Zhe Wang

Modular multilevel converter (MMC) is regarded as one of the most potential topologies for high‐voltage direct current transmission system. However, the multi‐frequency behaviour and complex control structure of MMC bright great challenges to the stability analysis, identification of factors affecting stability, and controller design. To address this problem, the harmonic state‐space (HSS) theory is applied to MMC in this study. First, the small‐signal HSS model is established to describe the multi‐frequency behaviour of MMC. Capacitor voltage control, circulating current control, and output current control are also taken into account in the modelling. On this basis, the stability analysis is performed by evaluating the eigenvalues of the HSS model, and participation factor is introduced to identify the factors affecting stability. In addition, the proposed approach reveals the influence of different controllers on the system stability, providing guidance for the tuning of multiple controller parameters. Finally, the effectiveness and accuracy of the proposed approach are verified by simulation and experiment results.

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.

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.

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.

Compared to the conventional half-bridge sub-module (HBSM) based modular multilevel converter (HB-MMC), the full-bridge SM (FBSM) capacitance of FB-MMC can be reduced significantly with consideration of negative voltage state of the FBSM, which has benefits to cost and size reduction. This paper analyzes the characteristics of the FBSM voltage fundamental frequency and second-order harmonic fluctuations of the FB-MMC without and with zero sequence voltage injection (ZSVI) comparatively. The analyses show that ZSVI has almost no impacts on the FBSM voltage fundamental frequency fluctuations, while the second-order harmonic components become 5/6 times of the one without. Based on these analyses, with the same fluctuation constraint as the case without ZSVI, the FBSM capacitance dimensioning method is also presented. The correctness of the presented analyses is verified by comparative simulation results on a ±200 kV FB-MMC system and experimental results on a laboratory platform. The simulation and experimental results indicate that the FBSM capacitance of the FB-MMC can achieve a reduction about 10% with ZSVI.