This chapter illustrates the use of hysteresis control techniques applied to multilevel inverters. First, a general classification of different methods applied to control multilevel inverters is presented from three main points of view: that of the switching frequency (fundamental frequency and high frequency), the related application (grid-tie and stand-alone), and the kind of control they use (linear and nonlinear). Into this frame, the hysteresis-based control methods are presented, with an overview of the existing literature. Thus both hysteresis current control (HCC) and hysteresis voltage control (HVC) are considered to illustrate the use of the technique in grid-connected and stand-alone applications, respectively. In the case of HCC, the three most relevant techniques are detailed: multiband (MB) HCC, multioffset-band (MOB) HCC, and time-based (TB) HCC. Similarly, for HVC, two methods are explained: multiband (MB) HVC and hysteresis voltage regulation (HVR). Finally, based on the overview of the existing techniques, an alternative HVC method is proposed for the first time in this chapter. This control can be classified as a low-frequency adaptive hysteresis band technique suitable for stand-alone applications. The theoretical fundamentals of the control are provided, using the particular topology of a transformer-based cascade multilevel inverter selected as a case study. Validation of the technique’s effectiveness is supported using simulation results in which the control ensures the output voltage shape by changing the number of output levels from 27 to 35 and simultaneously adapting the width of the hysteresis band according to the input voltage and load conditions.
This paper introduces a method to enforce balanced power distribution between the stages of a single-phase transformer-based cascaded multilevel inverter using the new asymmetric ratio 6:7:8:9 between stages. Since the inverter is fed by a single DC source, asymmetry is enforced by means of the transformer turns ratio providing multiple redundant switching patterns to synthesize an output signal of until 35 levels. As it is developed in the paper, optimum switching patterns for the proposed ratio allow reducing typical power unbalance produced by commonly used ratios in four stage multilevel inverters (1:2:4:8 and 1:3:9:27). The proposed method consists on determining off-line the best switching patterns for minimizing deviation error, and then, storing them as lock-up tables in the digital device controlling the inverter. By permanently reproducing the selected switching patterns, balanced power distribution is achieved. A closed-loop control approach to regulate the RMS value of the output voltage compatible with the proposed method is also developed. The experimental results using a laboratory prototype are presented validating the entire approach.
This work describes in detail a computational tool designed to study performance indicators of a four-stage transformer-based single-phase cascaded multilevel inverter. The proposed system integrates simulation, on-line measurement, control and signal processing providing automating testing functionality to optimize the performance of the inverter with base on indicators such as Total Harmonic Distortion (THD), partial and global efficiency and power balance between the stages. The computational component of the tool was developed in LabVIEW providing not only didactic interactivity with the user through the Human-Machine Interface (HMI) but also a reliable interconnection with the power converter and the instruments of the experimental setup. The hardware component was developed integrating the power converter prototype, an acquisition card and electronic circuits providing measurement, conditioning, digital control and gate driving functions. Experimental results obtained from automatic tests are presented showing potentiality of the tool to support research activities related with this type of power converters.