Research On Improved Synchronous Space Vector Modulation Strategy For Three-level Inverter

Neutral point clamped(NPC)three-level inverter has the advantages of low switching loss,low harmonic content of output waveform,low voltage stress of single power device,and is widely used in medium-high voltage and high-power driving occasions such as locomotive traction,mine hoisting,ship propulsion,etc.With the increasing of power level of inverter,the influence of common mode voltage and switching loss of the drive system becomes more and more prominent.Therefore,how to reduce the common mode voltage and switching loss of the inverter at the same time under the condition of low switching frequency,to ensure the stable and efficient operation of the inverter,has become a research hotspot in the field of medium-high voltage and high-power drive.This thesis first introduces the basic principle of the neutral point clamped three-level inverter,including the constraints of synchronization and symmetry of pulse signal,the distribution of reference vectors,the calculation of the action time of the basic voltage vectors and the design principle of the switching sequence of the traditional synchronous space vector modulation strategy.And on this basis,the common mode voltage,switching loss,neutral point voltage and the quality of output waveform of three-level inverter are analyzed in detail.Aiming at the problem that the common mode voltage amplitude and switching loss of inverter are higher under the effect of traditional synchronous discontinuous pulse width modulation(SDPWM)strategy,the switching times of inverter in the unit fundamental cycle are reduced by re-dividing the small area and making full use of the redundant switching state,and the common mode voltage amplitude is reduced by introducing the sum of switching algebraic and selecting the redundant switching state as the evaluation index,based on the principle of traditional switching sequence design,an improved switching sequence design method is proposed,and an improved synchronous discontinuous space vector modulation strategy which can simultaneously reduce the common mode voltage amplitude and switching loss is obtained.In addition,the neutral point voltage of the inverter under the improved strategy is analyzed by the numerical calculation method of the neutral point voltage,and the quality of output waveform of the inverter is analyzed by the calculation method of the amplitude of mean current ripple vector.In order to verify the correctness and effectiveness of the proposed modulation strategy,this thesis uses MATLAB/Simulink simulation software to analyze the output performance of the inverter under the effect of the traditional modulation strategy and the improved modulation strategy proposed in this thesis.On this basis,the NPC three-level inverter experimental system is built,and the traditional modulation strategy and the improved modulation strategy proposed in this thesis are verified and the results are compared.The simulation and experimental results show that the improved modulation strategy proposed in this thesis can effectively reduce the switching loss,the common mode voltage amplitude and the neutral point voltage ripple amplitude of the inverter.