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Redox flow batteries (RFBs) are one of the most promising techniques to store and convert green and renewable energy, benefiting from their advantages of high safety, flexible design and long lifespan. Membranes with fast and selective ions transport are required for the advances of RFBs. Remarkably, two‐dimensional (2D) materials with high mechanical and chemical stability, strict size exclusion and abundantly modifiable functional groups, have attracted extensive attentions in the applications of energy fields. Herein, the improvements and perspectives of 2D materials working for ionic transportation and sieving in RFBs membranes are presented. The characteristics of various materials and their advantages and disadvantages in the applications of RFBs membranes particularly are focused. This review is expected to provide a guidance for the design of membranes based on 2D materials for RFBs. [ABSTRACT FROM AUTHOR]

The regenerated LiNi0.6Co0.1Mn0.3O2 (NCM613) cathode material was synthesized from electrode scraps through separation, impurity removal, and relithiation. ICP, XRD, SEM, and electrochemical performance were used to study the composition, crystal phase structure, and surface morphology of commercial, cycled, and regenerated NCM613. After separation and impurity removal, the regenerated NCM613 was obtained by using lithium supplement and calcined at 950 °C for 11 h. At 0.1 C, the discharge capacity reaches 201.7mAh/g. After 200 cycles, the capacity is still maintained 175.6mAh/g at 1C, capacity retention is 95.1%. The results show that the regenerated NCM613 has high capacity and excellent cycle stability. This method provides a method for the recovery of spent lithium ion batteries. [ABSTRACT FROM AUTHOR]

This paper mainly aims at recycling the spent lithium-ion battery. The process is very innovative, simple, and feasible. It directly regenerates the degraded cathode material NCM613. In the air atmosphere, the low eutectic mixture LiNO3-LiOH is used as lithium salt for lithium supplements. The results of ICP, XRD, and SEM show that the components and crystal structure of the degraded cathode material can be regenerated to the pristine. The electrochemical results show that the discharge capacity can reach 175.25 mAh/g (0.1 C), and the charge–discharge coulomb efficiency is 87.8%. After 1000 cycles under the condition of 1 C, the capacity retention rate is 94.5%. Compared with the degraded cathode material, the cycling performance is greatly improved, which is similar to the Pristine NCM613 material. This research provides a unique idea for the green recycling of lithium batteries. [ABSTRACT FROM AUTHOR]

LiNi0.5Co0.2Mn0.3O2 (NCM523) has become one of the most popular cathode materials for current lithium-ion batteries due to its high-energy density and cost performance. However, the rapid capacity fading of NCM severely hinders its development and applications. Here, the single crystal NCM523 materials under different degradation states are characterized using scanning transmission electron microscopy (STEM). Then we developed a neural network model with a two-sequential attention block to recognize the crystal structure and locate defects in STEM images. The number of point defects in NCM523 is observed to experience a trend of increasing first and then decreasing in the degradation process. The space between the transition metal columns shrinks obviously, inducing dramatic capacity decay. This analysis sheds light on the defect evolution and chemical transformation correlated with layered material degradation. It also provides interesting hints for researchers to regenerate the electrochemical capacity and design better battery materials with longer life. [ABSTRACT FROM AUTHOR]

The worldwide consumption of lithium ion batteries grows year by year, and the process of recycling batteries is complex and harmful today. To solve this problem, a novel method which is green and simple is proposed. The process for recycling and regeneration from spent lithium ion batteries is acid-free, which is different from conventional chemical leaching and solid-state synthesis approach. The impacts of reaction temperature and time, solution concentration on the regeneration of the cathode materials are investigated systematically. Lots of characterizations, such as XRD, SEM, and ICP, are adopted to investigate the properties of the regenerate NCM523, which conclude it has ideal stoichiometry and phase purity; the electrochemical results certify that the regenerated NCM523 meet the commercial requirements for reuse. [ABSTRACT FROM AUTHOR]

This paper investigates an offshore wind farm based on modular dual-active-bridge (DAB) isolated DC-DC converter. The DAB converter is adopted due to its compact design and the galvanic isolation between its input and output terminals. The synchronous generator in each wind turbine is operated individually with the generator-side rectifier to maximize the power production. The output terminals of multiple DAB converters are connected in series so that the high voltage for DC transmission is achieved and the expensive offshore step-up substation is eliminated. The DC transmission voltage is excluded from the primary side of the DAB transformer by its galvanic isolation. To balance the power sharing among the DAB converters, their input terminals are connected in parallel. In addition, the droop voltage balancing control is presented for the series offshore wind farm. As a result, the input and output voltages of each DAB converter are governed by the same droop characteristic, and the DAB converters’ output voltages are kept balanced. The time-domain simulation results show that the DC voltages of each DAB converter and the high voltage direct current (HVDC) transmission system remain stable during the temporal and spatial variations in the wind power generation. [ABSTRACT FROM AUTHOR]

The traditional proportional-integral control for a modular multilevel converter (MMC) design based on a two-level voltage source converter does not consider the dynamics of arm circulating currents and sub-module capacitor voltages, which will lead to the incomplete decoupling between powers and circulating currents for an MMC. In view of this, a nonlinear decoupling control strategy, which applies for two scenarios, e.g., active/reactive power control for a power controlled station (PCS) and dc voltage/reactive power control for a dc-voltage controlled station (DVCS), is proposed for an MMC station based on feedback linearization. First, the nonlinear feedback laws for PCS and DVCS are derived based on the nonlinear equations of an MMC. Next, the complete nonlinear decoupling control strategies for PCS and DVCS are developed. Following that, the zero dynamics stabilities of the two control schemes for PCS and DVCS are investigated. In addition, the method to design parameters of the outer linear controller is also proposed to satisfy the requirement of dynamic performance. Taking a two-terminal MMC-HVDC for example, the simulation in Simulink validates the good transient performance of the proposed nonlinear control, and its robustness is also validated against the changing of system parameters. [ABSTRACT FROM AUTHOR]

The modular multilevel converter-based unified power flow controller (MMC-UPFC) is able to operate under unbalanced grid conditions with symmetric component decoupling. However, the constraint of the voltage limit of UPFC is not considered and no protection schemes are investigated to protect the UPFC from overmodulation under unbalanced grid conditions. To solve this problem, this paper proposes the cascaded control scheme for MMC-UPFC based on voltage limit control and symmetric component decoupling to balance the ac current of the transmission line. With appropriate transformer connections for MMC-UPFC, the negative- and zero-sequence currents are suppressed by the corresponding inner current loops. Considering the voltage limit of MMC, the operating ranges of UPFC-MMC under balanced and unbalanced grid conditions are investigated in both mathematical derivation and point scanning methods. The results are depicted and analyzed in 3-D vision. Based on the analysis of operating regions, the voltage limit control is proposed to protect the MMC from overmodulation and to maximize the controllable region under unbalanced grid conditions. Finally, the final cascaded control structure is constructed. The simulation results obtained in PSCAD/EMTDC are provided to validate the effectiveness and robust performance of the proposed control strategies. [ABSTRACT FROM AUTHOR]

Nowadays interconnections of non-synchronous AC grids with VSC-HVDC links become attractive in preventing cascading outages. To share the spinning reserves, the VSC-HVDC link is expected to balance the frequencies of the interconnected grids under tolerable disturbances. Based on the synchronous generator emulation control (SGEC) strategy, this paper presents a communication-free scheme for the frequency regulation of interconnected grids with a VSC-HVDC link. Under a tolerable disturbance, the scheduled powers of the rectifier and the inverter are changed to their actual powers, respectively. As a result, the grid frequencies could be balanced and the adjacent grid will provide frequency support to the disturbed grid. If the disturbance becomes larger, the scheduled powers of the two converters are changed until the maximum or minimum limits and the frequency support to the disturbed grid is limited. Thus, the large disturbance results in a deeper frequency drop in the disturbed grid, which could be arrested by its under-frequency load shedding. As a result, the disturbance propagating to the adjacent grid is suppressed, which does not jeopardize security. The frequency regulation scheme coordinates well with the automatic generation control (AGC) of the grids. Time-domain simulation studies are performed in the PSCAD/EMTDC software environment. [ABSTRACT FROM AUTHOR]

The voltage source converter (VSC) station is playing a more important role in modern power systems, but the dynamic behavior of the VSC station is quite different from that of the synchronous generator. This paper presents the synchronous generator emulation control (SGEC) strategy for the VSC-HVDC station. The SGEC strategy is divided into the inner control loop and the outer control loop. The inner controller is developed for fast current and voltage regulations. An inertia element is introduced into the frequency-power droop to determine the command reference of the frequency, and the inertia response and the primary frequency regulation are emulated. In addition, the secondary frequency regulation can be achieved by modulating the scheduled power in the SGEC strategy. The time-domain simulation results demonstrate the VSC station with the proposed control strategy can provide desired frequency support to a low-inertia grid. Therefore, the SGEC strategy provides a simple and practical solution for the VSC station to emulate the behavior of a synchronous generator. [ABSTRACT FROM PUBLISHER]

Commutation failures (CFs) in inverter stations disturb the power system greatly. Open-conductor fault (OCF) is a longitudinal fault. When it occurs in the alternating current (AC) system, the three-phase voltages are unbalanced. Under this circumstance, it is generally considered that inverters can maintain normal commutation order for the voltage support provided by other transmission lines in normal operation. In this paper, the possibility of the inverter CF during OCF is established. The sensitivities of the CF to the AC voltage disturbances caused by the changes of the AC superposition voltage are analyzed. It is concluded that the sensitivities of the CF with different transformer wiring schemes and fault types are diverse. Before the CF happens in the inverter station during OCF, the CF may occur in other inverter stations of the multi-terminal direct current systems with the constant current and constant extinction angle control strategies. Simulation studies are performed in the PSCAD/EMTDC software (Copyright(C)2006, Manitoba HVDC Research Centre, Canada), and the theoretical analysis is validated by the simulation results. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This study presents a novel protection criterion of fault locating in multi‐terminal DC (MTDC) transmission systems. When a fault occurs on the DC transmission line of the MTDC system, the traditional protection cannot locate the fault because there is no smoothing reactor (equipped at the converter output) in the transmission lines. In the circumstance of AC/DC conductor contact (ADCC) fault, the varieties of the transient voltage and current components are given based on the symmetrical fault analysis. According to the Bergeron equation, the travelling wave characteristic is analysed. A new criterion of fault locating using the change rate of the current travelling wave is presented which can reduce the impacts of the related fault on the AC system. Finally, simulation results verify the effectiveness of the proposed protection criterion. [ABSTRACT FROM AUTHOR]

SUMMARY This paper presents a new concept for the grid integration of large-scale offshore wind farms. In the offshore wind farm, the wind turbines are integrated into the offshore DC collector grid by a cascaded converter. The high-voltage level for the high-voltage direct current (HVDC) transmission purpose is built by series connections of low voltage modules, without the use of an offshore step-up substation. The output power of each module is controlled by the switching of the related half-bridge, and a modified straightforward voltage balancing strategy is proposed to balance the storage capacitor voltages in the turbines. Supplementary balancing strategies are also presented to assist in balancing the capacitor voltages, when the differences in the wind velocities flowing to the turbines become larger. Because of the stable capacitor voltages, self-commutated voltage source converter (VSC) is adopted as the onshore inverter station connecting the HVDC link and the onshore AC grid. The VSC station not only regulates the DC-bus voltage of the HVDC link, but also provides reactive power compensation to the onshore grid. The time-domain simulation results show that the proposed offshore wind-power collection and transmission system provides a new option for integrating large-scale offshore wind farms. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The modular multilevel converter (MMC) is an emerging and attractive topology for the high-voltage direct-current (HVDC) transmission system. This paper presents a generalized mathematical model for MMC in HVDC applications under balanced and unbalanced grid conditions. The dynamics of the positive-, negative-, and zero-sequence components are derived from the model. Then, a dual current control scheme with positive- and negative-sequence current controllers is applied to MMC. The power controller to eliminate negative-sequence current components and the other one to eliminate double-line-frequency voltage ripple are compared. Moreover, a zero-sequence current controller is proposed in addition to the positive- and negative-sequence current controllers. Time-domain simulations on a 61-level MMC-HVDC test system are performed in the PSCAD/EMTDC software environment. The results demonstrate that the MMC-HVDC system with or without converter transformer is able to operate under unbalanced conditions by the use of the proposed control scheme. [ABSTRACT FROM PUBLISHER]