Circulating Current Suppression And Neutral-Point Balance Control For Parallel-Connected T-Type Three-Level Grid-Connected Inverters

The three-level inverter has been widely applied due to its advantages of high quality output waveform and low switching stress.The three-level inverter should work under the condition of neutral-point(NP)balance,otherwise,neutral-point fluctuation will cause poor output quality,such as unbalanced voltages or currents,and it has a bad influence on the normal operation of the system.It is hard for one inverter to be used in high power application owing to its low capacity,hence parallel connection of multiple inverters is commonly applied to improve system capacity.However,the zero-sequence circulating current(ZSCC)will occur when the parallel connection structure is employed,which results in current distortion and system loss increasing.On the basis of one T-type three-level inverter,firstly,the working principle and mathematical model of the topology are analyzed,and the application of SVPWM to three-level inverter is introduced.Based on the above analysis,a few methods about neutral-point balance control are introduced.Secondly,in accordance with the mathematical model,the zero sequence circulation excitation sources are classified,and the control method is presented to suppress the ZSCC caused by different excitation sources.According to the average model of parallel T-type three-level inverters,it is found that the ZSCC can be regulated by adjusting duration time of N-type vectors of SVPWM.Then,the coordination control strategy to suppress ZSCC and balance neutral-point potential is discussed based on the distribution factors of SVPWM for two parallel inverters.Besides,a feedfoward control method is also used to suppress ZSCC and balance neutral-point potential.Finally,the simulation model is built by Matlab/Simulink to validate the effectiveness of the methods,and an experimental platform of two parallel T-type three-level inverters with the control core of STM32F407 ZG is built to verify the control strategy.The effectiveness of the proposed method on the ZSCC suppression and neutral-point balance control has been verified by experimental results.