Research On Synchronized Space Vector PWM For NPC Three Level Inverters

With the increasingly wide applications of voltage source inverter,pulse width modulation(PWM)technique has gradually become a common and core technology of the converters.Space-vector-based synchronized PWM strategies are able to maintain the synchronization and various symmetries in waveforms,and reduce the number of switchings.Due to these features,it has been increasingly used in medium voltage high power applications.In such applications,the device switching frequency is very low.This is mainly because the switching losses of semiconductor devices contribute to a major portion of the total device losses,and excessive switching losses will limit the current-carrying capability of semiconductor devices and thus inverter power output and efficiency get reduced.On the other hand,low switching frequency operation leads to increase in harmonic distortion of output voltage,which will cause some detrimental effects.Therefore,reducing the harmonic distortion of output voltage at low switching frequency and minimizing the total number of switchings to reduce device switching losses are two challenges that the PWM strategies of medium voltage high power inverters are faced with.The paper firstly detailed analyzes the basic working principle and implementation method of conventional space-vector-based synchronized PWM strategies.The performance of the conventional strategy has been tested through simulation and experiment.Then the paper presents an improved three level synchronized space vector PWM strategy based on modulation index(m)segmentation.The modulation index is segmented in the linear modulation region according to the relation between the value of m and the subsectors in which the reference vectors fall,for three level inverter.On this basis,the switching sequence for each segmentation of m is designed independently satisfying both the condition of minimizing harmonic distortion and minimizing the total number of switchings.In addition,for any number of reference vectors,the switching sequences are designed under two different distribution modes(Mode I and Mode II),respectively,such that more options are provided for optimization of sequences.Then the optimal switching sequences are further selected for each m segmentation by comparing the line voltage weighted total harmonic distortion(WTHD)of the proposed sequences with two different reference vector distribution modes(Mode I and Mode II).The performance of proposed strategy is evaluated through simulation and experiment on a constant v/f open loop induction motor drive at both steady-state and transient conditions in linear modulation region.In addition,the behaviors of proposed strategy in overmodulation region are illustrated.The experiment results match with the simulation results,showing that the redesigned sequences will result in significant reduction in harmonics while minimizing the device switching frequency in linear modulation region.The proposed strategy is also applicable to overmodulation region.In addition,the behavior of proposed strategy at transient condition illustrates that it can be employed to constant v/f open loop induction motor drives directly.It should be noted that,although the three level inverter is considered in the proposed algorithm,the design method of switching sequences is also useful for other multilevel converter PWM with low switching frequency.The proposed algorithm gives good performance at both steady-state and transient conditions.