Design Of T-Type Three-Level Modular Grid Connected Inverter

With the adjustment of China's energy structure,the development and utilization of new energy has been rapid development,photovoltaic power plant installed capacity increased year by year.However,with the increase of the output power of the PV power plant,due to the limitations of the semiconductor device manufacturing process,a single photovoltaic grid-connected inverter has been unable to meet the requirements of the user's power requirements.The output capacity of the inverter can be increased by the parallel or series connection of the switching elements,but the inverter integration is not high and the control is complicated and the cost is increased.Inverter parallel technology has become a hot issue.In this paper,three-level T-type inverter parallel technology has been studied in depth,and the corresponding problems in the parallel connection of the inverter are analyzed and the corresponding solutions are put forward.Firstly,the topology of the three-level three-phase inverter is compared and analyzed.It is concluded that the T-type three-level three-phase inverter has the advantages of structure and control.The commutation process of T-type three-level three-phase inverter is studied,and the loss model in the process of inverter operation is proposed.Secondly,the two-level inverter and three-level inverter control method are compared.The influence of voltage vector interaction time on the output of the inverter under the control of space vector control is studied emphatically.Three-level inverter space vector control is more complicated than two-level vector control,but the three-level inverter can effectively reduce the output harmonics,reduce the size of the filter circuit and reduce the cost.The correctness of the three-level vector control is verified by simulation.Thirdly,the problem of unbalanced point potential in the voltage stabilization voltage of the DC bus is studied in the process of the three-level inverter.Compared with the control method,the equilibrium factor algorithm has the advantages of simple realization and small steady-state fluctuation of the midpoint potential.The simulation results show that the balance factor algorithm is correct,the dynamic performance is good and the steady-state ripple is small.Finally,a scheme of synchronous communication driving of the inverter is proposed on the problem of circulation in the parallel process of the inverter.Because there is a grounding capacitance between the PV panel and the earth,it is easy to circulate between the inverter to reduce the efficiency of the system.Through the synchronous communication between the inverter,the driving signal can be fully synchronized and the circulation of the inverter can be reduced effectively.In order to verify the correctness of the proposed scheme,the 500 KW T-type three-level three-phase grid-connected inverter prototype design was completed.Experiments show that the prototype has good soft start characteristics,steady state can be very good to track the grid current.Through the balance factor control algorithm is a good solution to the problem of circulation between the inverter,with high efficiency,low output ripple advantages.