The deep integration of DC and AC is an important morphological feature of the new power system, and the DC power distribution system has the functions of flexible and controllable power flow, active voltage regulation and comprehensive management of power quality, which can effectively support the construction of the new power system.
1. demand analysis
(I) Background of development
On March 15, 2021, the ninth meeting of the Central Finance and Economics Committee pointed out that the "14th Five-Year Plan" is a critical period and window period for carbon peaks, and a new power system should be built.
The Guangdong-Hong Kong-Macao Greater Bay Area is one of the regions with the highest degree of openness and the strongest economic vitality in China, and it has an important strategic position in the overall development of the country. The construction of the Guangdong-Hong Kong-Macao Greater Bay Area is not only a new attempt to promote the formation of a new pattern of comprehensive opening up in the new era, but also a new practice to promote the development of the "one country, two systems" cause.
(II) technical requirements
With the carbon peak, carbon neutral target and the construction of new power system, the distribution system has been transformed into a new type of power distribution system with multiple functions such as collection, transmission, storage and distribution of power, and its safe and stable operation and reliable power supply are facing great challenges.
First, the local rapid growth of electricity load. There are some problems, such as the tension of regional distribution corridor, the high load rate of distribution transformer, the unbalanced distribution of feeder load, the excessive short-circuit current and so on. Two is the lack of distributed new energy carrying capacity. The rapid development of distributed new energy has caused problems such as uneven distribution of power flow in power distribution systems, line overvoltage, and reverse transmission of power flow in substations. Three is the rapid development of new load and energy storage. Electric vehicles and other loads, distributed energy storage rapid growth, distribution system power two-way flow, network congestion and other issues are becoming increasingly prominent. Fourth, the power supply capacity is unbalanced. The "closed-loop design and open-loop operation" structure of the traditional AC distribution system leads to the lack of power flow control capacity of the distribution system. Five is the power supply reliability and power supply quality requirements to improve. With the development of high-precision industries, the requirements of users for power supply reliability and power quality are increasing day by day.
(III) technical features
Smooth and reliable. The DC power distribution system runs more smoothly and the power supply reliability is higher. The DC power distribution system is connected to the AC power grid through the DC converter, and has certain fault ride-through and protection capabilities. When the voltage of the AC distribution system drops slightly, the converter can control the output current at the rated value through the output voltage tracking regulation.
Flexible and controllable. DC distribution system power mutual, trend control, power supply capacity is stronger and convenient for network load interaction. DC interconnection does not require consistency in phase, frequency and amplitude, which can realize interconnection of power supplies of different nature and ensure load transfer. The power supply capacity of DC line is about 1.6 times that of AC, which provides an effective solution for the improvement of power supply capacity in high density and narrow channel load area, and can realize flexible control of network load.
High quality and efficient. The DC distribution system has better power quality and higher energy efficiency, which is more conducive to the consumption of new energy. The converter in the DC distribution system can flexibly and dynamically compensate the reactive power of the AC bus and the user side, stabilize the AC bus and the user side AC voltage, reduce the AC/DC conversion link, facilitate the access of distributed new energy and energy storage, improve the comprehensive transmission efficiency of energy, and provide a certain support capacity for the distribution network.
II. Current Status of Development
In view of the problems existing in the traditional power distribution system, the use of DC power distribution system is a feasible idea. China has built Zhuhai Tangjiawan three-terminal flexible DC system and a number of DC power distribution demonstration projects, and achieved good results. However, the DC power distribution system and application scenarios are still in the stage of scheme discussion and demonstration verification, and there are still many technical bottlenecks in system planning, control and protection, key equipment, core devices, etc., which require the whole industry chain, such as power grid, scientific research institutes, equipment manufacturers and users, to jointly tackle key problems and breakthroughs.
3. core technology
The DC power distribution system must realize the voltage matching and energy interaction between different networks and between internal links through the DC converter. Among them, the commutation and transformation links of the DC power distribution system are also mainly based on power electronic technology. Therefore, the core technology of DC power distribution system can be divided into three levels: device, equipment and system.
(I) Device Technology
Power electronics technology is based on power semiconductor devices. The mainstream high-power semiconductor devices mainly include IGBT (insulated gate bipolar transistor), IGCT (integrated gate commutated thyristor), etc., but some devices still rely on imports. Therefore, the independent research and development of high-power semiconductor devices is of great significance to ensure the safety of China's industry and build a safe, reliable and economical new distribution system.
(II) Equipment technology
At present, the equipment of DC power distribution system mainly includes DC converter, DC transformer, DC circuit breaker and so on. There are still problems such as high cost, large volume and low practical degree, which need to be optimized and upgraded to support large-scale popularization and application.
(III) System Technology
Different from DC transmission, DC distribution network is closely combined with load and power supply, and the interaction with AC system is obvious. The ultra-fast fault protection and flexible optimization control technology of DC distribution system need to be further improved.
IV. Application Scenarios
DC power distribution technology can be applied to many scenarios, such as flexible interconnection, various types of parks, distributed new energy access and building buildings, etc., will serve China to achieve carbon peak, carbon neutral and the goal of building a new power system.
(I) Flexible interconnection
This scenario usually uses DC to realize the interconnection of AC substation medium voltage buses or feeders, and extends to the low-voltage power supply system, including AC distribution network combined thermal backup, improve power supply reliability, AC distribution network load active balance, improve equipment utilization rate, distribution network transformation capacity, reduce land occupation and investment.
(II) various parks
This scenario generally includes data centers, industrial high-quality power supply and other applications. According to the analysis of Lawrence Berkeley National Laboratory, compared with the AC system, the use of DC power supply can improve the transmission efficiency of about 3%, save about 6% of hardware costs and save about 33% of space.
(III) Distributed New Energy
The National Development and Reform Commission issued the "14th Five-Year Plan for Renewable Energy Development" and proposed that "actively develop renewable energy-based microgrids and DC distribution networks" can enhance the carrying capacity of the distribution system for new energy.
(IV) building building
This scenario is an extension of DC technology to the field of civil buildings, and plays a key role in reducing building energy consumption and achieving carbon emission reduction in the building field. According to the analysis of Lawrence Berkeley National Laboratory, photovoltaic households use DC electricity to save 5% of electricity compared with AC systems, while optical storage households use DC electricity to save 14% of electricity compared with AC systems.
5. Technology Outlook
Looking forward to the future, the key technologies and practical engineering problems that still need to be overcome in DC power distribution and utilization are as follows: 1) It is necessary to propose DC power distribution and utilization system planning principles and quantifiable planning evaluation methods to solve system planning problems; 2) It is necessary to propose a flat control architecture and control strategy for DC power distribution and utilization system based on "integration of regulation" and "primary and secondary integration" schemes to reduce the difficulty of system construction and operation and maintenance; 3) It is necessary to develop low-cost, compact and highly reliable DC circuit breakers and power converters to improve the practical level of medium-voltage DC distribution systems; 4) It is necessary to develop economic and intelligent key equipment for low-voltage DC power systems and DC electrical products with complete pedigree to meet the DC power demand in different scenarios and promote the ecological formation of low-voltage DC industry; 5) It is necessary to build a safe, intelligent and efficient DC power distribution project with multiple regions and scenarios and carry out operational evaluation to improve the standard system.
Guangdong Power Grid Corporation will rely on the technological innovation policy and environment of the Greater Bay Area, adhere to the technological development route of the combination of production, education, research and application, and build a complete technical system of the DC power distribution system based on research and development, innovation and application, and build a DC power distribution simulation and testing, Control and other series of R & D platforms. Drive key technological breakthroughs with innovative basic research, promote engineering applications with technology industrialization, form a batch of iconic original cutting-edge technological achievements, help the country's new energy development, and support the construction of a clean, low-carbon, safe and efficient new energy system. Strive to build the DC power distribution research center into a key laboratory of Guangdong Province in 3-5 years, become a domestic leading and world-leading DC power distribution technology research and development institution, and the technology engine of the DC power distribution industry, and help the Guangdong-Hong Kong-Macao Greater Bay Area Build an international technology innovation center.
苏公网安备32010402001819