With the last of 26 generators installed late last year, the Three Gorges Project (TGP) in China is now the largest capacity hydroelectric plant in the world. It has an installed capacity of 18,200MW, with each generator rated at 700MW. The second largest is Brazil's 12,600MW Itaipu plant. There are also plans to install six more turbines in an underground power house by 2012, giving it a capacity of 22,500MW.

However, the planned 85TWh/year from the scheme is unlikely to be realised because of the wide head range which varies from 61m to 113m according to the season - the units are only designed to give 852MW at 91m. It is understood, therefore, that the annual energy output will not reach Itaipu's 95TWh/year, where the river flow and head are almost constant.

The TGP has been under discussion for more than 90 years, with various studies and proposals put forward. Political instability stopped the studies moving forward until about 1950 when, supported by a strong central government and propelled by the urgent and perennial need for flood control plus the growing demand for electrical energy, the studies were resumed in earnest.

By 1993, final plans were approved and construction started in 1994. The planned completion date was 2009, but by the end of 2008 all 26 units had been installed and were generating.

China's annual power generation

By the end of 2006, the last date for which figures are available, China's installed capacity was 622GW, almost three times the level of 1994/5 at 217GW. The yearly power generation was 2.83×106GWh, making China second in the world for the total installed capacity and annual power generation.

Installed capacity per capita was 0.391kW and the average power generation per capita was 1,903kWh, less than half of the world average and one-sixth to one-tenth of that of developed countries.

The energy from the TGP, along with other hydropower sources now being vigorously pursued, will improve these figures slightly but, despite the substantial contribution the project will make to the country's installed capacity, flood mitigation is still its major function. However, the potential rewards of a large non-polluting source of energy have provided a strong financial basis for building the project. 

Yangtze navigation

Apart from flood control and electricity generation, another commercially valuable effect of the project will be the improved navigation along the Yangtze.

With regulation of the downstream section that follows from water impoundment, sections of the river - which were shallow during the dry season - have become deeper, which facilitates river transport. Larger vessels can now be used and for longer periods throughout the year, which means that transport of goods along the river is up to 50 million tonnes, five times greater than before the project began

The shiplocks themselves are on a scale commensurate with the project. They consist of two channels for the two-way passage of vessels of up to 10,000t capacity. Each channel has five chambers which are 320m long and 38m wide.

The Yangtze rises in the Qinghai-Tibet plateau and flows across 11 provinces and municipalities, as well as the Autonomous Regions which include Qinghai, Tibet, Sichuan, Yunnan, Hubei, Hunan, Jiangxi, Anhui, Jiangsu and Shanghai.

With a total length of 6,300km, a drainage area of 1.8×106km ² and a yearly flow of 976×109m ³ , the Yangtze falls 500m from its source to the estuary in the East China Sea. It has a catchment area of about 1.8 million km ² , which accounts for nearly 19 per cent of the country's land area where about 31 per cent of the population lives. It is the largest river in China, and is the third largest in the world.

The concrete gravity dam, 44km from city of Yichang and about 40km upstream from the existing Gezhouba Project, has a crest length of 2.3km and a height of 181m. The reservoir, impounded by the structure, will have a volume of 39.3 billion m ³ when the water level reaches the operating level of 175m.

At its maximum level the reservoir will form a 600km-long lake, which is about the distance from London to Glasgow in the UK. Its average width is 2.3km and the lake will inundate 632km ² of land. These figures will, of course, vary according to the season and flood conditions.

Tackling devastation caused by flooding

Two cities and 116 towns will be submerged by the TGP, and 1.13 million people have already been affected.

This resettlement programme sounds immense, but needs to be taken in context of the loss of life and livelihoods over the last 2,000 years due to flooding.

When the river flooded in 1931, 145,000 people were drowned and many millions more affected by loss of land and homes as a result of the disruption to the infrastructure.

In 1954, 40,000 people died as a result of flooding, while in 1998 flooding killed 1,520 people and 239,000 hectares of land were submerged.

There was a flood soon after construction on the TGP began but before the reservoir had been built, resulting in 1,500 casualties and losses of more than RMB 200bn ($29.2bn).

Floods occur on average every ten years in the Yangtze river basin, with a severe one every 100 years and a disastrous one every 1,000 years - these averages are only valid taken over long periods of time.

In theory, a severe flood could occur two or three times over five or six years, or even more frequently, but this is unlikely. TGP storage is adequate for a 100-year return period, and it could substantially mitigate a 1,000-year flood (storing some of the flood water in combination with controlled discharge).

The river flow is characterised by a heavy silt load that can cause erosion of the turbine blades. Wharfs, jetties and buildings on the river bank can also be affected by silt deposits. About 22,000m ³ /s of silt is discharged at the river delta, although this value fluctuates according to the season and may also be affected by silt trapped by Three Gorges and other dams.

The civil works are designed to reduce silt intake to the turbines, with the spillway being in the centre and the two powerhouses on either side. Because silt tends to move to the lower strata of flow, the intake to the powerhouses, which are higher than the spillway, will have some mitigating effect.

There are other measures in place to reduce the silt trapped by the dam, such as periodic discharge through sluice gates at appropriate times. However, this could affect the turbines at the Gezhouba plant downstream of the TGP, which have been operating for ten years and have already been significantly damaged by silt abrasion.

The silt load also has a critical effect on the life-span of the reservoir and its storage capacity, the navigation channels, and the development of a harbour area and civil works related to the river transport.

Earthquake safety

Any dam can be affected by earthquakes, and the effect of dam failure is particularly serious for those dams that impound large reservoirs. However, risks of such an event are always factored in at the design stage.

The efficacy of such designs was demonstrated during the earthquake in China in May 2008, which reached 7.8 on the Richter scale. None of the four large dams close to the epicentre failed. Although many dams were affected or damaged throughout the area, all the reservoirs held. The biggest problems were associated with landslides that temporarily dammed some rivers. Three Gorges was unaffected - it has been designed to withstand a
6.5 magnitude earthquake occurring on a fault 17km away.

However, there have been some criticisms of the studies on which this design is based. Silt load in the river increased substantially following the earthquake, giving further impetus to the construction of other dams on the Jinsha river, the Yangtze's longest tributary, specifically planned to trap silt before it enters the main river.

Power plants

The 26 Francis units are in two surface power plants - one on either side of the dam, with plans to build an underground one in the near future.

The left bank comprises 14 units. The design strategy allowed for these units to start generating while construction and installation continued on the right bank, comprising 12 units, and on the rest of the civil works. The machines in the left bank power house began generating in 2003 but, as these units were operating at below the design head of the reservoir, they were equipped with temporary runners suitable for the lower head range of 71m to 94m. 

Each powerhouse feeds a 500kV switchyard and +/- 500kV converter station. Power transmission from the project is by 15 transmission lines. Central China and Chonquing City are supplied via 500kV AC lines, while a DC line feeds eastern China.

Eight of the 14 units on the left bank were supplied by a group consisting of ABB, GEC (UK) and Alstom, now known collectively as Alstom Power. Kvaerner Energy AS of Norway, provided the hydraulic design.

The contract for the other six units was given to a consortium of GE Canada and Voith Siemens (the VGS consortium). Both contracts were for total design and manufacture of the first two units together with progressive technology transfer of the remaining six Alstom units to Harbin of China and of the 4 VGS units to Dongfang. After the first units had been manufactured, the work was progressively transferred to the Harbin and Dongfang factories.

Four units for the right bank powerhouse were supplied by Alstom, while the remaining eight were supplied by the Chinese partners Harbin and Dongfang.

Between signing the contracts and their completion, both Voith Siemens and Alstom built manufacturing plants in China so that a substantial part of the first four units were made there.

Again, by the time the contracts were fulfilled both consortia had changed their composition through mergers and acquisition so that by the end of the work the contractual mix was not the same as at the beginning.

Construction of the generators

The generators are rated at 20kV at 840 MVA continuous. Rated speed is 75 revs/min, while the runaway speed is 150 revs/min. The stators are water-cooled, and the rotor and associated components are air-cooled.

Because of the danger of leakage in the stator conductors, which has been experienced on other similar machines, ABB used stainless steel instead of copper for the hollow conductors. Copper has a tendency to develop deposits on the surface in contact with the running water, which can lead to clogging and then to intermittent vibration.

The weight and size of the units posed unique problems for their support, concentricity and circularity. The masses of the rotor, stator and turbine are 2,000t, 715t and 3,380t respect-ively. The diameter of the machines totals 21,233mm.

Because of the large masses involved and the machines' dimensions, ABB used oblique flexible plates to fix the stators to the foundations, rather then the traditional straight and stiffer fastenings. The flexibility of these plates, as well as their oblique orientation, allows for concentric expansion caused by thermal stress - this also maintains concentricity and minimises the chances of core buckling.

The same technique of oblique fixing was used to secure the upper guide-bearing bracket to the stator, again allowing for thermal expansion of the stator.

The rotor also uses the same technology and for the same reasons; oblique, rather than the conventional radial arms bridge the air intake region between the discs and the hub.

The long-term benefits of the project are certain, particularly in terms of flood relief and displacement of carbon-based energy sources. However, the short-term effect has been painful for the local population and has drawn criticism across the world.

In the medium-term, problems of siltation will need to be addressed if the benefits of TGP are to be maximised.