Research On Key Control Technology For Non-isolated Three-level T-type Photovoltaic Inverter

Renewable energy is an important approach to solve the environment pollution and energy crisis.Solar energy has large reserves,convenient installation and flexible size,which is acknowledged as the mainstream direction of the new energy development in the 21st century.And photovoltaic inverter is the key equipment of the photovoltaic power generation system.The three-level T-type inverter(3LT2I)is very important for reducing the system cost and improving the efficiency of the system account of its high efficiency and low harmonic.Therefore,3LT2I is the first choice of the inverter topology.However,the corresponding theories and technologies of the three-level photovoltaic inverter has not been formed,such as three-level T-type new topology and its modulation strategy,parallel 3LT2Is zero-sequence circulating current(ZSCC)suppression,leakage current suppression.In this thesis,the research object is non-isolated three-level T-type photovoltaic inverter.The researches on three-level new topology,ZSCC and leakage currents have been done,which lays the foundation for large-scare use in the photolytic system.The main content and innovation of this thesis are as follows:Aimed at the problem of more components and lower efficiency,in this thesis,a new modulation method is proposed for Z-source 3LT2I.The method can realize the Z-source 3LT2I boosting,NP voltage balance and inverter.The modulation method simplifies the space-vector diagran of a Z-source 3LT2I into that of a Z-source two-level inverter.In order to obtain NP potential balance and different boost ratio,the duty ratio and state transition time is adjusted.The validity of topology and modulation has been verified by simulation and experiment.Aimed at the problem of low efficiency of grid inverter with light load,module parallel 3MT Is can increase power rating,reliability and efficiency of system.However,the topology of transformerless inverter and with common dc-link would cause the zero-sequence circulating current(ZSCC)problem.The circulating current will result in current distortion and degrade the overall performance of the parallel 3LT2Is.Also,the coupling problem of the circulating current and NP potential will increase the difficulty of the circulating current suppression.Consequently,it proposed a novel SVM method to suppress the circulating currents and control the NP potential.This thesis proposes a novel deadbeat control scheme for the ZSCC suppression.A generalized model for analyzing ZSCC in parallel 3LT2Is is developed for designing the deadbeat control method.The ZSCC can be suppressed by real-time adjusting the dwell time of small vectors in each SVM.Compared to the conventional PI control method,the influence of different sectors is eliminated,thus,improving the output current waveform and reducing the ZSCC.The 3LT2I existes the inherent problem of unbalance DC-link capacitor NP voltage.Therefore,the time of the conventional symmetric SVM was changed by adding or subtracting the minimum dwell time to balance the NP voltage.The novel SVM and deadbeat control strategy can realize ZSCC suppression and NP voltage control under different current conditions by simulation and experiments.On the research results of the above,the ZSCC of module parallel 3LT2Is based on LCL filter can be suppressed.The averaged model of ZSCC is detailed analyzed.A small vectors feed-forward control method with PI controller is proposed to reject disturbance in zero-axis current system.Moreover,a double sampling method is employed to expand the bandwidth of zero-axis current system.Therefore,the ZSCC can be suppressed satisfactorily when output currents are different by applying the proposed scheme.The effectiveness of the proposed method is verified by the experimental results.Transformerless topologies are popularized in the photovoltaic grid-connected system because these topologies have the advantages in terms of the efficiency,weight and size.However,the leakage current will be generated in the photovoltaic 3LT2I.Moreover,the coupled problem between the leakage current suppression and the NP balance is difficult to deal with.To solve two problems,this paper proposes a model predictive current control to suppress the leakage current,balance the NP voltage and track the reference current.Model predictive current control presents fast dynamic response and reference tracking than other control methods,but working at lower switching frequency.The proposed method has been proven in simulation and experiment from the prototype 3LT2I.