Circulating current control and evaluation of carrier dispositions in modular multilevel converters

Abstract

This paper presents two control techniques to minimize the circulating currents of the modular multilevel converter. The control techniques reduce the amplitude of each capacitor voltage ripple. Multicarrier level-shifted pulse-width-modulation is applied and the performance of interleaving and non-interleaving the carrier waveforms between the upper and the lower arms is reported. The total harmonic distortion of the output common voltage and rms value of the arms currents are benchmarked against the case when such techniques are not employed. The techniques have been verified by simulation using MATLAB/Simulink and PLECS Blockset software. Experimental results from a low power phase-leg prototype built with five sub-modules per arm are also provided.Peer ReviewedPostprint (published version

Circulating Current Injection Methods Based on Instantaneous Information for the Modular Multilevel Converter

“…In [21], a closed-loop control strategy based on eliminating the second order harmonic of the arm currents was proposed. Similarly in [22], two control strategies were introduced to eliminate the ac components in the circulating current. Although these strategies reduce the rms value of the arm currents and therefore the power losses of the MMC, the capacitor voltage ripples can be reduced further.…”

A modified voltage balancing sorting algorithm for the modular multilevel converter: Evaluation for staircase and phase-disposition PWM

“…PD-PWM uses N carriers displaced to occupy equal bands within the modulation index range [19] and both modulation techniques generate N+1 voltage levels. Both modulation techniques can also produce 2N+1 voltage levels by interleaving the transitions between the upper and lower arms [19], [22], but this modulation technique is not considered in this work. …”

“…Detailed analysis of the output and circulating current control can be found in the available technical literature [5], [9], [11], [26]. In this paper, a direct control of the circulating current and elimination of the ac components [22] is applied.…”

“…Refs. [19], [22] have investigated the application of CB-PWM without and with carrier interleaving and the presented results show that carrier interleaving improves the THD of the output voltage at the expense of higher rms arm current. CB-PWM was also investigated in [20] for HVDC applications where a PS-PWM with voltage sorting algorithm was shown to provide lower capacitors voltage ripples compared to the PD-PWM and phase-opposition-disposition (POD-PWM).…”

“…Various modulation techniques have been applied to the MMC. Carrier-based pulse-widthmodulation (CB-PWM) techniques are the most common ones [19]- [22], but other techniques such as selective harmonic elimination (SHE-PWM) [23] or staircase modulation [24] can also be applied to the MMC. SHE-PWM can reduce the switching frequency of the SMs and it also provides good harmonic performance [23].…”