Design Of Three-phase Active Clamped Three-Lever Inverter And Research On Smooth Transfer Strategy Between Grid Mode And Off-grid Mode

The continuous consumption of traditional energy and the environmental problems that brought about makes the development and utilization of new energy increasingly become an important research topic.Distributed generation and Micro-grid are new generation systems for new energy.As the connection port between new energy and grid,inverter,plays a very important role in micro-grid system.Based on the active clamped three-level(ANPC)topology,a 60 kVA three-phase grid-connected inverter is designed in this paper.The switching combinations and common working modes of the topology are analyzed.Based on the three-level space vector modulation strategy,the basic principle and control idea of neutral-point voltage balance are given.The control mode of LCL grid-connected inverters is described.The difference between grid-side current closed-loop control and inverter-side current closed-loop control is analyzed.Finally,the analysis of topology principle,simulation modeling,programming and prototype debugging are completed.By analyzing the voltage and current of power devices in switching period and fitting the voltage-current relationship of their ports,a loss model based on superposition and integration is established.The losses of power devices in ANPC topology under different working modes are compared and calculated,and the relationship between the conduction loss,switching loss and power factor angle is given,so as to analyze the mode selection principle under different working conditions.The control method of microgrid inverters in grid model and off-grid mode is analyzed,and the causes for overshoot of load voltage and grid side current during mode switching are analyzed.On this basis,the smooth switching strategy is discussed from two aspects-unified mode control structure and clear switching timing.A time-sharing switching strategy based on current control is proposed.An experimental platform is built to test the ANPC prototype,the control and modulation strategies are verified by experiments.Furthermore,in order to study the mode smooth switching strategy for inverter,a three-phase half-bridge two-level AC microgrid experimental platform with RT-LAB as the control core is built.The relevant experiments are completed by designing the mode switching strategy,which verifies the feasibility and effectiveness of the time-sharing switching strategy.