Modular multilevel converter with integrated battery energy storage system has been verified as a better choice for large-scale battery energy storage system. However, battery power and its unbalanced distribution in submodules lead to significant increase of capacitor voltage ripple. Larger submodule capacitance is required to maintain the ripple within reasonable limits, increasing the cost and volume of the system. To address the problem, this article proposes a novel battery integration method. Each battery pack is connected with two submodules that belong to upper and lower arm, respectively, through an isolated three-port dc-dc converter. The proposed method can ensure the power balance between upper and lower arm to eliminate fundamental circulating current and reduce capacitor voltage ripple. Three-port converters provide power channels for the fundamental power fluctuation of arms; thus, capacitor voltage ripple is further reduced. Then, the small-signal model of the proposed structure is built, and the control strategy is designed to realize both capacitor voltage ripple reduction and battery power control. Finally, a downscaled prototype is built and detailed experiment results verify the effectiveness of the proposed battery integration method and control strategy.

Modular multilevel converter with integrated battery energy storage system (MMC‐BESS) has been proposed for energy storage requirements in high‐voltage applications with large‐scale renewable energy resources. The MMC‐BESS is essentially a three‐port converter which can transfer energy between any two of the ac port, dc port, and BESS, constituting multimode operation of the system. Different from conventional MMCs, the state‐of‐charge (SOC) inconsistency among batteries would magnify the submodule capacitor voltage unbalance issue in the MMC‐BESS. In this study, an improved capacitor voltage balancing method applicable for multimode operation of MMC‐BESS is proposed by adjusting ac and dc modulation indexes simultaneously. After that, the ratio of ac and dc modulation indexes is optimised to enhance the tolerance of unbalanced power among SMs within phase arm, greatly improving the SOC equalisation rate. Based on the proposed capacitor voltage balancing method, the control structure is given and the dynamic model is conducted for the analytic design of the closed‐loop controller. Finally, the simulations and experimental results validate the effectiveness and feasibility of the proposed control strategy.

Modular multilevel converter with integrated battery energy storage system (MMC-BESS) has been proposed for energy storage requirements in high-voltage applications. The state-of-charge (SOC) equilibrium of batteries is essential for BESS to guarantee the capacity utilization. However, submodule voltage regulation can lead to over-modulation of individual submodules, which will limit the efficiency of SOC balancing control. Focusing on this problem, a modified SOC balancing control method with high efficiency is proposed in this paper. The tolerance for battery power unbalance is defined to quantize the convergence of SOC balancing control. Both the DC component and AC component are considered while regulating submodule voltage. The linear programming method is introduced to realize the maximum tolerance for battery power unbalance in different operation modes. Based on the analysis, by choosing appropriate submodule voltage regulation method, the efficiency of SOC balancing control is improved greatly. In addition, the SOC controller is also optimally designed to avoid over-modulation of submodules. Finally, the detailed simulation and experiment results verify the effectiveness of the analysis and proposed control strategy.

A modular multilevel converter with an integrated battery energy storage system (MMC-BESS) has been proposed for high-voltage applications for large-scale renewable energy resources. As capacitor voltage balance is key to the normal operation of the system, the conventional control strategy for the MMC can be significantly simplified by controlling the individual capacitor voltage through a battery side converter in the MMC-BESS. However, the control strategy of the MMC-BESS under rectifier mode operation has not yet been addressed, where the conventional control strategy cannot be directly employed due to the additional power flow of batteries. For this defect, the rectifier mode operation of the MMC-BESS based on a battery side capacitor voltage control was analyzed in this paper, proposing a control strategy for this application scenario according to the equivalent circuit of MMC-BESS, avoiding passive impact on the state-of-charge (SOC) equalization of batteries. Furthermore, the implementation of a battery side converter control is proposed by simplifying the capacitor voltage filter scheme within phase arm, which enhances its performance and facilitates the realization of control strategy. Finally, simulation and experimental results validate the feasibility and effectiveness of the proposed control strategy.

Modular multilevel converter (MMC) is regarded as one of the most potential topologies for high-voltage direct current (HVDC) transmission system. However, the distributed capacitors lead to significant submodule voltage ripples, which have great influence on the terminal characteristics of MMC. In this paper, an equivalent model considering submodule capacitor voltage ripples and circulating current suppression control (CCSC) is established to explore the terminal characteristics of MMC. In the model, MMC is equivalent to the cascade of buck/boost converter and 2-level voltage source converter (VSC) from a terminal perspective. The influence of capacitor voltage ripples is represented by the equivalent series impedances, whose characteristics are investigated in detail with harmonic linearization method. Then the simplified equivalent series impedances are obtained. Based on the equivalent model, the study of terminal characteristics is greatly simplified by directly applying conclusions of 2-level VSC and buck/boost converter to MMC. Furthermore, the control strategy design and impedance modeling of MMC have been carried out to show how the equivalent model can be used in practical applications. Finally, the effectiveness of the proposed model is verified by detailed simulation results.