Abstract
This paper presents the optimization technique for minimizing the voltage and current total harmonic distortion (THD) in a single-phase multilevel inverter controlled by staircase modulation. The previously reported research generally considered the optimal THD problem in the frequency domain, taking into account a limited harmonic number. The novelty of the suggested approach is that voltage and current minimal THD problems are being formulated in the time domain as constrained optimization ones, making it possible to determine the optimal switching angles. In this way, all switching harmonics can be considered. The target function expression becomes very compact and existing efficient solvers for this kind of optimization problems can find a solution in negligible processor time. Current THD is understood as voltage frequency weighted THD that assumes pure inductive load—this approximation is practically accurate for inductively dominant RL-loads. In this study, the optimal switching angles and respective minimal THD values were obtained for different inverter level counts and overall fundamental voltage magnitude (modulation index) dynamic range. Developments are easily modified to cover multilevel inverter grid-connected applications. The results have been verified by experimental tests.
Highlights
In last decades, multilevel inverters have been widely used for medium- and high-voltage/power applications [1,2,3,4]
(b) have have been been selected selected for for minimum minimum and and experimental two test points (a)current and (b) total harmonic distortion (THD) have been selected for minimum andrange maximum values of to maximum values of voltage and over the modulation index corresponding maximum values of voltage and current THDs over the modulation index range corresponding to voltage and current THDs(Figures over the 10 modulation range corresponding to seven(a) levels, seven levels, respectively and 11)
Fundamental components have been determined by built‐in scope infinite impulse response (IIR) filter function (M1), and ripple components have been calculated as the difference between the (IIR) filter function (M1), and ripple components have been calculated as the difference between the actual waveforms and the corresponding fundamental components (M2 = CH2(3) − M1)
Summary
Multilevel inverters have been widely used for medium- and high-voltage/power applications [1,2,3,4]. This paper presents the analysis and experimental verification of voltage and current THD minimization problems for a single-phase multilevel inverter over the whole modulation index range, using the time-domain problem formulations preliminarily introduced in [17]. THD minimization for voltage and current brings some advantages, such as results equivalent to unlimited harmonics content, avoiding Fourier trigonometric calculation of large number of harmonic magnitudes, reduced processor time, and reduced accumulated numerical errors. In this case, the only calculation based on the frequency domain is the Fourier calculation to determine the amplitude of a fundamental component (i.e., the modulation index), which is unavoidable.
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