With the fast development of the electric vehicle industry, the reuse of second-life batteries in vehicles are becoming more attractive, however, both the state-of-charge (SOC) inconsistency and the capacity inconsistency of second-life batteries have limits in their utilization. This paper focuses on the second-life batteries applied battery energy storage system (BESS) based on modular multilevel converter (MMC). By analyzing the power flow characteristics among all sources within the MMC-BESS, a three-level SOC equilibrium control strategy aiming to battery capacity inconsistency is proposed to balance the energy of batteries, which includes SOC balance among three-phase legs, SOC balance between the upper and lower arms of each phase, and SOC balance of submodules within each arm. In battery charging and discharging control, by introducing power regulations based on battery capacity proportion of three-phase legs, capacity deviation between the upper and lower’s arm, and the capacity coefficient of the submodule into the SOC feedback control loop, SOC balance of all battery modules is accomplished, thus effectively improving the energy utilization of second-life battery energy storage system. Finally, the effectiveness and feasibility of the proposed methods are verified by results obtained from simulations and the experimental platform.

A flexible group battery energy storage system (FGBESS) based on cascaded multilevel converters is attractive for renewable power generation applications because of its high modularity and high power quality. However, reliability is one of the most important issues and the system may suffer from great financial loss after fault occurs. In this paper, based on conventional fundamental phase shift compensation and third harmonic injection, a hybrid compensation fault-tolerant method is proposed to improve the post-fault performance in the FGBESS. By adjusting initial phase offset and amplitude of injected component, the optimal third harmonic injection is generated in an asymmetric system under each faulty operation. Meanwhile, the optimal redundancy solution under each fault condition is also elaborated comprehensively with a comparison of the presented three fault-tolerant strategies. This takes full advantage of battery utilization and minimizes the loss of energy capacity. Finally, the effectiveness and feasibility of the proposed methods are verified by results obtained from simulations and a 10 kW experimental platform.