The Evaluation Mechanism And Optimization Strategy Of Output Waveform Quality For Three-Level Inverter

Neutral point clamped?NPC?three-level inverters are more and more widely used in the field of medium-voltage high-power motor drive due to their advanges including low harmonic output voltage and low voltage sustainability of semiconductor devices.However,with the increase of the power,the contradiction between the harmonic loss and the switching loss of the three-level inverter is more and more critical.Therefore,in order to improve the output performance of the three-level inverter without the increase of switching loss,many contributions have been made in this dissertation.The fundamental principles of asynchronous and synchronous space vector modulation,continuous and discontinuous modulation are derived firstly,and then three output performance evaluation indexes of three-level inverter,including weighted total harmonic distortion of output voltage(V_WTHD),rms value of output current ripple,and rms value of stator flux ripple,are all introduced.Their relationship and computational complexity are analyzed to provide the necessary theoretical basis for the following research on the evaluation methods and the optimization strategies of the output waveform quality.In order to suppress the output current ripple of asynchronous discontinuous space vector modulated three-level inverter,the zero clamping mode is proposed and the distribution of positive,negative,and zero clamping modes are determined by the principle of minimized amplitude of the mean current ripple vector.Then an optimization strategy of asynchronous discontinuous space vector modulated three-level inverter can be proposed.The results show that,by the reasonable distribution of discontinuous intervals of the output valtage,the output voltage and current waveform qualities can be effectively improved.In order to improve the output performance of three-level inverter in the whole modulation range,the continuous and discontinuous switching sequences are both adopted and the optimal distribution of these switching sequences are determined by the principle of minimized rms value of output current ripple.Then an optimization strategy of asynchronous hybrid space vector modulated three-level inverter can be proposed.The results show that,by subdivision of active areas of each switching sequence,the output voltage and current waveform qualities can are optimal in the whole modulation range.In order to reduce the output current ripple of synchronous space vector modulated three-level inverter,the whole modulation range are divided into finite number of segments and the switching sequence combination are designed with constraints of synchronization and symmetry in each segment.Then based on the principle of minimized amplitude of the mean current ripple vector,the optimal switching sequence combination is determined in each modulation segment.The results show that,without the increase of switching loss,the output current ripple can be reduced in the whole modulation range.