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Research On Vscf Dfig System Grid Connection Control Strategy

Posted on:2011-12-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:X G ZhangFull Text:PDF
GTID:1102360332457978Subject:Power electronics and electric drive
Abstract/Summary:PDF Full Text Request
With the development of wind power, the penetration of wind power in power grid increases rapidly. The grid connection operation of wind generator is paid great attention now. Doubly Fed Induction Generator (DFIG) is the most popular wind generator type adopted in the world. Because of its characteristic, the large-scale application of DFIG will bring many problems to the generator safety and power grid stability. In order to satisfy the grid code requirement, the grid connection control strategy of DFIG is researched in this dissertation. The main contents of this dissertation include:The mathematical model of DFIG in the synchronous coordinate system and the DFIG stage-wise control method are introduced. Due to the operation characteristics of DFIG, two close-loop control strategies are adopted respectively to estimate the rotor speed and position based on the generator grid-connect condition. Before the grid connection, the rotor speed and position are estimated through the stator voltage. When the generator is connected to the grid, the model reference adaptive control (MRAC) method based on the electromagnetism relationship of DFIG is used to estimate the rotor speed and position. The control strategy is verified by simulation and experimental results.Low voltage ride through (LVRT) control of DFIG is a significant content in this dissertation. Based on the electromagnetic relationship in DFIG, the principle of DFIG short circuit current is analyzed when three-phase short circuit fault occurred in stator and the influence of generator pre-fault operation state on short current is also analyzed. The active crowbar control method in symmetrical grid fault condition is researched. Simulation results shows that with proper design and control, LVRT of DFIG can be achieved during serious grid fault. In unbalanced grid condition, dual vector control strategy in positive and negative synchronous coordinate frame is adopted. Different negative sequence rotor current references for special control targets are introduced.The transient DC flux component will emerge in DFIG stator because of the voltage change caused by grid fault, which makes the output power fluctuate and the electromagnetic torque vibrate. A novel flux damping control strategy of DFIG based on grid voltage oriented is proposed. Research result shows that the transient DC flux component can be eliminated quickly and the stability of system will be improved, so this method is helpful to achieve the LVRT operation of DFIG system.In grid-connected wind generation systems, the grid information is very important to the control of wind generators and converters. In grid distortion and fault condition, especially in unbalanced condition, the detection of grid synchronization information will be badly disturbed. A novel three phase PLL design method is proposed because of the drawbacks of conventional grid voltage detection method. The influence caused by unbalanced voltage is eliminated by a delayed signal cancellation (DSC) controller in double synchronous reference frame.A variable period sampling method is used to trace the frequency and eliminate phase shift offset which is caused by frequency deviation.Based on the conventional vector control strategy of the grid side converter, an improved positive and negative sequence double synchronous vector control method is proposed. Through this method, both positive and negative sequence current can be controlled respectively and the front-end voltage distortion introduced by double synchronous will be avoided. The LCL filter is widely used in three phase voltage source PWM converter for its good performance. A novel active damping control strategy is proposed and the capacitor current estimate method is adopted so no addition sensor is needed. The feasibility of the proposed method is verified through experimental results.An improved grid voltage control strategy of DFIG wind farm in distribution networks is proposed based on the DFIG operation characteristic. The voltage control methods of reactive power and negative sequence voltage compensation are included. A modified reactive power limit calculation method in different operation states of DFIG is analyzed and the reactive power control strategy is introduced. The negative sequence voltage compensation control strategy of DFIG is proposed for unbalanced grid voltage condition. The compensation limit is given based on the current margins and DC voltage limitation of grid side converter. The distribution algorithms of negative sequence current in the wind farm and in the generator system are also introduced. Research result shown that the stability of wind farm and DFIG ysytem is improved through the grid voltage method.An experimental platform of DFIG wind power generation system is developed. The grid-connection control, VSCF operation between sub-synchronous and super- synchronous, active and reactive power decoupled control and bidirectional operation of converter system are achieved. Based on the experimental platform, a 1.5MW DFIG control system is designed for a project, and the on-ground test is carried out in the test bench.
Keywords/Search Tags:wind power generation, DFIG, grid fault, LVRT, vector control, PWM converter, voltage control of wind farm
PDF Full Text Request
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