Research On Zero-Voltage Transition PWM Soft-Switching Technology Of Three-Level NPC Inverter

The two-level inverter is widely used in low voltage and low power applications,because it has a simple structure and uses few power devices.However,in the application of photovoltaic inverter system,the DC bus voltage is higher,the inverter requires strict performance.Therefore,the two-level inverter structure is gradually replaced by the three-level inverter structure.The three-level Neutral Point Clamped(NPC)inverter has high output voltage quality,small voltage stress of the switching tube,and low electromagnetic noise.It has a very wide application prospect in medium and high voltage motor drive,microgrid construction,photovoltaic grid connection and various inverter power systems.It is also one of the main directions for the development of photovoltaic power generation systems.When the three-level NPC inverter is operated,there is almost no switching loss of the bridge arm inner switches,but the switching loss of the bridge arm outer switches and the reverse recovery loss of the clamp diodes are high.For this problem,a Zero Voltage Transition(ZVT)PWM soft-switching technique for a three-level NPC inverter is developed.The circuit has a simple control method and is suitable for modulation methods such as Space Vector Pulse Width Modulation(SVPWM).The proposed scheme realizes the zero voltage opening of the main switching tubes and the natural shutdown of the clamp diodes,and reduces the turn-off loss of the main switching tubes.In this thesis,the SVPWM modulation method and the converter commutation process of ZVT-PWM soft-switching three-level NPC inverter are deeply analyzed.The design method of soft-switching resonant circuit parameters is given by calculation.The switching loss and commutation process of the inverter are simulated and verified by PLECS simulation.On this basis,this thesis builds a 1 kW three-level NPC inverter experimental prototype,and the prototype adopts the SVPWM modulation method.The experiment compares the hard-switching operating state and the soft-switching operating state.The experimental results verify that the proposed so ft-switching strategy can be applied to modulation methods such as SVPWM,and the efficiency of the inverter is improved compared to the hard-switching operation.