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Xiangjiaba HPP, located in Yibin County of Sichuan Province and Shuifu County of Yunnan Province, has a concrete gravity dam (maximum height 162m) and an installed capacity of 6400MW. A series of key technical problems have been solved, including dam stability against sliding for deep and shallow layers, high-head large-unit-discharge flood discharge and energy dissipation, underground powerhouse with largest-span in the world, replacement of tailrace surge chamber by tailrace tunnel with sloping ceiling, one-stage vertical ship lift, largest caisson group in the world, largest artificial aggregate system in China, and long-distance belt conveyor.
The power station is a multi-purpose development with power generation as its main purpose, and with other purposes such as improving the navigation conditions of the upper and lower reaches, flood control, sediment retaining, irrigation, wood-drifting and re-regulation for the upper cascade hydropower stations. The project has a normal pool level (NPL) of 380.00m, total reservoir storage of 5.163 billion m3, and a live storage of 903 million m3. Its construction was commenced in November 2006.
Main Technical Features:
(1) Energy dissipation by hydraulic jump with high head and large unit discharge was adopted. At check flood level, the inflow to the reservoir is 49,800m3/s, the upper-lower water level difference is about 85m, and the maximum discharge power is about 40,000MW. To avoid adverse environmental impact of flood discharge on the lower neighboring county town and factories, energy dissipation by hydraulic jump was employed. The maximum unit discharge of the stilling basin is up to 225m3/(s.m), the inflow velocity 35m/s. Because of the high head and large unit discharge, the safe operation of the stilling basin is one of the critical technical problems to be tackled for the project development, and it has been well solved by the scheme of energy dissipation by hydraulic jump with drop-sill proposed on the basis of lots of theoretical analyses, hydraulic model tests and studies and numerical simulation calculations by several Chinese scientific research institutes.
(2) A tailrace system without surge shaft was employed. The right-bank underground powerhouse accommodates four sets of 750MW turbo-generator units, featuring in large unit capacity, large unit dimensions, and large quotative discharge. According to the analyses, for the scheme with surge shaft, a huge surge shaft would have to be built to make the draft tube negative pressure up to the requirements of the specification. That would be inefficient economically and cause great difficulties to adapt to the geological conditions and to construct the works. Therefore, it was proposed to study the scheme without tailrace surge shaft. Through hydraulic transit process analyses by mathematical modeling and physical model testing with model unit, it was finally determined to adopt the scheme of tailrace tunnel with sloping ceiling instead of tailrace surge shaft.
(3) One-stage vertical ship lift was adopted as the navigation structure. To solve the problem of navigation, the scheme of one-stage vertical ship lift was proposed through all-round comparison of several ship lift and shiplock alternatives. Its maximum cross head is up to 114.70m, one of the greatest in the world.
(4) The 750MW turbo-generator units are the largest in the world in terms of unit capacity. In the turbine design, we used the latest experience and lessons of China and other countries for reference and introduced new design concepts and technologies such as hydraulic design and analysis with CFD, blade profile design with negative inclination to change stress distribution, change of runner outlet circular rector to prevent vortex strip, and determining turbine parameters with new ideas, so as to improve the comprehensive performance and operation stability of the turbines.
(5) Large-current generator circuit breakers were provided. Since the power system requires flexible operation of the units, generator circuit breakers were furnished at the generator outlet to adapt to frequent startup/shutdown operation of the units, reduce the number of operations of HV circuit breakers, increase flexibility of operation dispatching, ensure continuous, reliable and flexible station service power supply, and meet the requirements of rapid short-circuit protection for generator-transformer group and synchronization operation at generator voltage side.
(6) A long-distance belt conveyor system was adopted to transport concrete aggregate. The system consists of five belt conveyors connected together with a total length of 31.1km, a transport capacity of 3000t/h, and a belt speed of 4.0m/s. There was no precedent for the design and use of 31.1km-long belt conveyor system, so the design was quite difficult and involved heavy workload.