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This article 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 one-third, 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 article, 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 paper proposes a novel STATCOM by cascading packed U-cell (CPUC) and using low capacitance. Compared to cascaded H-bridge, the CPUC-STATCOM can largely reduce switch counts and their related gate driver circuits. Then, skillfully utilizing the principle that the second-order harmonic component of capacitor voltage is in phase with the absolute value of grid voltage in capacitive mode, low capacitance is used in CPUC-STATCOM. It can reduce cost, weight and volume of system compared to usually using large capacitance to limit voltage ripple within 10% of nominal capacitor voltage. However, it has limited compensation capability in inductive mode by using low capacitance. Aiming at overcome the aforementioned drawback, a flexible third harmonic control method is proposed, which can reduce capacitor peak voltage and averaged voltage. Finally, the simulation results verified the performance of the proposed topology and control method.1

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.

This paper proposes a novel switched-capacitor five-level T-type (SC-SLT) inverter with reduced number of switches and with self-voltage balancing and boosting voltage ability. It consists of eight switches, two diodes, two capacitors and a single de-source, The proposed inverter does not adopt a load-end H-bridge to change output voltage polarity. Besides, the modulation scheme only using one triangular carrier is proposed for the proposed topology. Moreover, self-voltage balancing ability of capacitor and capacitance calculation are discussed in detail. Finally, the performance of the proposed inverter has been validated by various simulation results using Matlab software.

This paper proposed a novel hybrid nine-arm modular multilevel converter (9A-MMC) based on half-bridge submodule (HBSM) and unidirectional current full-bridge submodule (UC-FBSM). In hybrid 9A-MMC, the UC-FBSMs are employed in middle arms, while HBSMs are employed in upper and lower arms. Compared with conventional full-bridge submodule (FBSM), the number of required semiconductor devices in UC-FBSM is half of conventional FBSM, but the capability of generating negative-voltage is still preserved. The proposed hybrid 9A-MMC can extend the operation range and improve the utilization of dc-side voltage compared with standard 9A-MMC where only HBSMs are used, and this is benefit from the negative-voltage generated by UC-FBSM in middle arm of hybrid 9A-MMC. Besides, the mathematical model of hybrid 9A-MMC is also studied. The operational analyses including the principle of determining the number of SMs in per arm are discussed in this paper as well. Subsequently, the control scheme of hybrid 9A-MMC is presented. Finally, simulation and experimental results are displayed to verify the feasibility and characteristics of the proposed hybrid 9A-MMC.

For battery energy storage system (BESS), accurate state-of-health (SOH) testing is critical for determining which battery should be replaced and it is also the foundation for battery SOH balancing, fault prevention and maintenance, especially when using retired batteries from electric vehicle. However, the traditional SOH testing method requires the BESS to quit operation and takes a long time, which results in profit loss. The modular multilevel converter (MMC) can improve this situation to some extent, but the battery pack still needs to be disassembled and redundant sub-modules are needed. In order to obtain the real-time SOH and keep the BESS still be able to work at the same time without battery disassembling, this paper further proposes an online SOH testing method for MMC-BESS with DC-DC converter control, voltage balancing control of half-bridge and the power distributing method. Simulation and experimental results are presented to verify the validity of the proposed method.

The modular multilevel converter (MMC) is an emerging and highly attractive multilevel converter topology for medium and high voltage applications. In order to enhance power handling capacity and power ratings, this paper proposes a novel parallel-connected modular multilevel converters topology, which assumes two modular multilevel converters directly connected at both AC and DC sides. However, two key problems should be solved in the parallel-connected MMCs: voltage balancing of all sub-modules and the circulating currents between two MMCs. Firstly, this paper focuses on circuit configurations and voltage-balancing control of parallel-connected modular multilevel converters (PMMCs), and the combination of average and individual voltage control methods enables the parallel-connected MMCs to achieve the voltage balancing of all sub-modules without using any external circuit. Next, a zero-sequence model is developed to predict the dynamics of the zero-sequence current, and then an effective zero-sequence current control loop is designed to attenuate the circulating current. Finally, Matlab simulations verified the performance of the proposed control strategy.

This paper investigates the novel modulation method of six-switch back-to-back converter for the application of an online uninterruptible power supply. With an additional current direction judgment logic and related modulation adjustment, the total modulation index can be increased and consequently the overall losses and the voltage stress across semiconductors will decrease significantly, being much different from other modulation methods of the six-switch converter. Matlab simulation and experimental prototype are constructed to verify the performance of the proposed modulation method.