In this paper, a railway power conditioner (RPC) based on a modular multilevel converter (MMC) with a split supercapacitor energy storage system (SCESS) is studied. In this case, the MMC-SCESS-based RPC could not only provide normal negative-sequence current compensation, but also reduce the impact of power fluctuations caused by the locomotive braking or startup on the electric grid. First, this paper analyzes the power flow patterns and deduces the reference circulating current under different operation modes of the MMC-SCESS-based RPC. Then, the control objectives of the MMC-SCESS-based RPC are divided into two categories—the balance control and the current tracking control. The balance control methods are developed for the submodule capacitor voltages and the state of charge of the supercapacitor, which are associated with the operation modes. To ensure the current tracking performance, a model-predictive direct current control method is presented for the MMC and the bidirectional energy converter. Finally, the effectiveness of the proposed control methods is verified by the experimental results of a downscaled prototype.

Modular multilevel converter (MMC) has become a promising topology applied in medium/high-voltage applications due to its advantages. However, there are several challenges in the MMC, such as the appearance of harmonic circulating current. In this paper, a novel two-stage model predictive control (TSMPC) is proposed to suppress the harmonic circulating current. First, the mathematical model of the MMC is discretized to describe steady-state objectives including output currents, circulating currents, summation, and imbalance of capacitor voltages. Then, an optimal control options set consisting of inserted numbers is built by the front stage of TSMPC, while the inserted number applied to the MMC ultimately is selected from the above-mentioned set by the back stage of TSMPC. The proposed TSMPC can not only improve the control performance of harmonic circulating current but also ensure the overall control performance of the MMC system. In addition, a simplified rolling optimization method is proposed to reduce computation from the aspect of simplifying each control option prediction. An optimized search space method is also proposed to design the control options set to be evaluated from the aspect of ensuring the output current tracking and harmonic circulating current suppression. Finally, the effectiveness of the proposed method is validated by experiment results.

To manage the power quality of high-speed V/v traction system, a multilevel railway power conditioner (MRPC) is researched. According to the structure and equivalent models of MRPC, basically the MRPC is a symmetrical system, and its four clusters can be divided into two cluster-groups. According to the ac equivalent mathematical models of cluster-groups, it can be considered as the relationships in the αβ frame, but not completely symmetrical, so the positive- and negative-sequence dq models for cluster-groups can be derived. As the fluctuation and randomness of the locomotive loads, a sliding mode control system based on the exponential reaching law in the dq frame is explored to control the cluster ac currents, which can improve the dynamic performance and keep a good tolerance against parameter mismatches. Finally, an experimental prototype is developed, and the results show the feasibility and the robustness of the system.

This article proposes a hybrid model predictive control (HMPC) for a modified modular multilevel switch-mode power amplifier (M3-SMPA). The M3-SMPA consists of two parts connected in series including modular multilevel converter (MMC) and full-bridge converter (FBC). Different from available MPC methods that only apply either finite control set MPC (FMPC) or modulated MPC (MMPC), the proposed HMPC method synthesizes the merits of both FMPC and MMPC on different timescales. First, the optimal control option (CO) for MMC is calculated by FMPC with the multiple control objectives, including output voltage control, circulating current control, and submodules (SMs) capacitor voltages balance control. Then, based on the above-mentioned optimal CO, the optimal duty cycle for FBC is calculated by MMPC with a single objective of output voltage control. In this case, the FMPC achieves the multiobjective control of MMC while the MMPC eliminates the tracking error of output voltage using FBC with a fixed switching frequency. Furthermore, a circulating current injection method is presented to balance the SMs capacitor voltages and an improved adjacent search method is introduced to reduce the evaluated COs for MMC in each control period. The effect of dc-link voltage configuration for FBC on output steady-state performance is also analyzed. Finally, the effectiveness of the proposed HMPC method is verified by experimental results.

As for the reactive power and negative sequence compensation caused by the unbalanced load, the modular multilevel converter based static synchronous compensator (MMC-STATCOM) is adopted and analyzed. To reduce the complexity of control system, the three-phase current error signal is transformed into αβ frame and proportional-resonant controller is adopted to implement the static current tracking. In order to reduce the operation performance of MMC-STATCOM, a system small-signal model is established and an efficient circulating current suppressing method is proposed. As the circulating current of each phase is controlled individually, this method can be widely applied in diverse MMCs. The simulation and experiment results have validated the feasibility of the proposed control method.

Yunhe Feng, Cheng Guo, Bingbing Wen, Peng Sun, Yufei Yue, Dingwen Tao. Proceedings of the The Fifth International Workshop on Emoji Understanding and Applications in Social Media. 2022.