Study Of A New Multi-level Converter Topology And Modulation Algorithm

In the field of power electronics, especially in high-power applications, themulti-level converter has got more and more attention and has been more widelyused. Compared to the conventional two-level converter, the output waveform ofthe multi-level converter is closer to sine wave and has a better harmonic spectrumwith the increase in the number of level. At the same time, the voltage stress ofeach switching device is relatively small.Based on the analysis of advantages and disadvantages of traditionalmulti-level converter, a new cascaded multi-level topology is proposed in this paper.Different transformers of different ratio were used in each H bridge, so multi-leveloutput can be achieved by changing the ratios. One dc power was shared by theseH-bridge units. The problem that large numbers of DC sources, switching tubes andassistant devices are demanded can be solved. And the output waveform is muchcloser to the sinusoidal wave because of the short-circuit impedance of transformer.In the grid of single-phase for220V, three H-bridges are cascaded, variable ratiosof transformers are:9:1、3:1、1:1respectively, then the27-level step wave is output.In the grid of three-phase for35kV, four H-bridges are cascaded, variable ratios oftransformers are:1:1、1:3、1:9、1:27respectively, then the81-level step wave isoutput. These two kinds of topologies and their working principles are analyzed intheoretical detail, at the same time, simulation models are built to verify theeffectiveness of these two topologies.Common multi-level modulation strategies are summed up as follows: carrierdisposition PWM、 carrier phase-shifted SPWM、 SVPWM、 method of switchfrequency optimization modulation、 method of optimization step wave widthmodulation. These modulation strategies will be simply introduced and analyzed inthis paper. On the basis of analyzing these strategies, a modulation algorithm basedon FPGA is proposed. This algorithm is suitable for topology which can outputmore levels. Compared with the traditional modulation algorithm, this algorithmhas low complexity and high adaptability. The process of programming in Veriloglanguage to obtain27-level step wave is introduced in detail and the frequency ofthe27-level step wave is synchronous with the grid. Experimental platform is built based on theoretical analysis of the newtopology and the new modulation algorithm on FPGA to obtain the27-level stepwave. The fundamental harmonic of the output voltage is220V.