Research And Realization Of Discontinuous Pulse Width Modulation For Three-level PV Grid-connected Inverter

With the intensification of the global energy crisis,photovoltaic(PV)power generation technology,as one of the new energy industry,has been paid more attention and developed rapidly.PV grid-connected inverter is the core device of PV power generation system,the grid-connected power efficiency and the grid-connected power quality of which are the key to measure a system,and the PWM modulation technology is the important factor that affects both of them.Therefore,it is great significance to analyze the optimization performance of PWM modulation strategy in different inverter applications.In this thesis,a non-isolated PV grid-connected inverter with better structure,more efficient system and more stable operation is studied,the three-level T-NPC topology with low cost and low loss is taken as the research object,focusing on the loss of inverter,grid-connected power quality and filter design.(1)Research on three-level discontinuous pulse width modulation(DPWM)strategy.The principle of DPWM is analyzed in detail from the angle of space vector,and the DPWM is divided into 6 categories according to the spatial clamping mode of the switch.The realization method of DPWM and the rule of zero sequence voltage injected are given as a simple and feasible method of zero sequence voltage injection.The harmonic characteristics of SVPWM and DPWM are analyzed and compared.Then,aiming at the problem of neutral point potential unbalance of three-level topology,a hybrid modulation strategy is proposed,which can achieve the neutral point balance control by injecting zero sequence voltage and reduce the loss of the device at the same time.The simulation results show that the control of neutral point balance is good.(2)The design of the grid-side filter of the PV grid-connected inverter.In the non-isolated PV grid-connected inverter,the common-mode current generated by photovoltaic cell board will cause interference,increase the grid current harmonics,affect the normal operation of the inverter and even endanger the personal safety.The common-mode equivalent model of PV grid-connected inverter is established by analyzing the common-mode circuit.According to the common-mode current and LCL filter common-mode resonance,a simple and practical common-mode and differential-mode decoupling LCL filter design scheme is adopted,this thesis gives the design method of filter parameters under DPWM,and the theoretical correctness and practical feasibility of the design verified by the simulation.(3)The loss of the device using DPWM under different power factor.In order to ensure that the device always had the minimum loss under different power factors.Based on the detailed analysis of the conduction and switching losses of power devices under various DPWM,this thesis presents a method to realize the minimum loss of DPWM in real time with the change of power factor angle,and verified its feasibility by simulation.(4)The proposed DPWM strategy and filter design are verified by experiments.A 20kW three-level T-NPC PV grid-connected inverter is established,The THD of grid current and efficiency of SVPWM and DPWM were compared and the suppression effect of the neutral point balance control strategy was verified.This thesis analyzes the experimental results of the improved filter design and DPWM switching method,and proves the feasibility of the two.