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This paper proposes an improved SOC balancing strategy for the modular energy storage system (ESS) based on low bandwidth communication (LBC) technology, aiming at solving the drawbacks of the conventional balancing strategy based on droop control. The proposed control scheme can improve the SOC balancing efficiency and remove the current deviation by using the SOC slope-adjusting and current-shifting approaches simultaneously. The average value of SOC can be derived through the LBC line, and the droop coefficient based on SOC can be controlled in the local control layer. Thus, the SOC of each sub-module can converge to the average SOC value in system dynamic process, and the balancing speed under the condition of low SOC difference between sub-modules can be improved. Furthermore, the current regulation performance in steady and transient states can be enhanced by using the proposed method. At last the effectiveness of the proposed strategy is verified by experimental results. [ABSTRACT FROM AUTHOR]

This letter presents a fast open circuit diagnostic method for modular multilevel DC/DC converter (MMDDC), which is applied in super capacitor energy storage system. The proposed method is derived based on the analysis of submodule output characteristics under both normal and fault conditions. For MMDDC, different types of open circuit fault are reflected by the output voltage of submodule in certain operating modes, thus a simple and effective diagnostic method by the aid of a hardware circuit and a fast fault location algorithm is proposed. The hardware circuit is responsible for detecting submodule output voltage and its output result is transmitted to a processor to accomplish the final fault diagnosis along with drive signals. The proposed approach avoids complex mathematical operations and minimizes fault diagnostic time; moreover, it can achieve fault diagnosis under a wide super capacitor operating voltage range. Finally, the proposed diagnostic method is verified experimentally based on a laboratory-scale MMDDC energy storage prototype. [ABSTRACT FROM AUTHOR]