Research On Control Strategy Of Full Power Converter For Permanent Magnet Direct-driven Wind Turbine

With the rapid development of wind generation, since permanent-magnet windturbine features running stability, high power density, high efficiency, good reliabilityand potential for further large capacity, which has become one of the main types in windgeneration. Moreover, as the number growth of permanent-magnet and the highrequirement to specification of synchronization with the electrical network, whichpromotes researchers at home and abroad to switch to controlling strategy undernon-ideal electrical network from the normal electrical network conditions.In this paper, based on non-ideal electrical network situation, full-power converterof permanent-magnet wind generation system is main investigation object, whichincludes the two cases of grid voltage drop and power grid voltage three-phaseunbalanced. The performance of permanent-magnet wind turbine which the non-idealelectrical network impacts on and controlling strategy under the stability areinvestigated from the aspects of theoretical analysis and simulation.Firstly, line-side PWM converter and generator-side PWM converter mathematicalmodels of permanent-magnet wind generation system are introduced in three-phasestationary frame, two-phase stationary coordinate frame and two-phase rotatingcoordinate frame respectively. The relational expressions of active power, reactivepower and DC bus voltage at the out of line-side converter are calculated based onmathematical models, and the line-side converter and generator-side convertercontrolling strategy are studied based on stator voltage orientation vector controlcombining with relational expressions. Furthermore, in order to solve the problem thatDC bus voltage vastly increases because of electrical network voltage drop, oneimprovement strategy of generator-side converter is proposed above all, i.e. acompensational coefficient is added at the active power of generator-side converter.When electrical network voltage drops, reference value of active power ofgenerator-side converter will be add with this compensational power term so as todecrease the output of generator-side active power. When electrical network is normal,this coefficient will be zero, which will not influence original controlling strategy.Secondly, performance of permanent-magnet wind power generation system isinvestigated under the three-phase unbalanced conditions of electrical voltage. And then,the line-side and generator-side models are built in the unbalanced condition, according to the theory of three-phase asymmetric decomposition. And the controlling strategiesbased on no fluctuations in the output power and DC bus voltage are proposedcombining with the positive and negative thought, i.e. when electrical network isunbalanced, the second harmonic caused by unbalanced is eliminated with negativesequence current controller.At last, in order to validate the correctness and feasibility of models,Matlab/Simulink is employed to simulate these two controlling strategies.