The unit suction height of pumped storage power station will have a direct impact on the diversion system and powerhouse layout of the power station, and a shallow excavation depth requirement can reduce the corresponding civil construction cost of the power station; However, it will also increase the cavitation risk during the operation of the pump, so the accuracy of the elevation estimation during the early installation of the power station is very important. In the early application process of pump turbine, it was found that the runner cavitation under pump operating condition was more serious than that under turbine operating condition. In the design, it is generally believed that if the cavitation under the pump working condition can be met, the turbine working condition can also be met.
The selection of suction height of mixed flow pump turbine mainly refers to two principles:
First, it shall be carried out according to the condition that there is no cavitation under the working condition of the water pump; Second, the water column separation cannot occur in the whole water conveyance system during the transition process of unit load rejection.
Generally, the specific speed is proportional to the cavitation coefficient of the runner. With the increase of the specific speed, the cavitation coefficient of the runner also increases, and the cavitation performance decreases. Combined with the empirical calculation value of suction height and the calculation value of draft tube vacuum degree under the most dangerous transition process conditions, and taking into account that on the premise of saving civil excavation as much as possible, the unit has sufficient submergence depth to ensure safe and stable operation of the unit.
The submergence depth of the high head pump turbine is determined according to the absence of cavitation of the pump turbine and the absence of water column separation in the draft tube during various transients. The submergence depth of pump turbines in pumped storage power plants is very large, so the installation elevation of the units is low. The suction height of high head units used in power plants that have been put into operation in China, such as Xilong Pond, is – 75m, while the suction height of most power plants with 400-500m water head is about – 70 to – 80m, and the suction height of 700m water head is about – 100m.
During the load rejection process of the pump turbine, the water hammer effect makes the average pressure of the draft tube section drop significantly. With the rapid increase of the runner speed during the load rejection transition process, a strong rotating water flow appears outside the runner outlet section, making the center pressure of the section lower than the outside pressure. Even though the average pressure of the section is still greater than the water’s vaporization pressure, the local pressure of the center may be lower than the water’s vaporization pressure, causing water column separation. In the numerical analysis of the pump turbine transition process, only the average pressure of each section of the pipe can be given. Only through the full simulation test of the load rejection transition process can the local pressure drop be determined to avoid the phenomenon of water column separation in the draft tube.
The submergence depth of high head pump turbine should not only meet the requirements of anti erosion, but also ensure that the draft tube does not have water column separation during various transition processes. The super high head pump turbine adopts a large submergence depth to avoid the separation of water column during the transition process and ensure the safety of the water diversion system and units of the power station. For example, the minimum submergence depth of Geyechuan Pumped Storage Power Station is – 98m, and the minimum submergence depth of Shenliuchuan Pumped Storage Power Station is – 104m. The domestic Jixi pumped storage power station is – 85m, Dunhua is – 94m, Changlongshan is – 94m, and Yangjiang is – 100m
For the same pump turbine, the farther it deviates from the optimal working condition, the greater the cavitation intensity it suffers. Under the working conditions of high lift and small flow, most flow lines have a large positive angle of attack, and cavitation is easy to occur in the negative pressure area of the blade suction surface; Under the condition of low lift and large flow, the negative angle of attack of the blade pressure surface is large, which is easy to cause the flow separation, thus leading to cavitation erosion of the blade pressure surface. Generally, the cavitation coefficient is relatively large for the power station with large head change range, and the lower installation elevation can meet the requirement that no cavitation will occur during operation at low lift and high lift conditions. Therefore, if the water head varies greatly, the suction height will increase accordingly to meet the conditions. For example, the submergence depth of QX is – 66m, and MX-68m. Because the variation of MX water head is greater, it is more difficult to realize the adjustment and guarantee of MX.
It is reported that some foreign pumped storage power plants have experienced water column separation. The full simulation model test of the transition process of Japanese high head pump turbine was carried out in the manufacturer, and the phenomenon of water column separation was studied in depth to determine the installation elevation of the pump turbine. The most difficult problem for pumped storage power plants is the safety of the system. It is necessary to ensure that the spiral case pressure rise and tail water negative pressure are within the safe range under extreme working conditions, and ensure that the hydraulic performance reaches the first-class level, which has a greater impact on the selection of submergence depth.
Post time: Nov-23-2022
