What are the main frontier research hotspots of power electronics application in power system at present?

The power generation link of power system involves all kinds of equipment of generator set, and the application of power electronics technology is mainly to improve the operating characteristics of these equipment.

(1) Static excitation control of large generators. Static excitation adopts thyristor rectifier self-shunt excitation mode, which has the advantages of simple structure, high reliability and low cost, and is widely used in major power systems in the world. Because the intermediate inertia link of vibration exciter is omitted, it has its unique rapid adjustment, which provides favorable conditions for the advanced control law to give full play and produce good control effect.

(2) Variable-speed constant-frequency excitation of hydro-generator and wind-driven generator. The effective power of hydropower generation depends on the head pressure and flow. When the water head changes greatly (especially for pumped storage units), the optimal speed of the units will also change. The effective power of wind power generation is directly proportional to the cubic power of wind speed, and the speed at which windmills capture the maximum wind energy varies with the wind speed. In order to obtain the maximum effective power, the unit can run at variable speed. By adjusting the frequency of the rotor excitation current, the stator frequency, that is, the output frequency, remains constant after being superimposed with the rotor speed. The technical core of this application is frequency conversion power supply.

(3) Frequency conversion speed regulation of fans and pumps in power plants. The average auxiliary power consumption rate of power plant is 8%, and the power consumption of fans and pumps accounts for about 65% of the total power consumption of thermal power equipment, so the operation efficiency is low. Using low-voltage or high-voltage frequency converter to implement frequency conversion speed regulation for fans and pumps can achieve the purpose of energy saving. Low-voltage inverter technology is very mature, there are many manufacturers at home and abroad, and there are a complete series of products.

(4) Solar power generation control system. Developing and utilizing inexhaustible clean new energy-solar energy is an important strategic measure to adjust the future energy structure. High-power solar power generation, whether it is an independent system or a grid-connected system, usually needs to convert the direct current emitted by the solar cell array into alternating current, so the inverter with the maximum power tracking function becomes the core of the system. The Sunshine Plan implemented in Japan is mainly a 3 ~ 4 kW household grid-connected power generation system, and most of the power transmission projects implemented in China are independent systems of 10 ~ 15 kW, and the large-scale system is Siemens Solar Power Station (7.2MW) in California, USA.

Application in transmission link

(1) Flexible AC transmission technology (FACTS) The operational performance of AC transmission or power grid. The applied FACTS controllers include static var compensator (SVC), static capacitor (STATCON), static fast exciter (PSS) and series compensator (SSSC). In recent years, flexible AC transmission technology has been applied in important EHV transmission projects in the United States, Japan, Sweden, Brazil and other countries. FACTS has also been deeply studied and developed in China.

(2) HVDC transmission stations can be relocated, which can make medium-sized HVDC transmission projects competitive in a short transmission distance. In addition, the converter composed of turn-off devices can avoid commutation failure because of the use of turn-off power electronic devices, and there is no requirement for the capacity of the receiving system, so it can be used to supply power to isolated small systems (offshore oil platforms and islands), and can also be used for urban distribution systems and access in the future.

In recent years, DC transmission technology has made new development. In hvdc light, IGBT and other power electronic devices are used to form the converter, and the passive inverter adopts pulse width modulation technology, which solves the problem of dc transmission to the load point without AC power supply. At the same time, the equipment is greatly simplified and the cost is reduced.

(3) Static Var Compensator (SVC) SVC is a solid-state switch with thyristor as the basic element rather than an electrical switch, which changes the admittance of the transmission system quickly and frequently by controlling reactors and capacitors. SVC can have different loop structures, which are called Thyristor Switching Capacitor (TSC), Thyristor Switching Reactor (TSR) or Thyristor Controlled Reactor (TCR) according to different control objects and control modes.

Application in power distribution link

How to strengthen the reliability of power supply and improve power quality is an urgent problem for distribution system. Power quality control should not only meet the requirements of voltage, frequency, harmonics and asymmetry, but also suppress various transient fluctuations and interference. The application of power electronics technology and modern control technology in distribution system, that is, customized power technology. Customer Power Technology (CP) and FACTS technology are sister new power electronic technologies that are developing rapidly. The core of adopting FACTS is to strengthen the controllability of AC transmission system and increase its transmission capacity; The purpose of developing cleaner production is to strengthen the reliability of distribution system and improve the quality of power supply. The basic technologies of CP and FACTS are both power electronic technologies, and their controllers are the same in structure and function, the only difference is the difference in rated electrical values. At present, the two technologies have gradually merged into one, which is the so-called DFACTS technology. Representative consumer power technology products include: Dynamic Voltage Restorer (DVR), Solid State Circuit Breaker (SSCB), Fault Current Limiter (FCL), Unified Power Quality Regulator (PQC) and so on.

Development of Power Electronics Technology in China

1. Prospect of distribution automation

Distribution automation is the most important investment of smart grid: as the last link of transmission and distribution system, the automation degree of distribution network is closely related to the quality and reliability of power supply and demand. Distribution automation is one of the important foundations of smart grid. From the perspective of investment composition, we predict that distribution automation will account for about 40% of smart grid investment composition, which is the top priority of smart grid investment. China's distribution automation is in the primary stage; China's distribution automation is in the primary stage. The feeder automation rate of domestic urban distribution network is less than 10%, and the proportion of distribution automation in foreign countries has reached 60%-70% before the project. China has just started the pilot project, and there is a vast market space in the future.

2. Introduction of distribution automation

Distribution automation refers to combining modern electronic technology, communication technology, computer and network technology with power equipment, organically combining the monitoring, protection, control and measurement of distribution network and the work management of power supply department under normal and accident conditions, improving the quality of power supply, establishing a closer and more responsible relationship with users, meeting the diversity of users' needs at reasonable prices, and striving to achieve the best power supply economy and more effective enterprise management. Distribution automation is a huge, complex and comprehensive system engineering, including all related functional data flow and control of power enterprise distribution system. From the perspective of ensuring the quality of power supply to users, improving service level and reducing operating costs, distribution automation is a unified whole.

Distribution automation includes the following four aspects: ① feeder automation. Feeder automation completes feeder monitoring, control, fault diagnosis, fault isolation and network reconfiguration. Its main functions are: operation state monitoring, remote control and local autonomous control, fault area isolation, load transfer and power supply recovery, reactive power compensation and voltage regulation. ② Substation automation. Substation automation refers to an automation system that uses automatic control technology and information processing and transmission technology to monitor, measure and operate substations through computer software and hardware systems or automation devices instead of manual work. Substation automation marked by signal digitization and computer communication technology has entered the field of traditional substation secondary equipment, which has made great changes in substation operation and monitoring and achieved remarkable benefits. The basic functions of substation automation include: data acquisition, data calculation and processing, over-limit and state monitoring, switch operation control and locking, information exchange with relay protection, coordination and cooperation of automatic control, information exchange with other automation devices in substation and communication with dispatching control center or centralized control center. Substation automation technology is one of the key points of distribution automation. ③ Power distribution management system. Distribution management system (DMS) refers to the use of modern computers, information processing and communication technologies and related equipment to monitor, manage and control the operation of the distribution network. It is the nerve center of the distribution automation system and the monitoring, control and management center of the whole distribution automation system. The main functions are: data acquisition and monitoring (SCADA), distribution network operation management, user management and control, automatic drawing/equipment management/geographic information system (AM/FM/GIS) and so on. ④ Demand side management. Supply and demand sides participate in power supply and demand management through a series of economic policies and technical measures. Includes load management, power consumption management and demand side power generation management. Several contents of DSM involve both power supply and demand, and even the power management system, which must be regulated by legislation and corresponding rules, and finally by the power market. It can be seen that the power supply and demand sides are not only a power buying and selling relationship, but also a cooperative partnership with the interests of both sides as the link. In the electricity market environment, demand side management will be paid attention to.

3. Development trend of distribution automation

According to the research on the development trend at home and abroad, the development of distribution automation technology presents the following characteristics:

1) Diversification Although the development of distribution automation technology has gone through three stages, from the application situation in Japan and other countries, the technology in each stage is in use, and each has its own scope of application: feeder automation system based on automatic switching device is suitable for rural power grid and other areas with low load density, long power supply radius, many obstacles and poor power supply reliability; The second phase of distribution automation system is suitable for small and medium-sized cities and counties. The three-level distribution automation system based on artificial intelligence is suitable for big cities and important parks. Even the distribution network information system with remote communication and telemetry function but no remote control function has its application prospect, mainly because it can directly use public communication resources (such as GPRS) without building a special communication network.

2) Integrated distribution automation involves a wide range. It not only has its own real-time information collection part, but also has a lot of real-time, non-real-time and real-time information that needs to be obtained from other application systems. For example, obtain the information of main power supply network and substation from the ground dispatching automation system; Obtaining the extension model and related graphics of distribution lines in GIS system; Obtaining parameters of power distribution equipment from PMS system; Obtain user information from power consumption marketing system/load control system, etc. Therefore, the distribution automation master station is no longer a single real-time monitoring system, but a comprehensive application system that integrates multiple application systems related to distribution. In order to standardize the integration and interface between application systems, IEC has formulated IEC 6 1968 series standards, and proposed that several relatively independent and parallel application systems can be integrated by using information exchange bus (enterprise integration bus), so that each system can continue to play its own characteristics while realizing information exchange and form an effective application whole.

3) Intelligent distribution system is an important part of smart grid, and intelligent distribution system is the development direction of distribution automation. Therefore, distribution automation is closely related to the realization of smart grid, mainly in: self-healing distribution technology. This is the basic function of fault diagnosis, location, isolation and power supply recovery of feeder automation in distribution automation system, which needs to be further upgraded to adapt to the two-way energy flow of distributed generation under the background of smart grid. Efficient operation technology. This is the function of high-level application software in distribution automation system. Under the background of smart grid, it is necessary to further upgrade the business functions of asset optimization and intelligent scheduling considering the whole life cycle of equipment. Access technology of distributed power supply and energy storage system. This is a new requirement faced by distribution automation system, especially involving the calculation and analysis of distribution power flow and the influence of distributed generation on power grid. Custom power supply technology. According to the relevant standards of power quality, different technologies and prices are provided for different grades of power quality to meet the needs of different users for power quality grades. Distribution automation system is one of its technical support means. User interaction technology. This is a power outage management function in distribution automation system, which needs to be further upgraded to adapt to the two-way interaction of users under the background of smart grid.

At present, China's electric power is developing towards smart grid, and its technology and development are gradually becoming intelligent. The application of power electronics technology in power field can accelerate the intelligent development of power system.