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.

Abstract:As a critical subsystem in electric vehicles and smart grids, a battery energy storage system plays an essential role in enhancement of reliable operation and system performance. In such applications, a battery energy storage system is required to provide high energy utilization efficiency, as well as reliability. However, capacity inconsistency of batteries affects energy utilization efficiency dramatically; and the situation becomes more severe after hundreds of cycles because battery capacities change randomly due to non-uniform aging. Capacity mismatch can be solved by decomposing a cluster of batteries in series into several low voltage battery packs. This paper introduces a new analysis method to optimize energy utilization efficiency by finding the best number of batteries in a pack, based on capacity distribution, order statistics, central limit theorem, and converter efficiency. Considering both battery energy utilization and power electronics efficiency, it establishes that there is a maximum energy utilization efficiency under a given capacity distribution among a certain number of batteries, which provides a basic analysis for system-level optimization of a battery system throughout its life cycle. Quantitative analysis results based on aging data are illustrated, and a prototype of flexible energy storage systems is built to verify this analysis.

No abstract

No abstract

No abstract

This paper is mainly about the wind energy conversion system with a rated output of 1.2MIW which applied to the variable speed constant frequency wind energy system (VSCF). It utilizes the AC-DC-AC power converter to deliver the high-level power energy from permanent-magnet generators to the grid. As the important section of DC energy delivering link in the system, a multiple boost converter controlled by DSP with average current-control strategy is proposed and specifically analyzed in this paper. As the voltage and frequency of turbine generator's output vary along the wind speed change, the multiple boost converter is utilized to maintain constant DC link voltage level high enough for the PWM inverter to transmit energy to the network. The experimental results verify the feasibility of the multiple boost converter used in the wind energy conversion system and confirm the analysis we discussed.

Abstract: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.

The output power of a photovoltaic (PV) array is affected by insolation and temperature. It is important to improve the output efficiency of a PV array as the output power of the PV array is affected by intensity illumination and temperature. The simulation model of PV module in this paper based on Simulink is built in accordance with PV module's physical and mathematical model. The Simulink model can be used to simulate output characteristics of the PV module under different insolation and temperature．Meanwhile，this model can be applied to cases of other powers and used to study module's series and parallel characteristics. The simulation results show the characteristics of series and parallel PV array under non-uniform insolation and temperature conditions. The analysis will help to design optimum configuration of PV array as well as choose suitable MPPT methods.