Application of power electronics technology in distributed power generation (Electronic Information Engineering, School of Electricity, Zhejiang University
3080 104394) Abstract: Distributed power generation has attracted the attention of all countries in the world because of its high efficiency, cleanliness and flexibility, and has become one of the most important research directions of power system in 2 1 century. This paper mainly introduces the application of power electronics technology in distributed power generation through the conversion of power electronics technology to electric energy and the improvement of power quality by power electronics technology. Keywords: power electronics distributed power generation distributed power supply power conversion power quality power electronics distributed application
Yi Dai yin Xiang (college)
Department of Electrical Engineering, Zhejiang University, Hangzhou) Abstract: Because of its
Efficient, clean, flexible and distributed
Power generation is paid more and more attention by many countries in the world.
It has become one of the most important studies of power system in 21ST. This paper briefly introduces the application of power electronics technology in
DG's energy conversion and power improvement through power electronics
Quality of power electronics. Keywords: power
Electronics; Distributed generation; Distributed source; Power quality
0 Introduction Distributed Generation (DG) technology is an important direction of energy technology in the future, that is, the electric power field. It has the characteristics of high energy utilization rate, improved energy supply reliability and good economic benefits. Especially for countries with large population and limited resources, distributed power generation technology is the best choice for sustainable development. [1] Although distributed power generation technology has great application potential, it has not been widely accepted by the power sector at present. This is mainly because there are a large number of distributed resources in distributed power generation technology. On the one hand, the technology of how these distributed power sources are connected to the power grid through electric energy conversion is not very mature; On the other hand, when a large number of distributed power sources are connected to the power grid, the fundamental changes in the distribution network have caused profound changes in various protection settings and mechanisms of the power grid. At the same time, the grid-connected operation of distributed generation may cause power quality problems such as voltage and frequency offset, voltage fluctuation and flicker. [2] and a large part of these problems can be solved by power electronics technology. The definition of 1 distributed generation 1. 1 distributed generation DG is relative to the traditional centralized power supply mode, which refers to the modular power generation facilities located at or near the load and compatible with the environment. They are either connected to the distribution network or run independently to generate electricity economically, efficiently and reliably. Its main structure is shown in figure 1.
Huang Shengli, Kong Li, Zhang Guowei. Application of power electronics technology in microgrid [J]. Electrical Applications, 2008,27 (9): 55-58. [2] Mo Yingtao, Wu Weilin. Application of power electronics technology in distributed power generation [J]. North China Electric Power Technology, 2004,9.
Figure 1 1.2 Characteristics of distributed power generation DG system is small in scale and low in power; High efficiency, economy, reliability and little pollution; Operating independently or connected to the distribution network and located near the load; For renewable energy distributed power generation, the output power is intermittent. After DG was put forward and used, it was once regarded as a good medicine to solve the shortcomings of the existing large power grid, such as bloated structure and separation of power supply and consumption. Because of its inherent characteristics, there are still many problems to be further promoted. Compared with the traditional power generation mode, DG has small capacity and unstable energy output. These problems are caused by the inherent weakness of distributed generation, and it is difficult to overcome them independently. [3]2 Application of power electronics technology in distributed power generation electric energy conversion 2. 1 Basic classification of distributed power generation electric energy changes According to different primary energy sources used, distributed power sources can be roughly divided into two types: one is DC source type, such as solar energy, fuel cells and storage batteries; The other is the type of high-frequency AC power supply that needs rectification, such as wind turbines and micro gas turbines. Both of these power supplies need to be converted into standard power frequency alternating current to supply the load or connect to the power grid. Therefore, AC -DC, DC-DC and DC- AC converter technologies are used in the whole energy conversion process. 2. 1. 1 AC -DC conversion wind turbines and micro gas turbines are unstable AC power sources, which need to be converted into DC power sources first, and then converted into stable AC power sources through inverter technology. Diode rectification technology is usually used. 2. 1.2 DC-DC conversion solar energy, fuel cells and storage batteries are DC power supplies. Because of its low voltage level, the boost circuit is inverted before DC-DC must be used to boost the voltage to a suitable voltage level. In addition, the distributed power supply has the characteristics of long response time when the power output changes. For example, the response time of micro gas turbine is in seconds, while the fuel cell takes several minutes, so when the load changes suddenly or the given power changes, there will be insufficient active power supply. Solar energy and wind power generation have the characteristics of great fluctuation, so it is necessary to add energy storage units to the system. The energy storage unit can be a super capacitor or a battery, and a booster circuit is also needed to boost the bus voltage. On the contrary, when the bus voltage is too high, the step-down circuit is needed to charge the energy storage unit, so the energy storage unit often uses bidirectional DC-DC to charge and discharge. [4]2. 1.3 DC- AC conversion distributed power supply needs to use DC- AC to convert DC into standard AC for load power supply or grid connection after being converted into DC with appropriate voltage level by AC -DC or DC-DC technology. 2.2 Several power electronic technologies specifically applied to distributed power generation At present, several commonly used and low-cost systems are recognized as follows: [5] (1) wind power generation system; (2) photovoltaic cells; (3) Micro gas turbine; (4) Fuel cells. In these new distributed power generation systems, power electronic equipment plays a key role in energy conversion. Any form of distributed power generation must solve the problem of interface energy conversion between distributed power supply and power grid, and between users and energy storage system.
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2.2. 1 power electronics technology in wind power grid-connected system Denmark began to study wind power technology at the end of 19. It belongs to DGRs of AC nature. Wind power generation technology is a power generation technology that converts wind energy into electric energy, and its output power is determined by wind energy. The wind speed acts on the blades of the wind turbine to generate torque, which drives the wheel disc to rotate, and then is connected with the rotor of the asynchronous generator through the high-speed shaft, brake disc and coupling of the gearbox, so as to generate electricity and run. Wind energy is unstable because the magnitude and direction of natural wind speed are random. How to make the output power of wind turbine stable is an important problem of wind power generation technology. For an asynchronous generator system, diode rectifier is used for rectification first, then inverter is used for inversion, and then it is connected to AC power grid; The sum of mechanical frequency and rotor slip frequency is equal to the grid frequency, and the rated power of the inverter depends on the selected speed range. When the asynchronous generator runs above the rated synchronous speed, the conversion system can realize the reverse flow of power. [6] 2.2.2 Power electronics technology in photovoltaic power generation system The photovoltaic power generation system is a kind of DGRs with DC property, which is a system that converts DC electric energy generated by solar cells into AC electric energy with the same frequency and phase as the grid voltage, and at the same time realizes power supply to the load and power generation to the grid. The core of grid-connected system is grid-connected inverter, which should also have some functions and characteristics of independent photovoltaic power generation system. It is mainly composed of solar cell array and inverter. The general method of energy conversion in photoelectric system is to use DC- AC inverter to convert DC energy stored in photovoltaic cells into AC voltage synchronized with large power grid. [7] 2.2.3 Power electronics technology in fuel cell power generation system Fuel cell is a kind of DC DGRs, which converts chemical energy into electric energy through electrochemical process, and has the characteristics of high efficiency, cleanliness, no pollution, low noise, convenient installation and economy. The DC voltage generated by fuel cell is converted into AC voltage by DC- AC inverter, and its conversion process is similar to that of photoelectric system, which has many advantages compared with DC transmission and AC transmission. [8] Therefore, in the above-mentioned power generation types, the transmission of electric energy is in the form of DC transmission, but large power grids and people's life and production need AC with stable frequency, so power electronic interface devices such as rectifiers and inverter circuits, which are composed of power electronic devices, play an extremely critical role in the energy conversion and transmission of distributed power generation systems. 3. Application of power electronics technology in improving the power quality of distributed power generation 3. 1 Distributed power generation (DG) adversely affects the power quality (1), which affects the voltage flicker, that is, the visual experience caused by unstable illumination. The main reason of voltage flicker in traditional power grid is the instantaneous change of load. With the introduction of distributed generation, it will bring other factors that cause voltage flicker. These factors are mainly the following aspects: the start-up of large-scale distributed units, the short-term drastic changes in the output of distributed units, and the adverse effects caused by the interaction between distributed units and voltage feedback control equipment in the system. [9](2) Bringing a lot of harmonics to the system As we all know, there are a lot of nonlinear components in the power system, which leads to a lot of harmonics. The harm brought by the introduction of harmonics to the power system is: increasing the power loss of power stations and user equipment; Sensitive load or control equipment failure; The excessive proportion of harmonic components in power grid waveform will shorten the life of some power equipment. [10] due to the wide application of power electronic devices in distributed power generation, a large number of nonlinear loads are added to the power supply system, which inevitably brings a large number of harmonics to the system. As for the amplitude and order of harmonics, it is influenced by the power generation mode and the working mode of converter.
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3.2 Improving Power Quality through Power Electronics Technology The Electric Energy Research Association (EPRI) has done a lot of in-depth research in order to find advanced technologies to improve the performance of distributed systems. This customized power technology combines modern power electronic controller, distributed automation and complete communication to provide high-quality power for user terminals. Although very useful, the application of customized power supply equipment in distributed systems is limited. In recent years, some devices for rapid control have been developed one after another. Solid state circuit breaker (SSB), static var compensator (STATCOM) and dynamic voltage recovery (DVR) all belong to modern power electronic controllers. The cooperation of STATCOM, LTC and mechanical conversion capacitor can reduce the voltage fluctuation of the system. These power electronic devices for distributed system control, represented by STATCOM, have been fully demonstrated. These devices can not only realize continuous control, but also respond to system changes in real time. It is conservative to use power electronic equipment to control power quality in distributed systems, mainly because the cost is too high, and this method is only used for very important loads (such as hospitals). The most common power electronic equipment is UPS, which is widely used in computer systems. [1 1] With the further application of computer technology in life and production in the future, the demand for economical power electronic equipment will increase sharply, and some of them will be used to deal with instantaneous disturbance, voltage sag or other power quality problems. Conclusion Due to the disadvantages of current power generation mode, the exhaustion of non-renewable energy and the worldwide attention to environmental protection, distributed power generation will become the most important power generation mode in the future. From the analysis of distributed power generation in this paper, we can see that power electronics technology is widely used in distributed power generation, so vigorously studying and popularizing power electronics technology can open a new breakthrough for distributed power generation technology, thus further promoting the popularization and popularization of renewable energy. References [1] Huang Shengli, Kong Li, Zhang Guowei. Application of power electronics technology in microgrid [J]. Electrical Applications, 2008,27 (9): 55-58. [2] Mo Yingtao Wu Weilin. Application of power electronics technology in distributed power generation [J]. North China Electric Power Technology, 2004, 9:48-54. [3] An Mingrui, Wu Bingbing, Qiao Kun. Overview of distributed power generation and its application [J]. Electric Power Application Technology, 20 10, 13 (2): 40-43. [4] Liang Youwei, Chen,. Research on distributed generation and its application in power system.
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