The recycled batteries can be assumed for the cost-effective grid energy storage (ES) applications, where the state of health (SOH) of recycled batteries are hard to make equal when reassembling them in one ES system. Therefore, when implementing the traditional state-of-charge balancing control methods, the batteries integrated into the same ES system will demonstrate the SOH deviation characteristics during the long-term operation due to the different degradation they suffered, which will reduce the mean time to repair, increase the maintenance duty, and shorten the life time of the ES system. Alternatively, this paper proposes an SOH balancing control method for the modular multilevel-converter-based battery energy storage system (MMC BESS) by fully using the unique modular configuration. The relationship among SOH, depth of discharge, and life cycles is analyzed in the theory, which builds the criteria for power distribution among battery packs in the MMC BESS. In addition, instead of estimating the specific SOH value, a relative SOH evaluation method is presented for easier practical implementation. And then, multiple control loops are designed for achieving the expected operational performance. MATLAB simulation and experimental results are presented to verify the proposed control method.

This paper presents control methods for hybrid AC/DC microgrid under islanding operation condition. The control schemes for AC sub-microgrid and DC sub-microgrid are investigated according to the power sharing requirement and operational reliability. In addition, the key control schemes of interlinking converter with DC-link capacitor or energy storage, which will devote to the proper power sharing between AC and DC sub-microgrids to maintain AC and DC side voltage stable, is reviewed. Combining the specific control methods developed for AC and DC sub-microgrids with interlinking converter, the whole hybrid AC/DC microgrid can manage the power flow transferred between sub-microgrids for improving on the operational quality and efficiency.

Owing to the operational characteristic of Vienna rectifier, it is found that the traditional modulation method would result in the current distortion. This paper therefore proposes a novel zero-sequence component injection modulation method for improving the input quality of Vienna rectifier with balanced or unbalanced dc-link voltages. In specific, the degree of unbalanced dc-link voltages increases the size of abnormal regions in slant and asymmetrical space vector diagram, which leads to more serious current distortion. The proposed method identifies the abnormal intervals by average duties accurately without detecting the position of the reference vector. Considering the operational characteristics, three-phase compensation components are calculated respectively depending on unbalanced factor and added to three-phase average duties in abnormal intervals. As a result, the current harmonics of Vienna rectifier are mitigated significantly with balance or unbalanced dc-link voltages. The effectiveness and the performance of the proposed zero-sequence component injection method for Vienna rectifier are verified by simulation and experiments.

It is preferable for the retired batteries to balance their states of health (SOH) in the battery energy storage system (BESS) since it can prolong the system lifetime and reduce the maintenance burden. So far, the corresponding balancing techniques mainly focus on either the SOH balancing among packs or the SOH balancing of cells inside a pack. This paper further proposes the multilayer SOH equalization scheme to equalize all cells' SOHs of large-scale BESS by comprehensively combining the pack SOH balancing strategy and the commercial cell equalization techniques. It is noted that the balancing schemes for pack and cell cannot be simply superimposed since packs and cells have totally different connection modes and are equipped with different types of balancing circuits. In specific, the modular multilevel converter (MMC) is assumed to coordinate the pack and cell SOH equalization schemes, where the charging/discharging power per submodule is properly adjusted according to the extent of cell SOH deviation and the extent of pack SOH deviation together. Matlab simulation and experimental results of a 10kWh MMC-BESS prototype verified the performance of the proposed method.

This paper presents a decoupled power control strategy for a modular multilevel converter (MMC)-based hybrid ac-dc grid integrated with a hybrid energy storage system (HESS). This system can mitigate the active power fluctuation caused by intermittent renewable generation and also realize reactive power compensation as required by voltage regulation. The proposed hybrid ac-dc system is novel in that batteries and supercapacitors are distributed into the upper arm and lower arm of MMC, respectively. It is therefore possible to design fully decoupled power control and simultaneously optimize the performances of batteries and supercapacitors. Firstly, the system modeling and operational principles of MMC are briefly introduced in this paper. Based on these analyses, the power decoupling between upper arm and lower arm is presented to realize different control objectives. Moreover, battery state-of-charge (SOC) balancing control and supercapacitor voltage control are presented also, which are crucial for ensuring the normal operation of these energy storage components and the proposed MMC-based hybrid ac-dc grid. Finally, the proposed system and its control strategies are verified by laboratory experimental results.

Three-level inverter has an outstanding performance and is more advantageous in the switching vector selection than two-level inverter. In particular, the neutral-point voltage unbalance and common-mode voltage (CMV) reduction of three-level inverter should be carefully regulated for the appropriate operation, both of which, however, are mutually coupled resulting that the conventional space vector modulation (SVM) scheme cannot deal with them properly. To overcome this limitation, this paper proposes an improved space vector modulation (ISVM) technique to reduce the CMV and neutral-point voltage imbalance simultaneously. The generating mechanism of neutral-point voltage oscillation is derived. Based on the analysis, the proposed ISVM method adopts four voltage vectors (large, medium, small, and zero vectors) with adjusted dwell times to eliminate the ac unbalance of the neutral-point voltage. Considering the occurrence of neutral-point voltage disturbances, the dc neutral-point unbalance voltage is controlled by selecting the P-type or N-type small vector and adjusting the dwell times of small vectors for neutral-point voltage recovery. In addition, a novel switching sequence arrangement method with the minimal number of switches transition in one switching cycles and between switching cycles is proposed to reduce the total switching loss. Theoretical analysis and verification results show that the proposed ISVM scheme can reduce the magnitude of CMV to half of value using the conventional SVM, and an accurate control of ac and dc unbalanced neutral-point voltage can be obtained.