Optimal injection of harmonics in circulating currents of modular multilevel converters for capacitor voltage ripple minimization

Abstract

This paper proposes an algorithm to calculate the optimal amplitude and phase of the harmonic current components that can be injected in the circulating currents of a modular multilevel converter (MMC) to minimize the capacitor voltage fluctuations. An optimal second harmonic component and an\ud optimal set of second and fourth harmonic components are proposed. Simulation results are obtained in MATLAB/Simulink environment to study the effectiveness of the calculated optimal currents. Selected experimental results have been obtained from an MMC laboratory prototype, testing the effects of the circulating currents. The reported results demonstrate the effectiveness of using a fourth harmonic component in the\ud circulating current, which improves the effect of the second harmonic on reducing the capacitor voltage fluctuations.Postprint (published version

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

“…In order to evaluate the circulating current references studied in this paper, the results are benchmarked against those in [26]. The references for the circulating currents in [26] consider two cases; (i) injecting a second harmonic only and (ii) a combination of second and fourth harmonics.…”

“…The references for the circulating currents in [26] consider two cases; (i) injecting a second harmonic only and (ii) a combination of second and fourth harmonics. The values were obtained off-line by evaluating all the possible amplitudes and angles for the current harmonics injected into the circulating current.…”

“…In [24], a second harmonic is injected into the circulating currents of the MMC to achieve a reduction in the capacitor ripples. The injection of a fourth harmonic in addition to the second one is also considered in [25], [26]. The main drawback of these methods is that they rely on the determination of the amplitude and phase of the output currents of the MMC and the use of an extensive look-up table.…”

“…SM capacitor voltage ripple amplitudes and rms arm currents are presented and compared in Section VI according to the different circulating current references. In Section VII, the results obtained from the circulating current references studied in this paper are compared and benchmarked against those in [26]. In Section VIII a circulating current control is introduced.…”

The series bridge converter (SBC): Design of a compact modular multilevel converter for grid applications

“…The second harmonic voltage for inter-arm energy management does not appear in the AC or DC network and therefore does not affect the voltage or power quality in the HVDC system. The second harmonic injection concept explored in this paper is in all aspects different from that which has been explored for local capacitor voltage ripple minimisation in other MMC applications [15]. Now consider a system where the converter CL and SFB voltages are redefined with a proportion of second harmonic voltage as (9).…”