This letter proposes a novel nine-level T-type packed U-cell (9L-TPUC) inverter, which consists of one T-type neutral point clamped leg, one half-bridge, two switches and two identical dc sources. And the single carrier modulation scheme can easily produce the corresponding switching sequences, which makes the T-type leg switching under high frequency and other switches operating under low frequency. To further improve the operational efficiency, the T-type leg can be constructed by using SiC devices for withstanding the necessary high frequency switching. The experimental results verified the performance of the proposed topology.

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for high-voltage and high-power applications. This paper proposes the control method of parallel-connected modular multilevel converters (parallel-MMCs), which assumes that the multiple MMCs are directly connected at both ac and dc sides to effectively enhance the power rating as expected. Two key problems were first solved for the parallel-MMCs under the normal operation conditions: voltage balancing of submodules and mitigation of circulating currents, where the novel transformed third-order harmonic resonant controller in the synchronous reference frame was employed to mitigate the dominant second-order and fourth-order circulating currents and a sixth-order harmonic resonant controller is used to attenuate the zero-sequence sixth-order circulating current existed in all phase currents per MMC. Considering the high risk of switches fault in the parallel-MMCs, the fault-tolerant operation schemes were then proposed in this paper to address the major concerns of open-circuit and short-circuit switch fault in a submodule, respectively. Carefully controlling the healthy submodules and the corresponding phase arms, the parallel-MMCs can successfully maintain their balanced capacitor voltages and mitigate the circulating currents with the qualified output waveform obtained. In addition, the parallel configuration of MMCs provides the unique solution for the short-circuit switch fault operation which was seldom discussed in the published literature works with respect to the MMC fault-tolerant operation schemes. MATLAB simulations and the constructed experimental prototype have verified the performance of the proposed control strategy.

Battery energy-storage system (BESS) based on the modular multilevel converter (MMC) can flexibly manage the battery packs integrated into submodules, where the battery pack can directly or through a small capacitor connect to the rear-end half-bridge circuit for reducing cost and volume caused by an additional dc-dc converter. But the alternating current ripples will cause substantial additional charge throughput for battery packs and, consequently, reduce the battery lifetime in theory. Therefore, this article proposes a variable dc-link voltage regulation method for a single-phase MMC-BESS. In specific, the phase disposition (PD) modulation scheme is improved to properly bypass the gating signals with unnecessary conduction during operation and the modified gating signals will be rearranged for balancing the states-of-charge and states-of-health. Doing so, the additional charge throughput could be significantly reduced compared with the traditional phase-shift modulation and PD modulation. Even comparing with the ideal dc current charging/discharging, the proposed method will only increase slight additional charge throughput and nearly negligible capacity loss. The experimental results of a 6.6 kW/6.6 kWh single-phase MMC-BESS prototype verified the performance of the proposed method.

This paper proposes a new nine-arm modular multilevel converter (9A-MMC) with improved hybrid submodules for middle arms. In specific, the middle arms are formed by the unidirectional current full-bridge submodules (UC-FBSMs), while the upper and lower arms are formed by the traditional half-bridge submodules (HBSMs). Compared with the standard 9A-MMC where the upper, middle, and lower arms are all formed by HBSMs, the DC-side voltage of the new 9A-MMC can be reduced by 1/3, and whose switch count is the same as that of the standard 9A-MMC. Moreover, the DC offset in the common mode voltages (CMVs) of the new 9A-MMC is eliminated. In this paper, the middle arm current of the new 9A-MMC was thoroughly discussed, and the maximum operation range of the new 9A-MMC was also presented. To enlarge the operation range of the proposed 9A-MMC, a middle arm current reshaping control method was put forward as well. The feasibility and performance characteristics of the proposed 9A-MMC were finally verified by the simulation and experimental results.

This article proposes a thyristor-based T-type converter with modular multilevel dc-link (TTC-MMD), where the thyristor-based T-type converter provides the basic phase commutation capability and two arms of cascaded submodules in dc-link generate the multilevel switching line voltages. To guarantee the successful turn- on and turn- off of thyristors, one full-bridge submodule (FBSM) is embedded in each arm. In addition, the FBSM can also be employed to participate in the multilevel switching modulation. For real applications, the TTC-MMD can be employed as a medium-/high-voltage static synchronous compensator, which features triplen frequency fluctuation of capacitor voltages and easy balancing of capacitor voltage under various operation conditions due to the common dc-link shared for three phases. It can also be implemented in other medium-/high-voltage applications. Finally, simulation and experimental results verified the performance of the proposed converter.

With the number of installed high voltage direct current (HVDC) transmission systems increasing, there is a growing requirement to develop high voltage and power DC-DC converter for interconnecting the HVDC systems with different voltage levels, which can improve the flexibility and reliability of power system. This paper proposes a multiport DC-DC modular multilevel converter (MMC) for multiple HVDC systems interconnection. Besides inheriting the advantages of conventional MMC, the proposed multiport DC-DC MMC also presents several other advantages of fewer submodules amount, more attractive investment cost, and smaller volume. The topology and mathematical models are firstly presented in this paper. Next, the operation principle analyses including the required submodules amount and capacitor voltage balancing are also elaborately described. Moreover, the control scheme is put forward in this paper as well. Finally, the simulation and experimental results are displayed to verify the feasibility and performance of the proposed multiport DC-DC MMC and corresponding control strategy.