Research On Improved Control Strategy Of NPC Three-level Inverter

Neutral Point Clamped(NPC)three-level inverter was proposed by Nabee et al in 1981.Compared with traditional two-level inverter,NPC three-level inverter has many advantages in medium and high voltage AC drive,flexible transmission and distribution,and photovoltaic power generation and other fields.However,the midpoint potential imbalance of the voltage divider capacitor in the NPC three-level inverter is a hot issue.The unbalance of the midpoint potential will cause many problems such as output voltage distortion and excessive voltage on the switch tube.Therefore,measures must be taken to ensure the neutral point potential balance of the NPC three-level inverter.The traditional mid-point potential balancing method is mainly divided into hardware mid-point potential balancing method and software mid-point potential balancing method.Among them,the software mid-point potential balancing method does not require additional hardware equipment,the cost is lower,and the control is more flexible,which has caused Extensive research by scholars.The traditional software control methods are the recent three-vector method(NTV)and the virtual space vector method(VSVPWM).The NTV method utilizes the control effect of the redundant small vector on the midpoint potential to balance the midpoint potential,which has the advantages of clear principle and simple calculation.However,the system cannot balance due to the high modulation degree and low power factor.The midpoint potential deviation of the signal,resulting in large midpoint potential low-frequency fluctuations;and VSVPWM uses the principle of vector synthesis,using a synthetic vector that does not produce a midpoint potential shift to replace the middle vector,can fundamentally eliminate the cause of the middle vector The midpoint potential fluctuates at low frequencies.But the disadvantage of traditional VSVPWM is also obvious,that is,it can not actively control the existing midpoint potential offset,and the switching frequency is increased due to the increase of the vectors involved in the synthesis.In view of the shortcomings of traditional VSVPWM,this paper proposes an improved VSVPWM with active control of midpoint potential and a VSVPWM that reduces the switching frequency at low modulation.The improved VSVPWM with active midpoint potential control can quickly control the midpoint potential balance when there is a large midpoint potential offset;while VSVPWM with a low modulation degree reduces the switching frequency,when the modulation degree is less than or equal to 0.5,it can be quickly controlled While the midpoint potential shifts,the switching frequency is greatly reduced,and the structure is simple,the calculation amount is small,and it has strong practical value.The NPC three-level inverter adopts repeated control closed-loop control strategy to effectively suppress periodic disturbances,but the traditional repeated control has a slow dynamic response and cannot effectively suppress non-periodic disturbances.An improved control strategy combining proportional control and repeated control is studied.The improved control strategy can greatly improve the dynamic response speed of the system.The paper modeled and simulated the improved repetitive closed-loop control strategy and the two improved VSVPWM modulation methods under closed-loop.The simulation results show that the NPC three-level inverter system with the improved repetitive closed-loop control strategy has excellent dynamic and static Performance,And verify the correctness of the two improved VSVPWM.