Abstract
This paper presents a thorough prediction of DC-link voltage switching ripples in the three-phase four-leg inverters operating in balanced and unbalanced working conditions. The unbalanced modes examined here employ the highest degree of AC current imbalance while still preserving three-phase operation. This behavior can be found in many grid-connected or standalone grid-forming three-phase converters that supply “heavy” single-phase loads, comprising a recent trend in smart-grid, smart electric vehicle (EV)-charging applications. In this sense, for instance, the smart EV chargers might be employed in conditions when different power is drawn/injected from/to the grid, providing power conditioning services to the latter. The analysis of three-phase four-leg inverters is then extended to single-phase operations typical of home-charging or vehicle-to-home (V2H) applications. Their performances in terms of DC-link voltage switching ripple are demonstrated. Two of the most common carrier-based PWM modulation techniques are employed to drive the three-phase inverter—namely, sinusoidal PWM and centered PWM (carrier-based analogy of the space vector modulation). The derived mathematical expressions of peak-to-peak and RMS values of DC-link voltage switching ripple for balanced and unbalanced conditions are handy for designing the associated DC-link capacitor and estimating the overall efficiency of the converter. Extensive numerical simulations and experimental tests have been performed to validate the presented analytical developments.
Highlights
Three-phase voltage-source inverters (VSIs) are widely adopted in different applications
Different modulation strategies have been proposed in the literature to control these converters—namely, space vector modulation (SVM) and carrier-based techniques
The four-leg topology studied here will be considered employed as an inverter in which the power is transferred from the DC side to the AC side. This is what usually happens in electric vehicle (EV) charger applications when employed in V2G and V2H operations
Summary
Three-phase voltage-source inverters (VSIs) are widely adopted in different applications. In the case of unbalanced AC loads, four-wire topologies are employed in place of the classical three-wire inverters due to their built-in feature to handle zero-sequence components of both voltages and currents [1] Typical applications for this kind of topology are grid-forming converters [2,3], shunt power filters [4,5], active rectifiers [6,7,8,9,10], electric drives [11], and battery chargers for electric vehicles [12,13,14]. Different modulation strategies have been proposed in the literature to control these converters—namely, space vector modulation (SVM) and carrier-based techniques The latter are widely adopted because of their simplicity in analog and digital implementation, well-known harmonic spectrum that makes the filter design easier, and the fixed switching frequency.
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