Judging from the current energy structure of the world, the energy structure dominated by petrochemical energy with limited resources and serious pollution will gradually be transformed into an energy structure dominated by unlimited renewable energy with clean resources. Solar energy, wind energy, water energy, ocean energy, biomass energy, geothermal energy, fuel cells and other renewable energy sources are emerging green energy sources. With their advantages of never exhaustion, no pollution, and no limitation of regional resources, they are gaining rapid Promoting power electronics technology as the key to renewable energy power generation technology is directly related to the development of renewable energy power generation technology. Renewable energy sources emit direct current with varying sizes or alternating current with varying frequencies, requiring power electronic converters to convert electrical energy.
1 Application and development trend of power electronic technology in renewable energy power generation system 1.1 Renewable energy power generation system The development and expansion of renewable energy power generation technology make it gradually transition from supplementary energy to alternative energy. The following is a brief introduction of several types of fund projects: the six major talent projects of Jiangsu Province (06-A-045) funded projects.
The main renewable energy power generation system.
1 Photovoltaic power generation system Photovoltaic power generation system can be divided into independent photovoltaic power generation system and grid-connected photovoltaic power generation system. It is a schematic diagram of a solar photovoltaic grid-connected power generation system. The system consists of solar energy, photovoltaic array, bidirectional DC converter, battery or super capacitor and grid-connected inverter. In addition to ensuring the normal power supply of the load, the photovoltaic array stores excess energy in the battery or super capacitor through the bidirectional DC converter; when the photovoltaic array is insufficient to provide the power required by the load, the bidirectional DC converter works in reverse to provide power to the load .
112 The fuel cell of the wind power generation system is an electrochemical device that continuously converts the chemical energy in the continuously supplied fuel and oxidant into electrical energy. The biggest advantage of fuel cell power generation is high efficiency, cleanliness, no pollution, low noise, module structure, strong building block, not limited by Carnot cycle, high energy conversion efficiency, and its efficiency can reach 40% ~ 65%. The fuel cell is said to be It is the fourth generation power generation device and the power device to replace the internal combustion engine after water power, thermal power and nuclear energy.
114 The “wind-solar hybrid power generation system†composed of the natural complementarity of wind energy resources and solar energy resources in the hybrid energy power generation system can make up for the defect of low reliability caused by intermittent instability of wind energy and solar energy resources, to a certain extent Provide stable and reliable power.
The structure of the system is as shown.
The research and development of power electronic conversion devices as an important part of renewable energy applications have also become an important research topic. The power electronics technology applied in renewable energy power generation mainly includes inverter and grid connection technology, solar charging control technology, variable speed constant frequency wind power generation system, fuel cell power regulation system, harmonic suppression and energy management.
1.2 Inverter and grid-connected control technology The grid connection of renewable energy generation output power mainly uses ACAC converter grid-connected and inverter-connected grid-connected for variable-speed constant-frequency doubly-fed wind turbines. At present, the grid connection of renewable energy power generation mostly uses inverters to connect to the grid. The grid-connected inverter should have the characteristics of power factor 1, sinusoidal current on the grid side, and bidirectional flow of energy, so that it has excellent control. performance. When photovoltaic grid-connected power generation, grid-connected inverter must also have a fast dynamic response. In addition to ensuring the power quality and conditions required for grid connection, the inverter also needs to realize some functions of renewable energy power generation technology, such as solar maximum power output tracking control and maximum wind energy capture control, etc., requiring its main circuit topology to have Active and reactive power decoupling is adjustable and has high conversion efficiency. In addition, the seamless switching technology of the two modes of grid-connected operation and independent operation can reduce the impact on the power grid. At present, most researches in this area are focused on the circuit topology, and the control strategy adopted is mostly PI control, which is not robust to the external environment. There have been some achievements in using modern control theory to improve the performance of grid-connected inverters. For example, the nonlinear state feedback linearization method has been used to achieve the decoupling control of the active and reactive components in the line current, which has achieved the purpose of improving dynamic performance; Based on PI control, the introduction of predictive control can also improve the dynamic performance of the controller and reduce the capacity of the DC-side buffer capacitor; applying sliding mode control to the grid-connected controller of the wind turbine can achieve reliability at low speeds Power generation control; a grid-connected controller based on the principle of auto-disturbance rejection controller has significant improvement in dynamic performance and robustness, and is easy to implement. Although the above research has 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 to develop a unified Control algorithm. At present, China's solar photovoltaic power generation system is still dominated by an independent power supply system. The independently developed grid-connected inverter has the weakness of unstable system operation and low reliability, and the incomplete protection measures are prone to cause accidents and integration with buildings. Not well considered.
The "island" phenomenon in grid connection means that when the grid loses power, the photovoltaic grid-connected power generation system and the local load are in independent operation, and a self-contained power supply island will be formed by the solar grid-connected power generation system and the surrounding load. The "island" phenomenon will seriously affect the safe and normal operation of the power system and endanger the personal safety of line maintenance personnel. With the increase of photovoltaic grid-connected power generation systems and other decentralized grid-connected power sources, the probability of occurrence of "island" effect will also be higher and higher. In recent years, people in the countries and regions where renewable energy has developed rapidly have attracted widespread attention. . In general, an inverter equipped with over-voltage, under-voltage, over-frequency and under-frequency relays has the basic protection function of "island". But in the case of source-load power balance, the voltage and frequency change is very small, these relays will fail, causing the system to enter the "island" operation. "Isolated island" detection methods are divided into two categories, namely passive detection methods and active detection methods. Passive detection is to judge whether there are "islands" by observing the changes of voltage, frequency and phase of the power grid. However, when the power of the photovoltaic power supply is basically close to the power of the local grid load, resulting in a small change in the local grid voltage and frequency when the power is cut off, the passive detection method will fail. In order to solve this problem, active detection method came into being. The active detection method is to add a small voltage, frequency or phase disturbance signal to the control signal of the grid-connected inverter, and then detect the output of the inverter. When the "island" occurs, the effect of the disturbance signal will appear, and when the output change exceeds the specified threshold value, the occurrence of the "island" can be predicted. However, in general active detection methods (such as frequency offset method, output power change measurement method, etc.), the load phase angle characteristics have a greater impact on the effectiveness of detection, and such methods have "detection dead zones". How to quickly, accurately and cost-effectively detect and control "islands" will become a research hotspot of grid-connected technologies.
1.3 In order to improve the reliability of solar power generation, the solar charge controller 191 needs to be equipped with a certain capacity battery pack. The lead-acid battery has a relatively high composition cost and a limited service life. If used improperly, it will seriously affect the service life. The cost of battery packs has become a major obstacle to the popularization and application of solar photovoltaic power generation systems. 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 based on the charging characteristic curve of the battery, but the charging control accuracy is easily affected by the external environment, and the adaptive control is adopted. The algorithm can take good care of battery charge control and solar cell maximum power tracking control.
1.4 Variable speed constant frequency wind power generation system 141 At present, wind power generation in China basically uses grid-connected asynchronous wind turbines. The operation mode is direct grid-connected operation without control. When the wind speed and wind direction change, it is easy to form shocks and inject harmonics into the power grid. , Causing pollution and even affecting the stability of local power grid operation. The solution to this problem is to use variable-speed constant-frequency control, that is, when the wind speed changes to cause the rotor speed to change, it can still ensure that the output power frequency is constant. There are many ways to achieve variable-speed constant-frequency power generation, and the doubly-fed generator scheme has the most advantages. The doubly-fed induction generator is also known as an AC excitation generator. Its structure is the same as the wound asynchronous motor. The three-phase symmetrical winding on the stator side is directly connected to the power frequency grid. The three-phase symmetrical excitation winding on the rotor side requires and can provide a controllable amplitude. , Phase and frequency power are connected. Since the AC converter only needs to supply slip power, the requirements on capacity are greatly reduced. The generator adjusts the rotor excitation voltage frequency according to the speed change of the wind turbine, realizes constant frequency output, realizes the independent adjustment of the active and reactive power of the generator, and then controls the generator set speed to realize the tracking and capture operation of the maximum wind energy. In the case of changing wind speed, the wind turbine speed is adjusted in real time, so that it always runs at the optimal speed, thereby improving the power generation efficiency of the unit and optimizing the operating conditions of the wind turbine. In addition, when the variable speed constant frequency wind power generation system is connected to the grid, there is almost no current surge, and there is no need to worry about the problem of excessive surge current when the asynchronous machine is connected to the grid. At the same time, the operating frequency of the doubly-fed generator and the grid frequency are independent of each other. When the speed of the wind wheel and the generator changes, there is no need to worry about the possible out-of-step problem when the synchronous motor is directly connected to the grid. The control scheme of the brushless doubly-fed generator can not only realize variable speed constant frequency control and reduce the capacity of the inverter, but also realize flexible control of active and reactive power, which can play a role in reactive power compensation for the power grid. At the same time, the generator itself does not have slip rings and brushes, which not only reduces the cost of the motor, but also improves the reliability of the system operation.
The AC-AC converter of the variable-speed constant-frequency double-fed power generation system is a four-quadrant converter, which can be divided into AC-AC converter, AC-AC converter and matrix converter according to its topology. The AC-AC frequency converter does not need intermediate DC filter link. The thyristor adopts natural commutation method, always absorbing reactive power, low power factor, large harmonic content, and low output frequency. The AC-DC converter is currently the most widely used ACAC converter, but its DC link has a large filter capacitor, a short life, and a large switching loss. The two main structures currently in use are ACDC-AC mode and AC-AC mode as shown.
The matrix converter is an ACAC direct frequency converter. It consists of 9 switch arrays directly connected between the three-phase power supply and the three-phase load. There is no intermediate DC link. The power circuit is simple and can output amplitude, frequency, phase and phase sequence. Controllable voltage. The harmonic content is small, the input power factor is controllable, and it can be operated in four quadrants, but its commutation process does not allow two switches to be turned on or off at the same time, and the control is more complicated. As an AC excitation power supply for the power generation system, the matrix converter can not only meet the bidirectional power flow necessary for AC excitation variable speed constant frequency power generation, but also its excellent input and output characteristics ensure the production of high-quality electrical energy that meets the requirements, while solving it in a true sense The issue of the best use of energy and environmental protection. At present, the control of matrix converters mostly adopts the space vector transformation control method. Borrowing the traditional cross-interchange control strategy, the robustness and realization need to be improved.
The issue of compatibility between wind power and the grid must also be addressed. Today's wind power generation systems are not yet able to adapt to large grid voltage and frequency transient changes. When the grid voltage drops, the wind turbine is disconnected from the grid, and when the grid is stable, the wind turbine is re-connected to the grid. This is acceptable for a grid with fewer wind turbines, but when wind power generation increases to 10% to 15% of the grid capacity, the wind power generation system must support grid stability and its behavior must be similar to traditional power generation equipment. This puts more stringent requirements on the power generation system.
1.5 Fuel cell power regulation system The fuel cell has an internal resistance. The output voltage changes with the change of the output current. Such an output voltage cannot be directly applied, and the output voltage increases as the temperature increases. For a DC load, generally only a constant supply voltage is required, while for an AC inverter fuel cell boost converter battery or a supercapacitor bidirectional DC converter, the load of the fuel cell grid-connected power generation power regulation system is also It is necessary to invert the direct current to the required alternating current, so the power generation system of the fuel cell must have a power regulation system to work properly. The structure diagram of the fuel cell grid-connected power generation power regulation system is shown as follows. The system is composed of a fuel cell, a battery or a super capacitor, a boost converter, a bidirectional DC converter, and a grid-connected inverter. Since the cost of fuel cell power generation is still relatively high, the conversion efficiency of the power regulation system is required to be high.
For the wide range of input voltage of the power regulator of the fuel cell power system, a converter topology suitable for a wide input needs to be selected as the front stage of the power regulator. In addition, the characteristics of slow dynamic response speed must be considered. Several key issues in the study of bidirectional DC converters are: (1) how to develop a simple and efficient topology of bidirectional DC converters; 2) exploring the soft switching technology of new bidirectional DC converters, thereby further reducing the switching losses of the converter and widening Soft switching load adaptation range; 3) Reduce the circulating energy in the bidirectional DC converter, reduce the on-state losses, and improve the overall efficiency; (4) Further improve the dynamic response of the bidirectional DC converter; 5) Establish the control of the bidirectional DC converter The model helps to optimize the design of the converter and improve the performance of the converter; (6) To realize the digital control of the bidirectional DC converter is conducive to system migration, data collection, display and monitoring.
1.6 Harmonic Suppression in Renewable Energy Renewable energy power generation mostly uses power electronic devices to achieve power conversion, which usually brings power harmonics to the power grid, deteriorates power factor, voltage waveform distortion, and increases electromagnetic interference. With renewable energy As the scale of power generation increases, the problem of power quality brought to the grid is getting more and more attention. At present, there are two main methods for harmonic suppression: passive filtering and active filtering. Passive filtering uses the characteristics of capacitor and inductor resonance to suppress the higher harmonic components 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 transients, and compensate reactive power, etc., and has been widely used in some industrially advanced countries. There is room for further improvement in terms of compensation performance, reliability, and cost and loss reduction. In response to the strong nonlinearity and high real-time requirements of active filters, many scholars have applied advanced control techniques to the control of active filters, and have also achieved many beneficial results. Although the above research results can improve the quality of active filters, they are not yet mature. In addition, the improvement of power quality should be a comprehensive multi-objective optimization problem, and the management of unified power quality should be strengthened.
1.7 With the continuous development of renewable energy power generation technology, the energy management of hybrid power generation systems, the integration and modularization of power electronic application systems, the stability of grid-connected operation, fault tolerance and redundancy, and power quality have been proposed. With high demands, the power electronics industry is facing good opportunities and severe challenges.
2 Conclusion This paper introduces solar photovoltaic power generation, wind power generation, fuel cell power generation and hybrid energy power generation systems in renewable energy power generation, and studies in detail the application and development trends of power electronic technology in renewable energy power generation systems. New energy power generation The system provides a new direction for power electronics technology, and also provides new ideas for the research on renewable energy systems.
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