Alternating Parallel Three-level Inverter Circulation And Mid-point Balance Control

Three-level inverter has been widely used in high-voltage and high-power field because of its good performance.Parallel technology can be divided into three phase inverter bridge arm parallel and level parallel inverter,improves the system capacity,has the advantages of high reliability and redundancy,based on the staggered parallel inverter and its composition of inverter parallel system as the research object,the research analyzes the staggered parallel inverter bridge arm due to carrier phase difference between single-phase high-frequency circulation problems,ac/dc busbar operation of zero sequence current problems and the midpoint clamp type three level inverter is inherently neutral voltage imbalance.1.Research on three-level inverter based on stagger parallel technology.The three phases of staggered parallel three-level inverter are composed of two parallel bridge arms with the same modulated wave and the carrier shifting 180 o.The output current ripple is analyzed,which provides a basis for the design of output filter.Between parallel bridge arm have differential mode voltage because of the switch sequence mutual crisscross,lead to single-phase high-frequency circulation,puts forward the coupling inductance are in phase two bridge arm output parallel suppression method,comparative analysis of the coupled inductor,ordinary inductance and output inductance connection circulation impedance characteristics of three kinds of cases,combined with the coupling inductance magnetic characteristics of output current and circulation,coupled inductor is verified by the simulation of single-phase high-frequency circulation present higher impedance characteristics,inhibition effect is better.2.Research on zero-sequence circulation problem of staggered parallel inverter and its component parallel inverter system.The circulation path of the zero-sequence circulation is analyzed,the equivalent model of the zero-sequence circulation is established,and the zero-sequence circulation is classified by introducing the inverter bridge arm voltage switch function.The inverter parallel method with Shared midline is used to suppress the on-state zero-sequence circulation,and the improved LC filter method is used to suppress the high-frequency zero-sequence circulation.The suppression method of PCI controller is proposed for the low-frequency zero-sequence circulation,and the controller parameters are designed according to the system performance requirements.The effectiveness of the proposed method is verified by simulation and experiment.3.The research on the potential imbalance of mid-point in three-level inverter.The mechanism of mid-point potential imbalance is analyzed qualitatively and quantitatively.The model of mid-point current switch is established and the control method of mid-point potential is analyzed.Aiming at the existence of traditional zero sequence voltage injection of neutral voltage low frequency fluctuation quantity based on the inhibition ability is limited,is put forward based on the zero sequence voltage injection adjustment of neutral current feedforward control,and the inhibition of neutral voltage low frequency harmonic wave of PCI controller negative feedback control,a control method of both the control volume injection to the three phase modulation wave,to realize the balance control of the neutral voltage.At the same time,the effectiveness of the proposed method is verified by simulation under different working conditions,and it is verified by experiments.