Research On Pulse Width Modulation Algorithm Of High Power Three-Level Converters

The modulation algorithm performance of high power three level NPC converter is evaluated in this paper,and the synchronous modulation strategy under low switching frequency is studied,finally the related experiments are verified on 5MW three level converter.Based on the analysis of basic mathematical model of Selected Harmonic Elimination(SHEPWM),the concept of multiple solutions is proposed,when the voltage waveform is 1/4,1/2 symmetry.Through the initial value analysis the possibility of multiple solutions,and found the feasible solution is more in the high modulation.In order to obtain more feasible solution,the two sets of solutions are obtained by random initial value.To solve the problems of dead time effect and dynamic narrow pulse,this paper gives the corresponding angle correction strategy.Multilevel topology brings more freedom degrees for modulation algorithm.A variety of new modulation algorithm can be got by the different combination of carrier amplitude and phase of modulation wave,and the redistribution of zero vector in switching sequence of space vector modulation.The advantages and disadvantages of different algorithms can be evaluated by two performance indexes,which are the total harmonic distortion(THD)and the weighted total harmonic distortion(WTHD).In practical application,a hybrid modulation strategy can be combined with the advantages of different algorithms.For the inherent neutral point unbalancing problem of the three-level,this paper presents a capacitance power control scheme by modifing the action time of the positive and negative small vectors,and its effectiveness is verified by experimental.According to the output characteristics of three level converter in the whole frequency range under low switching frequency,a new method is designed,which is based on asynchronous modulation in low frequency and synchronous modulation in high frequency.The selection of carrier mode,sampling mode,and carrer ratio influent the output voltage waveform symmetry and harmonic distortion.After analyse and compare of its influence,this paper select the double sampling quasi optimization PWM.During the switching of the modulation algorithm,the fundamental content is the same,but the change of the harmonic content will bring a great impact on the motor.A phase synchronization strategy based on phase locked loop(PLL)is proposed for the phase adjustment of triangular carrier and modulate waveform.A fixed point switch is proposed to avoid the current shock for synchronous modulation.In this paper,the full range of synchronous modulation strategy is verified by experimental.The modulation characteristics of the high power converter under rated operation is tested by refluxing experiment,and the effectiveness of synchronous modulation in the full range is tested through the motor experiment.