Control and power electronics technology in renewable energy

1 Energy situation and sustainable development Energy is the basic element of human survival. It is also the main material foundation for national economic and social development, and energy security is the basic support for national economic security. The energy shortage, especially the shortage of high-quality energy, has always plagued China's rapid economic development, and has increasingly become a bottleneck restricting China's economic development. At present, China's total energy production ranks third in the world, and its total energy consumption and greenhouse gas emissions are second only to the United States after the United States.

On February 28, 2005, the 14th Session of the Standing Committee of the Tenth National People ’s Congress formally adopted the Renewable Energy Law of the People ’s Republic of China, which listed the development and utilization of renewable energy as a priority area for energy development. The legal basis for planning and development of the renewable energy business' Generally, petroleum, natural gas, coal and other fossil energy are called primary energy. It is non-renewable, and there are various degrees of atmospheric, environmental pollution and ecological damage during its mining, transportation, storage, and use, which have seriously endangered the human living environment. Non-fossil energy sources such as solar energy, wind energy, water energy, ocean energy, biomass energy, and geothermal energy are all process energy. Not only can it be regenerated, but it is clean without pollution or low pollution, and the storage is huge.

The key breakthrough project in key areas of Guangdong and Hong Kong (2004A10506003) is a non-renewable high-quality energy source such as petroleum, compared with other forms of energy. In 2004, China's total oil demand was about 280 million tons, of which imports were about 120 million tons, equivalent to the annual production of two and a half Daqing oil fields. The price increase alone paid nearly 9 billion yuan. In fact, with the rapid economic development, China has become the world ’s second largest energy consumer, with energy consumption accounting for approximately 11% of the world ’s energy and foreign dependence of more than 40%. In recent years, international oil prices have continued to rise. Competition is far beyond the scope of pure commerce, and it has become the weapon of the world's economic, military, and political struggles. The world ’s top 20 largest international oil multinational companies in the world account for 81% of the world ’s proven high-quality oil reserves, which directly leads to an intensification of the monopoly in the international oil market. Origin, China's oil security has the potential to be subject to human threats. In fact, in order to solve the growing energy demand, in 1993, China began a long journey to search for oil overseas, with footprints throughout Africa, Latin America, and the Middle East. As of November 2004, China has carried out oil development with 27 countries. Cooperation.

In 2004, China ’s total energy output was equivalent to 1.7 billion tons of standard coal, of which coal accounted for 67%, petroleum 22.7%, and the rest came from hydropower. Coal in the U.S. energy consumption only accounts for 12 ° /., And in developed European countries it is as low as 5 ° / the long-term coal-dominated energy structure keeps China ’s emissions of S02, C02, soot, and solid waste at the forefront of the world. The consequence is air and water pollution, causing regional and even regional ecological damage. It is estimated that by 2020, China's energy consumption will reach 3 billion tons of standard coal. If the annual production of 4 billion tons is maintained, the proven coal reserves can only be mined for 30 years; when 2050, China will reach the level of moderately developed countries At that time, energy consumption required 455.2 billion tons of standard coal, which is equivalent to 60% of the current total world energy consumption. No energy in the world can meet the energy demand of this scale. What's more, China's oil and gas resources are insufficient, natural uranium resources are scarce, and hydropower resources are unevenly distributed. In 2004, China's GDP reached 13.65 trillion yuan, but at the same time more than 20 provinces, municipalities and autonomous regions experienced varying degrees of power shortage. The energy consumption per 10,000 yuan of GDP was 2.3 times the world average.

Vigorously developing and utilizing renewable energy sources to achieve sustainable energy development, with the benefit of the contemporary era and the benefit of all ages, is the only way to change our current extremely unreasonable energy consumption structure. Many experts pointed out. The fundamental way to solve China's energy shortage lies in the development of renewable energy.

In fact, China ’s renewable energy storage is very considerable, and the hydropower resources that can be developed with technology reach 378 million kWh; the annual sunshine time in most areas is more than 2,000 hours; China ’s coastline is long, and tidal energy resources reach 200 million kWh; The straw can be used as an energy source more than 300 million tons per year. The total theoretically developable wind energy (lm height) is 3.2 billion kW, and the actual installed capacity is about 253 million kW, second only to the United States and Russia, ranking third in the world, and 1.3 times the current total power generation in China The wind energy resources that can be developed offshore reach 750 million kW, with the potential for commercial and large-scale development.

As of the end of 2004, China ’s cumulative installed wind power capacity reached 764MW, ranking tenth, with a cumulative installed of 0.065GWp of solar cells. China plans to exceed 20 million kW of installed wind power capacity and a cumulative installed capacity of 10GWp of solar cells by 2020. It is conservatively estimated that annual wind power construction The market capacity should remain at least 7 billion yuan.

The purpose of sustainable energy development is development, and the key is sustainability. The development and utilization of various types of renewable energy represented by solar energy and wind energy has become a strategic choice for mankind to solve survival problems. It has also become one of the economic fields with the most innovative capabilities and technical know-how in developed countries in Europe and America. By installing a wind turbine with a capacity of 1MW, the actual usable wind energy in the world is very considerable, which is 10 times larger than the total amount of water energy that can be developed and utilized on the earth. As of the end of 2004, the cumulative installed wind power capacity in the world has reached 47,616MW, and Germany, Spain, and the United States rank among the top three in the world with installed capacities of 16628.8MW, 8263MW, and 6740MW. According to statistics. Each time the wind power regulation doubles, its electricity price drops by 30 ° /. It is expected that half of the European population will use wind power after 15 years. It is expected that wind power generation can meet the entire electricity demand of European residents in 2020. The German wind power equipment manufacturing industry has replaced the automobile manufacturing industry and the shipbuilding industry.

600TW is adopted, which is the only energy source that can guarantee the future energy demand of mankind. Solar energy utilization includes photovoltaic power generation, solar thermal power generation, solar water heaters, solar houses and other utilization methods, of which solar water heaters have achieved large-scale commercial production and utilization. As of the end of 2003, the total number of solar water heaters in the world was about 120 million m2, of which China accounted for about 40%: the cumulative installed capacity of photovoltaic power generation was 3.15GWp: the cumulative installed capacity of solar thermal power generation was 500,000 lcWp. The cumulative installed capacity has reached 3.99GWp, and Japan ranks first in the world with 12GWp installed. It is estimated that the cumulative installation of solar cells in the world will reach 920GWp in 2030, and Japan will reach 205GWp. The conversion efficiency of solar cells will also increase from the current 6-38.9% to 8 ~ 50%. 2 Key technologies in wind power generation 2.1 Construction of wind turbines Wind turbines are complex non-linear objects. Both aerodynamics and electromechanical energy conversion processes must be considered. Wind speed, speed, torque, voltage, current, power transfer, various physical quantities are coupled and influence each other, and it is not easy to establish accurate dynamic mathematical horizontal patterns. The more considerations. The higher the system order.

And because the dynamic responses of the various factors considered are very different, it may end up being a divergent morbid model. How to obtain a simplified horizontal type of a wind turbine that is reasonable, applicable and easy to design a controller is currently a research topic that has not been properly solved in wind power generation.

From the process of energy conversion, the wind turbine can be divided into three parts: â‘ Wind turbine: convert wind energy into mechanical energy, the theoretical maximum conversion efficiency is 59.3%; â‘¡Generator: convert mechanical energy into electrical energy: â‘¢Grid-connected device: The electrical energy generated by the generator is fed to the grid.

When modeling the mechanical characteristics of wind turbines. Mainly considering the balance of forces, any complex wind turbine system can be expressed by the following equations of motion: a system damping coefficient matrix, a transmission system stiffness matrix, F-total force (driving force) orientation matrix, these The matrices are all time-varying.

The methods used to establish the above system motion equations are mainly transverse analysis and finite leaf element theory: transverse analysis can reduce the degree of freedom of the wind turbine after it is built, so it is widely used: with the development of computers, digital The processing ability is greatly strengthened, and the finite leaf element theory has also received extensive attention.

The components of wind turbines are divided into two categories: autonomous systems and grid-connected operation. Autonomous systems are divided into independent operating systems and complementary operating systems. On-grid operation is also divided into single-machine on-grid and multi-machine on-grid.

In order to study the characteristics of independent operation of wind power generation systems, the earliest adopted horizontal construction method ft mechanism analysis method, the main principle is to start from the internal structure of the wind turbine system. The mechanism model of each link is analyzed separately. Select the appropriate model structure to simulate the characteristics of the system, and then combine them to get the mechanism model of the wind turbine. It is a structural block diagram used to establish the horizontal type of the wind turbine operating independently. The main consideration is the electromechanical balance relationship, which can be reduced to the following expression: The structural block diagram of the wind turbine-including the nonlinear vector of the system. y system output.

Although the horizontal shape established by the mechanism analysis method can reflect the physical nature of the dynamic process of the system, due to the complexity of the wind turbine system, certain relatively minor factors are often ignored when establishing the horizontal shape of the deterministic part of the wind turbine. This kind of neglect is sometimes to make the established horizontal shape easy to control. Sometimes it is because the dynamic behavior of a certain aspect of the system is not sufficiently understood, and the neglected factors are described in an uncertain form, so the horizontal mechanism often deviates from the actual one. Uncertainty factors of wind turbines mainly come from the following aspects: measurement noise in sensors and actuators, dynamic characteristics of adjustment mechanisms, nonlinear components, and unknown starting conditions of the system. So the test method model appeared. Use. The maximum output power tracking control adopts the principle of adjusting the voltage to maximize the output power, and various control algorithms have been developed. The early perturbation observation method and the added conductance method are both added adjustment methods that adjust the voltage according to the change of the power point, and have defects in tracking speed and accuracy. Although the differential method can accelerate the search speed of the output power, it is easy to cause the fluctuation of the output power point and the accuracy is affected. Using the characteristics of self-organization and self-learning of the neural network to predict the state of the maximum power output of the solar cell, the maximum power output can also be achieved, but because there are many factors affecting the output power of the solar cell, the data used to train the neural network has certain limitation. The gradient of solar cell output power versus voltage is selected as the switching function, and sliding mode variable structure control can be used to achieve maximum output power tracking control. This method has certain advantages in terms of control robustness, search speed and realizability to environmental changes. The defect of the lateral sliding control is that there is a flutter phenomenon near the maximum power point, and the nonlinear control based on the Boltzmann function can effectively solve this problem.

The current search control algorithm is immature, the new algorithm has not been applied in actual systems, and seeking more effective methods with adaptive capabilities and the use of nonlinear control will remain the focus of future research.

In addition, when the external conditions such as sunshine intensity, solar electromagnetic surface attachments, and air humidity change, the terminal voltage of the solar cell output maximum power point changes greatly, which will directly affect the power generation voltage, power output and other energy indicators and solar power generation control device Conversion efficiency. Therefore, the fixed wiring method must be replaced by the optimal wiring method under the above gizzard factor, that is, to carry out a study on the dynamic optimization of the configuration of the solar cell array. At present, both domestic and foreign are still in the blank.

Solar power generation systems generally have to be connected to the grid. In order to give full play to its efficiency, it is necessary to control the solar inverter to make its voltage, frequency and phase consistent with the power grid, and put forward new requirements for the grid-connected controller.

When the photovoltaic power generation system is connected to the grid, and the power grid is cut off due to a fault accident or power outage maintenance, if the solar grid-connected power generation system fails to detect the power outage in time and is separated from the mains, the solar grid-connected power generation system and the surrounding The load forms an island of self-contained power supply. With the increase of photovoltaic grid-connected power generation systems and other decentralized grid-connected power sources, the probability of islanding effects will also become more and more separated. In recent years, people and countries have attracted extensive attention in countries and regions where renewable energy has developed rapidly. The island effect will affect both power distribution system equipment and end user equipment. My country is in the power system. The "island effect" is to solve this problem by the power dispatch of the power system through communication. With the large-scale use of decentralized renewable energy, the traditional communication and power dispatch of the power system obviously cannot completely solve this problem. However, China's current research on the "island effect" of renewable energy grid-connected power generation systems is almost blank.

4 Control problem of wind-solar hybrid power generation system The wind-solar hybrid power generation system composed of the natural complementarity of wind energy and solar resources can make up for the shortcomings of low ftability caused by the instability of wind energy and solar resources , To a certain extent, provide stable ft power. The reasonable and coordinated operation of each power generation device can also effectively reduce the storage capacity of the configured battery.

The basic control problems listed in the schematic diagram of the structure of the wind-solar hybrid power generation system are all under the control of the bottom unit. To form an optimized operation of the wind-solar hybrid power generation system, energy management at the upper level is also required to achieve coordinated control of wind solar power . Each part of the complementary power generation system needs to be coordinated and controlled according to load requirements, energy resources, meteorological conditions, etc. The current wind-solar complementary power generation system is mostly a simple combination of two power generation devices and their energy storage equipment, and there is still a lack of optimal matching of battery capacity , Battery intelligent charge and discharge control, and complementary power generation system hybrid coordination control energy management system research and controller implementation, for such a complex nonlinear system using horizontal fuzzy control is an effective solution, but due to influencing factors Many, the general horizontal paste control strategy will face the problem of the explosion of paste combination rules. Based on the fact that the main degree of each output is different, and the coupling degree of some variables is not quotient, a hierarchical horizontal control strategy is adopted to reduce the number of control rules that have an exponential relationship with the number of input displacements to a linear relationship, a better solution The explosion problem of combination rules. Another difficulty in cross-border control is the formulation of rules and the selection of membership functions, which generally rely on the experience and knowledge of experts. For complex systems, human selection often lacks accuracy and systematicness. Based on genetic algorithms, sub-fuzzy control optimization can improve Solve these problems effectively.

From a structural point of view, the wind-solar hybrid power generation system is a distributed energy placement system, and each of its components has the function of unit control. Therefore, they are regarded as agents. Several management agents are added to form a decentralized energy management system. Make it when the external conditions such as load, wind, and light change. Carrying out coordinated control to achieve the optimal scheduling strategy has become a hotspot in future research. The comprehensive utilization of wind power and solar energy technology to form a complementary, controllable, high-quality, and dispersible distribution of new energy sources will be a new growth point in the energy structure of this century.

5 Power Electronics Technology in Renewable Energy Power Generation The development and expansion of renewable energy power generation technology has enabled it to gradually transition from supplementary energy to alternative energy. The research and development of power conversion devices as important components of renewable energy applications, which are as important as the aforementioned control technologies, have also become an important research topic. The power electronics technologies applied in renewable energy power generation mainly include inverters, solar chargers, matrix frequency converters, and active filters.

S.1 Inverter grid-connected control I The grid-connected renewable power generation output power mainly uses the matrix converter networking for doubly-fed wind turbines and the DCAC conversion unit. Matrix converters do not require capacitance and have reliable reliability. But the control is complicated. At present, the grid connection of renewable energy power generation mostly uses DCAC conversion units to connect to the power grid. In addition to ensuring the power quality and conditions required for grid connection, DCAC conversion units also need to realize some functions of renewable energy power generation technologies, such as wind energy. Maximum capture control and solar maximum power output tracking control, etc., require that the main circuit topology has active and reactive power decoupling adjustable, and high conversion efficiency.

The current research in this area is in the circuit topology, the control strategy adopted is mostly P control under linearization of small deviations, which is not robust to the external environment. There have been some achievements using modern control theory to improve the performance of grid-connected controllers. For example, the non-linear state feedback linearization method is used to achieve the decoupling control of the active and reactive components in the line current, and the purpose of improving dynamic performance is achieved; on the basis of PI control, the introduction of predictive control can also improve the dynamics of the controller performance. It can reduce the accommodating capacity of the buffer capacitor on the DC side; the sliding mode control is applied to the grid-connected controller of the wind turbine. Reliable power generation control at low speeds can be achieved; a grid-connected controller based on the principle of auto disturbance rejection controller has a significant improvement in dynamic performance and robustness, and is easy to implement.

Although the above studies have obtained some research results, they are solved separately for each problem. To obtain practical technical results, the problems of power tracking control, power factor control, and output current waveform control should be comprehensively considered and studied. Unified control algorithm.

5.2 Solar charge control is to improve the stability and reliability of solar power generation. A battery pack with a certain capacity is required. Lead-acid batteries have relatively long components and limited service life. If used improperly, they will seriously affect the service life. The cost of the battery pack has become a major obstacle to the popularization and application of solar photovoltaic power generation technology.

Conventional charging methods, such as the constant current charging method, the stage charging method, the constant voltage charging method, and the pulse charging method, are all based on the charging characteristic curve of the battery, but the charging control accuracy is easily affected by the external environment, and the adaptive search is used The algorithm can take good care of battery charge control and solar cell maximum power tracking control.

S.3 Matrix transformation and control Matrix converter is an important realization of variable speed constant frequency control power electronic converter device. It has the function of bidirectional flow of power required to control the winding during synchronous speed up and down operation. Its topology is as follows: The topology of the matrix converter has 9 bidirectional switches arranged in a matrix of 3 rows and 3 columns. The duty cycle within 9 bidirectional switches is used to form a 3-row and 3-column switching modulation matrix to determine the transformation relationship of the matrix converter. Matrix converter used in wind power generation, by adjusting its output frequency, voltage, current and phase, to achieve variable speed constant frequency control, maximum wind energy capture control, and decoupling control of active power and reactive power, etc. The control of the converter is mostly based on the space vector conversion control method. Using the traditional AC-AC control strategy, the transfer function of the matrix converter is equivalent to two parts: "virtual rectification" and "virtual quasi-variable". The link connects the two parts and is controlled by the spatial vector modulation technique, which is also better in terms of robustness and realizability.

At present, the research work of matrix converters at home and abroad is not yet mature, and there is still a considerable distance from practical use. The key reason is that the applicable bidirectional switch is not yet mature and commercialized, and the control technology has yet to be developed.

5.4 Harmonic Suppression in Renewable Energy Renewable energy power generation uses power electronic devices to control the speed, which will bring power harmonics to the power grid, deteriorate power factor, voltage waveform distortion, increase line loss and electromagnetic interference. The increase of renewable energy power generation regulations has brought more and more attention to the power quality problems brought to the grid.

There are two main methods for suppressing harmonics: passive filtering and active filtering. Passive filtering uses the characteristics of capacitance and inductance resonance to suppress the high-frequency harmonics of a specific frequency and improve the power factor. However, it has the defects of large size, fixed filter frequency and serial / parallel resonance, which limits its application. In recent years. Active filtering has become a research hotspot due to its ability to compensate for various harmonics, suppress voltage flicker and compensate reactive power, etc., and has been widely used in some advanced industrial countries. However, it still needs to be further improved in terms of compensation performance, reliability, and cost and loss reduction.

For the strong nonlinearity and high real-time requirements of active filters.

Many scholars apply advanced control techniques. Such as adaptive identification technology. In order to reduce the current tracking error: Based on the essential interoperability of the power electronic switching device and the sliding horizontal switching variable structure in the active filter, the sliding transverse technology is applied to the active filter control, which can reduce the total harmonic distortion rate, and Parameter perturbation and external interference are strongly robust; the nonlinear characteristics of active filters, especially the presence of switching devices. It is very difficult to make accurate modeling. The artificial intelligence method can effectively solve the contradiction between complexity and precise control. If the fuzzy control is applied to the optimization and control of the duty cycle of the switching device, the dynamic quality of the current tracking control can be improved from the characteristics of the switch itself; the artificial neural network technology is applied to the detection of harmonic current and reactive power components and switch control, Increased control accuracy and achievability: In addition. Methods such as auto disturbance rejection control, genetic algorithm and grey prediction theory are applied to the control of active filters. It also achieved meaningful results.

Although the above research results can improve the quality of active filters, they are far from mature. In addition, the improvement of power quality should be a comprehensive multi-objective optimization problem, and the research on unified power quality regulators should be strengthened.

Finally, it should be pointed out that the power electronic devices used in power systems include renewable energy power generation systems. It is usually used as a power amplifier or an actuator in the entire system. When modeling and controlling the power generation system, the conventional method is to omit their dynamics and express it with an equivalent gain. However, in order to improve the stability, anti-interference and parameter perturbation and response quality of power electronic devices, they need to be modeled and controlled. It is usually completed on the dq axis, so various control algorithms in modern control technology can find corresponding application sites according to the requirements of various power converter modeling characteristics and performance indicators.

6 Concluding remarks China's severe energy situation and the huge potential of renewable energy form a huge contrast. Since entering the 21st century, China's policy and industrial departments have paid great attention to the development and utilization of new energy.

Can be expected. Development of wind power, photovoltaic power generation, small and medium-sized wind-solar hybrid power plants, construction of various large-scale wind farms, etc. A new growth point of the national economy will soon be formed. With the process of industrialization of new energy, control and power electronic technology to improve its operation quality and efficiency will be widely used. Finally, on the occasion of the promulgation of China's Renewable Energy Law, I would like to write this article to express support and response.

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