About Voltage Total Harmonic Distortion for Single- and Three-Phase Multilevel Inverters

Abstract

Many recent multilevel inverter papers end up with voltage total harmonic distortion (THD) values obtained from numerical voltage spectrum calculations (measurements). Motivated by IEEE Standard 519, a part of the multilevel research community uses a limited harmonic count to evaluate the multilevel voltage quality. First, this causes significant voltage THD underestimation, particularly for relatively high frequency PWM. Second, for a three-phase star-connected balanced load with an isolated neutral and phase symmetric modulation strategy, the calculated load line and phase voltage THD become different. However, simple considerations show that line and phase voltage THDs are essentially the same in this case. It may be difficult to judge about the multilevel voltage quality given a numerically calculated (measured) voltage THD value that may be subject to computation errors. Presented are simple smooth hyperbolic voltage THD upper and lower bound approximations for single-and three-phase inverters with nearest synchronous switching. They are valid for arbitrary modulation indices and uniformly distributed level counts and may practically serve as good reference values.Index Terms-DC-AC power conversion, multilevel inverters, pulsewidth-modulated power converters, voltage total harmonic distortion (THD).

Multilevel Inverters Power Topologies and Voltage Quality: A Literature Review

“…According to the available literature, the only works defining and quantifying the THD bounds of multilevel inverters output voltages are those of Ruderman in [41][42][43][44]. The author argues that many recent multilevel inverter papers end up with voltage THD evaluation results that are typically based on voltage frequency spectra numerical calculations/measurements (FFT).…”

“…As mentioned previously, voltage quality for single and three-phase multilevel inverters with high switching frequencies was considered in [41][42][43][44]. The assumption of infinitely large switching frequency has led to an upper bound of voltage quality for synchronous nearest switching.…”

“…Referring to (14) and (15), the upper and lower bounds are calculated for 5L and 9L H-bridge converters where an m cascaded multilevel converter operating under a modulation index M equal to 1 gives a line to line voltage with L = (m-1)/2 non negative levels [41]. Indeed, the estimated THD values are within the interval [THD up , THD low ] as illustrated by Table 3, proving the veracity of the Ruderman formulas at least for M = 1.…”

“…Elsewhere, the present review paper is enhanced by introducing Ruderman works [41][42][43][44]. These latter allow the estimation of the total harmonic distortion based on a time study instead of frequency domain.…”

Selective harmonic elimination in hybrid cascaded multilevel inverter using modified whale optimization

“…Multilevel inverters (MLIs) have become popular in medium-voltage, high-power, and renewable energy applications because of their ability to work at higher switching frequencies while restraining the dv/dt stresses. [1][2][3][4][5] Moreover, they generate output voltage with lesser harmonics and thus reduces of filter requirement. In general, the conventional MLIs are categorized into diode-clamped MLI, 3 flying capacitor MLI, 4 and cascade H-bridge MLI (CHB-MLI) topologies.…”