Energy Internet has a promising prospect of "overcoming hardness with softness"

North Star Power Transmission and Distribution Network News: On December 17, 2015, a report on "using flexible electricity on the island" in Xiamen was widely circulated. In addition to local media, some mainstream media have interpreted this from the perspectives of people's livelihoods, economic development, and other aspects.
For the energy industry, the significance of the Xiamen ± 320 kV Flexible DC Transmission Technology Demonstration Project (hereinafter referred to as the "Xiamen Flexible DC Project") being put into operation is more profound. As the world's first flexible DC transmission project with true bipolar connection, rated voltage and transmission capacity both reaching the international top, it not only marks that China has once again achieved international leadership in the field of flexible DC transmission, but also provides a pioneering practice for the construction of the global energy Internet.
Excited, an electricity expert exclaimed on WeChat Moments, "Ultra high voltage direct current is like a large airplane, with large capacity, high efficiency, and long distance; flexible direct current is like a helicopter, flexible and convenient, a new tool for clean energy grid connection."
Currently, the development and utilization of clean energy has become a global consensus, but large-scale integration of clean energy into the grid remains a global challenge and a rigid constraint on the rapid development of clean energy.
Flexible DC transmission can rely on its own flexibility and controllability to facilitate the efficient collection, flexible transmission, and dispersed consumption of clean energy on a larger scale. Therefore, it has become a key technology to overcome the difficulty of grid connection of clean energy, which can be called "using flexibility to overcome rigidity". Flexible DC transmission technology is a new type of DC transmission technology that 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 power transmission lines are compared to water pipes, 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 water flow." Dr. Deng Chaoping, Deputy Commander in Chief of Xiamen Flexible and Direct Engineering Debugging, vividly described it. It can independently control active and reactive power, eliminating the need for large capacity reactive power compensation devices, making its operation more flexible, and greatly improving system controllability.
Feng Qingdong, a professor level senior engineer at State Grid Energy Research Institute, told our reporter that flexible DC transmission has broad application prospects in clean energy grid connection, power transmission to weak AC systems and 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 multi DC feeding, which can flexibly connect wind power, solar energy, small hydropower, etc. from multiple sites to the transmission corridor, further enhancing its adaptability to clean energy grid connection. New Choices for Island Networking
Xiamen is an international island city with a lack of power plant site resources on the island, and all power loads are transmitted from the external power grid. With the continuous increase in load on Xiamen Island, it is expected that by 2016, the existing incoming power lines will not be able to meet the power supply demand on the island, so there is an urgent need to open up new power transmission channels. The Xiamen Island power grid belongs to the isolated island power grid and has previously used AC transmission. The gradual maturity of flexible DC transmission technology has provided a new option for power supply in Xiamen Island. Moreover, the construction and operation of the Xiamen Flexible Power Supply Project can also provide a demonstration sample for solving the problem of island power supply. After the project is put into operation, it can effectively enhance the power grid structure in Xiamen, eliminate the disadvantage of Xiamen Island as a passive power grid, not only supplement the power shortage in Xiamen Island, but also have dynamic reactive power compensation function, which can quickly adjust the reactive power of the island's power grid, stabilize the grid voltage, and effectively improve the power supply reliability, transmission capacity, and stable operation level of Xiamen Island's power grid. In addition, Dr. Deng Chaoping introduced that flexible direct current transmission significantly reduces the harmonic content of the system, and converter stations no longer require a large number of filtering devices. The compact design of converter valve modules also reduces the footprint, resulting in a significant reduction in the overall footprint of the converter station. This is very suitable for Xiamen Island, where land is precious. It can be seen that settling the demonstration project in Xiamen is not only a good opportunity to improve and test the research and application level of flexible DC transmission technology, but also to effectively meet local objective needs, achieving multiple benefits with one action. New breakthroughs in energy Internet
The breakthrough and innovation of Xiamen's flexible and straight project are inherent, and the newly built Puyuan and Ludao converter stations are a concentrated reflection. According to Ouyang Jie, the chief engineer of the Ludao converter station project, the four combined buildings of the two converter stations are all made of full steel structures with a span of 47.2 meters, which is currently the largest span among converter stations in China and faces huge challenges in design and construction. From this, it can be seen that building this "world's first" is not easy.
In fact, from the first flexible direct current project in China in 2011- the ± 30 kV Shanghai Nanhui Flexible Direct Current Transmission Demonstration Project, to the current Xiamen Flexible Direct Current Project, China's flexible direct current transmission has been rapidly practicing and exploring in multiple research directions, such as large capacity, multi terminal, bipolar, and back-to-back, relying on independent innovation. "Filling multiple world related technological gaps" has become the "new normal". Heroes share the same view. On the road to exploring the consumption of new energy, flexible DC transmission has become the next high ground for global power competition. At present, at least 16 countries worldwide have flexible DC transmission projects in operation or under construction. In Europe, in order to connect offshore wind power, hydropower in northern Europe 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 is particularly urgent. Tang Guangfu, vice president of the State Grid Smart Grid Research Institute, said that in order to meet the construction demand of the global energy Internet, further efforts should be made in the fields of higher voltage level flexible DC converter technology and equipment, 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 DC transmission technology. It is reported that State Grid Corporation of China is planning and constructing a ± 500 kV four terminal transmission network based on flexible DC transmission technology in Zhangjiakou, Hebei. The maximum commutation capacity will reach 3000 megawatts, which will once again break the limit of flexible DC transmission. In the 13th Five Year Plan, according to the plan of 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 cross regional and large-scale allocation of northwest wind and solar resources.
It can be predicted that as the flexible DC transmission technology continues to leap over new heights, and the related equipment manufacturing, engineering design, operation and other key links continue to mature, the picture of the global energy Internet will become clearer and clearer.