Research On High-Performance Modulation And Control Technology Of Three-Level Photovoltaic Inverter System

The photovoltaic(PV)power generation is an effective solution for solving the problems of conventional energy shortage and environmental pollution.It is also an important part of building a clean,low-carbon,safe and efficient energy system for China.As the core equipment of the PV power generation system,the performance of the inverter directly affects the efficiency and reliability of the PV system.The conventional two-level inverter topology has disadvantages of high distortions in output waveforms and large filters,which is difficult to satisfy the requirements of high-performance PV generation system.The three-level inverter has several salient advantages,such as low voltage stress and low power losses of power devices,good quality of output waveforms,and high efficiency.It can solve the above-mentioned problems of the conventional two-level inverter topology,and has become a research focus in academic and industrial fields at home and abroad.Therefore,the research for the three-level inverter has important theoretical and practical significances.The scholars at home and abroad have conducted extensive research work about the modulation and control technology for the three-level inverter.However,sever-al key issues related to three-level inverters have not been well solved.For low-power high-boost three-level inverter system,the common-mode voltage(CMV)of the conventional space vector modulation(SVM)scheme is high,which results in common-mode leakage current and electromagnetic interference.Furthermore,there exists coupling between the inherent neutral-point(NP)voltage problem and CMV.For high-power modular parallel-operated three-level inverters,the circulating cur-rent among different modules distorts the output waveforms,increases the losses,and decreases the efficiency.In addition,the unbalanced grid voltages and unbal-anced filter parameters aggravate the circulating current problem.For high-power centralized three-level inverter,the mismatches in PV modules and partial shading decreases the system efficiency.The above-mentioned problems seriously restricts the large-scale application of three-level inverters in PV power generation systems.To this end,the research status at home and abroad is summarized by this the-sis,based on which the high performance modulation and control technology for the three-level inverter are researched,such as CMV reduction and NP voltage balance control,circulating current suppression,separate control for dc-link capacitor volt-ages and CMV reduction,etc.The expected performance indexes for the three-level inverter include high reliability,low electromagnetic interference,high efficiency,strong adaptability and low harmonics.The main research and innovations of this thesis consist of the following four points.1.For the high-boost quasi-Z-source three-level inverter,the conventional S VM method has high CMV magnitude and large common-mode leakage current.To solve this problem,a new modulation scheme is proposed to simultaneously reduce the magnitude and slew rate of the CMV.The new scheme adopts large,medium,zero,and shoot-through vectors to synthesize the reference voltage vector.By s-electing suitable value of the shoot-through phase,the shoot-through states can be inserted within the zero vector,and voltage boosting and CMV reduction can be re-alized.Simulation and experimental results are provided to validate the correctness of the theoretical analyses and the effectiveness of the proposed scheme.2.The quasi-Z source three-level inverter topology has great potential in s-mall and medium PV power generation systems.However,some non-ideal factors such as capacitor manufacturing error and inconsistent power device characteristics cause NP voltage unbalance,and there exists coupling between the NP voltage and CMV.In order to reduce the CMV and control NP voltage balance,a novel SVM scheme is presented,in which large vectors,medium vectors,small vectors with low CMV magnitudes,zero vector,and shoot-through vectors are employed.By consid-ering the sector number and the voltages across the dc-link capacitors,the proposed scheme properly selects the small vector with low CMV magnitude in the current or adjacent sector,so as to control the NP voltage balance with reduced CMV.This scheme can overcome the inherent drawback of slow recovery of NP voltage balance in conventional "nearest three vectors" method.Simulated and experimental results demonstrate the effectiveness of the proposed scheme.3.The modular parallel-operated three-level inverters is an effective solution to solve the problem of inefficiency in light load.However,the common connection of dc bus and that of ac bus result in circulating current problem,which reduces the quality of output waveforms,reliability,and efficiency.The unbalanced operating conditions that include unbalanced grid voltages and unbalanced filter inductances present more serious challenges for the modular parallel-operated three-level invert-ers.In this aspect,a hybrid control scheme is proposed for the parallel-operated three-level inverters under unbalanced operating conditions.The proposed hybrid control scheme adds quasi-resonant controllers to the conventional proportional in-tegral(PI)controller and introduces accurate feed-forward control terms.Doing so,the circulating currents between parallel-operated inverters are effectively sup-pressed,and the quality of output currents is significantly improved.The effective-ness of the proposed scheme is validated by simulations and experiments4.The presence of mismatches in PV modules and partial shading decreases the system efficiency of high-power centralized three-level inverter.The topology with separate PV modules can solve this problem.However,the unbalanced NP voltage will increase the harmonics in output currents.In addition,the conventional mod-ulation method generates high CMV,which causes common-mode leakage current and electromagnetic interference.An optimized zero-sequence component injection scheme for CMV reduction is proposed for the centralized three-level inverter with separate maximum power point tracking(MPPT)capability.The proposed scheme comprehensively considers the dc-link capacitor voltages,three-phase modulation waves,and the sign of zero-sequence component.The range of zero-sequence com-ponent is accurately calculated,so as to reduce the CMV and obtain high-quality out-put currents.The fast and accurate deadbeat controller for NP voltage is designed,so as to control the dc-link capacitor voltages with high speed and accuracy.The effectiveness of the proposed scheme is validated by simulations and experiments.