Power system has been developing for more than 100 years since the beginning of last century. Following the development of power generation technology and power plant in particular big remote hydro plants, the distance of power transmission becomes longer and longer and accordantly the voltage of transmission becomes higher and higher. As for the power grid, after 100 years development, from some isolated small power grids to numbers of large interconnected power system, power grid becomes an extra huge national and international energy transmission and distribution channel in recent world.

Based on the major technical and economic features of power grid in the history, the power grid can be divided into three generations like other technical products did.?The first generation of power grid was featured with the following essential points: smaller capacity of power generator with normally lower than 200MW, lower transmission voltage below 220kV, isolated and city power grid with higher loses and environmental effects, lower system security and reliability index. The duration of the first generation of power grid worldwide existed till 1950 years last century.?Following the economy growth and the electricity demand rising rapidly after the world war two, the power grid was beginning into the development of the second generation in world wide. The main features of the second generation of power grid are as following: fossil fuel and nuclear based high efficiency big capacity generators with 300MW,?600MW, 1000MW etc., huge hydro power plants with generator capacity reaching more than 700MW, transmission voltage with 330kV and higher like 345kV, 400kV, 500kV, 735kV, 750kV, 765kV AC transmission and 500kV, 600kV, 800kV DC transmission, interconnected large power systems, high efficiency with scale economy based theory, low transmission losses and controllable Sox and Nox emission, improved security and reliability ability. The big problems of the second generation power grid we meet today are that the CO2 emission by fossil fueled plants and the supply of the fuel?gradually dry up nowadays. In recent common knowledge that the second generation power grid can be seen as a non-sustainable grid. From 1950 years to the end of the last century, the second generation power grid was developed world wide. China first 330kV transmission line was built in 1971 just following the Liujiaxia hydro power plant in upstream of Yellow River was put in operation and the formation of Northwest power grid at the same time. These events denoted the beginning of the build of the second generation power grid in China which is later than the developed countries in 20 years. Above mentioned two issues have forced the power grid must be transitioned into new generation which we called the third generation of power grid. Large scale and/or distributed renewable and nuclear power generation must be intergraded and taken big role in the grid. And smart features including IT technology application must be adopted in the whole aspects of the power grid operation and management so that to solve the difficulties of the system operation in the new condition. The third generation power grid can also be called modern power grid or smart grid. The above mentioned two main features of the modern power grid: large percentages of green generation and intelligentizing ability of the system makes the realization of the third generation will take quite longer time.

Prof. Xiaoxin Zhou graduated from the Electrical Engineering Department at Tsinghua University in 1965 and in the same year joined the Electric Power Research Institute of China. He is now a Chief Engineer of CEPRI, a Director of the Engineering and Material Department, National Science Foundation of China. He was elected as a member of the Chinese Academy of Sciences in 1993 and IEEE Fellow in 1996. His research interests are power system analysis and control, application of power electronic technology in power system. In 2008, he received the Nari Hingorani FACTS Award from IEEE Power & Energy Society.