On June 9, 2020, in the artificial short circuit test of the world first EHV flexible DC power grid project, two EHV 535kV DC circuit breakers jointly developed by Tsinghua University and Beijing Power Equipment Group successfully completed the prescribed operation, broke the fault current in 3ms and then closed the circuit successfully, indicating that the 535kV/25kA/3ms DC circuit breaker with the highest technical index in the world meets the requirement for engineering application and lays a solid foundation for the formal operation of "Zhangbei Renewable Energy - Flexible DC Power Grid Test and Demonstration Project" (hereinafter referred to as "Zhangbei Project"). The innovative circuit breaker technology, with completely independent intellectual property rights, was developed by a DC technology research team composed of teachers, students and researchers from EEA, including Professor Zeng Rong, Associate Professor Yu Zhanqing and Associate Professor Huang Yulong.

"Zhangbei Project" is the world first EHV flexible DC power grid project, and it is also the flexible DC project with the highest voltage level and the largest transmission capacity in the world. As a key supportive project for the 2022 Beijing Winter Olympic Games, it will supply 100% clean power to the Winter Olympic venues. CIGRE believed that "large capacity DC interrupting is a worldwide difficulty". The bottleneck is to realize the interrupting of tens of thousands of amperes of current and the tolerance of megavolt voltage in microseconds ~ milliseconds.

Facing this "bottleneck" difficulty in the a high voltage DC power grid and to solve the worldwide technical difficulty in DC interrupting, the DC technology research team led by Professor Zeng Rong relied on the platform of "One Department & two Institutes" (EEA of Tsinghua University, Energy Internet Innovation Research Institute and the Sichuan Energy Internet Research Institute of Tsinghua University) and created the world first topology of coupled negative pressure hybrid DC circuit breakers, which broke through the low loss and high reliable commutation technology, and solved the difficulties in bi-directional controllable current and high efficiency and large capacity interrupting of power electronic switches. The research achievement has perfected the theory and technical system of ultra-fast interrupting of large capacity DC, and realized innovative breakthroughs in the topological structure, commutation mode, product development, engineering application and so on.

The research team created the coupled negative pressure hybrid topology with low loss and high reliability, and innovatively proposed the coupled negative pressure technology to realize fast commutation in the full current range, which broke through the bottleneck of the hybrid circuit breaker cooling mode, and is advantageous in low conduction loss and absence of water cooling, space saving, high reliability, low operation and maintenance cost.

This technology solves the problem of multi-unit series equalization of power electronic switches, and creatively proposes a clamped and modularized series connection method based on discrete metal oxide varistor (MOV), which completely eliminates the device breakdown risk caused by the imbalance of traditional series components, and can meet the direct series requirement of hundreds of EHV and even UHV power electronic units; it breaks through the bi-directional controllable current technology of DC circuit breaker under fast transient condition, proposes a bridge type power electronic unit topology based on conventional recovery speed diode and compound nonlinear impedance, and successfully constructs a bridge type bidirectional structure with the conventional recovery speed diode, which saves 40% of the cost on any traditional circuit breaker solution.

At present, the Tsinghua team has successfully developed 375V/ 480V/ 10kV/ 40kV/ 80kV/ 535kV serialized DC circuit breakers, the first set of three-port DC circuit breakers in the world, and the 535kV DC circuit breakers with the highest voltage and current level and the largest breaking capacity in the world. The R&D achievements have been industrialized with Beijing Power Equipment Group, Taikai Group and other enterprises, and put into large-scale application in Zhuhai Tangjiawan DC distribution network, Dongguan AC/ DC hybrid distribution network, Suzhou Wujiang DC distribution network, Zhangbei flexible power grid and other projects, ensuring the safe and reliable operation of many DC projects, and is of great academic significance and value of engineering application.

The successful demonstrative application of the 535kV DC circuit breaker in Zhangbei flexible DC project has not only realized a new breakthrough in scientific research to serve the national major strategy, but is also of great significance in promoting the independent innovation capability of Chinese equipment manufacturing and the continuous progress of Chinese flexible DC power grid technology.

(Photos supplied by Hebei North Maintenance Company and Beijing Power Equipment Group)

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