The prospect of global energy internet "using softness to overcome hardness" is promising

China Energy Storage Network News : The commissioning of the Xiamen Flexible DC Project not only marks that China has once again taken the lead in the field of flexible DC transmission, but also provides a pioneering practice for the construction of the global energy Internet.

Text: Peng Huiwen, Li Zhaohui, Lin Meimei

On December 17, 2015, a report about "flexible electricity being used on the island of Xiamen" was widely circulated. In addition to local media, mainstream media such as People's Daily, Xinhua News Agency, and CCTV interpreted this from the perspectives of people's livelihood and economic development.

For the energy industry, the commissioning of the ±320 kV Flexible DC Transmission Technology Demonstration Project in Xiamen, Fujian (hereinafter referred to as the "Xiamen Flexible DC Project") has a more far-reaching significance. As the world's first flexible DC transmission project that uses true bipolar wiring and has the highest rated voltage and transmission capacity, it not only marks that China has once again taken the lead in the field of flexible DC transmission, but also provides a pioneering practice for the construction of the global energy Internet.

An electric power expert was so excited that he expressed his feelings on WeChat Moments: "UHV DC is like a large airplane, with large capacity, high efficiency and long distance; FDC is like a helicopter, flexible and convenient."

A new tool for clean energy grid connection

At present, the development and utilization of clean energy has become a global consensus, but large-scale clean energy grid connection is still a global challenge and one of the rigid constraints on the rapid development of clean energy. Flexible DC transmission can rely on its own flexibility and controllability to promote the efficient collection, flexible transmission and decentralized consumption of clean energy in a larger range, thus becoming a key technology to overcome the difficulty of clean energy grid connection, which can be called "using softness to overcome hardness".

Flexible DC transmission technology is a new type of DC transmission technology. It upgrades semi-controlled power electronic devices to fully controlled power electronic devices. It not only has all the advantages of conventional DC transmission technology such as large transmission capacity, low power loss, and the ability to achieve asynchronous grid connection, but also takes the adaptability of DC transmission to a new level.

"If the transmission line is compared to a water pipe, the flexible transmission technology is equivalent to installing a flexible and controllable water pump in the water pipe, which can accurately control the direction, speed and flow of the water flow." Dr. Deng Chaoping, deputy commander of the Xiamen Flexible Direct Current Project Commissioning, described it vividly. It can independently control active and reactive power, no longer need to configure a large-capacity reactive power compensation device, the operation mode is more flexible, and the controllability of the system has been greatly improved.

Feng Qingdong, professor-level senior engineer at the State Grid Energy Research Institute, told our reporter that flexible DC transmission has broad application prospects in the fields of clean energy grid connection, power transmission to weak AC systems or even passive systems, and underground power supply in large cities. Especially in the field of clean energy grid connection, flexible DC transmission technology currently supports multiple DC feed-ins, which can flexibly connect wind power, solar energy, small hydropower, etc. at multiple sites to the transmission corridor, which further enhances its adaptability to clean energy grid connection.

New Choice for Island Networking

Xiamen is an international island city. There is a lack of power plant sites on the island, and all power loads are supplied by the power grid outside the island. With the continuous growth of the load on Xiamen Island, it is expected that by 2016, the existing lines into the island will not be able to meet the power supply needs on the island, so it is urgent to open up new power transmission channels. The Xiamen Island power grid is an isolated island power grid and has always used AC transmission. The gradual maturity of flexible direct current transmission technology has provided a new option for power supply on Xiamen Island. Moreover, the construction and operation of the Xiamen flexible direct current project can also provide a demonstration sample for solving the power supply problem on the island.

After the project is put into operation, it can effectively enhance the grid structure of Xiamen area and eliminate the disadvantages of Xiamen Island as a passive power grid. It can not only supplement the power shortage of Xiamen Island, but also has the function of dynamic reactive power compensation, which can quickly adjust the reactive power of the power grid on the island and stabilize the grid voltage, effectively improving the power supply reliability, transmission capacity and stable operation level of Xiamen Island power grid.

In addition, Dr. Deng Chaoping introduced that flexible direct current transmission has greatly reduced the harmonic content of the system. The converter station no longer needs to install a large number of filtering devices. The converter valve module has a more compact design and the footprint has also been reduced. Therefore, the footprint of the entire converter station has been greatly compressed, which is very suitable for Xiamen Island, where every inch of land is valuable.

It can be seen that establishing the demonstration project in Xiamen is not only a good opportunity to improve and test the research and application level of flexible direct current transmission technology, but also can effectively meet local objective needs, achieving multiple goals at one stroke.

New breakthrough in energy internet

Xiamen's flexible direct current project is born with breakthroughs and innovations, and the newly built Puyuan and Ludao converter stations are a concentrated manifestation of this. According to Ouyang Jie, chief engineer of the Ludao converter station project, the four joint buildings of the two converter stations are all made of full steel structure with a span of 47.2 meters, which is the largest span among converter stations in China. It faces huge challenges in design and construction. From this, we can see that it is not easy to build this "world's first".

In fact, from China's first flexible DC project in 2011 - the ±30 kV Shanghai Nanhui Flexible DC Transmission Demonstration Project to the current Xiamen Flexible DC Project, relying on independent innovation, China's flexible DC transmission is rapidly developing practices and explorations in multiple research directions such as large capacity, multi-terminal, bipolar, and back-to-back. "Filling many related technology gaps in the world" has become the "new normal."

Great minds think alike. In the exploration of new energy consumption, flexible DC transmission has become the next commanding height for global power. At present, at least 16 countries around the world have flexible DC transmission projects in operation or under construction. In Europe, in order to connect offshore wind power, northern European hydropower and solar energy from North Africa to the power grid, an energy Internet supported by flexible DC transmission technology has entered the implementation stage.

In the future, based on the Global Energy Internet Initiative, the demand for flexible DC transmission technology will be particularly urgent. Tang Guangfu, deputy director of the State Grid Smart Grid Research Institute, said that in order to meet the needs of building the Global Energy Internet, it is necessary to further develop flexible DC conversion technology and equipment at higher voltage levels, and flexible DC transmission technology based on overhead lines.

"Faster, Higher, Stronger" is an expression of the Olympic spirit, and can also serve as an excellent interpretation of the development direction of flexible direct current transmission technology.

It is reported that the State Grid Corporation of China is planning to build a ±500 kV four-terminal transmission network based on flexible DC transmission technology in Zhangjiakou, Hebei Province. The maximum conversion capacity will reach 3,000 megawatts, which will once again refresh the limit of flexible DC transmission. In the 13th Five-Year Plan, according to the State Grid Corporation of China, a ±800 kV ultra-high voltage flexible DC transmission project connecting Xinjiang, Qinghai and Hunan will play an important role in the cross-regional and large-scale configuration of northwest wind and light resources.

It can be foreseen that as flexible direct current transmission technology continues to reach new heights and key links such as related equipment manufacturing, engineering design, and operation continue to mature, the picture of the global energy Internet will become clearer and clearer.