Fault-tolerant Three-phase PWM Converter And Its Application For Renewable Energy

As the power capacity of the renewable energy continues to increase,the effects of the renewable energy on the stability and security of the power system are receiving more attention.Among the concerns about the stability and security of the power system,the reliability and availability of the three-phase grid-connected PWM(TPSS)rectifier are the primary issue.A substantial number of surveys and investigations have demonstrated that for the three-phase grid-connected PWM rectifier converter,the power switching devices,such as the insulated gate bipolar transistors(IGBTs)and metal-oxide field-effect transistors(MOSFETs),are the most fragile components.In order to improve the availability and productivity of the power converter,the reliability of the power rectifiers has gained increasing attention in the past decades.In view of it,the focus of this dissertation is on the situation in which the open fault or short fault occur in one leg of the converter,which is the most common situation of the power converter in fault.To achieve the excellent performance of the converter in healthy condition and stable operation of the post-fault converter,the detailed modeling,optimized space vector modulation(SVM)and the reliable control algorithms are conducted in this paper.To achieve the excellent performance of the converter in healthy condition,a simple and accurate control-oriented model for a digitally controlled three-phase PWM boost rectifier is proposed in this paper.The proposed discrete model,based on the modified Z-transform,is derived from a complete small-signal model in the Laplace domain,including the modeling of the power stage,the AD unit,the digital PWM module and the control time delay that inherently exist in the digital control system.The experimental results verify that the model is a simple and accurate control-oriented model and is applicable to the direct discrete design of the digital controller.To guarantee the stable operation of the post-fault three-phase PWM rectifier,the comprehensive design and optimization,including the optimized modulation approach,detailed modeling and reliable control algorithm,is presented in this paper.The AC current ripple,the common voltage(CMV),and the current stress on the DC-link capacitor are defined as the critical performance indicators of the three-phase four-switch(TPFS)converter,and the effect of the zero-voltage vector distribution on these crucial indicators is investigated comprehensively in the proposed paper.Due to the complexity of the Fourier analysis method,the RMS values of the crucial indicators are adopted to evaluate the relative merits of various zero-voltage vector distributions.The proposed RMS value equations are straightforward for the evaluation of the effect of the zero-voltage vector distribution on these crucial indicators.The degree of freedom of the zero-voltage vector distribution can be utilized to achieve the overall performance of TPFS converter or the optimized performance of one indicator the TPFS converter.In addition,to realize the excellent performance of the AC current,the comprehensive optimization scheme,including the high frequency current ripple reduction and low frequency current distortion reduction,is proposed in this paper.The relationship among the AC current,DC voltage and linear modulation is investigated in detailed,and a novel linear modulation index function is defined to reject the low frequency harmonic current introduced by the overmodulation.Experimental results demonstrate that excellent current performance is achieved with the comprehensive optimization scheme.Furthermore,to avoid a secondary fault in post-fault operation,the comprehensive design and optimization,including the optimized modulation approach,detailed modeling and reliable control algorithm,is presented in this paper.The proposed reliable algorithm do not require the phase lock loop(PLL),and the elimination of the PLL means that the post-fault rectifiers can never lose synchronization with the grid,which prevents the secondary fault.Finally,to achieve a reduced current stress on the DC-link capacitors,a hybrid PWM strategy for the post-fault grid-connected three-phase rectifier is proposed in this paper.A novel distribution of the vector plane is developed for the proposed HBSVM strategy,where the size of sector I is the same as that of sector ?(?/3 radians)and the sizes of sectors II and IV are 2?/3 radians.The analytical results demonstrate that the proposed HBSVM strategy results in the reduced capacitor current RMS,which avoids a failure of the DC-link capacitors.The experimental results validate the effectiveness of the proposed HBSVM strategy.