Browsing by Author "Konstantinou, G."
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Item Controllers for eliminating the ac components in the circulating current of modular multilevel converters(2016-01-20) Darus, R.; Pou, Josep; Konstantinou, G.; Ceballos, Salvador; Agelidis, V.G.; POWER ELECTRONICS AND SYSTEM EQUIPMENTControl of the circulating currents in a modular multilevel converter (MMC) is an integral part of its operation as the circulating currents have a significant impact on the rating of the power devices, losses and also capacitor voltage ripples. This study proposes two methods to control the circulating currents in the phase-legs of the MMC to their minimum dc value through elimination of the low-order harmonic components in the circulating current. The first method acts directly on the ac harmonics of the circulating current allowing the dc component to establish naturally, whereas the second one directlym controls the current to its dc reference. Simulations and experimental results from a laboratory prototype demonstrate the operating principles of the two methods and validate their performance.Item Switching Frequency Analysis of Staircase-Modulated Modular Multilevel Converters and Equivalent PWM Techniques(2016-02) Konstantinou, G.; Pou, J.; Ceballos, Salvador; Darus, R.; Agelidis, V.G.; POWER ELECTRONICS AND SYSTEM EQUIPMENTThe large number of voltage levels in a modular multilevel converter (MMC) make staircase modulation a feasible alternative, particularly in high-power applications. However, staircase waveforms do not necessarily mean operation of the MMC submodules (SMs) at the fundamental frequency. This paper presents an analysis of SM switching frequencies in staircase-modulated MMCs and their correlation to the modulation index and load phase angle. A carrier-based pulsewidth-modulation (CB-PWM) equivalent technique is also developed. This analysis demonstrates that CB-PWM techniques provide a similar switching frequency with superior harmonic performance and improved voltage balancing characteristics at all modulation indices compared to staircase modulation. The theoretical analysis is verified with extensive simulation results for MMCs with different SMs and experimental results from a laboratory prototype.