Abstract
Nearest-level control (NLC) is a popular technique used in modular multilevel converters (MMCs) with a large number of submodules (SMs) owing to the NLC's flexibility and ease of implementation. However, in medium-voltage applications, MMCs contain a relatively low number of SMs, and the drawbacks of the NLC methods emerge, wherein the poor quality of output voltages and currents result in high total harmonic distortion, large ripples in SM capacitor voltages, and unsuppressed circulating currents. Several NLC methods have been proposed to handle these problems, but they do not satisfy all the control objectives simultaneously. This paper proposed a modified NLC capable of enhancing the output quality of MMCs with low number of SMs without deteriorating the control objectives. Unlike previously reported NLC methods, instead of directly calculating the numbers of SMs from the upper and lower arm voltage references, the difference and total number of SMs are obtained from the output voltage reference and circulating current control, respectively. Hence, the numbers of SMs in the upper and lower arms are acquired by simply solving a system of first-order two-variable equations. The simulated and experimental results for a single-phase MMC system were used to verify the appropriateness and effectiveness of the proposed modified NLC method.
Highlights
Multilevel converters have gained wider acceptance over twolevel converters owing to their improved waveform qualities, reduced semiconductor losses, low electromagnetic interferences (EMIs), and common-mode voltages (CMV)
This paper proposes a modified nearestlevel control (NLC) method to improve the output quality of the modular multilevel converters (MMCs) with a low number of SMs and to solve the problems observed in previous NLC methods
It can be noted that the total number of inserted SMs from the predictive NLC method varied with high frequency to ensure regulation of the output and circulating currents and to increase the output voltage level
Summary
Multilevel converters have gained wider acceptance over twolevel converters owing to their improved waveform qualities, reduced semiconductor losses, low electromagnetic interferences (EMIs), and common-mode voltages (CMV). The limit controller allowed the peak-to-peak value of the circulating current to be significantly reduced compared to the conventional NLC method, but the adjustment of the number of inserted SMs in the upper and lower arms with this method could not be controlled This might result in deterioration of the output voltage quality. An NLC method with circulating harmonic current suppression with deadbeat control was proposed in [21], where the numbers of inserted SMs in the upper and lower arms from the levelincreased NLC II were adjusted based on the deadbeat controller to regulate the circulating current This control scheme inherited the merits of both the level-increased NLC II and deadbeat controller to improve the output quality and control the circulating current.
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