State-of-Charge Balancing Control of a Modular Multilevel Converter with an Integrated Battery Energy Storage

Abstract

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

“…Since E b_kji may be different in individual SM due to the age of battery, the SOC equalization should take the capacity of individual battery into consideration. According to the research in Reference [15], battery capacity should be employed to produce the coefficient for corresponding SOC equalization control. For the sake of simplicity, it is assumed that all batteries are of the same capacity, which would not affect the following investigations in this paper.…”

“…In Reference [11], the control strategy of a MMC-BESS operating as an inverter was researched, proposing the three-level SOC equalization control structure. In Reference [15], the SOC equalization Due to the characteristics of randomness and intermittency of renewable energy resources, the normal operation and power quality of the power system would be remarkably affected, reducing the voltage and frequency stability. To attenuate the passive impact caused by renewable energy resources, a battery energy storage system (BESS) is a reasonable and efficient solution for grid-connected renewable energy generation systems, as shown in Figure 1a,b.…”

“…In Reference [11], the control strategy of a MMC-BESS operating as an inverter was researched, proposing the three-level SOC equalization control structure. In Reference [15], the SOC equalization of MMC-BESS considering the capacity inconsistency of batteries was proposed. In References [10,16], the MMC-BESS was applied in battery electric vehicles and the efficiency of this topology was assessed.…”

Modified State-of-Charge Balancing Control of Modular Multilevel Converter with Integrated Battery Energy Storage System

“…In Figure 3, the battery power injected into submodules is controlled by regulating the submodule voltage. As an example, with the method in [20,25], the submodule voltage is calculated as…”

“…In [24], the DC component is regulated to balance SOC in AC-side fault mode and the AC component is regulated to balance SOC in DC-side fault mode. In [25], a power factor is introduced to regulate both the DC component and AC components. However, the problem is that the regulation of submodule voltage can lead to over-modulation of submodules when the battery power unbalance exceeds certain limit.…”