Abstract
The three unit nine-level inverter can output more voltage levels with fewer h-bridge units, while having better output waveform quality. However, in the conventional hybrid frequency modulation strategy, only one low-voltage unit adopts pulse width modulation (PWM), which causes the problem of switching loss and uneven heat distribution between the two low-voltage units. At the same time, the output power of the conventional modulation strategy is unbalanced. Aiming to resolve the above problems, a modified hybrid modulation strategy and a power balance control method under the strategy is proposed in this paper. The modulation strategy achieves output power balance between the three units and an even distribution of switching losses between the two low voltage units while maintaining the same output power quality. Simulation and experimental results verify the feasibility of the modulation strategy.
Highlights
The cascaded multi-level inverter becomes one of the most widely used inverter topologies in medium voltage high power transmission system for it can output a higher level of voltage and a higher number of levels without increasing the withstand voltage of the device. [1,2]
The Cascaded H-bridge (CHB) type multilevel inverter topology can be divided into type I, type II, and type III according to the DC side voltage level of each unit
(HF-pulse width modulation (PWM)) technology has emerged at the historic moment, which can enable switching devices of different voltage levels to work together in the same topology, so the characteristics of each switching device can be fully utilized
Summary
The cascaded multi-level inverter becomes one of the most widely used inverter topologies in medium voltage high power transmission system for it can output a higher level of voltage and a higher number of levels without increasing the withstand voltage of the device. [1,2]. For asymmetric cascaded multilevel inverter topologies with different DC power supply voltage levels, it is difficult to output an ideal multilevel voltage waveform only by the modulation strategy of phase shift PWM (PS-PWM) [12] or phase disposition PWM (PD-PWM) [13]. A modified carrier disposition modulation strategy (MCD-PWM) is provided for the power balance problem of the three-unit cascaded seven-level. PWM the output voltage, achieve continuous modulation of the output voltage, but themodulation modulationofstrategy using only but themodulates modulation strategy using only unit, other two one low-voltage unit,PWM whilemodulates the other one two low-voltage units still use thewhile levelthe comparison units stilltouse the level comparisonsquare method to output low-frequency waves. The output output power power balance balance balance control of the inverter unit at full modulation depth is realized
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