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Overview
As a next-generation power system technology, flexible transmission functions like a precise scheduler for power delivery. It enables accurate control of current and can improve a grid's transmission capacity and operational security.
Definition
Flexible transmission covers technologies for both flexible AC transmission and flexible DC transmission. Developed since the 1980s, it is an interdisciplinary field that integrates power electronics, power system engineering, communications, and control technologies.
Flexible transmission devices can adjust power flows, voltages, and other parameters with flexibility and precision, allowing transmission networks to be controlled according to predefined objectives and strategies.
Flexible AC Transmission
Flexible AC transmission technologies and customer-side power technologies include static var compensator, controllable series compensator, static synchronous compensator (STATCOM), static synchronous series compensator, unified power flow controller (UPFC), dynamic voltage restorer, and unified power quality conditioner. These technologies use series or shunt power electronic equipment on AC lines to enhance AC grid stability, increase transmission capacity, suppress system oscillations, and improve power quality and efficiency for generation, transmission, distribution, and consumption.
The Unified Power Flow Controller (UPFC) is a typical representative of flexible AC transmission. UPFC functions include controlling line power flow and independently controlling active and reactive power, improving dynamic and transient stability and transmission capability, and regulating system voltage and reactive balance to enhance voltage stability. UPFC can control current and voltage on AC transmission lines and has broad application prospects in load-dense areas and renewable energy transmission corridors.
Flexible DC Transmission
Flexible DC transmission uses voltage-source converter technology for DC transmission. Integrating a flexible DC system into an existing grid is analogous to adding a controllable valve and power source: it can effectively control transferred power, isolate fault propagation, and rapidly absorb or inject energy as required by the grid.
Key technologies include VSC-HVDC (flexible DC transmission), multi-terminal VSC-HVDC, and DC grids. These systems can regulate voltage, current, phase angle, and reactive power for connected AC systems, enable asynchronous interconnections between AC systems, and provide fast, flexible, and comprehensive control of power transfer. Transmission distance and capacity are not constrained by synchronous stability requirements of AC systems.
Flexible DC transmission offers advantages for large-scale integration of renewables, flexible interconnection of grids, and urban grid construction, and it is currently a preferred method for integrating offshore wind power.
500 kV UPFC Demonstration Project
On September 14, the 500 kV Unified Power Flow Controller demonstration project in southern Suzhou, Jiangsu province, achieved 1,000 days of safe operation. The project, located in Wuzhong District adjacent to the 500 kV Mudu substation, began construction in November 2016 and entered operation in December 2017.
Main components include converters, series transformers, shunt transformers, starter circuits, thyristor bypass switches (TBS), control systems, and auxiliary systems. The project can automatically adjust load distribution in the 500 kV corridor according to power flow, improving transmission efficiency. Over 1,000 days, the demonstration project increased Suzhou grid supply capability by more than 1.3 million kW and ensured safe, stable transmission of hydropower from Sichuan to the Yangtze River Delta, delivering nearly 100 billion kWh—equivalent to the annual consumption of over 20 million households. Compared with constructing new transmission corridors, UPFC can save capital investment and land resources.
The project achieved flexible, precise control of 500 kV power flows for the first time globally, marking China as the first country to master 500 kV UPFC complete-equipment technology. On this basis, the State Grid Jiangsu Electric Power Company has explored an upgraded version of UPFC, the interline power flow controller (IPFC).
Flexible DC Grid Demonstration
On June 29, the Zhangbei flexible DC grid demonstration project began operation. It is the world's first major technology demonstration to construct a DC grid and a landmark project for large-scale clean energy integration. The project is rated at ±500 kV with a total line length of 666 km. It delivers about 14 billion kWh of clean energy annually to the Beijing-Tianjin-Hebei region and supported supplying 100% clean energy to venues for the Beijing Winter Olympics.
The demonstration overcame challenges in constructing flexible DC grids, increasing flexible DC capacity, and improving DC reliability, achieving an effective integration of flexible DC transmission technology and new equipment. By building the world's first DC grid with network characteristics, the project addressed difficulties in large-scale renewable integration, long-distance transmission, and source-load balance. Core technologies and key equipment were world firsts, achieving 12 world-firsts in total.
Expert Interview
In an interview, Pang Hui, deputy director of the DC Transmission Technology Research Institute at the Global Energy Interconnection Research Institute, discussed the role and future development of flexible transmission.
Question: What role can flexible transmission play in ensuring grid security, and what development trends do you expect for flexible transmission?
Answer: Flexible transmission uses controllable power devices composed of high-power, high-performance power electronic components to flexibly adjust voltage, current, phase angle, and power, making previously uncontrollable networks controllable. This increases transmission capacity, reduces fault impact, and significantly enhances system flexibility and stability.
Flexible transmission has become essential for large-scale renewable integration, energy internet construction, and energy transition. By interconnecting wind, hydro, thermal, and other sources on the generation side, and combining multi-source complementary generation with the fast and flexible regulation and stability support provided by flexible transmission, it is possible to smooth variability and intermittency of renewable generation over wide areas, reduce impacts on the grid, and enable large-scale power flow regulation and control to improve renewable generation reliability. On the load side, flexible transmission can improve system stability and controllability, thereby enhancing the safety, stability, and renewable integration capacity of downstream networks.
In the future, flexible transmission will move toward higher performance and more compact equipment and system design. The State Grid Corporation is accelerating development of high-power devices such as IGBT and SiC, and components such as power capacitors and AC/DC cables, to realize full domestic production of flexible transmission equipment. With domestic core components and technological upgrades, the techno-economic performance of flexible transmission will improve, its application scope will expand, and deployment will increase. Flexible transmission will be an important technical support for large-scale renewable development and grid construction.
