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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
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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
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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
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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
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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.
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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.
Acknowledgement
This research work has been supported by the Centre of
Advanced Research in Electrified Transportation (CARET), Aligarh
Muslim University, Aligarh, India.
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