On-Line Pulse Step Modulation For Cascaded Multi-Level Inverter

The appearance of multilevel inverter topology effectively solved the problem of more requirements desired for high voltage and power electronic equipment. Because of the special structure of multilevel topology, different demand is need in different occasion for modulation methods. Choosing cascaded multilevel inverter as main circuit, this paper studied the step modulation of multilevel inverter for high voltage and low frequency occasion.Firstly output voltage of full bridge inverter (basal module of cascade multilevel inverter) is studied when it is used step modulation method. The mathematical expression of output voltage is deduced and concluded. Take it as foundation, traditional step modulation and pulse step modulation is analyzed. Conducting angle arithmetic is discussed for traditional step modulation. The main pulse step modulation method is explained and simulated. Since off-line disadvantage of the two modulation methods is discovered, a new on-line modulation method called on-line pulse step modulation is proposed based on-line conducting angle arithmetic, combined the merit of pulse step modulation. Then two kinds of on-line conducting angle arithmetic are derived basis of single-phase five unit cascaded multilevel inverter. Choosing simple and appropriate conducting angle arithmetic, theoretical analysis and simulation verification of three-unit cascaded multilevel inverter and five-unit cascaded multilevel inverter are accomplished by using the two on-line modulation methods. Finally advantage and disadvantage of proposed modulation method are showed after comparison.Two kinds of modulation methods are both experimented by using TMS320LF2407DSP as main control unit, single-phase five-unit cascaded multilevel inverter as main circuit to verify the result of simulation. The results indicate the proposed method is correct and feasible.