| With the increasement of the capacity of the direct-driven permanent magnetsynchronous generator (D-PMSG), its importance to the grid has become increasinglyevident so that it would have a direct impact on the stability and power quality of theconnected grid. Therefore, considering the security of the grid, power authorityproposed that wind turbines should have the ability to operate without disconnectfrom the grid and ride through the grid fault when external grid was fault. Currently,the existing D-PMSG control methods are limited to running under the balanced gridvoltage, while the real grid always exists voltage transients and asymmetric fault. Thevariable speed constant frequency (VDCF) operation of D-PMSG uses traditioncontrol strategies under the condition of ideal large power grid, which could not meetthe reality that wind turbines in China was usually connected to the weak power grid,neither could ensure the security operation of the wind power system from thecontrolling. In that case, a new control model must be established to adapt the realgrid and new control strategies must be discussed to ride through the grid fault. In thiscontext, a D-PMSG was chosen for the study and research on its operation and controlstrategy under unbalanced grid voltage.In this dissertation, the unified mathematical model of D-PMSG was establishedby using space vector method. Then based on the model, the paper discussed theD-PMSG control strategy under such conditions: normal grid voltage, the small valueunbalanced grid voltage, and large value unbalance grid voltage caused by the gridasymmetric fault.By taking the2MW permanent magnet synchronous generator (PMSG)(XE-82)made by the Xiangtan Electric Manufacturing Group (XEMC) as an example, thethesis analyses the simulation of the operation and control strategy of the PMSGunder unbalanced grid voltage. At the same time, the thesis analyzed the experimentresult by taking a2kW back-to-back PWM converter as an example. And it achievedthe operation and control of the D-PMSG under small and large value unbalanced gridvoltage. The simulation and experiment results have shown that the proposed controlstrategy applied to the D-PMSG were correctness and availability, which has referentvalue to realize the asymmetry fault ride-through of the PMSG. The main researchcontents and innovation results of this thesis are listed as follows:(1) Based on the PMSG mathematical model, a steady-state control strategy suitable for the system under normal voltage was proposed and validated bysimulation, which laid the foundation of further study of the D-PMSG operation whenthe grid voltage under abnormal conditions. Then, the low voltage ride through withthe grid voltage symmetrically fall to the20%of rated voltage was achieved insimulation by adding an energy discharge unit on the DC bus.(2) By deducing the mathematical model of interior permanent magnetsynchronous wind turbine, the thesis proposed a new universal sliding mode observermodel which applied to conditions of both Ld=Lq and Ld≠Lq. Then starting fromthe factors affecting the observer’s precision of position from the observation system,the principle of filter selection and angle compensation in observer was deeplyanalyzed. Moreover, a simulation was carried out to prove the correctness of theproposed algorithm.(3) Dual current control loop by using PI controller in positive and negativesynchronous rotating coordinate and single current control loop by using PI-REScontroller in positive synchronous rotating coordinate were adopt respectively indetailed comparative study on inhibiting the negative sequence current under smallvalue unbalanced grid voltage. In this dissertation, a motor-side converter currentcontrol strategy to suppress generator torque pulsating caused by dc-bus voltagefluctuations under small value unbalanced grid voltage.(4) A control strategy in the asymmetry drop fault of grid voltage was putforward. By theoretical analysis, an energy discharge unit on the DC bus was usedrespectively with dual current control loop strategy by using PI controller in positiveand negative synchronous rotating coordinate, single current control loop strategy byusing PI-RES controller in positive synchronous rotating coordinate and a SVM-DPCcontrol strategy by using PI-RES proposed by this dissertation to achieve50%unbalanced grid voltage fault ride through.(5) In this dissertation, the control strategies of grid side converter and thenon-position-sensor control of machine side converter of the D-PMSG were verifiedby the experiment on a2MW PMSM experiment platform under the symmetricvoltage condition. The experiment proves the effectiveness of the proposed controlstrategy. At the same time，combining the energy discharging device of DC side withthe control strategy under large value unbalanced grid voltage，this thesis validatedthe proposed method by detailed experiment on a established2kW back-to-backdouble PWM converter. The experimental results verified the correctness of the correlation analysis. |
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