**A Control Strategy of Modular Multilevel Converter with Integrated Battery Energy Storage System Based on Battery Side Capacitor Voltage Control**

### **Zhe Wang, Hua Lin \* and Yajun Ma**

State Key Laboratory of Advanced Electromagnetic Engineering and Technology, Huazhong University of Science and Technology, Wuhan 430074, China; zhe\_wang@hust.edu.cn (Z.W.); mayajun@mail.hust.edu.cn (Y.M.) **\*** Correspondence: lhua@mail.hust.edu.cn; Tel.: +86-135-4423-772

Received: 17 April 2019; Accepted: 3 June 2019; Published: 5 June 2019

**Abstract:** 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 ye<sup>t</sup> 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 e ffectiveness of the proposed control strategy.

**Keywords:** renewable energy; battery energy storage system (BESS); control strategy; modular multilevel converter; state-of-charge (SOC) equalization
