The Grid Connection Research On Double-Fed Induction Generator

The wind power as a renewable clean energy has extensively attracted the attention of people.Wind power generation,which is of most potential and commercial value,has also become one of the ways of new energy generating electricity.Because the doubly fed VSCF wind power generating system has the advantages of having simple controlling method,the frequency converter in small volume as well as of low cost,it has been widely used in industrial production and also become the mainstream electrical power generating system.This paper has studied VSCF wind power generating system,focusing on the technology of the grid connection applying in doubly-fed generator,this thesis has done the following studies.Firstly,this paper introduces the operation principle of doubly-fed wind power generator,establishing the mathematical model of doubly-fed generator under the three-phase static coordinate system and the two-phase synchronous rotating coordinate system.Secondly,mobility of power of doubly-fed generator and three different working condition are analyzed.Then,the study analyzed the grid-side PWM-converter based on the control strategy in grid voltage oriented vector as well as introducing the technology of Space Vector Pulse Width Modulation(SVPWM)and constructing the mathematical model for grid-side PWM converter.The rotor side PWM converter control strategy is analyzed on the choice of control strategy in double closed loop-oriented stator flux linkage,meanwhile,the thesis establishes the mathematical model for doubly-fed generator under the dq axis.The no-load grid-connection control strategy of DFIG is investigated and the control block diagram for no-load grid connection is presented.The simulation model has been built based on the above theories under the Matlab/Simulink simulation environment and the simulation result has verified the correctness of theoretical analysis.Finally,transient characteristics are analyzed when grid voltage sags and the findings show that there will be large overvoltage and overcurrent generated in rotor side when grid voltage sags.As a result,the system must be protected by Crowbar circuit.Two mainstream Crowbar circuit structure of active crowbar and passive crowbar are introduced in this study.The system adopts the rectifier-bridge active crowbar with detecting and controlling functions to protect the system.The circuit with crowbar has been simulated.The study also verifies the effectiveness of the protectiveness of Crowbar circuit.The above research work has laid a good foundation on the following experiment on the hardware platform.