Charge Of Energy Storage System Research Articles

Convex Relaxation of Grid-Connected Energy Storage System Models With Complementarity Constraints in DC OPF

Including complementarity constraints in energy storage system (ESS) models in optimization problems ensure an optimal solution will not produce a physically unrealizable control strategy where there is simultaneous charging and discharging. However, the current approaches to impose complementarity constraints require the use of non-convex optimization methods. In this paper, we propose a convex relaxation for a common ESS model that has terms for both charging and discharging based on a penalty reformulation for use in a model predictive control (MPC) based optimal power flow (DC OPF) problem. In this approach, the complementarity constraints are omitted and a penalty term is added to the optimization objective function. For the DC OPF problem, we provide analysis for the conditions under which the convex relaxation of the complementarity constraint ensures that a solution with simultaneous ESS charging and discharging operation is suboptimal. Simulation results demonstrating ESS behavior with and without the penalty reformulation are provided for an MPC-based DC OPF problem on multiple IEEE test systems.

Adaptive Control Strategy of Energy Storage System Participating in Primary Frequency Regulation

In order to solve the capacity shortage problem in power system frequency regulation caused by large-scale integration of renewable energy, the battery energy storage-assisted frequency regulation is introduced. In this paper, an adaptive control strategy for primary frequency regulation of the energy storage system (ESS) was proposed. The control strategy combined virtual droop control, virtual inertial control, and virtual negative inertial control. The virtual inertial control was used to reduce the frequency change rate, and the virtual droop control was used to reduce the steady-state frequency deviation. The virtual droop control and the virtual inertia control were adopted in the frequency deterioration stage to slow down the frequency drop. While in the frequency recovery stage, the virtual negative inertia control worked together with the virtual droop control to accelerate the frequency recovery. Besides, the coefficients of the control methods were related to the state of charge (SOC) of ESS to avoid over-charging and over-discharging of the battery. Finally, a simulation model was built in MATLAB/SIMULINK, and case studies were conducted to verify the proposed control strategy. Results showed that the proposed method could meet the demand for frequency regulation and was beneficial to the cycle life of ESS.

Control of battery charging based on reinforcement learning and long short-term memory networks

In an electricity market with time-varying pricing, uncontrolled charging of energy storage systems (ESSs) may increase charging costs. A novel battery charging control methodology based on reinforcement-learning (RL) is proposed in this paper to minimize the charging costs. A significant characteristic of this method is that it is model-free, with no need for a high-accuracy battery/ESS model. Therefore, it overcomes the challenges brought by limited types of battery models and non-ignorable parametric uncertainties in reality. Additionally, since an accurate prediction of fluctuating electricity prices can promote the control performance, a long short-term memory (LSTM) network is leveraged to improve the prediction precision. The final control objective is to seek an optimal charging portfolio to minimize charging costs. Moreover, the presented control algorithm provides a basic framework for a more complicated electricity market where various types of ESSs, generators, and loads exist.

Stabilization of DC Nanogrids Based on Non-Integer General Type-II Fuzzy System

This brief proposes a dynamic consensus algorithm-based nonlinear I-V droop control for the balancing state of charge of energy storage systems (ESSs) in DC Nanogrids (DCNGs). The dynamic consensus algorithm (DCA) provides a coordinated secondary control with sharing information between Distributed Generation (DG) units to regulate the output power of each DG based on the capacity and state of charge (SoC) of the ESSs. Furthermore, a novel high bandwidth fractional order general type-2 fuzzy logic proportional-integral-derivative (FOGT2FPID) controller is applied in the secondary control level to ensure fast and accurate voltage regulation, and SoC balancing in the DCNG. In the primary control level, a nonlinear I-V droop control approach provides fast dynamic and accurate power-sharing among DGs. Moreover, the proposed control method can provide reliability, modularity, and flexibility. Compared with conventional methods, over-current failures and abruptly disconnection of DGs are prevented with the proposed controller. Experimental results are shown to verify the effectiveness of the proposed control scheme under different scenarios using facilities at the Microgrid Laboratory, Aalborg University.

Enhanced Low-Voltage Ride-Through Coordinated Control for PMSG Wind Turbines and Energy Storage Systems Considering Pitch and Inertia Response

Low-voltage ride-through (LVRT) requirements are defined by grid operators, and they vary based on power system characteristics. Coordinated LVRT control methods have been proposed for wind turbines (WTs) and energy storage systems (ESSs). ESSs can successfully help achieve LVRT by regulating DC-link voltage during a grid fault. During LVRT, WTs cannot transfer power to a grid because of their low voltage and current limit. Moreover, as grid operators typically require reactive power support during a grid fault, active power cannot be properly transferred to the grid. This results in fluctuation in the DC-link voltage in wind power generators and it can induce significant damage in the systems. ESS have been used for achieving better LVRT response to protect WT systems and meet LVRT grid requirements. Previous coordinated control methods have mainly focused on DC-link voltage regulation based on ESS charging and discharging control. As ESSs have high installation cost and limited charging capacity, it is better to coordinate the LVRT response properly considering the state of charge of ESSs and the rotor speed and pitch angle of WTs. In this work, an enhanced coordinated LVRT control method is proposed based on a fuzzy-logic algorithm. Fixed torque of the rotor control and a fixed ramp rate of the pitch control are employed for power analysis which is used in formulating fuzzy-logic controller. The effectiveness of the proposed method is validated by modeling a WT and an ESS topologically and performing simulations using the MATLAB/Simulink SimPowerSystems toolbox.

Optimal planning of energy storage systems in active distribution networks considering multiple operation optimisation measures

An original three-layer planning model of energy storage systems (ESSs) in active distribution networks is proposed in this study, taking demand response (DR) and network reconfiguration (NR) into account. In order to handle the uncertainties of distributed generations and loads, a practical scenario-generation method based on seasonal characteristics is put forward. Based on the time of use price, an ESSs charging and discharging strategy is presented. The mixed-integer nonlinear programming problem is converted to a mixed-integer second-order cone (SOCP) and the SOCP and an improved hybrid intelligent algorithm are applied to solve the ESSs planning model. Finally, the feasibility and effectiveness of the proposed model and solution methods are proven through the improved IEEE 33-bus system. The case study demonstrates the positive effect of DR and NR on optimising system operation and reducing total cost.

Practical Power Management of PV/ESS Integrated System

This study proposes the practical power management of integrated system with photovoltaic (PV) and energy storage system (ESS) to solve the line congestion and voltage problems in a distribution system while considering the electricity price and state of charge (SOC) of ESS. In addition, the proper power management of PV/ESS integrated system (PEIS) enables to increase the hosting capacity of renewable energies in a distribution system and to maximize the profit of independent power producer (IPP). In this study, the real power management of PEIS of 100 kW is implemented in practice and tested with the actual measurements for 3 days. The results show that the proposed method deals with the line congestion problem effectively and efficiently while increasing the profit of IPP by 11%. In particular, the reactive power control based on the international electrotechnical commission (IEC) standard, IEC 61850-90-7 is applied to the PEIS, and it can successfully mitigate the variations of voltage in a distribution system.

An optimal scheduling model of an energy storage system with a photovoltaic system in residential buildings considering the economic and environmental aspects

In terms of sustainable development, the Energy Storage System (ESS) with a photovoltaic (PV) is an efficient technology for residential distributed generation to raise its economic and environmental benefits, which are differentiated by charge and discharge scheduling of ESS. Accordingly, it is essential to optimize the scheduling of ESS with PV to maximize economic and environmental benefits for residential buildings. Therefore, this study developed an optimal scheduling model of ESS with PV in residential buildings considering the economic and environmental aspects. The optimal scheduling model was developed in the following two steps: (i) definition of the variables; and (ii) development of the optimal scheduling model of ESS with PV. Despite the rule-based scheduling could only raise lifecycle environmental benefit by sacrificing economic benefit, the optimal scheduling could raise both lifecycle economic and environmental benefits increasing the net income and self-sufficiency rate by up to US$2.8 per month and 2.7%, compared to the case of applying the PV system without ESS. This study has significant contributions in facilitating the wide use of ESS with PV by not only improving its economic and environmental benefits for the ESS owner but also reducing the cost for constructing additional power plants for the government.

Refined ramp event characterisation for wind power ramp control using energy storage system

With the advantages of fast response and bidirectional charge/discharge, an energy storage system (ESS) plays a promising role in wind power ramp control. In this study, an optimisation model based on refined ramp event characterisation is proposed to achieve continuous wind power ramp control using ESS. Firstly, four kinds of ramp scenarios are characterised considering both the wind power ramp event prediction and the charge/discharge state of ESS. State of charge of ESS is managed within its limits during ramp control, based on the classified ramp scenarios. Secondly, for the classified ramp scenarios, an active adjustment strategy is proposed to decide the expected charging/discharging energy of ESS according to the conditions of wind power and ESS. Thus, an appropriate energy storage reserve can be determined for anticipated ramp events. Refined ramp event characterisation is able to achieve better control performance with higher satisfaction of ramp requirement, less wind energy curtailment as well as promising adaptability to different ramp event predictions, wind conditions and changes of ESS parameters. The effectiveness of the proposed method is verified through case studies with real-world data from a 100 MW wind farm in China.

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.

Time Transient Electrochemical Monitoring of Tetraalkylammonium Polybromide Solid Particle Formation: Observation of Ionic Liquid-to-Solid Transitions.

Energy storage systems (ESSs) using a Br-/Br2 redox reaction such as a Zn/Br redox flow battery (RFB) or a redox-enhanced electrochemical capacitor (Redox-EC) suffer from self-discharge reactions resulting in significant Coulombic loss. To inhibit the self-discharge, quaternary ammonium (Q+) and tetraalkylammonium (T+) bromide are added to form ionic liquid (QBr2 n+1) and solid (TBr3) polybromides during the ESS charging process. The electrochemical formation of liquid QBr2 n+1 and its electrochemical properties have been examined. The detailed mechanisms of ionic solid TBr3 formation, however, have not yet been explored. In this article, we analyzed the ionic liquid-to-solid phase transition of TBr3 particles using a time transient electrochemical method. We suggest the formation of ionic solid TBr3 particles via hydrated TBr3 droplets as an intermediate phase, which are generated by electro-oxidation of Br- in an aqueous TBr solution. We found the phase transition time of TBr3 particles is strongly dependent on the chemical structure of T+ and the concentration of TBr in an aqueous solution.

Impact of Electric Vehicles on the Expansion Planning of Distribution Systems Considering Renewable Energy, Storage, and Charging Stations

Energy storage systems (ESS) have adopted a new role with the increasing penetration of electric vehicles (EV) and renewable energy sources (RES). EV introduce new charging demands that change the traditional demand profiles and RES are characterized by their high variability. This paper presents a new multistage distribution expansion planning model where investments in distribution network assets, RES, ESS, and EV charging stations are jointly considered. The charging demand necessary for EV transportation is performed using a vehicle model based on travel patterns. The variability associated with RES along with the demand requires the incorporation of uncertainty, which is characterized through a set of scenarios. These scenarios are generated by the ${k}$ -means++ clustering technique that allows keeping the correlation in the information of the uncertainty sources. The resulting stochastic program is driven by the minimization of the present value of the total expected cost including investment, maintenance, production, losses, and non-supplied energy. The associated scenario-based deterministic equivalent is formulated as a mixed-integer linear program, which can be solved by commercial software. Numerical results are presented for an illustrative 54-node test system.

Coordinated charging and discharging strategies for plug‐in electric bus fast charging station with energy storage system

Plug-in electric bus (PEB) is an environmentally friendly mode of public transportation and PEB fast charging stations (PEBFCSs) play an essential role in the operation of PEBs. Under effective control, deploying an energy storage system (ESS) within a PEBFCS can reduce the peak charging loads and the electricity purchase costs. To deal with the (integrated) scheduling problem of (PEBs charging and) ESS charging and discharging, in this study, the authors propose an optimal real-time coordinated charging and discharging strategy for a PEBFCS with ESS to achieve maximum economic benefits. According to whether the PEB charging loads are controllable, the corresponding mathematical models are, respectively, established under two scenarios, i.e. coordinated PEB charging scenario and uncoordinated PEB charging scenario. The price and lifespan of ESS, the capacity charge of PEBFCS and the electricity price arbitrage are considered in the models. Further, under the coordinated PEB charging scenario, a heuristics-based method is developed to get the approximately optimal strategy with computation efficiency dramatically enhanced. Finally, the authors validate the effectiveness of the proposed strategies, interpret the effect of ESS prices on the usage of ESS and provide the sensitivity analysis of ESS capacity through the case studies.

Dynamic simulation of two-tank indirect thermal energy storage system with molten salt

Thermal energy storage system, which can effectively store solar energy and make a solar power plant generate electricity in cloudy or rainy weather and nighttime, is a key part of a concentrating solar power plant, which makes solar power technology have unique advantages compared with other renewable energy power technology. Two-tank indirect thermal energy storage system with molten salt is most widely used and has been successfully commercialized in the field of solar power. In this passage, a universal dynamic simulation model of two-tank indirect thermal energy storage system with molten salt used for trough solar power plants based on the lumped parameter method is built, and the dynamic processes of thermal energy storage system charge and discharge, and the changes of heat transfer oil outlet temperature in the oil/salt heat exchanger, molten salt temperature and height in the molten salt tank when heat transfer oil mass flow rate has step disturbances are simulated on the STAR-90 simulation platform. The simulation results show that the model can simulate the charge and discharge dynamic characteristics of two-tank indirect thermal energy storage system with molten salt well, which can be good references for the system design, system control and system debugging.

A Superconducting Magnetic Energy Storage-Emulator/Battery Supported Dynamic Voltage Restorer

This study examines the use of superconducting magnetic and battery hybrid energy storage to compensate grid voltage fluctuations. The superconducting magnetic energy storage system (SMES) has been emulated by a high-current inductor to investigate a system employing both SMES and battery energy storage experimentally. The design of the laboratory prototype is described in detail, which consists of a series-connected three phase voltage source inverter used to regulate ac voltage, and two bidirectional dc/dc converters used to control energy storage system charge and discharge. “DC bus level signaling” and “voltage droop control” have been used to automatically control power from the magnetic energy storage system during short-duration, high-power voltage sags, while the battery is used to provide power during longer term, low-power undervoltages. Energy storage system hybridization is shown to be advantageous by reducing battery peak power demand compared with a battery-only system, and by improving long-term voltage support capability compared with an SMES-only system. Consequently, the SMES/battery hybrid dynamic voltage restorer can support both short-term high-power voltage sags and long-term undervoltages with significantly reduced superconducting material cost compared with an SMES-based system.

Expansion Planning of Active Distribution Networks With Centralized and Distributed Energy Storage Systems

A multistage active distribution network (ADN) planning model that is integrated with the application of energy storage system (ESS) is presented in this paper. Both the long-term investment cost and short-term operation conditions of ADN are considered in the proposed model. The power supply reliability improvement brought by ESS is also analyzed. At each planning stage, the operation conditions are divided into several typical day scenarios and an extreme condition scenario which are based on the load forecast data. The long-term expansion planning decisions including those for replacing and adding circuits, introducing ESS, and considering ADN short-term operation strategies for the ESS charging and discharging are optimized together in the proposed model. The ESS benefits pertaining to peak load shaving and power reliability enhancement are demonstrated using numerical cases, in which the centralized and the distributed ESS are considered as options for the ADN implementation. The effectiveness of the proposed model is demonstrated through various discussions in the paper.

Cooperative Controls for Pulsed Power Load Accommodation in a Shipboard Power System

To isolate the negative impacts of pulsed power loads (PPL) in a shipboard power system (SPS), energy storage systems (ESS) are usually equipped for online ESS charging and offline ESS discharging for PPL deployment. In order to achieve fast and smooth ESS charging, the generation control and ESS charging control need to be well coordinated. In addition, control constraints, such as generation and charging current bounds and ramp rates, and changes of operating conditions during charging, should also be properly considered. To better solve this important but insufficiently investigated problem, two control algorithms are presented in this paper. The first PI-based control algorithm is designed based on analysis of the desired performance and rules for coordination. The second feedback linearization based control algorithm is designed based on a simplified SPS model and ramp rates difference of supply and demand. The algorithms are simple to implement and can effectively reduce the negative impacts during supercapacitor charging. The algorithms are tested with detailed single-generator and multiple-generator SPS models. Power hardware-in-the-loop and real-time digital simulation studies are performed to demonstrate the effectiveness of the proposed algorithms.

Indirect Load Control for Energy Storage Systems Using Incentive Pricing under Time-of-Use Tariff

Indirect load control (ILC) is a method by which the customer determines load reduction of electricity by using a price signal. One of the ILCs is a time-of-use (TOU) tariff, which is the most commonly used time-varying retail pricing. Under the TOU tariff, the customer can reduce the energy cost through an energy storage system (ESS). However, because this tariff is fixed for several months, the ESS operation does not truly reflect the wholesale market price, which could widely fluctuate. To overcome this limitation, this paper proposes an incentive pricing method in which the load-serving entity (LSE) gives the incentive pricing signal to the customers with ESSs. Because the ESS charging schedule is determined by the customer through ILC, a bilevel optimization problem that includes the customer optimization problem is utilized to determine the incentive pricing signal. Further, the bilevel optimization problem is reformulated into a one-level problem to be solved by an interior point method. In the proposed incentive scheme: (1) the social welfare increases and (2) the increased social welfare can be equitably divided between the LSE and the customer; and (3) the proposed incentive scheme leads the customer to voluntarily follow the pricing signal.

EV Charging Facilities and Their Application in LV Feeders With Photovoltaics

Low-voltage (LV) grid feeders with high penetration of photovoltaics (PVs) are often affected by voltage magnitude problems. To solve such issues, previous research has shown that reactive power methods, active power curtailment and grid reinforcement can be used for voltage support, yet showing several limits. We introduce the use of electric vehicle (EV) public charging stations with energy storage system (ESS) as a solution for voltage regulation in LV feeders with PV. A novel method is proposed to determine the ESS charging load required for voltage regulation and compare the results for the different locations in the feeder. With time-series simulations, we quantify the energy size required for a station ESS. A Belgian LV residential grid, modeled using real PV generation and load profiles, is used as case study. The method and simulation results show the effectiveness of using public EV charging facilities with the additional function of voltage regulation in feeders with PV.