Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx Contents lists available at ScienceDirect Engineering Science and Technology, an International Journal journal homepage: www.elsevier.com/locate/jestch Full Length Article Fast EV charging station integration with grid ensuring optimal and quality power exchange Wajahat Khan, Furkan Ahmad ⇑, Mohammad Saad Alam Department of Electrical Engineering, Aligarh Muslim University, Aligarh, India a r t i c l e i n f o Article history: Received 29 October 2017 Revised 12 July 2018 Accepted 9 August 2018 Available online xxxx Keywords: Fast charging station Electric vehicles (EVs) Power quality Optimal power flow a b s t r a c t Increased problem of air pollution has led automotive industry to develop clean and efficient fuel based transportation and Electric Vehicles (EVs) appear to be the most suitable alternatives to conventional IC engine based vehicles. Fast charging of EVs is required to make EVs widely accepted as charging time is the key barrier standing in the way of widespread acceptance of EVs. Different strategies have been proposed for the deployment and integration of public fast charging, emphasizing on the power quality aspects and charging load management techniques. This paper presents the model of a fast electric vehicle charging station connected to the grid ensuring quality power transfer with reduced harmonic currents. The charging station consists of a converter connecting grid to a DC bus where EVs get connected through battery chargers. The control of individual vehicle charging process is decentralized and a separate control is provided to deal with the power transfer from AC grid to the DC bus. An energy management strategy based on optimal power flow is also proposed by integrating a solar PV generation system with charging station to alleviate the impact of fast charging on the grid. The combined system along with the power output of EV fleet batteries available at the charging station reduces the net energy provided by the grid, thereby decreasing the overall load on the grid as well as minimizing the conversion losses. Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction In recent years, the research and development activities associated with the automotive sector have laid down emphasis on the development of highly efficient and emission free means of transport [1–5]. Keeping this in view, electric and hybrid vehicles appear to be the best alternatives for replacing the conventional IC engine powered vehicles [6]. However, there are certain critical aspects which need to be looked upon in order to make Electric Vehicles (EVs) a commercial reality. Range anxiety can be considered as the most important factor impeding the widespread acceptance of EVs. Also, charging time reduction is considered to be a key goal in making electric vehicles (EVs) accessible to a larger population. In this perspective, fast DC charging provides a fascinating opportunity. DC fast charging reduces charging time to the range of 20–30 min [7]. Three different levels of fast charging are defined according to SAE J1772 standard which are classified ⇑ Corresponding author at: Department of Electrical Engineering, Aligarh Muslim University, Aligarh 202002, India. E-mail addresses: wajahatkhan@zhcet.ac.in (W. Khan), furkanahmad@zhcet.ac.in (F. Ahmad), hybridvehicle@gmail.com (M.S. Alam). as DC Level-1 (200/450 V, 80 A, up to power rating of 36 kW); DC Level-2 (200/450 V, 200 A, up to power rating of 90 kW) and DC Level-3 (proposed) (200/600 V DC, 400 A, up to power rating of 240 kW) [8]. All the three fast DC charging levels use an offboard charging equipment known as Electric Vehicle Supply Equipment (EVSE) which acts as an interface between the vehicle and supply. Fast charging of electric vehicles has detrimental effects on the power quality of the network. The main problems contributing to the degradation of the power quality include harmonics in line currents, phase imbalance, voltage deviations, dc offset, phantom loading and stray fluxes [9]. Nonlinear nature of EV chargers introduce higher order harmonics in the line current drawn by the them [10,11]. These problems are bound to affect the performance as well as endurance of the distribution network equipments. Moreover, the component of harmonic current induces additional I2R losses in the windings of the power transformers and cables. A lot of research has been done concerning the power quality problems caused by the AC/DC converters used in EV chargers. The impact of different charging rates of the batteries used in EVs on the power quality of the distribution system is studied in [12]. The effect of the harmonic currents on the system with fast charging of multiple EVs is studied in [13]. Due to the Peer review under responsibility of Karabuk University. https://doi.org/10.1016/j.jestch.2018.08.005 2215-0986/Ó 2018 Karabuk University. Publishing services by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 2 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx Nomenclature EVs EVSE ESS EMS RESs SR Nslot cosø Pev kload v dc V bat min mmin C dc t n Dp Dv SOC V2G V2V AER Ppv g Electric Vehicles Electric Vehicle Supply Equipment Energy Storage System Energy Management Scheme Renewable Energy Sources Rated capacity of the charging station in VA Number of available slots for charging of individual vehicle Power factor of the system Maximum charging power rate of an EV Overload factor DC bus voltage Minimum voltage of battery Minimum modulation index DC bus Capacitance Period of AC voltage wave multiple of ‘t’ DC power range of change in percentage, during transient Allowable DC bus voltage range of change in percentage, during transient State of Charge of battery Vehicle-to-Grid Vehicle-to-Vehicle All Electric Range of EV Power output of the installed solar PV PV array efficiency presence of these harmonic components, the commercially available on-board chargers give poor power quality [14]. The presence of lower order harmonics in the line current leads to low power factor operation and ineffective use of the volt-ampere rating. The problem of harmonic distortion deteriorates with increase in charging load. A solution to the high harmonic current injection in the distribution network is proposed in [15]. Some standards have been formulated to regulate the amount of harmonics that can be injected into the system such as IEEE 519–1992, IEC 61000-3-12/2–4 and EN 50160:2000 [16]. The quality of the input current can be enhanced by incorporating certain modifications in the control system of the charger by using an interim voltage source inverter (VSI) which prevents the harmonic currents to be fed back in the feeder. Moreover, the current control of the converters is more effective as compared to the voltage control in ensuring enhanced power factor operation and in suppressing the transients in current [17]. Apart from this, the integration of fast charging stations with the grid has some adverse impacts on the distribution network also [18]. One major effect can be in the form of increase of network peak load [19]. As charging load exhibits large volatility, it is difficult to confine the charging behavior to low load periods, leading to greater system peak differences [12]. This eventually results in poor utilization efficiency of distribution network equipments. Some other effects include an increase in energy losses [20], adverse effects on voltage profile and the distribution transformer [21,22]. Impact in terms of overloaded conductors and cables, low voltages at consumer end and violation of planning limit are prominent if the charging is uncoordinated [23]. Various demand side management schemes have been suggested to tackle the high-power demanded by fast charging stations [24,25]. Some strategies include the use of energy storage systems [26,27]. In [28] a hybrid energy storage scheme is proposed which uses a superconducting magnetic energy storage (SMES) system along with a battery storage for a fast charging Apv Surface area of solar PV G Incident solar radiation (kW/m2) on solar panel Surface temperature of solar panel Tc K o and K 1 Constant values Iph Solar-induced current Ipho Value of solar-induced current at 300 K Diode saturation current Isat Rs Series resistance in model of solar panel Rp Parallel resistance in model of solar panel k Boltzmann’s constant N Quality factor of diode q Charge on an electron T Operating temperature of solar PV PV2G PV to Grid Ep Net energy to be purchased Es Net energy to be sold DAM Day Ahead Market MCP Market Clearing Price for one day at DAM kp Purchasing price per unit of electricity ks Selling price per unit of electricity G2V Grid to Vehicle PV2V PV-to-Vehicle Q Maximum battery capacity PL Load demand at the charging station PD Power available for discharging Power taken from the grid PG station which limits the power magnitude and power change rate of a charging station by compensation of hybrid storage. Flywheel Energy Storage System (ESS) is used in [29] for power balancing in a fast charging station to lessen the impacts of fast charging on the utility grid by ramping the power peak. In this paper, model of an electric vehicle charging station with fast DC charging is presented. Power quality issues related to the source end harmonics are dealt with along with the implementation of a charging strategy using constant-current and constantvoltage modes. An optimal energy management scheme is presented in the end to mitigate the load on utility grid by use of renewable energy systems. Rest of this paper is ordered as follows. In Section 2, the system architecture and design aspects of the charging station are considered in detail. Control strategies used for the control of AC/DC converter and battery charger are discussed in Section 3. In Section 4 the simulation results are presented for the given model of charging station. Section 5 discusses an optimal Energy Management Scheme (EMS) to minimize the conversion losses and reduce the impact on grid. Finally, conclusion is presented in Section 6. 2. Design of charging station The schematic diagram of proposed fast EV charging station is shown in Fig. 1. As shown, the given architecture uses only one AC-DC Grid Tied converter to realize a DC bus, connecting the charging EVs through DC-DC converters. The DC bus makes it possible to connect Renewable Energy Sources (RESs) generation systems directly through a simple DC-DC converter. It is estimated that DC bus architecture reduces the overall conversion losses from about 32% to less than 10% when compared with the AC bus architecture [30]. Three phase supply is taken from grid. Three phase transformer is used to step down the voltage from the distribution Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 3 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx transient. Table 1 gives the input parameters and the resulting parameters of the modeled charging station. Grid Transformer 3. Control system design AC DC DC Bus DC DC DC DC Fig. 1. Schematic diagram of proposed Fast charging station. grid voltage level to EVs battery voltage levels. Three phase AC/DC converter transforms the ac power into dc power and forms a dc bus. EVs get connected to the DC bus for charging through DC/DC converters. A number of aspects have to be considered while designing a charging station such as Available area for parking of electric vehicles; this determines the number of vehicles which can be charged. Demand estimation for fast charging slots in a particular area. Network constraints like nominal voltage level and permissible power levels at the point of common coupling. Rate of allowable charging power to be supplied to each vehicle. The rated capacity of the charging station SR in VA is estimated according to Eq. (1): SR ¼ kload Nslot Pev cos£ ð1Þ where, Nslot defines the number of available slots for charging of individual vehicle, cosø defines the power factor of the system, Pev denotes the maximum charging power rate of an EV, kload defines an overload factor to take into account overloading during transients. The DC bus voltage v dc is generally decided according to the voltage of the grid. However, the connection to the grid through transformer makes selection of bus voltage unrestricted from the grid voltage level. But the battery’s minimum voltage V bat min and minimum modulation index mmin of battery charger, put an upper limit on the DC bus voltage as given in Eq. (2): v dc V bat min mmin ð2Þ The stability of DC bus directly depends on the size of DC capacitance which has to sustain the DC current ripples. As number of chargers have to be connected with the DC bus, DC ripple current may be quite high, thus, requiring a large value of capacitance. In this work, the capacitance of the DC bus is calculated using the method given in [31] and taking into account the rated active power and the rate of change of capacitor energy during the transient. Capacitance value is calculated according to Eq. (3): C dc ¼ SR 2nt Dp cos£ V 2dc Dv ð3Þ where, ‘t’ denotes the period of AC voltage wave, ‘n’ is a multiple of ‘t’, ‘Dp’ is the DC power range of change, and Dv defines the allowable DC bus voltage range of change, in percentage, during The three phase AC supply taken from grid is rectified using a rectifier. The problem with conventional uncontrolled rectifiers includes the power quality issues associated with the source side. Undesirable line current harmonics are drawn by the rectifiers. Due to the presence of harmonics in the line current, distortion of voltage occurs at point of common coupling. Voltage distortion may lead to malfunctioning of other connected loads, power system protection and other monitoring equipments. Due to the presence of low-order harmonics in source current, power factor also comes down. Poor power factor results in ineffective use of the volt-ampere (VA) rating. Therefore, a number of organizations have formulated standards to limit the magnitude of harmonic currents that can be injected into AC line. Various passive and active power factor correction techniques have been designed to reduce line current harmonics. 3.1. Converter control The basic strategy used in implementing the converter control is shown in Fig. 2. For proper operation of the converter, the dc voltage Vdc at any instant should be more than the peak value of AC source voltage Vs (peak). Initially during turn on, the capacitor charges to the peak of source voltage through the anti-parallel diodes and then the control circuit maintains the reference voltage at the desired value. A voltage controller (PI) is used to produce the reference current proportional to the input power needed to maintain the voltage of dc link as constant. The output of PI controller is multiplied by a sinusoidal unit vector derived from the Phase Locked Loop (PLL) and thus the reference currents for each phase are generated. The controller forces the actual current (ia) to follow the predefined reference current (i*a). The comparators switch the line current between a fixed bandwidth. The reference current, bandwidth and the source current wave shape are shown in Fig. 3. The bandwidth is fixed irrespective of the dynamic nature of the current. The bandwidth along with the current dynamics decides the switching instants and hence the switching frequency. This method provides fast dynamic response, reduces steady state error, minimum hardware and software is required for implementation and there is no need of acquiring information about the system parameters. Table 1 Charging Station parameters. input and resulting Parameters Values EV charging current cos£ kload mmin Battery Capacity t n fgrid VGrid X/R ratio 100 A 0.95 1.1 0.125 100 Ah, 48 V 1/50 s 0.5 50 Hz 415 V ph-ph 8 112 V 10% 5% 5 mF v dc Dv Dp C dc Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 4 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx VDC (ref) AC/DC Converter 3-phase ac input VDC PI Controller Switching Signals Current controller 3-phase Source Current Reference Current signals Sinusoidal unit vector derived from PLL Fig. 2. Control loop for converter control. for the DC/DC converter. This duty ratio command is given to the PWM generator circuit which accordingly generates the gating signal for the switch of the converter. The flow chart for the program of CC-CV controller is given in Fig. 5. 4. Simulation results Fig. 3. Source Current wave shape. 3.2. EV charger control A schematic diagram for the control of EV charger is shown below in Fig. 4. The battery charger is a DC/DC converter which connects the electric vehicle to the DC bus. The Charging scheme used for the charging of EV battery is Constant Current-Constant Voltage (CC-CV) charging scheme. In this charging scheme the battery current is kept constant initially and the battery voltage is allowed to increase until it reaches at a predefined value. This mode is called Constant Current (CC) mode. Once the voltage reaches this value, current is allowed to decrease and voltage is maintained constant at the predefined value. This is known as Constant Voltage (CV) mode. Most of the charging is done in constant current mode. Controller designed for CC-CV charging controls the switching of DC/DC converter and accordingly generates the output suitable for the EV battery. Feedback of battery voltage and current is given as input to the controller. The reference signals for the voltage and current are generated using CC-CV program written in Matlab codes. The error signals are processed through two P-I controllers, one for each mode. The output of PI controller gives the duty ratio Simulation study was conducted in Matlab Simulink. Results of simulation are shown in the subsequent figures. Fig. 6 shows the waveforms for voltage and current drawn by the charging station from the grid. Three phase sinusoidal input current with less distortion is drawn by the charging station. The harmonic spectrum for the source current gives Total Harmonic Distortion (THD) of START Measure Battery Voltage (Vbat) Vbat < Vmax NO Yes CV Mode CC Mode DC Bus Change duty cycle to keep Ibat constant EV Baery Charger Change duty Cycle to keep Vbat constant EV Baery Ibat < = IThsld Baery Parameters Gate Driver CC-CV Controller Fig. 4. Control scheme for EV Charger. NO Vbat > = Vmax NO Yes Yes STOP Fig. 5. Flow chart for CC-CV charging. Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx (a) (c) (e) 5 (b) (d) (f) Fig. 6. Waveforms of (a) Input three phase current (b) Phase voltage and current (c) Harmonic spectrum of source current (d) DC bus voltage (e) Input three phase current with load change (f) Dynamic characteristic of DC bus voltage on load change. 1.47%. There is no phase lag between the source current and source voltage as can be seen from Fig. 6(b) which shows that a high power factor near to unity is obtained. The DC bus voltage characteristic is shown in Fig. 6(d) which settles down at steady state value. Input current waveforms for change in load is shown along with the DC bus voltage in Fig. 6(e) and (f) respectively to show the dynamic performance of the designed model. Fig. 7 shows the SOC characteristic of the EV battery along with the waveforms of battery voltage and current in CC mode. Fig. 7(a) shows the characteristic of battery current which remains almost constant during this period, Fig. 7(b) shows the characteristic of battery voltage which increases continuously up to a certain level and Fig. 7(c) shows the change in SOC of the battery during the simulated period. Most of the charging (up to 90%) takes place in this mode. Fig. 8 gives the EV battery characteristics while it changes from CC to CV mode. The current starts to decrease as shown in Fig. 8(a) while the voltage stops rising and settles down at a constant value and from here onwards charging takes place at this constant value of voltage as shown in Fig. 8(b). The transition from CC to CV mode takes place at around 89% SOC shown in Fig. 8(c). Fig. 9(a)–(c) show the waveforms of battery current, voltage and SOC in CV mode. The current in CV mode continues to decrease until it reaches at a minimum specified threshold level after which the charging stops or takes place in trickling mode. 5. Optimal ems for proposed charging station Additional load in the form of EVs is bound to affect the grid adversely, if proper scheduling is not done in advance [32,33]. The charging demand of electric vehicles in a fast charging station can lead to a significant rise in the peak load of the network. It may lead to imbalance in voltage and frequency [34,35]. Thus, it is necessary to monitor the system continuously while charging large number of EVs, in order to ensure grid balancing. Demand side Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 6 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx (a) (a) (b) (b) (c) Fig. 8. Characteristics of EV battery (a) current (b) Voltage and (c) SOC at transition from constant current (CC) mode to constant voltage (CV) mode. 5.1. PV generation (c) Fig. 7. Characteristics of EV battery (a) current (b) Voltage and (c) SOC in constant current mode. management can play a key role in this regard [36,37]. Different demand side management strategies can be adopted to overcome these situations [38]. One way to reduce the impact of fast charging on the grid is to encourage the use of renewable energy sources like solar PV along with the grid [39]. Also, a bidirectional flow of energy can be established between the system and the charging station by using the concept of Vehicle-to-Grid (V2G) and Vehicle-to-Vehicle (V2V) charging. For that an optimal power flow strategy has to be implemented. In this work, an optimal Energy Management Scheme (EMS) has been proposed which involves the flow of energy between the grid, installed solar PV and fast charging station. The installed solar PV capacity is calculated using the base demand of charging station by considering that on an average 100 vehicles return to the charging station for charging in a single day. The SOC values of the vehicles when they arrive and leave are supposed to be in the range of 0–10% and 90–100% respectively. Five different vehicles types are considered with different All Electric Range (AER) and battery capacities [40] as shown in Table 2. The available output power from the installed PV plant throughout the day is calculated using the data available for solar irradiance throughout the day and single-diode model of PV system as given in Eqs. (4)–(7). Based on the computations performed, the required capacity of solar PV comes out to be 110 kW. Fig. 10 gives the graph of output power available from solar PV. T c 25 Ppv ¼ gApv G 1 200 ð4Þ q V pv þ Ipv Rs Ipv ¼ Iph Isat exp ðV pv þ Ipv Rs Þ=Rp NkT pv ð5Þ Iph ¼ Ipho ð1 þ K o ðT 300ÞÞ ð6Þ Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 7 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx (b) (a) (c) Fig. 9. Characteristics of EV battery (a) current (b) Voltage and (c) SOC in constant voltage mode. Table 2 Various Types of PHEVs [40] 120 AER (mile) Battery Capacity (kWh) Car Car Van, SUV, Light Truck Van, SUV, Light Truck Other Truck 40 20 20 10 10 11.2 5.6 6.94 3.47 4.34 Isat ¼ K 1 T 3 e qV g kT 100 PV Output (kW) Vehicle Type 80 60 40 20 ð7Þ where, Ppv is the power output of the installed solar PV, g is the PV array efficiency, Apv is the surface area, G is the incident solar radiation (kW/m2) on the panel, T c is the surface temperature, K o and K 1 are constant values, Iph is solar-induced current, Ipho is the value of solar-induced current at 300 K and Isat is the diode saturation current, Rs and Rp are the values of series and parallel resistances respectively, k is the Boltzmann’s constant, N is defined as the quality factor of diode, q denotes charge on an electron, and T denotes the operating temperature of solar PV. 0 0 2 4 6 8 10 12 14 16 Time (Hour) 18 20 22 24 Fig. 10. Solar PV Output. 5.2. Load demand A typical charging demand profile for a fast charging station for one day is shown in Fig. 11. As evident from the charging demand curve, the peak of charging demand is observed at times during the day when there is peak load on the network too. So, the charging Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 8 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx 6.5 Electricity Price (INR/kWh) Charging Demand (kW) 120 100 80 60 40 20 MCP Selling Price Purchasing Price 6.0 5.5 5.0 4.5 4.0 3.5 3.0 2.5 2.0 0 0 0 2 4 6 8 10 12 14 16 18 20 22 24 Time (Hour) 2 4 6 8 10 12 14 Time (Hour) 16 18 20 22 24 Fig. 13. One Day Energy Pricing at DAM. Fig. 11. Expected charging demand of a fast charging station on a weekday [41]. demand peak is most likely to coincide with the network peak load and increase the net peak of the system. This excess demand on the system in the form of charging load can be met through installed solar PV plant as the output of solar PV is sufficient to meet the charging demand during peak daylight hours. This would eliminate the conversion losses which are involved when EVs are charged from the grid. In case, the charging demand is not too high, the energy available from solar PV can be given to the grid. This would further enhance the system operation. Apart from the energy available from the grid and solar PV, the surplus energy available at the charging station in the form EVs that come for discharging, can also be utilized to meet the charging demand by using the concept of V2V charging. This V2V charging can also eliminate need for AC/DC conversion, thereby, reducing the losses incurred in conversion, and the charges paid to utility grid will also be minimized. Fig. 12 shows the characteristic of the available energy in the form of EVs in a charging station on a sample weekday (See Fig. 13). 5.3. Energy pricing Available Discharging capacity (kW) The aggregator of the charging station can also be benefitted by participating in the energy market [42] and supplying the excess energy available back to the grid using the concept of Vehicle to Grid (V2G) and PV to Grid (PV2G). Using the data available for the load demand, PV output and discharging capacity available at the charging station, the net energy to be purchased (Ep) and net energy to be sold (Es) can be calculated based on the preference order that the available capacity from installed solar PV and surplus energy available for discharging are fully utilized before taking energy from the grid. The historical data for Market Clearing Price (MCP) for one day at Day Ahead Market (DAM) is obtained from energy exchange and the net selling and purchasing price are estimated by taking into account the transmission losses, transmission charges and exchange charges. Eq. (8) is the objective function which governs the net transaction between the grid and charging station. XT min t¼1 ðEp kp Es ks Þ ð8Þ where, kp is purchasing price and ks is the selling price per unit of electricity. 5.4. Optimal power flow The proposed optimal power flow scheme gives the distribution of power flow between the charging station, solar PV, utility grid and the vehicles available for discharging. The distribution is classified into five different modes i.e., Grid to Vehicle (G2V), PV-toVehicle (PV2V), Vehicle-to-Vehicle (V2V), Vehicle-to-Grid (V2G) and PV-to-Grid (PV2G). The scheme is designed in such a way that the available energy from solar PV and the surplus energy available for discharging are fully utilized while minimum energy is taken from the grid. The governing equations of the problem are shown in Eqs. (9)– (11) R SOC ð%Þ ¼ SOC i ð%Þ 100ð SOC ðtÞ ¼ SOC ðt Dt Þ Ibat :dt Q PD ðtÞDt Q PG ðtÞÞ ¼ PL ðt Þ Ppv ðtÞ PD ðtÞ ð9Þ ð10Þ ð11Þ System is constrained by limits given in Eqs. (12)–(14). SOC i SOCðtÞ SOC f ð12Þ PDmin P D ðtÞ PDmax ð13Þ 140 Ppv min Ppv ðtÞ P pv max ð14Þ 120 where, Q is the maximum battery capacity, PL is the load demand at the charging station, P D is the power available for discharging and PG denotes the power taken from the grid. The resulting power flow based on the proposed scheme is shown below in Fig. 14. The net profit gained by the aggregator in selling the energy to utility is Rs 3556. This type of power flow management scheme can prove to be beneficial for both the utility and the aggregator. Such a demand side management can help in smooth running of power grid with less disturbances. The grid operation is enhanced by injection of power back to the grid using V2G and PV2G concept. Further enhancement in the system can be realized with addition of a 100 80 60 40 20 0 2 4 6 8 10 12 14 16 Tim e (Hour) 18 Fig. 12. Available discharging capacity. 20 22 24 Please cite this article in press as: W. Khan et al., Fast EV charging station integration with grid ensuring optimal and quality power exchange, Eng. Sci. Tech., Int. J. (2018), https://doi.org/10.1016/j.jestch.2018.08.005 W. Khan et al. / Engineering Science and Technology, an International Journal xxx (2018) xxx–xxx References 140 G2V PV2G V2G PV2V V2V 120 Flow of Power (kW) 9 100 80 60 40 20 0 2 4 6 8 10 12 14 16 Time (Hour) 18 20 22 24 Fig. 14. Distribution of power flow. backup energy storage system which would help in getting a flatter load profile with less system peak differences. 6. Conclusion In this paper a model of charging station for fast DC charging is proposed. A DC bus is realized using grid connection through an AC/DC converter. The converter is so designed that near to unity power factor operation is obtained and minimum line current harmonics are drawn. Good performance is observed with change in load. Results show a proper dynamic behavior of the DC bus voltage, the battery voltage, and the battery current. The line current harmonics are greatly reduced by the use of proposed control technique. The control is relatively easier to implement and also gives good dynamic performance in terms of DC bus voltage stability. The controller designed for CC-CV charging is effective in controlling the charging modes. The proposed model is also effective in reducing the impact on grid by reducing the net energy drawn from the utility. The advantage of the coordinated operation of Electric utility, solar PV generation and available reserve capacity is highlighted in terms of the net profit earned by participation in the energy market. The proposed power flow management using renewable energy source like solar PV would prove to be beneficial to the utility as well as to the aggregator of the charging station. Proposed energy management scheme also minimizes the conversion losses and is effective in reducing the overall load on the grid. Based on discussion, it can be concluded that it is necessary to develop and extend the fast charging infrastructure for the benefits of both the users and the manufacturers. The energy management system of EVs needs a more specific framework to deal with the multilevel uncertainties associated with renewable energy sources, arrival and departure pattern of xEVs and variable market price etc. From the market framework point of view, there is a potential in researching EV market design. New market models that enable active and reactive EV-power system services such as load shifting, peak shaving, valley filling, voltage regulation, and reactive power control at the distribution system level can be investigated. Moreover, the methods and systems for remote management of electric, hybrid and plug-in hybrid electric, vehicle charging system using vehicle to cloud (V2C) strategy operated in coordination of the vehicle communication portal, vehicle human machine interface system, vehicle battery management system and the dedicate cloud based app needs immense research. 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