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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.

Circulating current is an undesirable but inherent phenomenon in the modular multilevel converter (MMC), which is usually suppressed to reduce the current rating of semiconductor devices, the submodule (SM) capacitor voltage ripples, and power losses. Nine-arm modular multilevel converter (9A-MMC) is deemed as a compact MMC with two sets of three-phase terminals, and there is also circulating current in 9A-MMC. This paper proposed an exclusive circulating current control method for 9A-MMC based on the combination of the conventional proportional resonant (PR) controller and the flexible design of the number of SM and the rated SM capacitor voltage in each arm. First, this paper analyzed circulating current in 9A-MMC in detail. Subsequently, the circulating current control method suitable for 9A-MMC was proposed in this paper. Then, the control scheme of 9A-MMC was briefly described. Finally, the simulation and experimental results verified the effectiveness of the proposed circulating current control method.

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 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.

This paper proposes an improved topology of unified power flow controller (UPFC) based on nine-arm modular multilevel converter which could realize the same function as conventional UPFC composed of two back-to-back modular multilevel converters (MMCs). The proposed improved UPFC topology based on nine-arm MMC (9AMMC-UPFC) could offer significant advantages, that the number of required submodules and arm inductors could be reduced by 25% and 50%, respectively over the traditional UPFC based on MMCs. Therefore, the system volume and cost could be drastically reduced. The structure and operation principle of proposed 9AMMC-UPFC are firstly illustrated in this paper. Next, phase-shifted carrier-based PWM technique including capacitor voltage balancing control is briefly described for the operation of proposed 9AMMC-UPFC. Then, in order to suppress the exclusive odd-order harmonics in circulating currents of proposed 9AMMC-UPFC, circulating current suppression control method is put forward. Finally, the performance of the proposed 9AMMC-UPFC are evaluated by the simulation and experiment results.

Nine-arm modular multilevel converter (9A-MMC) could be applied in many applications, such as dual-motor driving and unified power flow controller, which usually include two sets of ac output terminals. Generally, two conventional modular multilevel converters (MMCs) are applied in such application, which would increase the volume and cost of system. Compared with two conventional MMCs, 9A-MMC could reduce 25% of the required submodules and 50% of the arm inductors, which means that the system volume and cost could be extremely reduced. This paper proposes a complete control scheme for better operation of 9A-MMC. To well elaborate the proposed control scheme, the operating principles of 9A-MMC, especially for capacitor voltages fluctuation and circulating currents, are firstly analyzed in detail. Subsequently, the control methods including capacitor voltage balancing control and circulating current suppression control are systematically presented in this paper. Finally, the simulation results verify the performance of 9A-MMC and the effectiveness of control methods.

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 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.

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.