With the continuous development of society, a large number of DC loads appear in modern social life. In daily life, fluorescent lamps, energy-saving lamps, LED lamps and other lighting systems, televisions, set-top boxes, routers and other audio-visual equipment, as well as rapidly developing electric vehicles, data processing centers, etc., ultimately use direct current. At present, the conventional method is to use alternating current, which is inseparable from AC/DC conversion. It is necessary to implement a rectification conversion at each power consumption terminal, and at the same time, some electrical energy losses during the conversion process are dissipated in the form of heat energy. If DC power supply is adopted, a large number of rectifier transformers can be reduced, the heat emitted during the conversion can be reduced correspondingly, and the capital investment of ventilating fans and air conditioners can be reduced.

　　Similarly, a large number of frequency conversion electrical appliances have entered the lives of ordinary people, such as frequency conversion refrigerators, frequency conversion air conditioners, and frequency conversion washing machines. In order to achieve low frequency conversion power consumption, the essence of its internal is the conversion process of AC/DC/AC. If the DC distribution network is adopted, DC/AC conversion can be directly realized, and reducing intermediate links means reducing energy loss.

With technological advances in distributed power generation, energy storage, and power electronics, the proportion of user-side distributed renewable power generation capacity in the distribution network will continue to increase. The demand for distributed energy storage and DC load power supply is increasingly emerging, and the trend in the distribution network There is an increasing demand for two-way interaction between power grids and users, high-voltage power distribution networks and medium-voltage power distribution networks, and medium-voltage power distribution networks and low-voltage power distribution networks. DC power distribution has the advantages of two-way flexible and controllable power, fault isolation, and fast response. The evolution of the public distribution network to the physical form of AC and DC hybrid power supply may become the development direction of the future distribution network.
　　

Theory

　　Converting AC and DC at the same time

　　 Flexible substation is a combination of power electronic technology and substation technology. It is characterized by the power electronics of primary equipment and simplifies the types and quantities of substation equipment. Flexible substation is not only an energy transmission node in the grid, but also a grid regulation node and load regulation node. It aims to improve grid state parameters and enhance the precise and flexible control of power flow. It can achieve smooth regulation of grid active power, reactive power, and voltage, and improve The power grid has functions such as safe and stable operation level and efficient operation. Flexible substations are applied to the distribution network, which can improve the source-grid-load interaction capability, realize the friendly, effective reception and coordinated control of distributed power sources and diversified AC and DC loads, realize the rapid isolation of faults, and improve the power quality of users' power supply. .

Compared with conventional substations, flexible substations have outstanding advantages in system flexible control, multi-information flow interaction, improved power quality, rapid fault removal, improved operation and maintenance efficiency, and flexible access to the environment. At present, they save construction costs and save land. There are still shortcomings, which need to be further improved in the future.

The application of the power electronic converter in the flexible substation integrates the functions of voltage transformation, breaking, and reactive power compensation, which can save the original main transformer high and low voltage outgoing circuit breaker, and the low voltage reactive power compensation device in the station, simplifying the electrical main wiring And the layout of electrical equipment. Based on the real-time power situation of the power supply and load, the flexible substation comprehensively coordinates the power grid, distributed power supply and controllable load, realizes the multi-directional on-demand distribution of power flow, optimizes power flow path, and realizes multi-energy complementation and economic operation. The flexible substation enhances the management and control functions of the original substation. Through the information interaction with the distributed power supply on the outgoing side, the controllable and adjustable load, and the upper-level grid, from the perspective of control and management, it effectively organizes the power resources on the outgoing side to ensure the area The power supply and demand balance within the range. Power electronic equipment can achieve 100% isolation of the high and low voltage side of the harmonic related cross transmission, while effectively suppressing voltage sags, controlling the sag time, and improving the voltage qualification rate of the grid. The breaking time of ordinary AC circuit breakers is about ten milliseconds to tens of milliseconds. Power electronic converters and solid state switches greatly shorten the fault removal time, improve system stability, and protect important electrical equipment.
　　

The flexible substation adopts a modular design method. Each module can be independently checked and repaired when other modules are working normally. This avoids the problem of overall outage for maintenance when the existing substation equipment fails, and improves the power supply capacity and operation and maintenance efficiency. Flexible substation adapts to the development of a variety of distributed power sources and large-scale DC loads. In addition to ordinary AC outlets, it also provides DC interfaces, which can be flexibly connected to various AC and DC loads such as electric vehicles, distributed energy storage and distributed power supply.
　　

 practice

　　Support the flexible and stable operation of the power grid
　　 

The AC/DC distribution network and flexible substation demonstration project is one of the key demonstration projects built by the State Grid Corporation of China during the 13th Five-Year Plan period. It is an important part of the Zhangjiakou Renewable Energy Demonstration Zone and the State Grid Corporation’s Global Energy Internet Demonstration Zone. The company plans to build the world's first fully controllable power electronic flexible substation and an AC/DC hybrid distribution network with flexible control. The AC-DC hybrid distribution network has a flexible and highly controllable structure, high transmission efficiency, and good power quality. It can simplify the access method of distributed power sources, and is highly compatible with distributed wind power and photovoltaic DC power grids, and supports energy storage and DC loads. Access. The flexible substation mainly realizes power supply of 10 kV voltage level, can realize low-voltage AC and DC mixed output, and can be flexibly connected to distributed power generation, decentralized energy storage devices and AC and DC loads to achieve high-quality consumption and flexible access of clean energy And flexible deployment, improve the coordination and interaction level of regional energy "source-grid-load", and support the flexibility and system stability of the future grid operation mode.
　　 

It is expected that after the completion of the project in 2018, it will play a demonstrative role in the development of the future distribution network. The project will realize the integration of smart grid and cloud computing industry for the first time, and will realize the high reliability requirements of data center for the first time, and the load will be directly supplied by the DC grid. The construction of the world's first fully controllable power electronic flexible substation AC/DC distribution network and flexible substation demonstration project can support the flexibility of the future distribution network operation mode and system stability. The demonstration project plans to build a DC power distribution network with a two-terminal topology, combined with the ability of flexible substations to achieve power flow regulation, and exert the advantages of long transmission distance, large transmission capacity, and small transmission loss of the DC grid. Provides solutions for the system construction, operation control and protection configuration of AC and DC distribution network, and explores the shape of the future distribution network. The demonstration project can realize the DC transmission of the photovoltaic power station, and realize that the megawatt-level capacity of the centralized photovoltaic power station is directly boosted and connected to the DC distribution network without the DC/AC inverter link, eliminating the inverter link of the photovoltaic power station access system. It is a useful attempt to absorb new energy power generation, especially photovoltaic power generation.
　　

 Flexible substation is mainly characterized by the power electronics of primary equipment, with medium/low voltage, AC/DC multi-terminal interfaces, integrated power flow flexible management and control, multi-form energy access, fault current limiting, fault isolation and other functions. Flexible substation is no longer just an energy transmission node, but also a grid power flow and state parameter control node, as well as a load control node. It can realize the flexible and rapid adjustment of variables and parameters such as grid injection power, line power flow, bus voltage, and "source-network". The coordinated control of "-load" realizes the rapid isolation of short-circuit faults, and provides strong technical support for the safe, stable and economic operation of the power grid. With the gradual increase of new energy penetration rate and large-scale access of diversified loads, the gradual development and maturity of high-voltage, high-frequency, large-capacity, and low-loss SiC power devices, the efficiency of flexible substations will be further improved, and the volume will be greatly reduced , Can realize the efficient operation and intensive construction of flexible substations. In the future, flexible substations will be more widely promoted in the distribution network, and will be applied in higher voltage transmission grids, which will strongly support the development of smart grids and the global energy Internet.