According to the Code for Anti freezing Design of Hydraulic Structures, F400 concrete shall be used for the parts of structures that are important, severely frozen and difficult to repair in severe cold areas (the concrete shall be able to withstand 400 freeze-thaw cycles). According to this specification, F400 concrete shall be used for the face slab and toe slab above the dead water level of the upper reservoir face rockfill dam of Huanggou Pumped Storage Power Station, the water level fluctuation area of the upper reservoir inlet and outlet, the water level fluctuation area of the lower reservoir inlet and outlet and other parts. Prior to this, there was no precedent for the application of F400 concrete in the domestic hydropower industry. In order to prepare F400 concrete, the construction team investigated domestic research institutes and concrete admixture manufacturers in many ways, entrusted professional companies to carry out special research, prepared F400 concrete by adding silica fume, air entraining agent, high-efficiency water reducing agent and other materials, and applied it to the construction of Huanggou Pumped Storage Power Station.
In addition, in severe cold areas, if the concrete in contact with water has slight cracks, the water will penetrate into the cracks in winter. With the continuous freeze-thaw cycle, the concrete will be gradually destroyed. The concrete face slab of the main dam of the upper reservoir of the pumped storage power station plays the role of water retaining and seepage prevention. If there are a lot of cracks, the safety of the dam will be seriously reduced. The construction team of Huanggou Pumped Storage Power Station has developed a kind of crack resistant concrete – adding expansion agent and polypropylene fiber when mixing concrete to reduce the occurrence of concrete cracks and further improve the frost resistance of face slab concrete.
What if there are cracks on the concrete face of the dam? The construction team has also set up a frost resistance line on the surface of the panel – using hand scraped polyurea as a protective coating. Hand scraped polyurea can cut off the contact between concrete and water, slow down the development of freeze-thaw scaling damage of face slab concrete, and also prevent other harmful ingredients in the water from eroding concrete. It has the functions of waterproof, anti-aging, freeze thawing resistance, etc.
The face slab of concrete face rockfill dam is not cast at one time, but constructed in sections. This results in a structural joint between each panel section. The common anti-seepage treatment is to cover a rubber cover plate on the structural joint and fix it with expansion bolts. In winter in severe cold areas, the reservoir area will be subject to thicker icing, and the exposed part of the expansion bolt will be frozen together with the ice layer to cause ice pullout damage. Huanggou Pumped Storage Power Station innovatively adopts a compressible coating type structure, which solves the problem of structural joints damaged by ice pullout. On December 20, 2021, the first unit of Huanggou Pumped Storage Power Station will be put into operation for power generation. A winter operation has proved that this structure type can prevent the damage of panel structural joints caused by ice pulling or frost expansion extrusion.
In order to complete the project construction as soon as possible, the construction team tried to carry out winter construction. Although there is almost no possibility of winter construction outdoors, the underground powerhouse, water conveyance tunnel and other buildings of the pumped storage power station are deeply buried underground and have construction conditions. But how to pour concrete in winter? The construction team shall set insulation doors for all the openings connecting the underground caverns and the outdoor, and install 35kW hot air fans inside the doors; The concrete mixing system is completely closed, and heating facilities are set indoors. Before mixing, wash the concrete mixing system with hot water; Calculate the amount of coarse and fine aggregates in winter according to the amount of concrete earthwork required for winter pouring, and transport them to the tunnel for storage before winter. The construction team also heats the aggregates before mixing, and puts “cotton padded clothes” on all the mixer trucks transporting concrete to ensure that the temperature is maintained during concrete transportation; After the initial setting of concrete pouring, the concrete surface shall be covered with thermal insulation quilt and, if necessary, covered with electric blanket for heating. In this way, the construction team minimized the impact of cold weather on the project construction.
Ensuring the safe operation of pumped storage power plants in severe cold regions
When pumped storage power station pumps water or generates electricity, the water level of upper and lower reservoirs will change constantly. In the cold winter, when the pumped storage power station has operation tasks every day, a floating ice sheet will be formed in the center of the reservoir, and a ring of crushed ice belt will be formed outside. The ice cover will not have a great impact on the operation of the pumped storage power plant, but if the power system does not need the pumped storage power plant to operate for a long time, the upper and lower reservoirs may be frozen. At this time, although there is enough water in the reservoir of the pumped storage power station, the water body cannot flow due to the inability to connect with the atmosphere, and the forced operation will bring safety risks to water supply structures and unit equipment and facilities.
The construction team carried out a special study on the winter operation mode of pumped storage power plants. The research shows that the dispatching operation is the key to ensure the safe operation of pumped storage power plants in winter. In cold winter, at least one unit generates electricity or pumps water for more than 8 hours every day, which can prevent the reservoir from forming a complete ice cap; When the power grid dispatching cannot meet the above conditions, anti ice and ice breaking measures shall be taken.
At present, there are three main anti ice and ice breaking measures for reservoirs and gate wells of pumped storage power plants: artificial ice breaking, high-pressure gas inflation, and water pump flushing ice breaking.
The cost of artificial ice breaking method is low, but the operation time of personnel is long, the risk is high, and safety accidents are easy to occur. The high-pressure gas inflation method is to use the compressed air ejected by the air compressor in the deep water to eject a strong warm water flow, which can melt the ice layer and prevent the formation of new ice layer. The Huanggou Pumped Storage Power Station adopts the method of water pump flushing and ice breaking, that is, the submersible pump is used to pump up the deep water, and then the water is ejected through the jet hole on the jet pipe to form a continuous flow of water, so as to prevent local water surface from icing.
Floating ice entering the flow passage is another risk of pumped storage power plant in winter operation, which may damage hydraulic turbines and other mechanical equipment. At the beginning of construction of Huanggou Pumped Storage Power Station, model tests were carried out, and the critical velocity of floating ice entering the channel was calculated to be 1.05 m/s. In order to reduce the flow velocity, Huanggou Pumped Storage Power Station has designed the inlet and outlet section to be large enough, and set the flow velocity and temperature monitoring sections at different elevations of the inlet and outlet. After a winter monitoring, the staff of the power station did not find the floating ice entering the flow passage.
The preparation period of Huanggou Pumped Storage Power Station starts from January 2016. The first unit will be put into operation for power generation on December 20, 2021, and the last unit will be put into operation for power generation on June 29, 2022. The total construction period of the project is six and a half years. Compared with the same type of pumped storage power station projects in China, the construction period of Huanggou pumped storage power station has not lagged behind because it is located in severe cold areas. After experiencing the test of a cold winter, all hydraulic structures, equipment and facilities of Huanggou Pumped Storage Power Station operate normally. In particular, the maximum leakage behind the concrete face rockfill dam of the upper reservoir is only 4.23L/s, and the leakage index is at the leading level among earth rock dams of the same scale in China. The unit starts with dispatch, responds quickly, and operates stably. It undertakes the tasks of Northeast Power Grid to meet the peak in summer, winter, and important festivals, and ensures the safe and stable operation of Northeast Power Grid.
Post time: Nov-17-2022
