Grid-connected Battery Research Articles

Testing the Performance of the Digital Modular Protection for Grid-Connected Battery Storage Systems

This paper presents the performance evaluation of the digital modular protection for grid-connected battery storage systems (BSSs). The tested digital protection is designed using multiple digital relays that are located at different parts of the protected BSS. Each digital relay is featured with a phaselet-based fault detection to ensure accuracy and response speed. The outputs of the digital modular protection are control signals that operate circuit breakers in the charging and discharging circuits, battery units, and the point of common coupling. The tested digital modular protection is implemented for performance evaluation using a 250-kW grid-connected BSS that is charged through a $3\phi$ ac–dc power electronic converter (PEC) and discharged through a $3\phi$ dc–ac PEC. Performance results show that the digital modular protection can initiate fast, accurate, and reliable responses to faults occurring in different parts of the protected BSS. These response features have a negligible sensitivity to the type and/or location of faults, the charge/discharge mode of operation, and/or levels of power exchange with the host grid.

Combined Economic and Experimental Evaluation of Energy Storage for Grid Applications

Batteries will undoubtedly play critical roles in modernizing energy grids, allowing greater penetration of renewable generation and providing services that help grids function. Applying storage technology to grid applications is a business decision that requires potential revenues to be accurately measured to determine economic viability. Using models of storage connected to the California energy grid, we simulate grid-scale applications to estimate such revenues under current market conditions. All calculations are undertaken over two full years of operation of the storage device where a simulation of the grid-connected battery is undertaken so that control and decision are made in real time. Control and decision are made with data available at the time and mimics actual use as it might have happened. The use of a long period of data provides precise estimates as to the values of different applications of the battery. Each simulation results in duty cycles (power profiles) on which the cell-level batteries are tested. We examine the effects of these differing duty cycles on the efficiencies of batteries with various chemsitries. For the first time, we reveal critical tradeoffs between the battery chemistries and the applicability of their energy content, and show that accurate revenue measurement can only be achieved if realistic battery operation in each application is taken into account. The findings in this work could call for a paradigm shift on how the true economic values of energy storage devices could be assessed.

Improving optimal control of grid-connected lithium-ion batteries through more accurate battery and degradation modelling

The increased deployment of intermittent renewable energy generators opens up opportunities for grid-connected energy storage. Batteries offer significant flexibility but are relatively expensive at present. Battery lifetime is a key factor in the business case, and it depends on usage, but most techno-economic analyses do not account for this. For the first time, this paper quantifies the annual benefits of grid-connected batteries including realistic physical dynamics and nonlinear electrochemical degradation. Three lithium-ion battery models of increasing realism are formulated, and the predicted degradation of each is compared with a large-scale experimental degradation data set (Mat4Bat). A respective improvement in RMS capacity prediction error from 11% to 5% is found by increasing the model accuracy. The three models are then used within an optimal control algorithm to perform price arbitrage over one year, including degradation. Results show that the revenue can be increased substantially while degradation can be reduced by using more realistic models. The estimated best case profit using a sophisticated model is a 175% improvement compared with the simplest model. This illustrates that using a simplistic battery model in a techno-economic assessment of grid-connected batteries might substantially underestimate the business case and lead to erroneous conclusions.

Efficiency analysis for a grid-connected battery energy storage system

Efficiency is one of the key characteristics of grid-scale battery energy storage system (BESS) and it determines how much useful energy lost during operation. The University of Manchester has been commissioned with 240 kVA, 180 kWh lithium-ion BESS. This paper investigates round-trip efficiencies, comparing energy extracted from and returned to the grid, of the BESS for different operation modes. This is done by developing simulation switch-level model of power components of the BESS. The simulation is validated against hardware test results and used to identify optimum operation modes, which vary depending on SOC and charging and discharging power rates of the BESS.

A Comparison of Grid-Connected Battery Energy Storage System Designs

This paper presents a method for evaluating grid-connected battery energy storage system (BESS) designs. The steady-state power losses of the grid interface converter, the battery pack, and the balancing circuit are calculated. The reliability of each complete system is calculated using a Markov-based modeling approach that takes into account the built-in redundancy of the system as well as performance degradation caused by faults. Finally, a simple economic analysis based on capital cost and efficiency is used to provide a basis for direct comparison between competing system designs. Three design options for a 1-MW 1-MWh BESS connected at 11 kV are compared: a conventional BESS using parallel low voltage power blocks, a BESS using a high-voltage intelligent battery pack, and a BESS using a cascaded H-bridge converter. The results of the analysis indicate that additional power electronics included in the battery pack as part of the intelligent battery pack and H-bridge designs can enhance the reliability of the BESS by an order of magnitude under typical conditions, without increasing the overall cost of the system.

Equivalent electricity storage capacity of domestic thermostatically controlled loads

A method to quantify the equivalent storage capacity inherent the operation of thermostatically controlled loads (TCLs) is developed. Equivalent storage capacity is defined as the amount of power and electricity consumption which can be deferred or anticipated in time with respect to the baseline consumption (i.e. when no demand side event occurs) without violating temperature limits. The analysis is carried out for 4 common domestic TCLs: an electric space heating system, freezer, fridge, and electric water heater. They are simulated by applying grey-box thermal models identified from measurements. They describe the heat transfer of the considered TCLs as a function of the electric power consumption and environment conditions. To represent typical TCLs operating conditions, Monte Carlo simulations are developed, where models inputs and parameters are sampled from relevant statistical distributions. The analysis provides a way to compare flexible demand against competitive storage technologies. It is intended as a tool for system planners to assess the TCLs potential to support electrical grid operation. In the paper, a comparison of the storage capacity per unit of capital investment cost is performed considering the selected TCLs and two grid-connected battery storage systems (a 720 kVA/500 kWh lithium-ion unit and 15 kVA/120 kWh Vanadium flow redox) is performed.

Achieving the Dispatchability of Distribution Feeders Through Prosumers Data Driven Forecasting and Model Predictive Control of Electrochemical Storage

We propose and experimentally validate a control strategy to dispatch the operation of a distribution feeder interfacing heterogeneous prosumers by using a grid-connected battery energy storage system (BESS) as a controllable element coupled with a minimally invasive monitoring infrastructure. It consists in a two-stage procedure: day-ahead dispatch planning, where the feeder 5-minute average power consumption trajectory for the next day of operation (called \emph{dispatch plan}) is determined, and intra-day/real-time operation, where the mismatch with respect to the \emph{dispatch plan} is corrected by applying receding horizon model predictive control (MPC) to decide the BESS charging/discharging profile while accounting for operational constraints. The consumption forecast necessary to compute the \emph{dispatch plan} and the battery model for the MPC algorithm are built by applying adaptive data driven methodologies. The discussed control framework currently operates on a daily basis to dispatch the operation of a 20~kV feeder of the EPFL university campus using a 750~kW/500~kWh lithium titanate BESS.

Distributed Control Scheme for Package-Level State-of-Charge Balancing of Grid-Connected Battery Energy Storage System

For the battery energy storage system (BESS) consisting of multiple battery packages, package-level state-of-charge (SOC) balancing can provide safety redundancy in protecting battery packages from overcharging or overdischarging, and maintain the maximum power capacity of the overall BESS. In this paper, a distributed control scheme is proposed for package-level SOC balancing of a grid-connected BESS. The proposed control scheme comprises three parts. First, each battery package shares its SOC with its neighboring battery packages over the communication network. Based on the SOCs of its own and its neighbors, each battery package is equipped with an energy coordinator. Second, for each battery package, a distributed observer is employed to estimate the average desired power output, which is used to determine the local reference power output together with the energy coordinator. Third, local current controller is synthesized for each battery package to achieve local reference power output tracking. Comprehensive case studies are conducted to evaluate the effectiveness of the proposed control scheme.

Predictive direct power control of multilevel direct current link converter for grid connected battery energy storage systems

This paper proposes the adoption of a multilevel dc link converter for a grid connected battery energy storage system in low voltage applications. The multilevel dc link converter is the simple upgrade of a typical two level voltage source converter with the incorporation of some cascaded modular chopper cells which constitute a switched-battery circuit. With the additional chopper cells, more candidates of voltage vectors are available for switching vector selection in power control. Compared to two level voltage source converter using a look-up-table direct power control or finite control set model predictive direct power control, the multilevel dc link converter which employed the latter power control tracks the reference power with much lower power ripple, as well as results in much lower total harmonic distortion in grid current. Therefore, the power quality can be improved significantly. Simulation results are presented to verify that a 3 level dc link converter is more effective than a two level voltage source converter for the grid-connected battery energy storage systems in both generating mode and charging mode.

Battery energy storage system for primary control reserve and energy arbitrage

Abstract The transition to high penetration of renewable energy sources brings about problems related to the security and reliability of the electric power system. For this reason, EU countries are considering extending participation in the provision of ancillary services to distributed generators. Grid-connected Battery Energy Storage Systems (BESS) are a promising technology for enabling this transition. Besides the research efforts to regulate and integrate BESS into the existing power systems, several studies have introduced improvements in BESS control for ancillary services provision. In this paper, attention is focused on primary control reserve (PCR). An introduction to the suitability of using BESS for PCR is followed by a literature review on BESS control strategies and controller models. Then the paper presents a model to investigate methods to increase BESS potential in providing PCR. The model is based on two different operating options: (i) variable-droop , meaning the droop-control is allowed to vary in time in order to avoid state of charge saturations and guarantee PCR availability; (ii) energy arbitrage , meaning that the battery is charged and discharged when economically favourable. A 1MW/1MWh BESS was simulated in MATLAB ® Simulink ® by implementing the two operating options via two fuzzy logic controllers that determine the droop and the arbitrage set points. The simulations rely on real metred data inputs (i.e. frequency and electricity prices) and demonstrate that both options improve BESS operations. Specifically, a study on the Italian case was applied to evaluate the feasibility of these applications in a real life scenario.

Finite Control Set Model Predictive Control of AC/DC Matrix Converter for Grid-Connected Battery Energy Storage Application

This paper presents a finite control set model predictive control (FCS-MPC) strategy for the AC/DC matrix converter used in grid-connected battery energy storage system (BESS). First, to control the grid current properly, the DC current is also included in the cost function because of input and output direct coupling. The DC current reference is generated based on the dynamic relationship of the two currents, so the grid current gains improved transient state performance. Furthermore, the steady state error is reduced by adding a closed-loop. Second, a Luenberger observer is adopted to detect the AC input voltage instead of sensors, so the cost is reduced and the reliability can be enhanced. Third, a switching state pre-selection method that only needs to evaluate half of the active switching states is presented, with the advantages of shorter calculation time, no high dv/dt at the DC terminal, and less switching loss. The robustness under grid voltage distortion and parameter sensibility are discussed as well. Simulation and experimental results confirm the good performance of the proposed scheme for battery charging and discharging control.

Techno-economic analysis of grid-connected battery storage

This paper presents a methodology to evaluate the technical and economic performance of a grid-connected system with storage under a time-of-use (TOU) electricity tariff. The storage can help smooth demand, reducing peak demand from the grid and, in some cases, also reducing the electricity bill for the consumer. The methodology is valid for any kind of storage, but it has been used for lead-acid or Li-ion batteries, technologies that could be applied in any kind of building (residential, commercial, or industrial). This kind of system could make sense with a TOU tariff: each day, electricity would be bought during off-peak hours (at a low price) to charge the batteries, and during peak hours (at a high price), the batteries would be discharged to supply the whole load or a part of it. We focus on the storage system’s profitability for the electricity consumer, analysing the total net present cost (NPC) of a system with storage and comparing it with a system without storage. The results show that even given a Spanish TOU special for electric vehicles (with a great difference between on-peak and off-peak prices of 0.135€/kWh), at the present cost of battery storage (battery bank+bidirectional inverter+control), the storage system is not profitable for the consumer. For the battery system to be economically profitable, the costs of batteries would need to be reduced to about 0.05€/kWhcycled in the case of low-efficiency lead acid batteries (with bi–di converter of 700€/kW) or to 0.075€/kWhcycled in the case of efficient Li-ion batteries (with bi–di converter of 300€/kW). The most critical parameters are the acquisition cost of the battery bank and the number of cycles to failure, which determine the acquisition cost of the battery bank per kWh cycled.

A Synthetic Discrete Design Methodology of High-Frequency Isolated Bidirectional DC/DC Converter for Grid-Connected Battery Energy Storage System Using Advanced Components

This paper reviews recent advances of key components in isolated bidirectional dc-dc converter (IBDC) and discusses potential of IBDC based on advanced components. The concept of safe operation area of IBDC is proposed and clearly defined, which is determined by the intersection of the effective operation areas of transmission power, current stress, and current root mean square. By analyzing efficiency and power density characteristic, this paper points out that the efficiency optimization of IBDC needs to give way to the power density optimization. On this basis, a novel synthetic discrete optimization design methodology of IBDC based on advanced components for battery energy storage system (BESS) is proposed. Finally, an experimental prototype is implemented, and some design suggestions for hardware optimization are provided. The experimental results verify the effectiveness of the designed BESS and show that the practical applications of next-generation high-frequency conversion system are expected with the appearance and application of advanced components.

A FACTS Based Hybrid Filter Compensator (HFC) for H2V Battery Charging Schemes

The paper presents a robust low impact FACTS based filter compensation scheme for V2H Battery chargers to, improve the power quality, reduce total harmonic distortion, decrease AC and DC inrush currents, and ensure effective Ac and DC Common bus voltage stabilization. The Neutral point Facts Filter Compensation Schemer (NP-HFC) ensures effective decoupling of the AC-DC Sides and minimal impact of inrush currents during fast charging modes. In the same time, the novel FACTS device ensures efficient energy utilzation and improved power factor at the common AC bus. A dynamic multi regulation multi-loop error driven control strategy is developed to ensure fast charging, minimal impact on host electric grid and efficient utilization of grid-connected battery charging scheme with effective AC-DC decoupling and stabilization of the DC Common Bus Voltage. The self regulating battery charging multi- regulator control scheme has been fully validated using Matlab-Simulink Software Environment. The FACTS-based Battery Charging V2H unit is controlled using modified multi-zonal error driven control strategies for fast dynamic action and minimal stead state error to ensure improved power factor operation, reduced Total Harmonic Distortion and decoupled AC- DC Grid Operation. The battery charger has a hybrid selected Voltage-Current Regulation strategy.

Economic Operational Planning of Grid-connected Battery Energy Storage

In this paper, we develop a framework for valuing a battery energy storage system as it participates in the wholesale electricity market. We utilize an economic optimization routine with an internal battery energy storage model to plan when a battery should offer its power into the marketplace to maximize its benefit to the electric grid-and maximize its value. We examine the potential value of a vanadium redox flow battery (VRFB) used for frequency regulation service in the Texas organized electricity market from 2007-2009. Our results show that the price point where a VRFB system used for frequency regulation in Texas would be a good investment is approximately $1500/kW. We show that the expected annual revenue from frequency regulation is highly dependent on variations in wholesale prices, and that longer-duration storage could provide increased revenue from frequency regulation up to approximately 3 hours of storage capacity.