Research On Generator-side Control Strategy Of Direct-drive Wind Power System

Direct-driven wind power generation system (DDWPGS) has the merits of highreliability, good power quality, high power generation efficiency and flexible control, andmeets the development trends of wind power system so as to show a good perspective in itsresearch and exploitation. Based on the analysis of composition structure and workingprinciple of DDWPGS, in order to improve the efficiency and reliability of the system, thisthesis has investigated on the control strategy of system’s generator side and verified thecorrectness and effectiveness of the control strategy by simulation.Firstly, this thesis has briefly introduced the development of wind power industryworldwide, analyzed the topology of mainstream wind power systems, and overviewed theresearch status of sensorless control and maximum power point tracking (MPPT) control.Secondly, the composition and working principle of DDWPGS are analyzed, and theback-to-back PWM topology is choosen as the object of study. Furthermore, themathematic models of DDWPGS’s generator-side components which include wind turbine,mechanical drive train, PMSG and pitch mechanism are established.Thirdly, in order to reduce the use of physical sensors and improve the reliability ofDDWPGS, a sensorless vector control of PMSG is investigated where model referenceadaptive control (MRAC) is used to estimate the rotor speed and position of PMSG; singlesynchronous reference frame software phase locked loop (SSRF-SPLL) is employed tolock the initial position of PMSG; the vector control of id=0and SVPWM modulationtechnique are used to realize the speed control of generator. Simulation results show thatthe sensorless vector control can quickly and accurately estimate the rotor speed andposition of PMSG, and make rapid response to a given speed signal so that it has a goodcontrol performance.Fourthly, in order to maximize the utilization of wind energy and reduce the speedfluctuation, MPPT control strategies are studied so that three MPPT algorithms are given,namely, variable step-size HCS based on slope, variable step-size HCS based ondichotomy, fuzzy logic searching. Their working principles, flowcharts and designmethods are analyzed in detail. Simulation results show that the three MPPT algorithms are able to implement the MPPT control of a wind turbine, and the fuzzy logic searching hasthe highest precision and least fluctuation.Finally, an integrated control strategy of pitch angle control and MPPT control isgiven according to the characteristics of wind turbine, which uses power hysteresiscomparator and mean sampling power to prevent the controller from frequent switchingbetween different control modes when the wind speed fluctuates near the rated value. Thesimulation results under the random wind speed show that the integrated control strategy isable to implement reliable switching between pitch angle control and MPPT control evenunder the random wind speed, and obtain satisfactory control performance.