Research On Asymmetrical Three-level Four-leg Inverter And Its SVPWM Algorithm

Three-level inverter has been received extensive research and application,which has been used a lot in the field of the active power filter,reactive power compensator,AC speed,due to its low output harmonic content,low switching frequency and low voltage stress of power device.In order to simplify the structure of three-level inverter and reducing the voltage stress of switch tube,a improved asymmetric topology of three-level three-phase four-leg inverter is proposed.The improved topology is consisted of asymmetric three-level legs and two-level legs.Compared with the conventional types of diode-clamped,capacitor-clamped and cascaded three-level inverter,the structure is simple,the volume is small,the number of power switching devices is reduced,the clamping diode and capacitor is not needed,the switch voltage stress is low.we can give full consideration to the switching frequency and voltage stress to select power switching devices reasonably and the cost of three-level inverter can be saved.For this novel structure,due to lack of switching vector,the synthetise of reference vector is difficulty,a improved three-level space vector pulse width modulation algorithm in abc coordinate is adopted,The algorithm adopts the way of vector expand,the reference vector synthesis area is expanded.Solve all the asymmetry structure of the multilevel inverter due to lack of vector,can not using space vector modulation algorithm.The algorithm can be adopted in modulation for arbitrary multilevel inverters with symmetrical or asymmetrical structure.Avoiding the ??? transformation and a series of operations,and the selection of switch states or switching time are simple and clear,complexity of the modulation is simplified.In order to verify the correctness of the improved asymmetric topology and the improved space vector modulation algorithm,A simulation model is built in MATLAB/simulink and in the DSP platform,in symmetrical three-phase voltage unbalanced three-phase voltage,unbalanced three-phase distortion voltage and asymmetric load conditions.Simulation results show that the improved topology is feasible,the proposed modulation strategy is superior,it can track the reference voltage and get a good output waveform.