Research On Control Strategy Of Neutralpoint-Clamped Three-Level Inverter

Among multilevel converters,three-level converters are more widely used owing to their low costs and less distorted output currents.What’s more,neutral-point-clamped(NPC)three-level inverter becomes a hot topic because of its simple topology and control strategy.But,it has the problem of unbalanced neutral-point potential due to its own topology structure.Therefore,this thesis focuses on researching NPC three-level inverter and solving the problem,and carries out the following work:This thesis firstly introduces the basic principle of NPC three-level inverter,and analyses the space voltage vectors and main factors affecting the neutral-point potential.Positive and negative small vectors have opposite effect on neutral-point potential,while the effect of medium vectors on neutral-point potential should be judged by the direction of the neutral-point current,which can be the basis of the following control strategy.Virtual vector strategy can’t control neutral-point potential within full index and effectively utilize medium vectors.And varied virtual vector strategy also has drawbacks,such as: whether its vector synthesis ratio is reasonable,and inflexible threshold.This thesis proposes an interval mapping control strategy utilizing small and medium vectors to control neutral-point potential within full index,and mapping way for analysis and determines new vector synthesis principle,and the effect of medium vectors on neutral-point potential for time optimization and solving the problem of complex calculation caused by too many variables.Compared with varied virtual vector strategy,the proposed method doesn’t need to set threshold and synthesis ratio,and has faster control speed and certain adaptability.The effectiveness of this strategy is verified through comparative simulations.Aiming at the problem that the interval mapping control strategy cannot control the neutral-point potential when open-circuit fault occurs to the inverter,a redundant switching states prediction control strategy is proposed.Firstly,the analysis of the voltages and currents waveforms caused by these faults can be applied to detect the fault switching state.Then,the fault switching states can be replaced with the normal one by using redundant vectors and model prediction control can suppress the fluctuation of neutral-point potential.According to the capacitor voltage model and vector synthesis method under fault conditions,the cost function is built to track target reference vector and keep neutral-point potential balanced.Simulations verify that the proposed strategy has certain capability of fault-tolerance and can control neutral-point potential.A NPC three-level experimental platform is established to verify the proposed control strategies.The experiment results show that the interval mapping control strategy has faster control speed,and the redundant switching states prediction control strategy has certain capability of controlling neutral-point potential when the opencircuit fault occurs.