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2009 3rd International Conference on Power Electronics Systems and Applications Digital Reference K2105090110 Power Electronic Technology in Wind Generation System of Variable Speed constant Frequency Xiaohong WANG 1 3 Jinming YANG 2 3 Jie WU 2 3 Yuan HU 2 1College of Automation Science and Technology South China University of Technology Guangzhou 510640 2College of Electric Power South China University of Technology Guangzhou 51064 3 Guangdong Key Laboratory of Clean Energy Technology Guangzhou 51064 Abstract This paper summarizes the latest development of power electronic technology in wind generating system of variable speed constant frequency VSCF Firstly the typical structure merits and demerits of general VSCF wind generating system are analyzed The relative power electric technology and application in the direct driven wind generating system and double fed wind generating system such as the full power converter dual PWM converter matrix converter are introduced in detail In addition some new converters in VSCF wind generating system such as rotor current controller are also discoursed Keywords Wind generation variable speed constant frequency power electronic technology full power converter matrix converter dual PWM converter I INTRODUCTION Along with the development of human society and the industrialization the global energy consumption will still at high speed conventional energy resources will be faced with growing increasingly exhausted From energy development strategy mankind must seek a sustainable energy development road But with the development of economy the improvement of living standard people pay more and more attention to survive the living environment improvement environmental protection consciousness unceasingly strengthens need some clean renewable new energy Development and utilization of various renewable energy represented by wind become the strategic choice of human existence problem solving Wind power generation technology is currently the most mature technology and one of the most widely used renewable power generation technology many countries focus on developing new energy technology II MAIN METHOD 1 Constant speed constant frequency and VSCF wind generating system According to the operation characteristics and control technology of the generator the wind generating set can be divided into constant speed constant frequency and VSCF generating set two categories Early wind generating set mainly uses the mouse cage induction generator the structure is simple reliable relatively low cost and can be directly connected grids Figure1 Constant speed wind power system structure On the assumption that under the premise of infinite power grid the output voltage and frequency are set by the power grid Once the wind generating set is parallel operation wind speed is constant namely constant speed constant frequency Its basic structure is shown in figure 1 Constant speed constant frequency generating set only run in a fixed speed can reach the highest efficiency when the wind speed changes wind generating set will be biased running away from the best speed resulting in decreased operating efficiency Asynchronous wind generating set output of electric power quality is poorer power factor is lower at the same time when the assumption of infinite power grid does not exist each wind generating set running state adjustment will have a great influence on local power grid the wind speed and direction of randomness instability will make wind generating set output voltage frequency and power change With the development of technology this type of wind generating set will gradually withdraw from the market To solve the constant speed wind generating sets power output of poor quality low power factor and improve the efficiency of generating set effectively relieve and even eliminate the stress and the torque oscillations of the driven chain VSCF wind generating set has attracted people concern and attention Although currently used in wind power systems mostly are constant speed constant frequency electric system based on the induction generator in the VSCF wind power generation systems the wind generating set can maintain efficient operation in a wide range of wind speed variation for its obvious advantages many states has invested a great deal of human resource and material resources for research Currently the realization of VSCF wind power generation has a variety of programs according to key components has several different classification methods In accordance with the availability of gear box can be divided into common type with gear box and direct drive based no gear box According to the type of generator can be divided into induction generator type permanent magnet synchronous generator type doubly fed generator type switched reluctance generator type etc in accordance with the form of power converter can be divided into full power AC DC AC type dual PWM converter type matrix converter type The above several specific combination between classification more practical and more research are the use of induction generator with a gearbox full power AC DC AC type VSCF wind power generation system the system structure in Figure 2 a shows the use of permanent magnet synchronous generators for direct drive based no gear box AC DC AC full power type VSCF wind power generation system the system structure 2009 3rd International Conference on Power Electronics Systems and Applications Digital Reference K2105090110 in Figure 2 b shows the use of double fed motor two way excitation control of power converter VSCF wind power system its system structure in Figure 2 c shows Below respectively the typical introduce the full power generator and double fed generator of power electronics technology a b c Figure 2 VSCF wind power generation system structure diagram 2 Power electronics technology in full power generating system To achieve the purpose VSCF generator stator winding connect with the power grid through the AC DC AC converter and will change variable frequency variable voltage electric power which is changed With the wind speed and direction changes into the same frequency voltage electric power in the power grid Since all power converts through the power converter therefore such systems of power electronic devices known as full power converter With the gearbox full power AC DC AC type VSCF wind power generation system contain gear box in the normal operation of the gearbox wear and tear and its larger size is also a lack of such systems Generator types can be cage induction generator synchronous generator electrical excitation permanent magnet synchronous generators and generator winding induction Direct drive based no gear box AC DC AC full power type VSCF wind power generation system uses a multi polar low speed permanent magnet or synchronous generator excitation power while eliminating the gearbox but the size of motor is very bulky And the same as the gearbox VSCF unit the generator stator windings also need to use a AC DC AC converter to connect with power grid Therefore the above two types of VSCF wind power generation system of power converter the structure and the body is approximately the same According to the stator side converter rectifier and network side converter inverter of the different options such systems typically have the following topology 2 1 Non controlled rectifier circuit voltage source inverter circuit topology Its topological structure as shown in Figure 3 this circuit structure is simple and sophisticated control methods The AC issued by generator is converted to DC through rectifier filter circuit and then by a PWM voltage type inverter it can output the AC whose voltage frequency and phase match the power grid line and can control the system output s active and reactive power to regulate the generator torque rotational speed so that fans work in the best condition Figure 3 Non controlled rectifier inverter circuit topology Due to a generator voltage and frequency variation therefore after non controlled rectified the dc voltage is variable although we also can adjust the modulation depth of PWM inverter to keep the output voltage invariable but its adjustment range is limited as dc bus voltage below the threshold inverter can t output power Therefore if you want to broaden the scope of use this device will need to join booster part according to the boost part isolated transformer of the type parallel grid inverter can be divided into low frequency link parallel grid inverter high frequency link parallel grid inverter and Non Isolated parallel grid inverter 3 2 2 Non controlled rectifier circuit non isolated voltage regulator circuit voltage type PWM inverter circuit topology Its topological structure is as shown in Figure 4 relative to shortcomings of the last topology of inverter input DC voltage instability voltage value low it added a level of non isolation DC DC chopper between non controlled rectifier circuit and inverter circuit After adding Chopper when wind speed generator output voltage changes it can make inverter input DC voltage stabilized at near expectations by the chopper circuit so that for the inverter control system the source effect input power of disturbance greatly reduced Figure 4 Non controlled rectifier circuit the non isolation chopper inverter circuit topology Chopper here generally use the boost circuit BOOST circuit can not only regulators boost at the same time can improve input circuit of the power factor reduce harmonic Taking into account that there is only one power tube in 2009 3rd International Conference on Power Electronics Systems and Applications Digital Reference K2105090110 the BOOST circuit when the capacity of power generation system is relatively large the single tube generally is very difficult to meet the requirements therefore most of the use is the multiple BOOST circuit so that it can not only reduce the power of governing capacity improve circuit reliability At the same time can also increase the equivalent switching frequency reducing the capacity of filter capacitor reducing the output ripple voltage etc 2 3 Non controlled rectifier circuit voltage type PWM inverter circuit power frequency step up transformer topology Its topological structure is as shown in figure 7 with a different topology there is a way to improve voltage by adding a step up transformer means in the inverter parallel grid output side to realize Figure 5 Non controlled rectifier circuit voltage type PWM inverter circuit power frequency step up transformer topology Adding a step up transformer the generator s operating voltage range can be further expanded and at the same time taking into account that the non isolated network is prohibited in a number of countries after the use of transformers the system and the city electric power grid are completely electrically isolate improving the safety standards of the grid This circuit structure is simple and efficient but the drawback is that the transformer volume and weight of large higher raw material consumption it is bound to result in increased costs at the same time brought about audio noise pollution 2 4 Non controlled rectifier circuit isolation voltage regulator circuit voltage type PWM inverter circuit topology The last topology use a frequency step up transformer to complete the wind power systems and power grids isolation but the power frequency transformer is bulky high cost if we adopt the high frequency transformer to replace the power frequency transformer to complete the mission of isolation and transformation it will greatly reduce costs improve power density Figure 6 Non controlled rectifier circuit isolation voltage regulator circuit voltage type PWM inverter circuit topology Such type of circuit topology as shown with the difference from the non isolated voltage regulator circuit such a circuit adds the high frequency transformer in the DC DC chopper circuit this converter is also known as the high frequency link power conversion browser in which typical Isolated DC DC converter is the full bridge converters this converter research more sophisticated technology at home and abroad and shall not repeat them here 2 5 Non controlled rectifier circuit high frequency link PWM inverter circuit Topology High frequency link power converter according to the direction of power transmission can be divided into one way and two way type last topology belongs to a one way type Since the energy needs to go through four Power Conversion That is low frequency rectifier High frequency inverter Rectifier and low frequency high frequency inverter Thus general configuration loss is bigger System structure is complex and is low efficiency Bi directional high frequency link voltage source power converter can not only bi directional power transfer only need to go through low frequency rectifier High frequency inverter and frequency converter 3 Power Conversion The structure of high frequency voltage source than one way power converters is simpler and state is low loss System is efficient high frequency link inverter circuit topology of the system is as shown in Figure 6 High frequency link inverter circuit is mainly composed by high frequency inverter and cycloconverter Figure 7 Non controlled rectifier circuit high frequency link PWM inverter circuit Topology 3 Power Electronic Technology in doubly fed Wind Power Generation System In full power VSCF wind power system Generator stator windings must be connected by power electronic devices and power grid connection While increasing the quality of the output power due to power electronic devices connected to the main circuit its capacity must be greater than or equal to generator capacity leading to the high cost of wind motor significantly In order to reduce the capacity of power electronic devices Doubly fed generator began to be used in wind power generation system Doubly fed motor in accordance with the availability of brush and slip ring can be divided into brush doubly fed motor and Brush doubly fed motor has an early start in research In VSCF wind power generation system it is in mainstream BDFM eliminates the vulnerability of the brush and slip ring has certain advantages therefore many researchers have also carried out on BDFM and its application in wind power research 3 1 Doubly fed machine theory and operation characteristic Doubly fed induction machine use for reference the structural characteristics of wound rotor type induction motor The motor stator directly is connected to the three 2009 3rd International Conference on Power Electronics Systems and Applications Digital Reference K2105090110 phase symmetrical grid the rotor is supplied power by variable frequency AC power and adopting appropriate control strategy to control the rotor current amplitude frequency phase and phase sequence Since the motor works by respectively feed power to the rotor and stator motor has also been known as the doubly fed motor Constituted by the doubly fed motor wind generation system is shown in Figure 8 Figure 8 Doubly fed induction wind generation system schematic Steady state speed formula of the doubly fed induction machine is formula 1 1 In the formula n for the motor speed f1 1 for the stator windings current frequency and magnetic potential rotation angular frequency f2 2respectively for rotor windings current frequency and magnetic potential rotation angular frequency When the motor speed is below the synchronous speed 1 Synchronous motor is operating in the sub state 1 2 are rotating in the same direction at this point Stator current and rotor current are in the same phase sequence in formula 1 in front of f2 is controller absorbs energy from the power grid to the rotor windings When the motor speed is above the synchronous speed 1 Motors are running in the super synchronous state 1 2 are rotating in the opposite direction at this point Stator current and rotor current are in the opposite phase sequence In formula 1 in front of f2 is controller absorbs energy from the rotor winding to the power grid as a special case when 2 for 0 Motors are running in synchronous speed at this point the rotor excitation converts from AC excitation into DC excitation motor works just like the common synchronous motor The above formula of motor steady state speed shows that When a certain p change in wind speed makes motor speed change We can make f1 constant by changing f2 due to the existence of control Windings no matter how the wind speed changes just change the corresponding doubly fed wind motor control winding current amplitude frequency phase phase sequence we can facilitate the achievement of the purpose of maximum power tracking of wind energy and VSCF constant pressure at the same time control winding power converter only flow turn bias power its capacity is greatly reduced than full power converter generally about one quarter of the total generating power this is exactly the advantages of the BDFM Over the years brushless AC motor has been the focus of one of the study for doubly fed motor human carry out the same effort BDFM is developed from a cascade of induction motor through constant evolution and improvement two coaxial series of induction motor are combined two independent stator simplified rotor winding so that it will have a solid cage rotor and reliability can meet the electrical requirements of the rotor magnetic field BDFM have the same characteristics with the brush doubly fed induction generator But there is no slip ring and brush improving the system reliability It is the optimization program in wind power generation in the VSCF power The BDFM working principle is shown in figure 8 Stator windings constitute of 2 sets of three phase symmetric windings known as the power winding and control winding respectively Power winding pole pair number is recorded as pp control winding pole pairs number is record