Research On Improvement Of Modulating Inverter Modulation Method

Cascaded topology with convenient modular implementation, without equalizing control, is easy to achieve soft-switching technology and many other advantages received extensive attention among multi-level topology. This paper analyzes the modulation algorithm of cascaded inverter. In order to make the best performance of the inverter output, the paper focus on improving the modulation algorithm of cascaded inverter on the basis of previous work.Firstly, the paper summarizes the development, classification and application of the multi-level inverter. At the same time, working principle and mathematical models of N-unit cascaded inverter is described in this paper. The paper analyzes the advantages and disadvantaged of the three stepped modulation to solve the switching angle. Lamination of the three carriers and carrier level phase shift methods is simulated in this paper, the result shows that in-phase stacking is the best way for degree of elimination of harmonics, followed by alternating inverse cascade method and the worst is negative inversion lamination method. Base on the thought of freedom control, the paper inject the third harmonic or zero sequence components into modulation wave of carrier wave phase-shift to increase the utilization of DC voltage and reduce the switching losses and frequency. This paper discovers the nature links between the topology of three-phase two-level inverter and main circuit of cascaded inverter by decomposing circuit of cascaded inverter.Secondly, the paper presents an improved modulation algorithm combined the advantage of ladder and carrier modulation phase shift method, it can always maintain a power unit carrier phase-shift control and the other unit ladder wave modulation according to the size of the modulation wave. The paper design the switching loss measurement model to verify that the algorithm can effectively reduce the switching losses by measuring switching losses of power transistor per unit. Because STS-SVPWM technology needs for left and right leg of each unit power sampling and vector synthesis, this complex algorithm will result in a large number of CPU's memory is occupied, therefore, the paper presents overlap time staggered SVPWM method which combine shifting principle and STS to simplify the STS-SVPWM algorithms.Finally, the paper builds simulation models to verify the correctness and effectiveness of the algorithm in Matlab simulation platform. Simulation results show that improved modulation algorithm can reduce the switching losses effectively; Although the Overlap time staggered SVPWM and STS-SVPWM is roughly the same for output performance, but the former is greatly simplified.