The solid-state transformer based on modular multilevel converter (MMC-SST) is usually composed of MMC-stage and isolated bidirectional dc/dc-stage, which is a preferred solution for realizing flexible interconnection of medium and low voltage hybrid ac/dc networking system. The large size of submodule (SM) capacitor is the key factor that limits the power density of MMC-type systems. This article proposes a four-terminal dual-active-bridge (FDAB), to obtain a new FDAB-based MMC-SST (FM ² C-SST) with high-frequency-link, to realize the inductive-filtering of SMs ripple-power without any control. The synchronous-switching modulation of primary FDAB can make the horizontal three SMs exhibit switched-capacitors characteristics, and based on the three-phase symmetry between ripple-powers, the inductive-filtering on multi-winding transformer can be achieved, which greatly reduces the size of SM capacitors, at the same time eliminating the arms second-order circulating current. In this article, the operating principle, control method and parameters constraints of inductive-filtering process are analyzed. A practical engineering case is used to evaluate the proposed FM ² C-SST, including devices design, volume, cost, power loss and thermal behaviors evaluations, and the operating characteristics under grid unbalance is also discussed. Finally, the correctness and effectiveness of the proposed FM ² C-SST inductive-filtering strategy are verified by simulation and experiment.

Two types of common mode voltage (CMV) exist in medium-voltage motor drive system based on modular multilevel converter (MMC) type topology, that are, arms CMV (ACMV) and system CMV (SCMV). The former will produce circulating current between phases, and the latter will do harm to the motor. This article proposes a suppression scheme of CMV with a dual dc bus MMC-based solid-state transformer (DBM ² C-SST) based drive system. The horizontal submodules (SMs) of MMC are cascaded with a quadruple-active-bridge (QAB), then a low voltage is formed. Plus the original medium voltage bus, the system is characteristic as a dual dc bus. The primary triple-active-bridge of QAB which characterized by low-impedance and synchronous-switching helps to achieve the natural suppression of the fluctuating voltage of the MMC horizontal three SMs within the entire operating frequency range under the relevant modulation strategy, thus realizing the full suppression of ACMV and low-frequency components suppression of SCMV simultaneously, and with the additional minimum method of SCMV high-frequency components, the DBM ² C-SST can achieve zero CMV. As the SMs fluctuating voltage and CMV are decreased, the values of SMs capacitance and arms inductance can be significantly reduced.

For the power electronic transformer based on cascaded H bridges with single inductor quad active bridge (SIQAB), this article proposes a submodule (SM) capacitance minimization scheme based on switching synchronization hybrid phase-shift modulation (SSHPSM) of high frequency link. Under SSHPSM, the primary side of SIQAB-stage operates in switched-capacitor mode to realize the automatic elimination of the second-order-frequency voltage ripple of SM capacitor without complex control, which helps to reduce the capacitance of SM and improve the power density of the system. The principle of low-frequency ripple transfer and cancellation is analyzed from the perspective of electric and magnetic coupling, and it shows that the scheme is still applicable under the influence of the nonideal factors. The power losses distribution and efficiency of dc–dc-stage and the capacitance constraints of SM are carried out. Furthermore, the operation feasibility of the proposed scheme in the case of the typical critical conditions are evaluated, including the low-frequency operation for motor drive and the fault ride-through ability of three-phase voltage asymmetry for grid-tied conditions. Finally, the proposed scheme is verified by simulation and experiment.

A triple-port solid-state transformer (TP-SST) topology based on the combination of modular multilevel converter (MMC) and resonant push–pull converter (RP ² C) is proposed in this article. For an MMC-based SST, large size capacitor caused by fluctuating power and voltage imbalance of MMC submodules (SMs) are two important problems. Based on a simple open-loop control scheme, the RP ² C-based high-frequency links (HFL) are characterized by a voltage source with fixed conversion ratio and low impedance, and then can form a power free-coupling channel between SMs and realize the automatic balance of capacitors voltage. Moreover, the fluctuating power can cancel each other based on their three-phase symmetry, which obviously reduces the capacitance size, and the second-order harmonic circulating current of the arms could be eliminated at the same time, greatly simplifying the control of system. In this article, the working principle of TP-SST is described, and the methods of HFL realizing SM voltage ripple elimination and self-balancing are analyzed in detail. In addition, a comparison with traditional SST in efficiency and cost is presented. Finally, simulation and experimental results verify the correctness and effectiveness of the proposed topology scheme.

For the modular multilevel converter (MMC) based photovoltaic system, when the submodules (SMs) of MMC are connected to distributed photovoltaic (PV), power mismatches between legs and arms are the inherent problem. To eliminate the power mismatches, this article proposes a lightweight MMC-based grid-connected energy router (MGER) integrated PV and energy storage (ES). Large-scale PV and centralized ES are connected to horizontal three SMs through four-terminal dual-active-bridges (FDAB). The primary three full-bridges of which adopt synchronous switching to achieve mismatch power elimination between horizontal SMs, so legs power balance can be realized. In addition, the SMs three-phase symmetrical ripple-power can also cancel each other out under synchronous switching of primary FDAB, so as to achieve low capacitance and lightweight of MGER, and the MMC arms second-order circulating current can be also eliminated from root cause. In this article, the power mismatches analysis, MGER configuration, power mismatches elimination, control method, and lightweight implementation are discussed in detail. Finally, the correctness and effectiveness of the proposed scheme are verified by simulation and experiment.

Cascaded H-bridge-based solid-state transformer (CHB-SST) has been the typical scheme in medium- and low-voltage ac/dc conversion applications, and large-size capacitance caused by second-order ripple of submodules (SMs) is the inherent problem of CHB-based systems. In this article, we propose a novel low-capacitance CHB-SST based on zero-impedance resonant push–pull (ZRPP) decoupling channel for ripple power. ZRPP presents zero-impedance characteristic at its switching frequency equaling to series resonant frequency, which makes the equivalent parallel of CHB ac-link capacitors, and provides a decoupling channel for ripple power. Therefore, the horizontal SMs ripple power can achieve mutual cancelation based on their own three-phase symmetry, which significantly reduces the size of ac-link capacitance. In addition, the elimination of SMs ripple-voltage achieves the elimination of grid zero-sequence harmonics from root cause, and the mutual clamping of parallel ac-link capacitors also achieves the voltages self-balancing, which reduces the complex of system control. The CHB-SST configuration, operating principle, ripple-power decoupling, SMs voltage self-balancing, system design, and evaluation are discussed in this article. Finally, the effectiveness of the proposed CHB-SST is verified by simulation and experiment.