Research On Common-mode Voltage Suppression Strategy For Non-isolation T-type Three-level Photovoltaic Grid-connected Inverter

Facing the decrease of primary energy sources,photovoltaic(PV)grid-connected power generation technology is more and more used in various fields.Users have higher requirements in reliability,safety,high efficiency.Non-isolation PV grid-connected inverter has the advantages of light weight,small size,low cost,high efficiency,etc,but change of common-mode voltage on parasitic capacitance which exists between PV array and the earth produces the common-mode current due to the direct electrical connection with power grid,the bigger common-mode current can cause such problems as grid current distortion,electromagnetic interference,and also threaten the user's personal safety.Common-mode voltage suppression strategy for the non-isolation T-type three-level PV grid-connected inverter is studied in this paper.The common-mode voltage mathematical model of the T-type three-level grid-connected inverter is made.The influence factors of common-mode voltage on non-isolation grid-connected inverter are analyzed,the effective method inhibiting common-mode voltage is extracted according to the analysis of the mathematical model.On the basis of comparison of the existing common-mode voltage methods,five common-mode voltage suppression strategies based on SVPWM are discussed from the view point of optimizing the modulation strategy.The change of common-mode voltage amplitud e and frequency is analyzed in different sectors for each modulation strategy,and compared with conventional SVPWM strategy.Ultimately,three SVPWM strategies which have high dc-bus voltage utilization ratio and small common-mode voltage are studied.The neutral-point voltage unbalance factor of the three-level inverter is analyzed in detail.The charge variation at neutral-point during a switching cycle is calculated to receive the change trend of neutral-point voltage when the above three different S VPWM strategies are used,thus neutral-point voltage balance control method with the low common-mode voltage is put forward.The method can reduce the change range of common-mode voltage,and effectively control dc-bus neutral-point voltage of the three-level inverter.The simulation model is built by means of MATLAB/Simulink and experimental platform based on the microcomputer STM32F407.Simulations and experiments of the above three low common-mode voltage modulation strategies and neutral-point voltage balance method are carried out respectively.C hange rules of the neutral-point voltage,common-mode voltage and leakage current based on the new algorithm are studied,the simulation and experimental results are consistent with the above mathematical analys is,the correctness and feasibility of low common-mode voltage suppression strategies are verified.