cr: Power Engineering Technology Magazine

State Grid Jiangsu Electric Power Co., Ltd. Electric Power Research Institute, etc. Wu Shengjun Wang Yi

1 Introduction

  Due to the limitation of the technical level of DC transmission, DC transmission and distribution are more difficult to achieve than AC. Today's power system is still dominated by AC transmission and distribution. In recent years, with the development of power electronics technology, the technical problems faced by DC transmission and distribution have been gradually solved, and the technical advantages of DC systems have also emerged. Compared with the AC system, the DC system can significantly improve the operation level of power transmission and distribution, make power transmission and distribution simpler and more efficient, and reduce the cost of power transmission and distribution.

  On the power user side, a large number of DC terminals such as photovoltaic power generation, energy storage batteries and modern power electronic loads are connected, making the DC power supply system more advantageous than the AC system. Most renewable energy power generation systems are DC power sources, such as photovoltaic cells and fuel cells. Although the wind turbine is an AC motor, it needs to undergo AC-DC-AC conversion to be connected to the grid through AC, and the double conversion can be avoided through DC grid connection, making the grid connection of the wind power system more convenient and efficient. Modern power electronic loads such as TVs, LED lights, phones, and computers are all DC loads. The popularity of electric vehicles in the future will increase the demand for DC power supply and promote the development of DC power supply.

  It can be seen that the development of DC power supply technology is mainly driven internally by the advantages of DC technology and externally promoted by the development of distributed energy and DC loads. This article first introduces the development status of DC power supply technology, then analyzes the key technology and equipment development of DC power supply, and sorts out the engineering research and application of DC power supply.

2. Current status of DC power supply technology

2.1 Development of low-voltage DC technology

  In the 1880s, Andison Electric Lighting Company used the "Juhan" DC generator to power thousands of incandescent lamps, forming the prototype of DC power supply technology.

  By the end of the 20th century, low-voltage DC power distribution has been successfully applied to data communications, aviation, ships and urban rail transit, which require high power supply quality.

  In 2010, Virginia Tech proposed the SBN (sustainable building and nanogrids) system, which has two voltage levels: DC 380V and DC 48V. The University of North Carolina in the United States proposed the FREEDM (the future renewable electric energy delivery and management) AC/DC hybrid distribution network for receiving and managing new energy. Scholars from the United Kingdom, Switzerland, and Italy proposed a similar function UNIFLEX-PM ( universal and flexible power management) system.

  Domestically, in the "863" project "Research and Application of Key Technologies for Smart Power Distribution Based on Flexible DC" undertaken by Zhejiang University, research has been carried out on the basic framework, power interface, converter configuration and economy of DC distribution networks. . The State Grid Corporation of China has also carried out research on DC power supply technology. The Jiangsu Provincial Electric Power Research Institute has established a new energy and intelligent distribution network coordination control laboratory integrating AC-DC hybrid and wind-solar storage to carry out distributed new energy grid-connected and AC-DC Research on hybrid power grids and other directions.

2.2 DC voltage level and power supply standard

  The voltage level and sequence of DC power supply are related to key issues such as power grid security, economy, and load adaptability, and have an important impact on the future development of the power grid. Special industries such as communications, transportation, shipping and aviation have relatively small DC loads, high requirements for power supply reliability, and demand for electrical energy storage. Therefore, DC power supply systems have been adopted earlier, among which communication and shipbuilding industries have adopted more voltage levels . Figure 1 shows the DC voltage levels of various industries.

Figure 1 DC voltage levels of various industries

Table 1 Voltage level sequence of medium and low voltage DC power supply system

  In terms of international standards, many international organizations have carried out research work on DC power grids, and China plays an important role in the formulation of low-voltage DC standards. The International Electro-technical Commission (IEC) established a research group TC-57 to study the management and information exchange of DC grids in the future. In January 2017, IEC established the Low-Voltage DC Power Supply Committee (SyCLVDC) to study the application scenarios, feasibility and safety of low-voltage DC. In October of the same year, the Low-Voltage DC Distribution Network Working Group (IEC TC8 WG9) was established. The institute led the convening to carry out the standardization of low-voltage DC distribution network.

3. Key technology of DC power supply

3.1 DC distribution network coordinated control technology

  DC distribution network control can be divided into unit level, microgrid level and distribution network level according to the system level. In general, there are more researches on the unit level and DC microgrid level of power electronic converters, and less research on the distribution network level. It is necessary to study related DC distribution network control theories and technologies on the basis of the former two to support DC Development of distribution network.

(1) Unit level control

  In the DC distribution network, unit-level control is mainly the control of power electronic converters. According to the operation requirements of the microgrid and distribution network, each converter controls the voltage, current and power to ensure the normal operation of each unit and system.

(2) Microgrid level control

  Microgrid-level control in DC distribution network can be divided into two types: bus voltage control and power quality management.

(3) Distribution network level control

  Distribution network-level control research includes distributed power generation forecasting and load forecasting technology, DC-based multi-terminal and multi-voltage distribution network operation control technology, DC distribution network economic optimization scheduling method, and DC distribution network two-way power flow quality control technology, etc. .

3.2 DC distribution network protection technology

  Due to the access to diversified distributed power sources, loads, and energy storage, the DC power distribution system has many different operating states. At the same time, the DC distribution network is completely different from the AC distribution network in fundamental technologies such as electrical characteristics and measurement methods. There is no low-cost, commercially applicable large-capacity DC circuit breaker, and related DC protection technologies and equipment lack standards and equipment. Lack of operating experience. Therefore, the protection configuration of DC power distribution system faces many challenges.

(1) Fault type of DC distribution system

  There are a large number of power electronic devices in the DC distribution network, and they are close to user terminals. The faults are complex and diverse. In addition to short circuits, ground faults and abnormal operation of insulation degradation, DC distribution networks also have AC and DC hybrid connections, DC ring networks, etc. malfunction.

Table 2 Failure types of medium and low voltage DC distribution system

(2) Principle and composition of DC active protection

   Active protection is based on the topological structure and control principle of power electronic converters, and integrates the protection action into the converter control logic. Based on multiple protection strategies, the isolation unit and power electronic devices of the power electronic converter are effectively used to realize the DC distribution system. The natural isolation of multiple faults and the breaking of serious fault circuits prevent minor faults from developing into serious faults and guarantee the normal operation of the system to the greatest extent.

Table 3 Types of active protection

(3) Protection equipment for DC distribution network

  At present, the switching methods of DC circuit breakers mainly include increasing arc voltage method, segmented series connection current limiting resistance method, magnetic field control gas discharge tube interruption method, superimposed oscillating current method, current transfer method, etc. Based on these methods, DC circuit breakers of various capacity levels have been researched and tested at home and abroad. At present, low-voltage DC circuit breakers below 400V have been industrialized. Although the research and development of medium and high-voltage DC circuit breakers have made some breakthroughs, they are still far from industrialized applications. In addition, when the commonly used AC-type multi-function wiring boards and plugs are applied to low-voltage DC distribution networks, large arcs will be generated at the moment of connection and disconnection, which brings safety hazards. Therefore, the research and development of DC switches, DC plugs and sockets is the basic work to promote the popularization and application of DC distribution networks.

3.3 Power quality control of DC distribution network

  The DC distribution network does not have the voltage phase and frequency problems in the AC system. Its power quality mainly refers to the balance of active power and voltage. Typical power quality problems include DC voltage sags and swells, voltage deviations, voltage fluctuations, DC modulation harmonics, etc. .

(1) The DC voltage dips and rises. Transient changes in the output power of the micro power supply in the DC distribution network, the momentary load on or off, the switching of the microgrid and off-grid, and the voltage interruption of the AC side system may all cause the DC feeder voltage to sag or rise.

(2) DC voltage deviation and fluctuation. Active power imbalance is the root cause of the voltage deviation of the DC distribution network. Changes in the output, operation mode, network structure, and load of the micro power supply will produce different degrees of active power imbalance, causing voltage fluctuations in the distribution network.

(3) DC modulation harmonics. The multi-source and multi-conversion equipment in the DC distribution network is a variety of harmonic sources, which inject harmonics into the grid in different ways.

  The power quality control methods of DC distribution network mainly include: using additional equipment for targeted management, using micro power sources to participate in power balance adjustment, and using power electronic interface control algorithms to improve the operating environment of the distribution network.

4. DC power supply key equipment

4.1 AC/DC converter

  AC/DC converter is the basic equipment of DC distribution network. Its control efficiency directly affects the stable operation of DC distribution network and the coordinated distribution of DC power. Converters are mainly divided into current source type and voltage source type.

  Current source converters have obvious technical advantages in high-voltage and long-distance DC transmission, and are widely used in DC transmission projects. However, in the DC distribution network, due to the large number of network terminals and frequent flow reversals, it causes great inconvenience to the application of current source converters.

  With the advancement of power electronic devices and control technology, voltage source converters are developing rapidly. Three-phase, two-level, and multi-level converter topologies are currently used in many projects.

4.2 Power electronic transformer

  Power electronic transformer is a kind of power electronic converter with transformer function, which integrates electrical isolation, voltage conversion, energy transfer and other functions, and can realize AC and DC interconnection and access functions. At present, power electronic transformers have been research hotspots at home and abroad, and certain results have been achieved in circuit topology, control and prototype development. The power electronic transformer indicators of various institutions are shown in Table 4.

Table 4 Comparison of power electronic transformer indicators

4.3 Low voltage DC circuit breaker

  The DC circuit breaker is a key device related to the protection and safe operation of the DC distribution network. It is of great significance to the flexible operation of the system and preventing the expansion of the fault range. According to the breaking principle, DC circuit breakers can be divided into three types: mechanical, solid-state and hybrid. Mechanical DC circuit breakers have the advantages of high reliability, low cost, and low on-state loss, but the breaking speed is slow and the controllability is not strong; the advantages of all solid-state DC circuit breakers are fast action and strong controllability. But at the present stage, the cost is relatively high, and the on-state loss is relatively large; the hybrid DC circuit breaker combines the good static characteristics of mechanical switches with the good dynamic performance of power electronic devices, and theoretically has short breaking time, small on-state loss, and no special cooling required. Equipment and other advantages, but the structure is complex, the technology is difficult, and the cost is high.

5. Research and application of DC power supply engineering

  With the development of power electronics technology, DC power supply and use usually appear together with power electronic converters. Due to its high controllability, reliability and economy, the current projects involving DC power supply and use are usually related to flexible DC and energy Internet related.

5.1 Research on typical foreign projects

  In 2011, the University of North Carolina in the United States proposed the FREEDM system structure. The system includes 400V DC bus and 120V AC bus plug-and-play interfaces, and proposed an intelligent energy management (Intelligent energy management, IEM) device, also known as an energy router, for Connect 12KV medium voltage AC power distribution bus and DC 400V and AC 120V low voltage power distribution bus.

  In 2006, Osaka University in Japan proposed a bipolar DC microgrid system structure. The system obtains 230V AC power through a 6.6KV distribution network and converts it to ±170V DC voltage through bidirectional rectification. A gas turbine is connected to the 230V AC bus, and distributed power sources such as super capacitors and photovoltaic cells are connected to the DC bus through a DC/DC converter.

  The typical structure of AC and DC distribution network designed by Finnish Elenia Oy is shown in Figure 2. This typical structure leads to a branch line from a medium-voltage AC 20 KV line, which is first stepped down to a low-voltage AC 0.63 KV through a 20/0.63 KV distribution transformer, and then boosted to a low-voltage DC 900 V through a rectifier converter, and then passed through a low-voltage DC 900V The distribution line is transmitted to the user, and converted into a low-voltage AC 500~600V by an inverse converter on the user side, and then output AC 400V through a 500~600/400V low-voltage isolation transformer to supply power to the end user.

Aachen University in Germany has completed the actual project of 10KV DC distribution network, and carried out 6 DC distribution research topics, including the medium voltage DC ring network distribution system and the experimental study of 10KV medium voltage DC distribution in campus.

5.2 Domestic typical projects

   Based on the research of DC power supply technology, the State Grid Corporation of China has carried out the construction of a demonstration zone for active distribution network application in Suzhou Industrial Park. The demonstration project for the application of AC-DC hybrid active distribution network technology based on flexible DC interconnection consists of ±20KV flexible DC interconnection. The system is composed of controllable units such as distributed photovoltaics, distributed wind power, micro gas turbines, batteries, electric vehicle charging and swapping loads, user-side distributed energy storage, and DC microgrid. The overall architecture is shown in Figure 3.

6. Conclusion

  The DC distribution network is the key to promoting the construction of smart grids and solving energy and environmental crises. Using the technical advantages of the DC distribution network, it can support the consumption of distributed renewable energy with high penetration rate and improve the reliability, stability and reliability of the distribution system. Economical, and build a clean, low-carbon, safe and efficient new energy system. This article introduces the development status of DC power supply technology, DC voltage levels and power supply standards, focusing on the analysis of key technologies such as coordinated control, protection and power quality control of DC distribution networks, as well as AC/DC converters, power electronic transformers and low-voltage DC The development of key equipment such as circuit breakers has also reviewed the research and application of DC power supply projects at home and abroad.

  At present, the research of DC power supply is still in the experimental and exploratory stage. DC power supply standards and specifications are being stepped up, and there are still a lot of problems that have not been resolved. Power supply technology research is concentrated on the DC microgrid level, and the performance of key equipment cannot meet the application requirements, and practical engineering applications are lacking. With the development of energy reform and the deepening of technical research, DC power supply will develop rapidly with strong technical and economic advantages, and will have a huge impact on the future energy structure.