Circulating current suppression control for modular multilevel converters based on restricted self-redundant states pred
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ORIGINAL ARTICLE
Circulating current suppression control for modular multilevel converters based on restricted self‑redundant states prediction Xing Hu1 · Jianzhong Zhang1 · Fujin Deng1 · Gui Chen1 · Qiang Chen2 · Zaki ud Din1 Received: 8 January 2020 / Revised: 8 May 2020 / Accepted: 20 May 2020 © The Korean Institute of Power Electronics 2020
Abstract Circulating current exists among phases or between the DC link and the three phases in a modular multilevel converter (MMC). Suppression control of the alternating components in circulating current is a critical issue for the stable and efficient operation of an MMC. Due to the redundancy and symmetry of MMCs, some of the self-redundant states of MMCs can be used to suppress the alternating components in circulating current without affecting the output performance of the converter. In this paper, a theoretical derivation of the boundary of redundant states is given. Then a suppression strategy for circulating current is proposed based on prediction control and the boundary of redundant states. The selection range of the redundant states is narrowed in this case, which reduces the computation burden when compared to the original method and ensures excellent performance in the suppression of circulating current. Simulations and experiments are carried out to verify the effectiveness of the proposed strategy. Keywords Circulating current suppression · Modular multilevel converter · Prediction control · Restricted self-redundant states
1 Introduction With the development of semiconductor switching technology and the ever increasing demand for excellent output of power quality in industrial fields, multilevel converters are playing an important role in medium and high voltage applications [1–3]. The modular multilevel converter (MMC) is one of the most promising of the multilevel converters. The MMC was first proposed by Prof. R. Marquardt [4, 5]. When compared to other multilevel converters, the MMC has a larger number of excellent features, including high modularity, scalability to meet any voltage level requirements, and superior sinusoidal characteristics at the AC output terminal [6–9]. Due to the continuous arm currents in MMCs, circulating currents exist between the DC link and the three phases under normal operating conditions. It has been verified that the * Jianzhong Zhang [email protected] 1
School of Electrical Engineering, Southeast University, Nanjing, China
School of Electric Power Engineering, Nanjing Institute of Technology, Nanjing, China
2
main components of the circulating current are the DC component and twice the fundamental component in a negative sequence [7, 8]. The DC component in circulating current contributes to the power transmission from the DC side to the AC side. However, the alternating component in circulating current does not transmit power. Instead, it enlarges the arm current, which increases the power losses, capacitor voltage ripples, and the required current rating of the semiconductors switching devices [10–12]. If t
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