Control Of PWM Converters In Doubly-fed Induction Generator Systems For Wind Turbines During Grid Fault

The stator of doubly fed induction generations (DFIG) is connected to the powergrid, it’s sensitive to the unbalanced voltage and grid voltage sags. Wind turbinesmight have to be disconnected with the network in the condition of grid faults, and thestability will be reduced. The newly proposed grid standard requires that windturbines must have the ability of compensation to unbalanced voltage and low-voltageride-through during grid faults. The control strategy of power converter is the key tothe wind turbines’ normal operation, and it has important significance to research thecontrol strategy to solve the problems. With the research for fault ride-through ofDFIG and the requirement of large scale wind turbines connected to grid this papermake research on the operating characteristic and control strategies of DFIG.The unmatched model established by traditional model predictive control (MPC)usually deteriorates the control performance and stability of control systems. In thispaper, the mathematical models of the grid-side converter for DFIG are derived.Besides, the operating mode and an error analysis have been given. The instantaneousvariation rates of d-and q-axis currents by applying each converter voltage vector insix different sectors are deduced. Based on the current prediction model, voltagevectors were selected according to minimizing current errors in a fixed time interval,which results in a MPC. The various components which deteriorate the performanceof a conventional model predictive current controller are regarded as disturbances.The relevant gains of the disturbance estimator are determined by root-locus analysis.Moreover, the stability of the disturbance estimator when the inductor filter parameterexist errors is analyzed. The proposed method has an inherent rapid dynamic responsedue to the conventional MPC current controller, as well as robust control performancewith respect to the disturbance and noise interference due to use of the combinedestimation algorithm. Simulation and experimental results are presented to validatethe effectiveness of the model predictive controller with disturbance estimator. Theproposed solution provides good control performance under the accurate model andthe unmatched model.Moreover, this paper concentrates on grid-side converter of the DFIG systemunder unbalanced grid voltage. Unbalanced grid voltage leads to the increase ofcurrent harmonic and violent fluctuation of active/reactive power. Method of symmetrical components is used to analysis the characteristic of grid-side converterinput current and output power under the unbalanced grid voltage. In order to avoidinvolving any sequential decomposition process, the mathematical model of doublyfed induction generations is established under the unbalancing power grid, and theproblem is transformed to the synchronous rotating coordinate system. As thetraditional solutions based on positive and negative phase sequence decompositionhave relatively complex structure, large calculation load and active/reactive powerfluctuations that can not be suppressed at the same time, a power resonancecompensation control strategy is proposed and unbalance control system for PWMrectifier is designed accordingly. The strategy added a compensation part in traditionalvoltage-oriented vector control strategy to solve the problem of power fluctuation. Aproportional resonant (PR) power control scheme implemented in the two-phasesynchronously rotating reference frame was proposed for the compensation. ThePower pulse compensating controller is designed, and influence of the parameters onthe controller is analyzed. The proposed strategy was finally verified by simulationand experimental results.In weak grid, the stator of doubly fed induction generations is connected tounbalance voltage, which causes a number of problems, such as unbalance currents,pulsations of active/reactive power and torque. Therefore, beyond a certain amount ofunbalance, wind turbines might have to be disconnected with the network. Accordingto system model of DFIG in the positive and negative synchronous reference framesunder unbalance grid voltage condition, this paper analyses the pulsation features ofstator active/reactive power and torque under this condition. Also, thedouble-frequency expressions of the stator/rotor current, active/reactive power andtorque are inferred. A new multi-target compensation control strategy withoutinvolving positive and negative sequence decomposition is proposed. The controllercan compensate for the problems caused by an unbalanced grid based on differenttargets. Compared with the dual current control design based on separate loops for thepositive and negative sequence components, the proposed control scheme is simplerand need not positive and negative sequence decomposition. It can not only limit thepulsation of the torque or active or reactive power with more flexibility according todifferent targets, but also eliminate active and reactive powers simultaneously whichcan not be realized in the dual current control scheme. The theoretical analysis andfeasibility of the proposed control scheme are validated by simulation study on a1.5 MW wind-turbine driven DFIG system.Additionally, the transient characteristics of the rotor of DFIG current duringsymmetry and asymmetry grid voltage dips are discussed in the paper. Withoutchanging the structure of hardware, a novel control strategy is also proposed for therotor side converter of doubly fed induction generators-based wind power generationsystem. The strategy consists of a traditional proportional-resonant controller andauxiliary controllers. They can limit direct inrush current and second harmonic whenfalse happens. The auxiliary controllers can compensate output voltage of RSC in thecondition of grid faults, limiting rotor inrush current of DFIG and meeting therequirements of low voltage ride through (LVRT).To improve the response of thesystem, sequential-component decompositions of current are not required in thecontrol system. Since the resonant compensator is a double-side integrator, theauxiliary controllers can be simplified by the coordinate transformation.Therelationship between rotor voltage grade and the LVRT operation area of DFIG isdiscussed and the robustness of the control system and the influence of controlparameters are also discussed. The theoretical analysis and feasibility of the proposedcontrol scheme are validated by simulation study on a1.5MW wind-turbine drivenDFIG system.