The Gansu Wind farm in China is the largest wind farm in the entire world.

The Gansu Wind Farm Project is a group of 18 wind farms under construction in the Gansu province in China. The construction is broken up into three phases, the first of which has been finished. When the project is complete in 2020, it will be the largest collective wind farm in the world!

The Gansu Wind Farm Project is located in desert areas near the city of Jiuquan in two localities of Guazhou County and also near Yumen City, in the northwest province of Gansu, which has an abundance of wind resources.

The project is divided into multiple phases. The first 3,800 MW phase consists of eighteen 200 MW wind farms and two 100 MW wind farms. The second 8,000 MW phase will consist of forty 200 MW wind farms. The planned capacity is 5,160 MW by 2010, 12,710 MW by 2015 and 20,000 MW in 2020.

In 2012, the approximate installed capacity was 6,000 MW, roughly equivalent to the United Kingdom's entire wind power capacity.

(Source)