Bidirectional Charger Research Articles

An Integrated DC Microgrid Solution for Electric Vehicle Fleet Management

The coordinated development of charging points reveals to be crucial in supporting the envisaged diffusion of electric vehicles, along with their proper design to ensure smart and fast charging performances and economic advantages for drivers. The integration of charging points with local generation and storage can enhance this process. In this paper, the structure of a modular electric vehicle supply infrastructure is proposed. It is based on the integration of photovoltaic rooftops, energy storage and electric vehicle stations in a dc microgrid, enabling coordination and control as well as the functions of bidirectional charging and the possibility to provide grid services at connection point. The main features of the technical demonstrator are described, along with the targeted innovations for system manager and final user, and the possible control functions involving operation planning, real-time control, and ancillary service market inclusion.

A Phase Synchronization Technique Based on Perturbation and Observation for Bidirectional Wireless Power Transfer System

For the bidirectional wireless electric vehicle charging system, when both the primary and secondary converters are active, the phase synchronization between the two converters are necessary to control the power flow direction. This article proposes a new phase synchronization method by tracking the maximum (or minimum, depending on the power flow direction) value of the output current, without auxiliary hardware or the real-time communication between the primary and secondary sides. First, the relationship between the output current and the phase difference of the control signals is derived. It is found that the maximum output current is determined by the relative phase-shift angle (time interval between the middle points of the primary and secondary voltages) and the internal phase-shift angles of the two converters (time interval when the output voltage equals to the dc-link voltage). Second, a new scheme for generating the control signals is proposed to ensure that the relative phase-shift angle is not influenced by the internal phase-shift angles. Third, based on the perturbation and observation method, a procedure is proposed to track the extreme value of the output current and obtain the target relative phase-shift angle for the phase synchronization state. Moreover, the relationship between the internal phase-shift angle and the transfer power is derived to regulate the magnitude of the transfer power, with the dead-time effect taken into consideration. Finally, experimental results verify the validity of the proposed method with different air gaps and power levels. The method is easy to be implemented, helpful for the optimal operation of the static and quasi-static wireless charging system and suitable for the bidirectional power flow.

Zero-Phase-Angle Controlled Bidirectional Wireless EV Charging Systems for Large Coil Misalignments

Misalignment between charging coils is an unavoidable problem in all electric vehicle (EV) charging systems that are based on wireless power transfer (WPT) technology. This is because variations in both self- and mutual inductances of charging coils due to coil misalignments significantly affect the charging rate and efficiency of the EV charger. As a solution, this article proposes a new control technique that maintains the rated charging power level at optimum efficiency in spite of large coil misalignments. To compensate for the variations in inductances, the technique uses the information extracted from input impedance to operate the system essentially at unique converter voltages with zero phase angle, ensuring that charging will take place at rated power with optimal efficiency despite any pad misalignments. The article presents a comprehensive mathematical model, describing the theoretical basis of the proposed control philosophy. To demonstrate the validity of the proposed control concept, a 1-kW bidirectional WPT EV charging system is built and results are presented under misaligned coil conditions. Results convincingly indicate that the proposed controller is efficient and capable of maintaining the rated charging rate despite large coil misalignments and without any dedicated wireless communication.

Expert perceptions of enhancing grid resilience with electric vehicles in the United States

Electricity and transportation systems in industrialized countries are undergoing transformations that, if coordinated, could improve the resilience and environmental performance of energy systems. The electrification of transportation and the expansion of renewable electricity can be leveraged by the bidirectional smart charging of electric vehicles, called “vehicle-grid integration” (VGI). Studies to date have focused on simulations and pilot studies to estimate the technical potential of VGI. We survey members of the U.S. Department of Energy’s Electricity Advisory Committee using a Delphi approach to assess VGI’s market potential and challenges. Building on the tradition of managing real-time demand with energy efficiency and load control, we extend the concept to consider grid resilience services with VGI. The survey results are examined in tandem with a targeted review of the literature, current policies, and the ecosystem of stakeholders. We find that experts rate grid-to-vehicle integration as the most valuable mode, and they rate battery degradation and warranty issues as the most important integration challenges. Our expert respondents also noted the need to create markets for ancillary grid services in vertically integrated utility systems. Managing electricity and transportation as complementary systems could help to address the growing need for grid resilience and carbon mitigation.

A Bidirectional Power Charging Control Strategy for Plug-in Hybrid Electric Vehicles

Plug-in Hybrid Electric Vehicles (PHEVs) have the potential of providing frequency regulation due to the adjustment of power charging. Based on the stochastic nature of the daily mileage and the arrival and departure time of Electric Vehicles (EVs), a precise bidirectional charging control strategy of plug-in hybrid electric vehicles by considering the State of Charge (SoC) of the batteries and simultaneous voltage and frequency regulation is presented in this paper. The proposed strategy can control the batteries charge which are connected to the grid, and simultaneously regulate the voltage and frequency of the power grid during the charging time based on the available power when different events occur over a 24-h period. The simulation results prove the validity of the proposed control strategy in coordinating plug-in hybrid electric vehicles aggregations and its significant contribution to the peak reduction, as well as power quality improvement. The case study in this paper consists of detailed models of Distributed Energy Resources (DERs), diesel generator and wind farm, a generic aggregation of EVs with various charging profiles, and different loads. The test system is simulated and analyzed in MATLAB/SIMULINK software.

Power Conditioning of Distribution Networks via Single-Phase Electric Vehicles Equipped

This paper presents the design of a single-phase electric vehicle (EV) on-board bidirectional charger with the capability of power conditioning. This charger can control its charging/discharging active power based on the demand of EV battery/network or load. Also, it controls reactive power and harmonic current based on the characteristics of the nonlinear and linear loads. The topology of the proposed charger consists of the bidirectional ac/dc and buck–boost dc/dc converters, where it can operate in four quadrants in the active-reactive power plane with the capability of harmonic compensation. In the next step, this paper presents a suitable control strategy for the bidirectional charger according to the instantaneous active and reactive power (PQ) theory. Based on the PQ theory, the active and reactive power that includes average and oscillatory components obtained, based on the demand of nonlinear/linear loads and EV battery. Then, the reference current of ac/dc converter of the charger and battery is obtained, and in the next step, the situation of the charger switches is determined using output signals of the proportional–integral and proportional–resonant controllers and pulsewidth modulation. Finally, the proposed approach is validated and implemented in the OPAL-RT to integrate the fidelity of the physical simulation and the flexibility of the numerical simulations.

Multi-Objective Reconfigurable Three-Phase Off-Board Charger for EV

This paper deals with the control and implementation of solar photovoltaic (PV) array-based multi-objective bi-directional charger beneficial for electric vehicle (EV), household load, and utility. For EV, the charger provides the charging facility. For household load, the charger acts as a standalone inverter, and for utility the charger acts as an active power filter. However, for providing the multi-functionality and the multi-mode operation capability of the charger, a reconfigurable control strategy is proposed. Moreover, an adaptive dc-link voltage strategy is proposed for achieving the optimum voltage at dc link under all voltage conditions for achieving minimum ripple in grid currents. The charger also uses a voltage synchronization strategy for achieving the seamless mode transition between the grid-connected and standalone modes. In grid-connected mode, the charger uses a double second-order generalized integrator-based positive sequence estimator along with the improved linear sinusoidal tracer-based algorithm for generating the balanced and sinusoidal reference grid currents, under the unbalanced and distorted grid voltages. The charger is implemented in the laboratory, and the performance is presented under various steady-state and dynamic conditions.

The Influence of Path Dependence on the Degradation of Lithium-Ion Batteries in Electric Vehicles

Gaining understanding into battery ageing is essential, especially for electric vehicles (EVs) where the reliability of the vehicle is dependent on battery performance, capacity and lifetime. Studying the causes of degradation, the conditions that accelerate degradation and the end of life criteria of batteries under realistic EV conditions enables us to make accurate predictions of the lifetime of a cell. During usage, the battery will experience ageing caused by multiple factors, but broadly, these can be grouped into two ageing types: calendar ageing when the cell is at rest and cyclic ageing when it is in use. Degradation path dependency is the impact on degradation caused by the order in which a cell is exposed to different ageing conditions. The order in which the cell experiences each type of ageing can affect the ageing of a cell by influencing the interactions between degradation mechanisms. Studying path dependency allows us to determine how the interactions between these degradation mechanisms impacts state of health [1]. An experimental study is being conducted which exposes groups of lithium-ion NCA 18650 cells to various blended load profiles consisting of calendar and cyclic ageing in various orders. The ratio of calendar and cyclic ageing in the profiles is constant through the tests to allow comparability between the degradation data collected. The profiles for groups 1 and 3 are designed to expose the cells to 1 day of cyclic ageing followed by 5 days of calendar ageing at C/2 and C/4 respectively. The cells in groups 2 and 4 experience 2 days of cyclic ageing and 10 days of calendar ageing at C/2 and C/4 respectively. The cyclic ageing portion of the profile is conducted using constant current charge/discharge and the calendar ageing portion is conducted at 90% state of charge (SoC). The stress factors that the cell is exposed to are controlled in terms of environmental temperature, depth of discharge, SoC and C-rate. This aims to ensure that the data obtained from this study are an accurate representation of the degradation effects due to path dependency only. Specifically, two groups of cells that are exposed to the same calendar/cyclic ageing in different orders are shown in the supporting image. Although group 1 and group 3 experience the same amount of ageing, the order in which the cell experiences the ageing leads to a difference in state of health in terms of capacity and resistance. The current lifetime estimation models superimpose calendar and cyclic aging however the early indications from the results obtained suggests that path dependency does exist, meaning that existing empirical ageing models may need to be reconsidered or modified in order to account for coupling between different ageing mechanisms [2]. Image Caption: Normalised capacity graphs highlighting the impact of path dependency on capacity fade rate between two groups of cells that are exposed to the same aging conditions in different orders. [1] M. Dubarry, A. Devie, and K. McKenzie, “Durability and reliability of electric vehicle batteries under electric utility grid operations: Bidirectional charging impact analysis,” J. Power Sources, vol. 358, pp. 39–49, 2017. [2] M. Schimpe, M. E. von Kuepach, M. Naumann, H. C. Hesse, K. Smith, and A. Jossen, “Comprehensive Modeling of Temperature-Dependent Degradation Mechanisms in Lithium Iron Phosphate Batteries,” ECS Trans., vol. 80, no. 13, pp. 147–170, 2017. Figure 1

A Vehicle-to-grid Framework Based on Electric Vehicle with Solar Panel

In this paper, we extend previous studies of vehicle-to-grid technology with electric vehicle equipped with solar panel. Instead of connecting a solar power system into the grid, we consider a distributed scenario in which every electric vehicle has the power of collecting solar power and bidirectional charging abilities. Through mathematical formulation and numerical simulations, we demonstrate that our framework has superior advantages of incentivizing electric vehicles to install solar panels and discharge to the grid.

Potential Benefits of Vehicle-to-Grid Technology in California: High Value for Capabilities Beyond One-Way Managed Charging

With Electric Vehicles (EVs) Rapidly becoming more popular, along with a need for flexible energy storage resources in the electric grid, the development of vehiclegrid integration (VGI) technology has taken on a new urgency. VGI encompasses any technology that helps EVs better integrate with the grid, including management of charging and control of bidirectional charging and discharging. The practice of managing the one-directional flow of power from the grid to vehicles (V1G) is currently being rolled out across California and in many places where EVs have become popular. V1G programs in California include San Diego Gas and Electric's Power Your Drive, EV-specific retail tariffs from Southern California Edison, and the Charge Forward smart charging pilot program from Pacific Gas and Electric in collaboration with automaker BMW.

Three-Phase Grid Connected Bi-Directional Charging System to Control Active and Reactive Power with Harmonic Compensation

Abstract The feasibility of integration of Battery Energy Storage System (BESS) with a three-phase AC grid is being investigated in this paper. A converter is an inevitable part of a modern DC generating system. The link between the grid and the BESS is established through a Voltage Source Converter (VSC). Therefore, the converter can be utilized to dispatch the DC generated power to the connected AC grid and at the same time provides reactive power compensation and load harmonic compensation throughout the day. The DC bus voltage control of the converter system is carried out to keep the power factor always at unity, irrespective of the charging state of the battery source. The charging and discharging of the connected battery energy storage system are carried out through a bidirectional DC-DC converter. Adaptive hysteresis band current control (AHCC) scheme is employed to produce the switching signals. Finally, its performance is compared with the traditional hysteresis band control technique.

Full-scale electric vehicles penetration in the Danish Island of Bornholm—Optimal scheduling and battery degradation under driving constraints

Abstract The paper proposes an analysis of a 100% electric vehicle (EV) scenario on the energy system of the island of Bornholm in Denmark. The paper intends to present challenges and opportunities that a realistic system would face when completely shifting to electric transportation. The EVs are subject to different charging strategies in order to assess the impact on the grid, the potential savings on the charging cost and the effects on battery degradation. In contrast to uncontrolled charging, smart charging strategies are designed not only to satisfy the same charging requirements at the EV departure time, but also maximize the savings on the charging cost and avoid interconnection congestions. Smart strategies bring a reduction in annual charging cost around 12%, on top of a reduction in the degradation because of lower average SOC and number of cycles. Moreover, results show a limited benefit in bidirectional charging because of a marginal increase in savings: this more demanding operation, which allows discharges, leads to higher battery degradation, due to the increase in the number of cycles.

Optimized Operational Cost Reduction for an EV Charging Station Integrated With Battery Energy Storage and PV Generation

A four-stage intelligent optimization and control algorithm for an electric vehicle (EV) bidirectional charging station equipped with photovoltaic generation and fixed battery energy storage and integrated with a commercial building is proposed in this paper. The proposed algorithm aims at maximally reducing the customer satisfaction-involved operational cost considering the potential uncertainties, while balancing the real-time supply and demand by adjusting the optimally scheduled charging/discharging of EV mobile/local battery storage, grid supply, and deferrable load. The chance-constrained optimization objective has been stated in stages: (1) stage I, optimization of day-ahead energy management schedules; (2) stage II, multitiered EV charging price update and optimization of discharging participation bonus; (3) stage III, optimization of hour-ahead energy management schedules; and (4) stage IV, real-time control. Such algorithm provides more resilience for unpredictable conditions, provides more incentives for EV users to participate, and better coordinates the integrated system including the building load to reliably serve the customers while lessening cost. Case studies are implemented and the comparison analysis is performed in terms of the use and benefit of each design feature of the algorithm. The results indicate that the proposed algorithm can reduce the operational cost and at the same time provide higher tolerability toward uncertainties.

Intelligent Energy Management Algorithms for EV-charging Scheduling with Consideration of Multiple EV Charging Modes

Electric vehicles (EVs) are now attracting increasing interest from both industries and countries as an environmentally friendly and energy efficient mode of travel. Therefore, the EV charging and/or discharging issue has become an important challenge and research topic in power systems in recent years. An advanced and economic EV charging process, however, should employ smart scheduling, which depends on effective and robust algorithms. To that end, a comprehensive intelligent scatter search (ISS) algorithm within the frame of a basic scatter search has been designed with both unidirectional and bidirectional charging considered. The ISS structure also supports both a flexible and constant charging power rate by respectively employing filter-SQP (sequential quadratic programming) and mixed-integer SQP as local solvers with module control. The detailed design of ISS is presented and the objectives of smoothing the daily load profile and minimizing the charging cost have been tested. Compared with methods based on GS (global search), GA (genetic algorithm), and PSO (particle swarm optimization), the outcome-verified ISS can produce attractive results with a significantly short computational time. Moreover, to handle a large scale EV charging scenario, a hybrid method comprised of a GA and ISS approach has been further developed. Simulation results also verified its prominent performance, plus superbly low computational time.

Advanced control of bidirectional three-phase BEV charger with V2X technology

In this paper, the control of a bidirectional three-phase BEV charger with vehicle-to-grid functionality is addressed. The charger structure is composed of two bidirectional power converters: a three-phase ac-dc converter and a dc-dc converter associated to an electric vehicle battery. The control objectives are the following: i) estimation of the battery SOC; ii) unity power factor during grid-to-vehicle (G2V); iii) adjusting the reactive power injected into the power grid during vehicle-to-grid (V2G); iv) dc-bus voltage regulation; and v) ensuring and safeguarding the battery charging and discharging process. To this end and based on the system modelling into dq coordinates, a nonlinear controller is designed. The fact is that the battery SOC is not accessible to measurement. Hence, a nonlinear observer is designed to estimate all state variables on the battery-side. The performances of the proposed controller are highlighted using theoretical analysis and numerical simulations, which clearly illustrate that all control objectives are achieved.

A Controller Design for a Stability Improvement of an Integrated Charging System in Hybrid Electric Vehicle

Abstract This paper proposes a controller design for a stability improvement of an integrated charging system in hybrid electric vehicle (HEV). Conventional HEVs include a starter generator system, which consists of generator and its drive inverter. Contrary to them, the proposed integrated charging system operates as a generator drive system and bi-directional battery charging system by installing some power relays and additional circuit in general HEVs. Therefore, the integrated charging system results in reduced system components in HEV by eliminating on-board charger (OBC), also the weight and system volume can be decreased. In addition, a feed-forward compensation method is proposed to improve the transient states characteristics of the battery charging system in this paper. The effectiveness and validity of proposed design and control method for the integrated charging system is verified by simulation results.

Small-Signal Stability Assessment and Active Stabilization of a Bidirectional Battery Charger

A two-stage electric vehicle (EV) battery charger typically consists of an ac–dc converter cascaded with a dc–dc converter. In such a cascaded system, maintaining stability at the intermediate dc link is imperative for reliable operation of the battery charger under different operating modes. This paper addresses the intermediate dc-link stability challenges that exist in a bidirectional two-stage grid connected single-phase battery charger. It is delineated that the small-signal load-dependent resistance of the ac–dc converter plays a crucial role in determining the stability of the bidirectional battery charger. A virtual-resistor-based active damping control strategy that does not require any additional sensors is explored for the ac–dc converter to stabilize the cascaded system under all operating modes irrespective of the power flow direction. Experimental results on a grid-connected single-phase battery charger hardware prototype are presented to validate the proposed models and showcase the improvement in the dc-link stability due to the virtual-resistance-based active damping approach.

Single-Phase Bidirectional On-Board Charger Using Starter Generator System in Hybrid Electric Vehicles

This paper presents the design and control methods of a single-phase bidirectional on-board charger (OBC) using a hybrid starter generator (HSG) and an inverter in a hybrid electric vehicle (HEV). In an HEV, there are a number of components, including the combustion engine, transmission, traction motor, motor controller, OBC, and HSG system. The proposed design reconfigures the HSG system to provide battery-charging capability instead of a conventional OBC based on the use of additional power relays. As a result, the number of power converters is effectively reduced through the replacement of the conventional OBC, and, thus, the power density is increased. This paper also proposes a control method for enabling not only battery charging but also a reactive power support depending on the grid command. Compared with a conventional reactive power compensation method, the proposed method has an advantage because it is located near the principal reactive power source. The simulation and experimental results verify the validity and feasibility of the proposed bidirectional OBC design and its control methods.

Optimal Minimization of Plug-In Electric Vehicle Charging Cost With Vehicle-to-Home and Vehicle-to-Grid Concepts

This paper deals with the problems of the Plug-in Electric Vehicles charging costs in housing sector. A review of the optimal strategies is proposed and as such six algorithms are presented: three smart unidirectional and three smart bidirectional charging algorithms where the Vehicle-to-Grid (V2G) and the Vehicle-to-Home (V2H) concepts were exploited. In addition an innovative V2G algorithm named Optimal Logical Control (V2G-OLC) is introduced in this paper. This latter is dedicated to the French energy billing system within the peak/base hour's contract. All the algorithms are tested over a set of 1000 data composed of real elements with four different daily energy price profiles. The results show a great efficiency of the V2G-OLC algorithm compared to the traditional optimal charging strategies. Indeed, the results show a reduction of 47.94% of the average charging cost with a unitary selling/buying price ratio in comparison to the simple charging at 230V-32A.

Vehicle grid integration for demand response with mixture user model and decentralized optimization

With the rapidly growing electric vehicle adoption rate and increasing number of public electric vehicle charging stations in recent years, electric vehicle becomes more and more critical in the demand response programs. Development in Vehicle to Grid technology has converted electric vehicle to distributed energy resources. Using the Electric Vehicle Smart Charging Infrastructures on UCLA campus and city of Santa Monica as testbeds, we have collected real-world datasets of electric vehicle usage, based on which, we proposed optimal bi-directional charging control strategies to integrate electric vehicle in commercial and public parking facilities into the power grid as distributed energy resources for demand response programs by two-stage distributed optimization and water-filling algorithm. Driver behavioral uncertainties have been considered in our approach. Specifically, electric vehicle users are clustered by their behavioral patterns using a modified Latent Semantic Analysis. The first-stage optimization is performed to minimize energy cost using day-ahead wholesale energy price with predictions on energy demand and electric vehicle availability which generated by a mixture user model. Decentralized optimization (second-stage) is carried out on the next day in real-time to control individual electric vehicle so that the aggregated load can follow the first-stage optimal profile. As an alternative, a fast converging water filling algorithm is proposed and compared with two-stage optimization. Extensive simulation results show that proposed charging controls can utilize electric vehicle as distributed energy resource to accommodate demand response program while satisfying electric vehicle energy demands and providing significant energy cost savings.