two kinds of surge chamber on the maximum and minimum pressure control effect.Tthe results show that the normal surge tank can control better in the terms of maximum pressure and the air cushion surge chamber can control better in the terms of minimum pressure.What is more,an example of hydropower station was setted up to compare with the results of the theoretical derivation.At last,the conclusion are verified right.(2)In the actual hydropower station,a water diversion tunnel often corresponds to more than one unit.It is much more complex about combined operation conditions.Therefore,this paper studies on the control of the extreme surge and extreme pressure in the upstream air cushion surge tank with double machine arrangement and puts forward some suggestions on the safe operation of the unit.The method is as follows:we choose the four typical combination conditions,the same as normal hydropower station;the non-linear progressive method is used to work out the time of worst superposition of surge wave and the maximum and minimum pressure of upstream air cushion surge chamber when combined operation conditions is occured.In addition,the influence of the impedance coefficient of surge chambers is analyzed.The results show that the maximum pressure was controlled by the condition that rejecting load at first and increasing load after adverse time and that the control of minimum pressure was changed form the condition that incresing load at first and regecting load after adverse time to continuous load rejection.At the same time,an example of hydropower station was setted up to compare with the results of the theoretical derivation.At last,the conclusion are verified right.(3)In this paper,an special extreme surge is proposed firstly,that surge of the initial time is the extreme surge.Based on the simple operation conditions,we find that the reason and control parameters of whether this situation occurred.Also,the influence of these parameters are analyzed.At the same time,this paper proposes that the normal surge tank can be regarded as a special air cushion surge chamber,and the results of the air cushion surge chamber can be used to normal surge tank too.In addition,on the basis of simple operation conditions,the situation that surge of the initial time is extreme surge is also occurred by the the change of the control parameters.Combining these researches above,the hydraulic performance of the air-cushion surge chamber and the normal surge tank is analyzed and compared in the process of large fluctuation transition.The two kinds of surge chambers are analyzed just from the view of the transition process.By comparing with normal surge chamber,more suggests can be provided for the use of air-cushion surge tank.