Zhangbei renewable energy flexible DC power grid test demonstration project, Xiamen ±320kV flexible DC transmission science and technology demonstration project, Zhoushan ±200kV five-terminal flexible DC transmission science and technology demonstration project, Shanghai Nanhui flexible DC transmission project... In the field of energy and power, these innovative projects with great scientific and technological demonstration significance all have applied a common advanced technology-flexible DC transmission technology. 

Flexible DC transmission technology is a new generation of DC transmission technology with voltage source converter as the core. It has the characteristics of fast response speed, good controllability, strong adaptability, flexible operation mode and small footprint with the same capacity. It is an important technical means to improve the acceptance capacity of renewable energy, support energy transformation and build a new type of power system.

For more than ten years of overcoming difficulties and scaling new heights, the State Grid Corporation of China has always adhered to independent innovation and made continuous efforts towards high-end technologies with  extremely challenging and promising development prospects in the electric power field. It has continuously turned the "stuck neck" technology list into a list of key projects, changed the technology "no man's land" into an innovative "new blue ocean", promoted the R & D and engineering application of flexible DC transmission technology in China to achieve innovative breakthroughs from scratch, and realized "China creation" and "China leading". Contribute to the power of the State Grid in the construction of a powerful country in energy science and technology and the realization of high-level scientific and technological self-reliance.

Adhere to independent innovation and realize "zero breakthrough" of flexible DC transmission technology in China

Flexible DC transmission technology is the "golden key" to solve the problem of large-scale grid-connected consumption of clean energy. It is one of the most critical technologies for effective control and efficient conversion of energy, and it is of great significance to the power grid.

However, conquering the peak of flexible DC transmission technology is not easy.

Due to the late start of relevant research in China, and the strict confidentiality of flexible DC transmission technology in the world, there has never published a article, a drawing, a photo, and a lack of relevant materials. The research and development of flexible DC transmission technology in China has been advancing slowly, and only carried out some follow-up research before 2004. Faced with the actual situation of weak research foundation and no technical experience for reference, the company, as a "national major project" and "pillar", organized a backbone R&D team to resolutely break the ice and set sail to face the difficulties, and started the systematic R&D work of flexible DC transmission technology in China.

IIn May 2006, the company organized experts and scholars in the field of electric power to hold a seminar on "Research Framework for Key Technologies of Flexible DC Transmission System", systematically proposed the overall planning for research on flexible DC transmission technology, and comprehensively launched major scientific and technological projects of flexible DC transmission. The prelude to the development of China's flexible DC transmission technology was officially played. In the same year, with the support of the company, the flexible DC technology research and development team led by Tang Guangfu, the director of the Institute of Power Electronics of China Electric Power Research Institute and deputy director of the Transmission and Distribution Engineering Center, formulated the key technology research framework of flexible DC transmission system and determined the overall technical route of "basic theory -- key technology -- equipment development -- engineering demonstration".

Relying on a series of scientific and technological projects, basic theory and key technology research have been gradually deepened. Through repeated verification by means of mechanism analysis, formula derivation, model building and simulation verification, the R&D team successfully discovered the law of power transfer in the AC-DC conversion process, created a series of simulation and test R&D means, mastered a series of key technical methods, and established the basic theoretical system for the flexible DC transmission technology research for the first time in China.

At the end of 2007, the company carried out the demonstration of flexible DC transmission technology project in Shanghai Nanhui Wind Farm, and began to explore and try the technical route suitable At the end of 2007, the company launched a demonstration project of flexible DC transmission technology in Shanghai Nanhui Wind Farm, and began to explore and attempt the flexible DC transmission technical route suitable for high voltage and large capacity. At that time, the technical route adopted by foreign enterprises had problems such as large operation loss and difficulty in capacity expansion. The company innovatively put forward the idea of modular multilevel converter topology. Siemens, ABB and other well-known multinational companies also started research on high voltage and large capacity flexible DC transmission technology almost at the same time.

After all the way to struggle and innovative practice, the company's R&D personnel were under pressure to carry out hundreds of experiments to develop converter valve prototype, to tackle the problems of high withstand voltage capacity and high resistance to electromagnetic interference of digital circuit in high potential occasions, and continuously to carry out more than 10 design iterations and more than 50 expert technical review meetings... After making comprehensive breakthroughs in basic theoretical research, key technology research, core equipment development, experimented capacity building and engineering system integration, the company built Asia's first flexible DC transmission project in Nanhui, Shanghai in July 2011, realizing a "zero breakthrough" in the field of flexible DC transmission technology in China. The project became a typical case of wind farm combined to the grid and was listed in CIGRE 370 guideline. The company has also become the third enterprise to completely master the full range of core technologies of design, testing, commissioning and production of flexible DC transmission equipment after ABB and Siemens, competing with world-renowned multinational enterprises.

At this point, the company's flexible DC transmission technology research began to move towards continuously the direction of large capacity, multi-terminal, bipolar, back-to-back and so on. The company continues to carry out exploration and innovation practice, fully promotes the application of key achievements, strives to make new breakthroughs in higher voltage levels, larger transmission capacity and higher technology levels, and strives to firmly grasp the initiative of competition and development in its own hands.

Continue to overcome difficulties and push equipment technology and engineering demonstration to a new level

"Foundation" is gradually firmer and deeper, flexible DC transmission technology in China has achieved a great leap from basic theory research to the development of high-voltage large-capacity complete equipment. The company continues to tackle higher-parameter flexible DC transmission equipment, accelerate the pace of project demonstration, and promote the implementation of more innovative projects.

Zhoushan,Zhejiang Province,is dotted with more than 1300 islands. On July 4th, 2014, the world's first five-terminal flexible DC transmission project---Zhejiang Zhoushan ±200kV five-terminal flexible DC transmission science and technology demonstration project was put into operation. This is a major scientific and technological demonstration project with completely independent intellectual property rights of the company. Two years after the project was put into operation, the world's first 200kV HVDC circuit breaker and fast recovery device independently developed by the company were successfully applied. The circuit breaker can disconnect fault currents of up to 15000 amps generated by a 200kV high-voltage DC transmission line within 3 milliseconds, which is 100 times faster than a human blink of an eye. This marks that the world-class technical problem, high voltage DC is difficult to disconnect quickly has been overcome, and major breakthroughs have been made in the research and development and manufacturing of DC transmission core equipment. China's flexible DC transmission equipment and technology have reached a new level and are at the forefront of the world.

At the commissioning site of Zhoushan flexible DC transmission system DC circuit breaker and damping recovery technology demonstration project, Zheng Jianchao, academician of Chinese Academy of Engineering, excitedly said: "DC circuit breaker has been successfully developed,passed all tests and put into engineering application, providing key technical support for the development of DC power grid with higher voltage level in the future."

This year, Zhoushan flexible DC project has made a new breakthrough. On April 10, Zhoushan Flexible DC Transmission System Zhousi Converter Station operated at full power for more than 24 hours, marking the overall success of the project's high-power test. It has five full-power operation conditions and will realize the full consumption of offshore wind power, photovoltaic power generation, tidal power generation and other new energy sources. Under the background of building a new type of power system, the carrying capacity of flexible DC system equipment and lines has been further enhanced, preparing for large-scale access of new energy to the power grid.

In Xiamen, which is more than 900 kilometers away from Zhoushan, the world's first flexible DC transmission project adopting true bipolar connection scheme --- Xiamen ±320kV flexible DC transmission science and technology demonstration project is implementing reliability improvement and transformation project to further improve the energy utilization rate of the system. The project was put into operation on December 17, 2015, marking that China has fully mastered the key technologies and engineering complete sets of high voltage and large capacity flexible DC transmission.

"If the project is compared to a bridge across Xiamen Island and the electric energy delivered is compared to traffic flow, then the true bipolar design is equivalent to adding a two-way backup lane to the regular two-way lane. If any lane fails, we can activate the backup channel to ensure the normal flow of current. Even if all lanes in either direction are blocked,the alternate channel can guarantee half of the electricity to pass smoothly." Zheng Guoshun, then head of the flexible DC converter station of the maintenance branch of State Grid Fujian Province Electric Power Co., Ltd. explained.

As a bridge between DC and AC, the converter valve is one of the main equipment with the highest technical content and the greatest challenge in flexible DC projects. The ± 320kV/1000MW flexible DC converter valve and valve control equipment applied to Xiamen Flexible DC Project are independently researched and developed by the company. They have been evaluated as "excellent performance, completely independent intellectual property rights, and the core technology and overall technical performance of the product are at the international leading level" by seven academicians, including Zhou Xiaoxin, Zheng Jianchao, Pan Yuan, Shen Guorong, Lei Qingquan, Wang Tianran and Yu Yixin. This technical achievement was subsequently popularized and applied to the world's first flexible DC back-to-back project with commercial operation, the highest voltage level and the largest capacity---Chongqing and Hubei back-to-back flexible DC project, which effectively guaranteed the safe and stable operation of regional power grid.

Build up fully and release sparingly, forward with strength. With the flexible DC system and grid coordination control technology, gigawatt flexible DC core equipment design technology, flexible DC grid construction technology and other problems have been overcome, flexible DC technology has begun to be more widely used in wind power grid connection, power grid interconnection and other scenarios, truly becoming a solution for large-scale renewable energy wide-area coordination and complementation and reliable transmission.

Realize "China's leadership" and accelerate the integration of technology, standards and equipment to "go global"

Zhangbei,on the dam has deep grass and thick woods, its sky is blue and the water is clear. Cool wind blows from the grassland, affecting the rotation of wind turbine blades and been turned into clean electricity.

On June 29,2020, Zhangbei Renewable Energy Flexible DC Power Grid Test Demonstration Project was put into operation. Clean electric energy from Zhangbei area can flow into Beijing Power Grid along this "green highway", and about 14 billion kilowatt hours of green electricity is sent to Beijing-Tianjin-Hebei region every year through this project. Beijing Winter Olympic venues realized 100% clean energy power supply for the first time in Olympic history.

Zhangbei Flexible DC Project is not only a landmark project to realize large-scale grid connection of clean energy, promote energy revolution and practice green winter Olympic concept, but also the world's first major scientific and technological test demonstration project to realize DC power grid construction. The core technology and key equipment of the project are the first in the world, creating 12 world firsts, and it has significant comprehensive benefits in accelerating the transformation of resource advantages in the sending area and promoting the transformation and upgrading of China's electrical equipment manufacturing industry. 

As the world's first flexible DC power grid project, Zhangbei flexible DC power grid project has little experience to draw on and original innovation is extremely difficult. From scheme demonstration to production preparation, from joint control and protection to field test, the company's scientific research personnel have always insisted on innovation and breakthrough on the basis of digestion and absorption. Finally, they successfully raised the flexible DC transmission voltage to ±500kV and the rated capacity of single converter to 1.5 million kilowatts, realizing the integration of flexible DC transmission technology and new equipment technology, and solving the problems of large-scale grid connection, long-distance transmission and source load balance of new energy. It further promotes the innovative development of China's flexible DC transmission technology and DC power grid, drives the upgrading of power grid technology and equipment manufacturing industry, and creates favorable conditions for DC technology and major equipment to "go global".

Over the years, the company has actively deployed and carried out research on offshore wind power flexible DC technology, and has achieved profound technical accumulation in system access, customized development of DC equipment, and complete engineering design. Since 2017, the State Grid Research Institute has carried out technical cooperation and business negotiations with European power grid enterprises, and has successively participated in and won the bid for technical demonstration and preliminary design projects of several flexible DC projects in Europe. This signing of this contract marks that the company's flexible DC transmission technology with independent intellectual property rights and high-end electrical equipment such as DC converter valves have international leading core competitiveness. It is an important achievement for the company to promote the integration of technology, standards and equipment to "go global". It is of great significance to build " Belt and Road" with high quality and deepen the green energy cooperation between China and Europe.

Nowadays, under the initiative of jointly building the " Belt and Road", flexible DC transmission technology threads a needle,it has not only promoted the deep participation of China's power equipment technology in international competition, but also helped China's power grid enterprises to successfully win overseas markets, bringing new historical opportunities for the high-quality development of China's power industry.