This article presents a modified modular multilevel converter (MMC) equipped with the middle-cells and control algorithm for medium voltage applications. It is intended to reduce the submodule (SM) capacitor voltage ripples by the third-order harmonic voltage injection (THVI) and second-order harmonic current injection (SHCI) without the common-mode voltage injected on the ac-side. On the one hand, using the proposed algorithm, the independent power degrees of freedom can be produced to eliminate the dominant fundamental part in arm powers, which can dramatically reduce the SM capacitor voltage ripples. On the other hand, the smaller amplitude of the SHCI can be obtained, which decreases the power losses. Meanwhile, by properly controlling the proposed middle-cells of the modified MMC, the compensated parts can be produced to eliminate the common-mode voltages caused by the THVI. Additionally, the arm power analysis of the modified MMC under different power factors is conducted, and the obtained results are discussed. The mathematical models of the SM capacitor voltages for the half-bridge based SMs and the proposed middle-cells in the modified MMC are also analyzed. Finally, the effectiveness of the theoretical analysis of the modified MMC is verified by simulation and experimental results.

This paper presents a modified modular multilevel converter(MMC) with the three-terminal middle-cells inserted between arms and ac-side for medium voltage applications. It is intended to reduce the sub-module(SM) capacitor voltage ripples by controlling the high frequency powers flowing among arm branches for low-frequency output operations without common-mode voltage injected in ac-side. Due to the physical connection among the upper arm, the lower arm and ac-side, the middle-cell can be regarded as the excess degree of freedom to modify the dc-side and ac-side characteristics. Therefore, the common-mode voltage in ac-side can be eliminated by the inverse common-mode voltage produced by the middle-cell in ac-side. Moreover, owing to the unique structure of the middle-cell, the middle-cell capacitor voltage also results in a small ripple due to the high frequency current flowing into it by proper control. The operation principle and control strategy of the middle-cell has been proposed in detail. Simulation and experimental results have verified the effectiveness of the proposed topology.

This paper introduces a hybrid modular multilevel converter(MMC) with the full-bridge sub-modules (FBSMs) inserted in ac-side. It is intended for redistributing arm powers of the MMC system to reduce the sub-module(SM) capacitor voltage fluctuation by injecting high frequency powers in upper and lower arms without disturbing the output voltage. New power distribution of the hybrid MMC system can be realized by controlling the SMs and FBSMs working under the high-frequency powers. The proposed FBSMs in the hybrid MMC system also performs as the harmonic power generator to eliminate the common-mode voltages in ac-side. Since the FBSMs locates in ac-side of the hybrid MMC system, the traditional MMC control method can be directly applied in arm control of the hybrid MMC system. Meanwhile, relationship between the arm powers and SM capacitor voltage has been analyzed in detail to reduce the SM capacitor voltage ripples by redistributing the frequency spectrum of arm powers. Simulation and experimental results have also verified the effectiveness of the proposed method.

In this article, we propose a time-sharing principle-based modular multilevel converter (TS-MMC) for medium- to high-voltage power transmission applications, which is designed with a lightweight running feature. The TS-MMC consists of cascaded switch stacks (CSSs) and alternative arm converters (AACs), integrating the merits of both the modular multilevel converter and the two-level voltage-source converter. In TS-MMC, the AACs shape the required multilevel arm waveforms, and the CSSs arrange the AACs operating at different intervals for producing the desirable output waveforms. By applying a “time-sharing” principle for the AACs, a dramatic reduction of the submodule number is achieved. Additionally, the TS-MMC does not require any arm inductors, whereas only three inductors are equipped in ac side. Moreover, featured by the specific structure, the dc fault blocking capability of the TS-MMC is also enabled. Furthermore, to explore the performance of the TS-MMC, mathematical model and control strategy of the TS-MMC are provided. Finally, the simulation and experimental results are carried out to verify the feasibility of the TS-MMC.

Owing to peak power demands of pulsed power load like radar and beam weapon is much larger than capability of generator, researches about energy storage equipment sizing optimization have been extensively carried out. However, these researches are mainly considered from static perspective without taking dynamic characteristic into consideration. To cover this gap and meet strict weight requirements of airborne platform, this paper proposed a hybrid energy storage system sizing process to satisfy peak power demand while dynamic standard set up by MIL-STD-704F is also studied by means of spectrum and impedance analysis. The overall filter capacitor on DC BUS is considered as a part of energy storage system for improving system dynamic characteristics. The research content is divided into four parts. First, the power waveform is discussed through spectrum analysis to give full play to hybrid energy storage system. Second, the influence of energy storage equipment on system dynamic characteristics is analyzed and the results are taken as constraints for optimization. Then, combined with dynamic and static constraints, a hybrid energy storage system sizing process depend on Non-dominated Sorting Genetic Algorithm II and decision-making method are proposed to obtain the most suitable result. Finally, simulation model is established in MATLAB/Simulink environment for verification.