State Of Charge Balancing Control Research Articles

State of Charge Balancing Control for Multiple Output Dynamically Adjustable Capacity System

<div>A multiple output dynamically adjustable capacity system (MODACS) is developed to provide multiple voltage output levels while supporting varying power loads by switching multiple battery strings between serial and parallel connections. Each module of the system can service either a low voltage bus by placing its strings in parallel or a high voltage bus with its strings in series. Since MODACS contains several such modules, it can produce multiple voltages simultaneously. By switching which strings and modules service the different output rails and by varying the connection strategy over time, the system can balance the states of charge (SOC) of the strings and modules. A model predictive control (MPC) algorithm is formulated to accomplish this balancing. MODACS operates in various power modes, each of which imposes unique constraints on switching between configurations. Those constraints are mathematically formalized so that MPC can be applied to minimize predicted SOC differences over a finite time horizon. In this article, several variations that vary in how freely strings can connect and disconnect from the bus bars are presented. Methods allowing more flexibility in configuration changes can balance SOCs more quickly but take more computation to resolve. In contrast, simpler schemes reduce computation and simplify implementation, but take longer to balance the SOCs. Simulation results illustrate the expected behavior.</div>

Model predictive and SoC balancing control of a CHB inverter in battery energy storage application using reward functions

SummaryThis article presents an improved model predictive current control algorithm combined with a novel state of charge (SoC) balancing approach for a three‐phase cascaded H‐bridge inverter in battery energy storage applications. Based on the model predictive control with elimination of redundant voltage vectors, neighboring vectors of the last voltage vectors are used to find the optimal ones in only one control cycle, which solves the problems of high computational burden and slow dynamic response. To overcome the unsuitability of the conventional cost function in SoC balancing, the reward functions that effectively balance the inter‐phase SoCs, inner‐phase SoCs, and inner‐phase dc voltages are presented respectively. Simulation and hardware‐in‐the‐loop‐based experimental results validate the effectiveness of the proposed strategy.

SOC Balancing Control Based on Predictive Power Model Amongst Supercapacitor Packs in MMC With Embedded Energy Storage System

Modular multilevel converter with supercapacitor (SC) packs-based energy storage system (MMC-SESS) can play a role in energy transition and renewable energy consumption. However, SESS's imbalance problem is caused by the inconsistency of its packs, which has a negative effect on the utilization of the MMC-SESS and has become a hot research topic. Confronting this problem, this paper proposes a state of charge (SOC) balancing control strategy amongst SC packs based on a discrete time-domain predictive power model in MMC-SESS based medium-voltage direct-current (MVDC). It is a double-loop control to accurately control the charging and discharging power. The outer power control establishes a discrete time-domain predictive power model for the SOC differences. The inner current loop designs a hybrid controller based on the continuous control set-model prediction control (CCS-MPC) and the feedback control strategy to accurately track the dynamic current reference. Simulations in PSCAD/EMTDC have shown that the proposed control can achieve SOC balancing amongst SC packs and improve the dynamic response as well as mitigate the SC pack current ripples.

SOC Balancing Control Method for Cascaded Energy Storage System under Extreme Working Conditions

To solve the problem of an unbalanced state of charge (SOC) between the in-phase sub-modules of the cascaded H-bridge energy storage system, this paper proposed a method based on carrier phase-shifting pulse-width modulation. Different from the traditional in-phase balancing method which is achieved by adding components, by setting the parameter related to the SOC difference to adjust the amplitude of the modulation wave, the new method could solve the problem that the traditional method is not applicable under extreme SOC conditions. The new in-phase SOC balancing method could expand the scenario, and the feasibility of the method is verified by simulation.

A Modular Active Balancing Circuit for Redox Flow Battery Applied in Energy Storage System

To improve the operation performance and energy conversion efficiency of the redox flow battery (RFB), a modular active balancing circuit for redox flow battery applied in the energy storage system is proposed in this paper. Detailed topology description, parameter design, characteristic analysis, operation principle and control strategy of the proposed circuit are presented and discussed in the paper. Different from the conventional battery balancing circuit, the key point of the proposed balancing circuit is that it integrates the circulating pump driving circuit and the state-of-charge (SOC) equalization circuit of the redox flow battery. It uses the energy consumption of the pump driving to balance the SOCs of different battery stacks, which provides a new SOC balancing mechanism for RFB. Based on the proposed circuit, an active balancing control strategy using the time-sharing energy transmission method is proposed, in which the sub-modules of the circuit are alternatively configured in charge-state and discharge-state for absorbing energy or releasing energy to achieve the SOC balancing control for RFB. Compared with the conventional balancing solution or circuit, the proposed modular active balancing circuit simplifies the complexity of the battery management system for RFB, which has the advantages of high efficiency, high reliability, and good scalability. Simulations on Matlab/Simulink and experiments on a downscaled prototype were carried out to verify the feasibility and effectiveness of the proposed circuit and control strategy.

A Decentralized SOC Balancing Method for Cascaded-Type Energy Storage Systems

As unbalance state of charge (SOC) of storage units usually leads to the decrease of lifetime, SOC balancing control is essential. In this article, a decentralized SOC balancing method is proposed to balance the SOC of cascaded-type energy storage systems. Since the method does not rely on any communication, it possesses higher reliability. As is well known, SOC is a slowly changing variable compared to other variables such as voltage and current. Thus, the studied system has obvious two-time-scale characteristic. In addition, the stability analysis of the system based on the singular perturbation theory is carried out. Simulation and experiments are performed to verify the validity of the proposed SOC balancing scheme.

Voltage-SOC balancing control scheme for series-connected lithium-ion battery packs

Abstract The basis for determining whether the cell needs to be balanced is generally the voltage or SOC (state of charge), the voltage balancing control scheme is simple but performs poorly, the SOC balancing control scheme performs well but needs to estimate the SOC of all cells. Based on the analysis why existing naive approaches are not effective for voltage balancing control scheme, the voltage-SOC balancing control scheme is proposed, which has the advantages of simple of voltage balancing control scheme and well performance of SOC balancing control scheme. By utilizing the difference of voltage of cell and SOC of battery pack, the difference of SOC between two cells can be obtained indirectly, and the transition from voltage balancing control scheme to SOC balancing control scheme can be realized, while the advantages of voltage balancing control scheme and SOC balancing control scheme are retained. The 4 experiments designed show that the estimated errors of the difference between the SOCs of the two cells are 0.24%, 5.3%, -5%, and 0.8%, respectively, which proves the excellent performance of voltage-SOC balancing control scheme.

Active SOC Balancing Control Strategy for Modular Multilevel Super Capacitor Energy Storage System

This paper presents an active state-of-charge (SOC) balancing control strategy for modular super capacitor energy storage system (ESS). The strategy has a master–slave structure, including a common dc bus voltage loop and individual slave current loop for each submodule. The common voltage loop ensures the ability of the system to stabilize dc bus voltage cannot be affected by SOC balancing control, and provides a same original current reference for all submodules. SOC information is introduced into current loops to achieve SOC balancing control. The highest SOC is taken as a reference for low SOC modules, thus, the low SOC modules can regulate the submodule average operation current according to the SOC reference and their own SOC. In order to minimize system current deviation caused by the introduction of SOC, a current correction method is proposed to improve system current regulate performance. The proposed strategy can achieve SOC balancing control during ESS charging and discharging process dynamically, in addition, the modularity of the system is improved and the proposed strategy has favorable stability. A modular ESS prototype consisting of three half-bridge submodules is tested in the laboratory, and the experimental results verify the effectiveness of the proposed control strategy.

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

Modular multilevel converter with integrated battery energy storage system (MMC-BESS) has been proposed for energy storage requirements in high-voltage applications. The state-of-charge (SOC) equilibrium of batteries is essential for BESS to guarantee the capacity utilization. However, submodule voltage regulation can lead to over-modulation of individual submodules, which will limit the efficiency of SOC balancing control. Focusing on this problem, a modified SOC balancing control method with high efficiency is proposed in this paper. The tolerance for battery power unbalance is defined to quantize the convergence of SOC balancing control. Both the DC component and AC component are considered while regulating submodule voltage. The linear programming method is introduced to realize the maximum tolerance for battery power unbalance in different operation modes. Based on the analysis, by choosing appropriate submodule voltage regulation method, the efficiency of SOC balancing control is improved greatly. In addition, the SOC controller is also optimally designed to avoid over-modulation of submodules. Finally, the detailed simulation and experiment results verify the effectiveness of the analysis and proposed control strategy.

Power conditioning system control strategy for cascaded H‐bridge converter battery energy storage system

Large capability for a cascaded H-bridge converter battery energy storage system is one of the effective tools to solve the grid-connection problem of renewable energy resource such as large-scale wind farm. The power conditioning system (PCS) control strategy is used proportional-resonant regulator to implement the control with decoupled current control for instantaneous power. The balancing control of the state of charge (SOC) is divided into individual balancing control between bridges is implemented by means of the proportional controller and cluster control between three phases is based on the zero-sequence-voltage injection. The system designed in high-power medium voltage 6.6 kV combining 30 LiFePO 4 (lithium iron phosphate) battery units with cascaded H-bridge multilevel inverter. The simulation results of SOC balancing control was carried out. As a result, a steady-state error of PCS can be smaller and three-phase output power balancing has been achieved, thus, a system simulation model has verified the effectiveness of control strategy. The waveforms of PCS in quadrant operation show voltage and current at active power 1 MW.

Hierarchical distributed control for decentralized battery energy storage system based on consensus algorithm with pinning node

A decentralized battery energy storage system (DBESS) is used for stabilizing power fluctuation in DC microgrids. Different state of charge (SoC) among various battery energy storage units (BESU) during operation will reduce batteries’ service life. A hierarchical distributed control method is proposed in this paper for SoC balancing and power control according to dispatching center requirement in DBESS. A consensus algorithm with pinning node is employed to allocate power among BESUs in the secondary control whereas in the primary control, the local controller of BESU adjusts output power according to the reference power from secondary control. Part of BESUs are selected to be pinning node for accepting command from dispatching center while other BESUs as following nodes which exchange output power and SoC information with the adjacent nodes through communication network. After calculating reference power of each BESU by adopting consensus algorithm, the power sharing in DBESS is achieved according to their respective SoC of BESUs. Meanwhile, the total output power of DBESS follows the varying requirements of dispatching center. The stability of DBESS is also improved because of having no center controller. The feasibility of the proposed control strategy is validated by simulation results.

The Study of a Battery Energy Storage System Based on the Hexagonal Modular Multilevel Direct AC/AC Converter (Hexverter)

This paper studies a battery energy storage system based on the hexagonal modular multilevel direct ac/ac converter with focus on a control method for state-of-charge (SOC) balancing of the battery units. First, the system structure and the operating principle of the hexagonal multilevel converter, including the branch voltage and the power, are analyzed and simulated, respectively. Then, the mathematical model of the energy storage submodule consisting of the battery energy storage unit and H-bridge submodule cell is developed in PSCAD/EMTDC software. After that, a SOC balancing control strategy is proposed, including the branch SOC balancing between the converter’s six branches and the individual SOC balancing between the 14 cascaded converter cells in each branch, which is realized by controlling currents in branch modules. Currents in the branches are controlled by the internal model principle. Finally, the effectiveness of the control strategy is verified in simulation.

Multiagent-Based Distributed State of Charge Balancing Control for Distributed Energy Storage Units in AC Microgrids

In this paper, a multiagent-based distributed control algorithm has been proposed to achieve state of charge (SoC) balance of distributed energy storage (DES) units in an ac microgrid. The proposal uses frequency scheduling instead of adaptive droop gain to regulate the active power. Each DES unit is taken as an agent and it schedules its own frequency reference given of the real power droop controller according to the SoC values of all other DES units. Further, to obtain the average SoC value of DES, the dynamic average consensus algorithm is utilized by each agent. A generalized small-signal model of the proposed frequency scheduling for the proposed frequency scheduling is developed in order to verify the stability of the control system and to guide control parameters design. The convergence characteristics for the dynamic consensus adopted in the multiagent system are also analyzed to choose the proper control parameter. Experimental results verified the effectiveness, the robustness against communication topology changes, and capability of “plug & play” for the proposed multiagent system through different case studies.

Inter‐module state‐of‐charge balancing and fault‐tolerant operation of cascaded H‐bridge converter using multi‐dimensional modulation for electric vehicle application

This study presents a method for module-level state-of-charge (SOC) balancing and fault-tolerant operation of the cascaded H-bridge (CHB) converter-based battery energy storage system for electric vehicle (EV) application. The proposed methods are based on the multi-dimensional pulse-width modulation (MD-PWM) strategy. It is shown that the proposed methods are conveniently integrated with the PWM generation algorithm. A conceptual design of modular battery pack (or micro-pack) system using lithium-ion cells for the CHB converter is described to show its feasibility for EV application. A brief review on the MD-PWM strategy and a detailed explanation on the method to integrate the inter-module SOC balancing and fault-tolerant control with the PWM generation are presented. Experimental results from the laboratory setup of a five-level CHB converter driving an induction motor with a battery system consisting of six 52.8 V 60 Ah (16 lithium-ion cells in series) modules are presented to verify the system operation for fault-tolerant control and the proposed SOC balancing method. The results demonstrate the need of SOC balancing, and efficacy of the proposed methods in achieving fault-tolerant as well as SOC-balanced operation to prolong the system operation.