Photovoltaic grid-connected power generation system[Copy link]
Human development and energy utilization are closely linked, especially today when industry is highly developed. Since the Industrial Revolution, human beings have made unrestrained demands on nature for their own development. The deteriorating ecological environment has gradually made people realize that human beings must take the path of sustainable development, and vigorously developing and utilizing renewable energy is the only way. As a huge renewable energy source, the radiation energy of solar energy reaching the earth's surface every day is equivalent to the energy of burning hundreds of millions of barrels of oil. The development and utilization of abundant and vast solar energy can produce no or very little pollution. Solar energy is not only an energy supplement urgently needed in the near future, but also the foundation of the future energy structure. Solar photovoltaic utilization technology has entered a stage of rapid development under this situation. The conversion and utilization of solar energy can be divided into three categories in a narrow sense: photoelectric conversion, photothermal conversion, and photochemical conversion. Solar photovoltaic power generation is one of the main aspects, that is, solar radiation energy is directly converted into electrical energy through photovoltaic cells, and matched with energy storage devices, measurement and control devices and DC-AC conversion devices to form a photovoltaic power generation system. Development Status at Home and Abroad In recent years, solar photovoltaic power sources have begun to transition from supplementary energy to alternative energy, and have developed from small and medium-power independent power generation systems in remote areas without electricity to grid-connected power generation systems. In 1979, with the support of the Department of Energy, the American Solar Associates developed a large photovoltaic module with an area of 0.9*1.8M and built a household rooftop photovoltaic test system. In 1980, the famous Carlisle was built at MIT. House", with a 7.5KW photovoltaic array installed on the roof, combined with passive solar houses and solar collectors to provide electricity, hot water and cooling for the building. More than 20 years ago, Japan's Sanyo Electric Company developed a tile-shaped amorphous silicon solar cell module that can output 2.7W of electricity per piece. By 1997, several megawatts had been installed. The United States and the European Union have successively implemented the "Million Roof Plan"; Japan plans to reach an installed capacity of 5GW for photovoltaic systems by 2010. The world's largest rooftop photovoltaic system was built at the Munich Exhibition Center in Germany. The capacity of the photovoltaic system installed in the first phase was 1MW, and now it has reached 2MW. France and India have also successively launched the "1-5KW Million Rooftop Photovoltaic Plan". Although China's photovoltaic technology has reached a certain level and foundation after 40 years of efforts. However, there is still a lot of gap compared with the world's advanced countries. At present, the market share of China's photovoltaic products is: household photovoltaic power sources and independent photovoltaic power stations account for 30%, the communication field accounts for 40%, other industrial fields such as railway and highway signal sources, meteorological station power sources account for 20%, and various civilian products account for 10%. . China has very rich solar energy resources. It is estimated that the solar radiation energy received by the land surface each year is about 50╳1018kj, which is equivalent to about 170 billion tons of standard coal. The total solar radiation across the country is 3340~8400MJ/m2?a, the national average annual sunshine hours are 2200h, and the average solar power is 1700TWh, which is about multiple times the current installed capacity. The total solar radiation in Tibet, Qinghai, Xinjiang, southern Inner Mongolia, northern Shaanxi and other vast areas of China is very large, especially the vast majority of areas in the Qinghai-Tibet Plateau have very rich solar energy resources, which have unique resource conditions for the development and utilization of solar energy. Household photovoltaic systems and independent photovoltaic power stations are important ways to solve the electricity consumption problems of residents and society in remote areas without electricity in China. For networked photovoltaic power generation systems, due to the high cost of photovoltaic applications in areas covered by the power grid, there is currently no competitiveness. There are only a few demonstration grid-connected photovoltaic power generation systems in China. In order to fill the domestic technical gap, the Ministry of Science and Technology of China issued the "1-5KW grid-connected inverter/control integrated machine" in November 1996. , "Ninth Five-Year Plan" National Key Scientific and Technological Research Project. The current status of photovoltaic power generation research in China is as follows: 1. Domestic photovoltaic grid-connected power generation is quietly rising, and 10 photovoltaic grid-connected power generation projects have been built; 2. National "Ninth Five-Year Plan" and "Tenth Five-Year Plan" key research projects: 5KW, 20KW and 50KW grid-connected photovoltaic power generation systems; 3. Beijing Jike Energy New Technology Development Company and Hefei University of Technology Energy Research Institute jointly cooperated to successfully develop a "3KW dispatchable grid-connected inverter" prototype, and formally passed the project acceptance organized by the Ministry of Science and Technology on July 13, 1999. 4. Shenzhen Garden Expo Park photovoltaic power generation system has been connected to the grid on the user side, with a capacity of 1000KW, the first in Asia and one of the few in the world. On Nan'ao Island in Guangdong, a beneficial attempt is being made to connect photovoltaic power generation and wind power to the grid. The first phase of 30KW has been connected to the grid and is operating well. The second phase of 70KW project is under construction. 5. 50KW butterfly rooftop grid-connected photovoltaic power generation system in Software Park, Shangdi Technology Development Zone, Beijing; 6. 10KW and 100KW grid-connected power generation systems of Beijing Tianpu Company; 7. 300KW rooftop grid-connected photovoltaic system of Capital Museum; 8. 10KW dispatchable grid-connected photovoltaic power generation system of Beijing Jike Company production base; 9. Rooftop power generation system of Beijing Ecological Park in Changping District; 10. Photovoltaic power generation projects of the upcoming Olympic venues and Olympic Park, etc. Structure and principle of dispatchable grid-connected system Photovoltaic grid-connected power generation system is divided into dispatchable grid-connected system (with a small amount of battery) and non-dispatched grid-connected system (without battery). The latter has a relatively low cost due to the lack of battery, but the effect of peak regulation is poor due to the inability to control the online time. This type of grid-connected system requires the inverter to have only a single grid-connected working mode and stop working when the grid loses power. The purpose of developing dispatchable grid-connected systems abroad is to regulate the peak of the power grid. Although it is equipped with batteries, its capacity is only required to meet the peak regulation of 3-4 hours per day, unlike the independent photovoltaic system, which requires the storage capacity to meet the use of 3-4 days. Therefore, the cost is much lower than that of the independent photovoltaic system. Since the online time can be controlled, the peak regulation effect of the dispatchable grid-connected system is greatly improved, which is very popular with the power department. The dispatchable grid-connected system requires the inverter to have both independent working and grid-connected working modes, which has greater flexibility and is more easily accepted by the power department as power peak regulation. At present, the development schemes of dispatchable grid-connected inverters mostly use 80C96 16-bit single-chip microcomputers with D/A conversion and MOSFET power modules to realize SPWM sinusoidal pulse width modulation current synchronous tracking grid-connected inverter/independent inverter switching control and other functions. The output of the grid-connected control inverter must be synchronized with the grid frequency and phase. At the same time, its AC ripple distortion should be able to meet the requirements of the power grid. It has perfect protection measures, such as anti-islanding effect, overvoltage, undervoltage, short circuit and lightning protection. There are three main aspects of the research on photovoltaic grid connection: the first is the maximum power point control; the second is the research on DC conversion circuits; and the third is the research on inverter circuits. 1. Maximum Power Point Tracking Since the maximum power output point of photovoltaic cells varies with light intensity and temperature. In order to make full use of solar energy, the system must track the maximum power point. The maximum power control method has also undergone a development process. Early photovoltaic systems used a constant voltage control method. The advantages of this method are that it is simple and easy to use, and it can basically track the maximum power point. However, with the development of power electronics and control technology, the simplicity of this method has become very uneconomical compared to the energy loss it causes. Therefore, some new control methods have emerged, such as the perturbation observation method (climbing method), the admittance incremental method, etc. (1) Constant voltage control (CVT) Under different light intensities, the silicon solar cell array has the volt-ampere characteristic curve shown in Figure 1 It shows that solar cells are neither constant voltage sources nor constant current sources, but a nonlinear DC power supply. The intersection points A, B, C, D, and E of the volt-ampere characteristic curve of the solar cell array and the load characteristic curve L are the working points of the photovoltaic system. If the working point can be moved to the maximum power points A', B', C', D', and E' of the volt-ampere curve of the photovoltaic array, the energy utilization rate of the photovoltaic array can be maximized. People have found that when the temperature is kept at a certain value, the maximum power point is basically on both sides of a vertical line, so the trajectory of the maximum power point can be approximately regarded as a vertical line with a constant output voltage. This is the theoretical basis for constant voltage control. However, this tracking method ignores the influence of temperature on the open-circuit voltage of the array. The factors that have the greatest influence on the junction temperature are the ambient temperature and solar irradiance. For conventional single-crystal silicon solar cells, when the ambient temperature increases by 1°C, the open-circuit voltage decreases by about 0.35% to 0.45%. Taking an array in Xinjiang as an example, the open-circuit voltage of the array is 363.6V when the ambient temperature is 25°C. , when the ambient temperature is 60℃, it drops to 299V, and the drop is 17.5%. This is an influence that cannot be ignored, and this cannot be overcome by using constant voltage tracking. (2) Maximum power point tracking (MPPT) In fact, the tracking of the maximum power point is a self-optimization process. By detecting the current output voltage and current of the photovoltaic cell, the current output power of the photovoltaic cell is obtained, and then compared with the photovoltaic cell power stored at the previous moment, the smaller one is discarded and the larger one is retained, and then detected and compared again. This cycle is repeated continuously, so that the photovoltaic cell can dynamically work at the maximum power point. ① Hill Climbing Method The main idea of the Hill Climbing Method is to periodically add disturbances to the output voltage of the solar cell and compare its output power with the output power of the previous cycle. If the power increases, add disturbances in the same direction in the next cycle, otherwise change the direction of the disturbance. ② Admittance Differential Hill Climbing Method The change in output power is simply considered to be caused by the change in the output voltage of the solar cell. This method cannot compare the output power of the solar cell with the actual maximum power point voltage, thus deviating from the actual maximum power point. Admittance differential method adjusts the output voltage of solar cells according to the voltage of maximum power, thus avoiding the occurrence of this phenomenon. dP/dV corresponds to the output voltage value one by one. (1) When dP/dV = 0, at the maximum power point (2) When dP/dV > 0, on the left of the maximum power point (3) When dP/dV < 0, on the right of the maximum power point and dP/dV = d(IV) / dV = I + VdI/dV, so by judging the sign of I/V + dI/dV, that is, G + dG, the position of the working point can be determined. 2. DC/DC converter part: DC conversion circuits are divided into direct conversion and indirect conversion. The former first converts DC voltage into AC voltage, and then converts it into DC voltage after transformer conversion; the latter does not involve the intervention of the intermediate transformer and directly changes the DC voltage. 3. DC/AC inverter part: The DC voltage output by the previous part is transferred to a sinusoidal AC power with the same frequency and phase as the power grid through this device and fed into the power grid. The sine wave inverter circuit adopts a full-bridge circuit, and the quality of the inverter output power directly depends on the SPWM signal. SPWM sinusoidal pulse width modulation can be divided into bipolar modulation (SPWM wave has two positive and negative levels), unipolar modulation (SPWM wave has three positive, negative and zero levels) and unipolar frequency multiplication modulation (SPWM wave has three positive, negative and zero levels, and the number of pulses of SPWM wave is twice that of unipolar modulation). The characteristic of the bipolar modulation method is that V1-V4 all work at a higher frequency (carrier frequency), and a sinusoidal output voltage waveform can be obtained, but at the cost of generating a large switching loss. The characteristic of the unipolar modulation method is that in one switching cycle, two power tubes switch complementary at a higher switching frequency to ensure that an ideal sinusoidal output voltage can be obtained; the other two power tubes work at a lower output voltage fundamental power, thereby greatly reducing the switching loss. The characteristic of the unipolar frequency multiplication modulation method is that the pulsation frequency of the output SPWM wave is twice that of the unipolar, while the switching frequency and unipolar modulation are unchanged, so the loss of a single switch tube is unchanged. In order to reduce the distortion of the output waveform, a higher carrier frequency should be used, but the increase in carrier frequency will inevitably produce more switching losses, and the dead zone effect will also be more obvious. From this point of view, the unipolar frequency multiplication modulation method is obviously better than the first two methods. The harmonic analysis of the modulated SPWM wave shows that the harmonics contained in the SPWM waveform under the unipolar and bipolar modulation methods are mainly near the carrier frequency and several times the carrier frequency. The harmonic component of the unipolar SPWM wave is smaller than that of the bipolar one, and the unipolar frequency multiplication modulation method increases the harmonic frequency of the SPWM waveform without increasing the switching frequency. The harmonics contained in the SPWM waveform are mainly near 2 times and 4 times the carrier frequency. The higher the harmonic frequency, the easier it is to filter, and the harmonic component of the output voltage is effectively controlled. Therefore, the performance of the unipolar frequency multiplication modulation method for the bridge voltage SPWM inverter power supply is superior to the other two modulation methods. The traditional bridge SPWM inverter uses analog discrete components or dedicated analog control centralized chips to generate the driving voltage of the four power tubes, but it is difficult for analog devices to implement complex and advanced control algorithms. Digital control power supply is the development direction of power supply today. The full comparison unit in the EV (event management) module of DSP TMS320F240 can generate control voltages for 4 power tubes. The working process can be briefly described as follows: after comparing the grid-connected current set value with the same frequency and phase as the grid with the actual grid-connected current, the difference is processed by PI control, and after triangular wave modulation, a sinusoidal pulse width modulation signal is output, which is amplified by the drive circuit to drive the IGBT to work and generate a sinusoidal current with the same frequency and phase as the grid. Among them, synchronous signal acquisition, triangular wave signal generation, PI controller operation, overcurrent, overvoltage, overheating protection and communication functions are all completed by DSP. IV. Conclusion This paper introduces the significance of photovoltaic power generation and its development status at home and abroad, and also introduces the composition of the dispatchable grid-connected system and the existing implementation scheme. Since photovoltaic power generation has become a reality and is developing rapidly around the world. Solar energy will eventually replace fossil energy as the main energy source for mankind.
提升卡
变色卡
千斤顶
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