The Three Gorges Dam is a hydroelectric power plant that spans the Yangtze River by the town of Sandouping, located in the Yiling District of Yichang, in Hubei province, China. It is the world's largest electricity-generating plant of any kind.

The dam body was completed in 2006. Except for a ship lift, the originally planned components of the project were completed on October 30, 2008, when the 26th generator in the shore plant began commercial operation. Each generator has a capacity of 700 MW.

Six additional generators in the underground power plant are not expected to become fully operational until 2011. Coupling the dam's 32 main generators with 2 smaller generators (50 MW each) to power the plant itself, the total electric generating capacity of the dam will eventually reach 22.5 GW.

The project produces electricity, increases the river's shipping capacity, and reduces the potential for floods downstream by providing flood storage space. From completion through September 2009 the dam has generated 348.4 TWh of electricity, covering more than one third of its cost.

The Chinese state regards the project as a historic engineering, social and economic success, with the design of state-of-the-art large turbines, and a move toward limiting greenhouse gas emissions. However, the dam flooded archaeological and cultural sites and displaced some 1.3 million people, and is causing significant ecological changes, including an increased risk of landslides. The dam has been a controversial topic both in China and abroad.

Layout and scale

The figure on the right faces upstream. The dam body in the middle left of the figure was divided into the left side (the right one-third of the dam body in this figure), the spillway (the middle of the dam body), and the right side (the left one-third of the dam body in this figure). The underground power plant under construction is to the left of the dam body, hidden in the mountains. The dam wall is made of concrete and is about 2,309 metres (7,575 ft) long, and 185 metres (607 ft) high. The wall is 115 metres (377 ft) thick on the bottom and 40 metres (131.2 ft) thick on top. The project used 27,200,000 cubic metres (35,600,000 cu yd) of concrete, 463,000 tonnes of steel, enough to build 63 Eiffel Towers, and moved about 102,600,000 cubic metres (134,200,000 cu yd) of earth. The ship lift is to the right of the dam body with its own designated waterway. The ship locks, which can be seen in the second figure, are to the right (northeast) of the ship lift.

When the water level is at its maximum of 175 metres (574 ft) over sea level (110 metres or 361 feet above the river level downstream), the dam reservoir is about 660 kilometres (410 mi) in length and 1.12 kilometres (0.70 mi) in width on average, and contains 39.3 km3 (31,900,000 acre·ft) of water. The total surface area of the reservoir is 1,045 km². The reservoir flooded a total area of 632 km² of land, compared to the 1,350 km² of reservoir created by the Itaipu Dam.

Power generation and distribution

Generating capacity

The Three Gorges Dam is the world’s largest hydroelectric power station by total capacity, eventually reaching 22,500 MW. It will have 34 generators: 32 main generators, each with a capacity of 700 MW, and two plant power generators, each with capacity of 50 MW. Among those 32 main generators, 14 are installed in the north side of the dam, 12 in the south side, and the remaining six in the underground power plant in the mountain south of the dam. The expected annual electricity generation will be over 100 TWh, 18% more than the originally predicted 84.7 TWh, resulting from the six generators added in 2002.

Generators

The main generators weigh about 6,000 tonnes each and are designed to produce more than 700 MW of power. The designed head of the generator is 80.6 meters (264 ft). The flow rate varies between 600–950 cubic metres (780–1,240 cu yd) depending on the head available. The greater the head, the less water needed to reach full power. Three Gorges uses Francis turbines. Turbine diameter is 9.7/10.4 m (VGS design/Alstom's design) and rotation speed is 75 revolutions per minute. Rated power is 778 MVA, with a maximum of 840 MVA and a power factor of 0.9. The generator produces electrical power at 20 kV. The outer diameter of the generator stator is 21.4/20.9 m. The inner diameter is 18.5/18.8 m. The stator, the biggest of its kind, is 3.1/3 m in height. Bearing load is 5050/5500 tonnes. Average efficiency is over 94%, and reaches 96.5%.

The generators are manufactured by two joint ventures. One of them includes Alstom, ABB Group, Kvaerner, and the Chinese company Haerbin Motor. The other includes Voith, General Electric, Siemens (abbreviated as VGS), and the Chinese company Oriental Motor. The technology transfer agreement was signed together with the contract. Most of the generators are water-cooled. Some newer ones are air-cooled, which are simpler in design and manufacture and are easier to maintain.

Here is a video animation of the Three Gorges Dam generators.

Generator installation progress

The 14 north side generators are in operation. The first (No. 2) started on July 10, 2003 and No. 9 completed things on September 7, 2005. Full power (9,800 MW) was only reached on October 18, 2006 after the water level reached 156 m.

The 12 south side generators are also in operation. No. 22 began operation on June 11, 2007 and No. 15 started up on October 30, 2008. The sixth (No. 17) began operation on December 18, 2007, raising capacity to 14.1 GW, finally surpassing Itaipu (14.0 GW), to become the world's largest hydropower plant.

The underground power plant and its six generators, were still under construction as of December, 2008.

Output milestones

By September 16, 2009, the plant had generated 348.4 TWh of electricity,. Its 18,300 MW installed capacity is about 4,300 MW more than Itaipu's. In July 2008 it generated 10.3 TWh of electricity, its first month over 10 TWh. On June 30, 2009, after the river flow rate increased to over 24,000 m3, all 28 generators were switched on, producing only 16,100 MW because the head available during flood season is insufficient. During an August 2009 flood, the plant first reached its maximum output for a short period.

During the November to May dry season, power output is limited by the river's flow rate, as seen in the diagrams on the right. When there is enough flow, power output is limited by plant generating capacity. The maximum power-output curves were calculated based on the average flow rate at the dam site, assuming the water level is 175 m and the plant gross efficiency is 90.15%. The actual power output in 2008 was obtained based on the monthly electricity sent to the grid).

Several factors limited power output in 2008. First, the dam was not operating at 175 m for most of the year, reducing the water flow's potential energy. During the flood season, in order to accommodate water surges, operators lowered the water level to 145 m. Also generators were still being installed—the plant did not reach capacity until the end of the year. Note that the diagrams on the right are based on the power sent to the grid, while the table displays the total generated.

Annual Production of Electricity
Year Number of
installed units		 TWh
2003 6 8.607
2004 11 39.155
2005 14 49.090
2006 14 49.250
2007 21 61.600
2008 26 80.812
2009 26 79.47
Total 26(32) 367.984

Distribution

The State Grid Corporation and China Southern Power Grid paid a flat rate of ¥250 per MWh ($35.7 US) until July 2, 2008. Since then, the price has varied by province, from ¥230.6-11.1 per MWh. Higher-paying customers receive priority, such as the city of Shanghai. Nine provinces and two cities consume power from the dam.

Power distribution and transmission infrastructure cost about 34.387 billion Yuan. Construction completed in December 2007, one year ahead of schedule.

Power is distributed over multiple 500 kilovolt (kV) transmission lines. Three Direct current (DC) lines to the East China Grid carry 7,200 MW: Three Gorges-Shanghai (3,000 MW), HVDC Three Gorges-Changzhou (3,000 MW), and HVDC Gezhouba - Shanghai (1,200 MW). The alternating current (AC) lines to the Central China Grid have a total capacity of 12,000 MW. The DC transmission line HVDC Three Gorges-Guangdong to the South China Grid has a capacity of 3,000 MW .

The dam was expected to provide 10% China's power. However, demand has increased more quickly than planned. Even fully operational, it would support only about 3% of 2006 requirements.