Research On Three-level Neutral-point-clamped Converter System

Three-level neutral-point-clamped converter has the advantages of low output voltage harmonics and low switching voltage stress.It has a wide range of applications in the field of high voltage and high power conversion.This dissertation introduced the implementation principle of the three-level SVPWM modulation strategies in different partition modes firstly.Aiming at the problem of neutral-point voltage imbalance in the topology,improved adjustment strategies are proposed based on the common adjustment strategy.The selection strategy of redundant small vectors is optimized to make the selection more reasonable.A novel adjustment strategy is proposed,which can adjust the action time of medium vector.Thus the influence of the medium vectors on the neutral-point voltage can be reduced.The performance of two proposed adjustment strategies is analyzed,and the simulation model is built in Matlab/Simulink to verify the effectiveness of the proposed adjustment strategy.For the backto-back three-level inverter,the operating range in which the neutral-point voltage fluctuation can be completely suppressed is analyzed under the condition that the inverter side does not control the neutral-point voltage.This dissertation studied the digital PI current control strategy of the three-level PWM rectifier.By adding a quasi-integral controller to the predictive deadbeat control strategy,a composite digital control strategy is constructed.The compound digital control strategy has the same dynamic performance as predictive deadbeat control,while the anti-jamming ability of the system is improved.The maximum flux linkage ripple of the three-level PWM rectifier under SVPWM modulation is analyzed to determine the minimum inductance value at the maximum current ripple limit.The simulation model is built in Matlab/Simulink to verify the effectiveness of the proposed compound digital control strategy.Finally,a prototype of three-level converter with capacity of 1500 VA was built.The design and fabrication of the main power circuit,the drive circuit and the sampling circuit are completed.Using DSP to realize the digital control of the system,the operation and debugging of the prototype is completed.The output waveform of the converter is good,and the function of balancing the neutral-point voltage is realized.