Resonant Control Of Doubly Fed Induction Generator-based Wind Energy Power Systems Under Unbalanced Voltage Conditions

With the increasing capacity of wind power into the power grid,grid-connected wind power generators have an increasing influence on the stability of power system.As a result,the grid codes are issued by different countries in the world.In the grid codes,the adaptive operation capability for the grid voltage of the grid-connected wind power generators are clearly defined.Since the large-scale wind farms are far away from the load centers in China,the network voltage at the point of common coupling(PCC)is in the presence of the unbalances.Therefore,based on Chinese Standard GB/T 19963-2011《Technical Rule for Connecting Wind Farm to Power System》,it is required that the wind power generators should have the adaptive operation capability during grid unbalances.Thus,further investigations are essential on the enhanced and reinforced control of the generators.Nowadays,the grid-connected wind power generations can be divided into two main categories,i.e.,the doubly fed induction generator(DFIG)-based system and the permanent magnetic synchronous generotor(PMSG)-based system.The full-power converter is used in the PSMG-based system between the generator and the grid with less network disturbances.However,the DFIG is more sentive to the network disturbances due to the direct connection of the generator and the part-power converter.Therefore,based on the theoretical analysis,the simulation study and the experimental validation,the DFIG-based system is investigated in this paper.The main contribution of this dissertation can be highlighted as follows.1.The second-order generalized ingeator(SOGI)-based vector control(VC)strategy for DFIG’s rotor-side converter(RSC)and grid-side converter(GSC)is proposed.From the viewpoint of the coordinated control of DFIG’s RSC and GSC,the RSC is controlled to eliminate the torque ripples caused by the voltage unbalance,while the GSC is controlled to rebalance the output current of the DFIG system.Based on the proposed SOGI-based vector control,the sequential separations of the measured currents and voltages can be avoided.Meanwhile,the complex calculations of the commanded negative-sequence currents are also removed.By this means,the system implementation can be obviously simiplified.Finally,the simulation and the experimental results demonstrate the effiectiveness of the SOGI-based vector control and the availability of the corrdinated control of DFIG’s RSC and GSC.2.The reduced-order generalized integrator(ROGI)-based vector control(VC)strategy for DFIG’s GSC is investigated.The GSC is controlled to eliminate the twice frequency active(or reactive)power pulsations caused by voltage unbalances.The ROGI-based vector control takes advantage of the the frequency and the rotating direction discrimination ability of the ROGI.As a result,the undesired 3rd harmonic current can be eliminated.Meanwhile,this method can also remove the sequentional seperations of the measured voltages and currents.Finally,the simulation and expremental results are given to validate the effectiveness of the ROGI-based vector control.3.The ROGI-based direct power control(DPC)for the RSC is proposed.In this control strategy,both the active power and the reactive power are directly controlled.The RSC is controlled to eliminate the twice frequency torque ripples of the generator during network unbalance.The positive-and negative-sequence currents/voltages seprations is of no need.Furthermore,the generator is not needed in the calculations of the commanded values,thereby resulting in the reduced parameter dependency.Similar to the ROGI-based vector control,the highly sinusoidal current can be guaranteed with no positive sequence 3rd harmonic currents.In the end,the simulation and experimental results illustrate the effectiveness of the ROGI-based DPC strategy.4.The small-rated test rig of the DFIG system with two-level back-to-back voltage source converter has been developed.The constration of this test rig is shown in detail.The explnations of the hardware and the software are also given.