Study Of Low-Carrier-Ratio Modulation Strategies For High Power Three-Level Grid-Connected Inverter

With the rapid development of new energy grid-connected power generation technology,high-power wind power converters and photovoltaic inverters have been widely used.The continuous improvement of inverter power level and the decrease of switching frequency have brought some challenges and made low-switching-frequency modulation strategy become a research hotspot.At present,the traditional modulation algorithm is becoming more and more mature.In order to further reduce the switching frequency than traditional modulation strategy,it is very necessary to study modulation strategies under low carrier ratio.Due to its low output harmonic and low switching loss,three-level inverter has been widely applied in high-power condition.In this paper,modulation strategies under low carrier ratio is studied for three-level inverter.This paper first expounds the significance and background of researches of modulation strategies under low carrier ratio,then introduces and compares main three-level modulation strategies.Taking three-phase three level grid-connected inverter with LCL filter as the research object,it first studies the principle and implementation of a typical optimized PWM strategy——Selective harmonic elimination pulse-width modulation(SHEPWM).Secondly,it studies the optimization of three-level SHEPWM.In view of the three-level neutral point potential migration problem,a neutral point balanceing control algorithm based on small vectors replacement is studied.Aiming at weakening the large harmonic peak which has not been eliminated by SHEPWM,the LCL filter is improved.In view of the poor dynamic response of optimized PWM,a hybrid modulation strategy of SHEPWM and model predictive control(MPC)——SHE-MPC is studied.First,the SHE-MPC strategy based on weight factor is studied,makes the SHEPWM output states and current tracking both as optimization objects.In view of the shortcomings of its quantization error,the SHE-MPC strategy based on dynamic switching is also studied.Finally,the effectiveness of the proposed strategy is proved by simulation and experimental verification.