Comprehensive Optimization For Fault-tolerant Control Of Converters In PMSG Based Direct-driven Wind Power Generation Systems

Wind power generation has become one of the most important ways to solve the fossil energy crisis and environmental pollution.Therefore,many researchers all over the world have paid their attentions to the wind power generation.Because of their high effifiency,high energy density and low maintenance costs,the permanent magnet synchronous generator(PMSG)based direct-driven wind power generation systems are widely applied.With the expansion of the wind turbine installed capacity,the wind power generation systems have more and more significant influences on the stability of the power grid.In order to maintain the safe and stable operation of the grid,the reliability and non-stop operation ability of the wind power generation system are required.Studies have shown thatthe power switches of the power electronics converters and their drive circuits are the most fragile part of the wind power generation system,severely restricting the reliability of the grid.Therefore,based on the PMSG based direct-driven wind power generation system,the three-phase four-switch(TPFS)inverter topology is taken as the fault-tolerant topology when the unpredictable failures occur to the power swithes in the grid-side converters or the motor-side converters.Consequently,the fault-tolerant and uninterrupted operation of the PMSG based wind power generation system can be realized to ensure the stability of the grid.In order to improve the post-fault performances of the PMSG based wind power generation system,in-depth theoretical analysis and experimental verifications have been conducted in this paper.As a result,several optimized control strategies have been proposed to reduce the negative impacts on the post-fault PMSG based wind power generation system,so that the uninterrupted operation after the failure of the power switches is ensured.Research efforts of this study are mainly focused on two parts:one is the non-stop operation of the grid-side converter after faults occurring;the other one is the performance improvement of the PMSG at post-fault operation.The main research contributions are organized as follows:1.In order to achieve a high performance control,this study develops a control oriented dual single-input-single-output model for the post-fault grid-connected TPFS converter of the wind power generation system in a novel non-orthogonal frame.Using given DSISO model,the coupling effect of the two axes is totally dismissed,and basic guidelines for the controller design are provided.To eliminate the grid current harmonics caused by the capacitor voltage fluctuations,a very simple modulation strategy is proposed based on the non-orthogonal coordinate system,which exhibit the merits of less computation and higher accuracy compared with traditional methods.2.Some crucial performance characteristics related to the operational reliability of the post-fault grid-connected TPFS converter in the wind power generation system,such as the line current harmonic distortion,common mode voltage(CMV),and current stress to the capacitors,are fully investigated.The aforementioned performance characteristics of the post-fault converters are highly dependent on the utilized space vector modulation(SVM)schemes,which are also examined.Detailed analyses of the three most commonly used SVM schemes for the post-fault grid-connected converters are provided,revealing the major differences in zero vector synthesis approaches.To compare the performances of the three SVM schemes,the operating principles of the post-fault grid-connected converters are presented with various SVM schemes.Using analytical and numerical methods in the time domain,the performances of the line current distortion,common mode voltage and capacitor current are evaluated and compared for each SVM scheme.The proposed analysis demonstrates that the zero vector synthesis approaches of the considered methods have significant impacts on the performance characteristics of the rectifier.In addition,the advantages and disadvantages of the proposed SVM schemes are discussed.3.The linear modulation range of the post-fault TPFS inverter-fed PMSG drive is derived considering capacitor voltage fluctuations,therein avoiding the low-frequency torque ripples caused by over-modulation.The analysis results show that the linear modulation range is affected by the rotor speed and output torque of the PMSG at post-fautl operation.Meanwhile,it is shown that the DC-link voltage offset deteriorates the linear modulation range severely,restricting the performance and adjustable speed range of the PMSG.Therefore,an adaptive control method is proposed in this study to suppress the DC-link voltage offset in TPFS inverter-fed PMSG drives.By investigating the effects of four basic voltage vectors on the voltage offset,the proper compensating vectors,whose duration times are obtained by the closed-loop control,are added in each control period.Moreover,an adaptive notch filter is proposed to extract the offset component,providing excellent dynamic and steady performances even at extremely low rotor speed.4.The torque ripples of PMSGs severely deteriorate the performance and reliability of the entire system.Hence,comprehensive considerations for torque ripple reduction,including high-and low-frequency torque ripples,are elaborated considering post-fault TPFS inverter-fed PMSG drives.The second-order torque harmonics produced by DC capacitor voltage fluctuations are first demonstrated,and a very simple compensation method is presented by introducing a novel non-orthogonal coordinate transformation.Due to the asymmetrical circuit topologies,the forward voltage drops of semiconductor power switches have a unique influence on the performances of post-fault TPFS inverter-fed PMSG drives,producing unbalanced motor currents and pulsating torques.In this study,the effects of forward voltage drops on the output voltages are investigated in depth through the proposed equivalent circuits of TPFS inverters,and the results indicate that unbalanced voltage harmonics are injected into the stator voltages of the PMSG.The distorted phase voltages produce unbalanced motor currents,resulting in low-frequency torque ripples.The torque ripples become tremendous when the PMSM is running at low speeds.According to the analytical results,a compensated modulation strategy is proposed to eliminate the torque ripples introduced by the forward voltage drops.Then,to evaluate the effects on the high-frequency torque ripples of space vector modulation(SVM)schemes,two commonly used switching sequences in TPFS inverter-fed PMSG drives are fully investigated based on the root mean square(RMS)value of the torque ripples,in which the effects of the different equivalent zero vectors on the torque ripples are presented.Then,a hybrid space vector modulation(SVM)strategy is proposed to minimize the torque ripples by alternatively using the two equivalent zero vector synthesis approaches during a fundamental period.The sector division of the proposed hybrid SVM strategy is determined by the location of the stator current vector,which is quite different from the methods used in other SVM methods.Then,a simplified sector identification method is proposed to reduce the computational burden.The comprehensive torque ripple reduction methods proposed by this study improve the performance of the PMSG at post-fault operation greatly,guaranteeing the stability and reliability of the grid.