Research On Z-source Three-level Four-leg Asymmetric Inverter And Its Middle Point Potential Control Method

Because of its flexible control method,high efficiency,low output voltage and current distortion,low common mode voltage,low switching frequency and low voltage stress,three-level inverter has a lot of research and application in the field of active power filter and reactive power compensator,AC motor,photovoltaic power generation and solar power generation etc.Because the three-level inverter is buck converter,in order to obtain higher output voltage,adding a boost circuit to improve the DC side voltage is needed in practical engineering applications,which will form a two-stage converter.No doubt it will increase the cost of system hardware and reduce the efficiency of the system.In order to overcome the above defects,a new Z-source asymmetrical three level four legs inverter is proposed in this paper.The shoot-through state is no longer a prohibited state because of introducing the Z-source network.By controlling the duty cycle,the purpose of lifting the output voltage can be achieved,and the Z-source network enhances reliability of the system,eliminating adverse effect of the dead time.The proposed inverter bridge composed of three-level and two-level bridge,compared to the traditional NPC structure,the asymmetrical bridge structure reduce the number of power switches and don't need clamped diode,so reducing the inverter size and saving cost of the system hardware.The NP voltage deviation will increase lower output harmonic and voltage of the power switches,even damaging the life of the DC side capacitor and power switches in serious situation.Therefore,this paper analyses the causes of the NP voltage deviation and research the method for controlling the NP voltage balance.Considering that the NP voltage can't be controlled in asymmetry three-level inverter,because of missing vector in the space vector modulation,we can't use redundant vector to control the NP voltage.According to its unique insertion rule of upper and lower shoot-through state which can influence the NP voltage,combined with PI control,a space vector pulse width modulation(SVPWM)strategy based on fuzzy control is proposed.By adjusting the up-and-down shoot-through time,the NP voltage is suppressed effectively while the boosting DC voltage is realized.This method minimizes the switching loss which is caused by the shoot-through operation.The feasibility and validity of the method are verified by Matlab/Simulink simulation.