1. Introduction
The transportation sector has the prime share in the global average per capita energy consumption. This condition raises concerns about the carbon emission and energy independency due to the reliance on fossil fuel. Substituting internal combustion engine (ICE) vehicles, which are powered by gasoline or diesel, by electric vehicles (EVs) is a potential approach to solve the environmental issues and energy crisis. Consequently, various countries have taken the initiatives to develop their own EV manufacturing, aiming to de-carbonize their transportation sectors. An EV battery can be charged by using either direct current (DC)charging technologies (known as off-board charging) or alternating current (AC) charging technologies (known as on-board charging) [
1]. Recently, a number of advanced approaches have been applied to EV charging. These approaches can be categorized into three groups namely, slow charging (up to 10 kW), fast charging (up to 24 kW), and quick charging (50 kW or higher) [
2]. The main challenge of slow charging is that it takes a longer time to fully charge the battery, typically around 8 to 12 h.
Nowadays, an EV with lithium-ion battery is sustainable for 100 kilometers range, where the battery could be fully charged within 30 min by using a fast charging approach. In this respect, a real implementation of EVs fast charging station equipped with an energy storage system is briefly presented in [
3], where multiple EV charging stations with AC distribution network has been analyzed accordingly. In order to decrease the high energy demanded from the grid for EV fast charging, a renewable energy can be installed in the charging stations [
4]. The modeling of a high-power density, efficient, and reliable fast charging has become a major challenge of expanding the number of EVs globally. In other words, one of the main issues with EV fast charging is the usage of power electronics devices; such as rectifiers, inverters, and converters that potentially generate harmonics distortion into the electricity grid. These harmonics have a major impact on the stability and power quality of the electricity grid. In addition, the charging currents and voltages with high total harmonic distortion (THD) can eventually damage other electrical and electronic devices around the charger station, such as conductors and transformers [
5]. Recently, the THD that are generated through an EV charging process is standardized by IEC 1000-3-2 and SAE J2894/1 standards, where these standards are highlight the limit of THD allowed by EV charging [
6].
One of the commonly used approach to eliminate these harmonics distortion is by using a passive filter, where this approach provides a low impedance path to the harmonics currents Several researchers generally utilized passive filter in order to reduce THD of EV fast charging stations [
7,
8,
9], and most typical ones is a single tuned filter. The single tuned filter is designed to mitigate a specific harmonic component from the charging system by a properly sized inductance, capacitance, and resistance values [
10]. The authors in [
11] proposed a band-pass inductor–capacitor (LC) filter integrated into an asymmetric synchronous reference of an EV charger connected to the grid to mitigate the 5th and 7th harmonic components. However, there is a lack of research in measuring both voltage and current harmonics in EV fast charging.
Previous researchers have designed active power filters (APFs) in EV fast charging to decrease the harmonic distortions created by the nonlinear components. AFPs can be classified into two types, namely, shunt active power filter and series active power filter. Shunt APFs are more conventionally used compared to series APFs in the EV fast charging stations. Shunt APFs have several advantages, such as easy installation and capable of improving the power factor by decreasing the reactive power drain on the distribution network [
6]. The researchers in [
12], developed a fast battery charger with shunt APF to improve the power factor correction and to decrease the THD by imposing the utility to supply the active current. However, the current approaches cannot regulate the charging precisely due to the fact that the interface between the harmonic current and harmonic voltage of the charging station is not considered, and an accurate approach of choosing the safety factor has not been proposed.
Recently, multiple control algorithms have been developed in the EV fast charging system in order to reduce the THD in power distribution networks, such as direct power control (DPC) [
13], hysteresis control [
14,
15], and model predictive control (MPC) [
16]. However, one of the main disadvantages of these algorithms is the capability to observe the harmonic distortion limits of specific order harmonic frequencies, (10
k ± 1, 8
k ± 1 6
k ± 1, 4
k ± 1; for
k = 1, 2, ...) of the output voltages/currents.
In order to address these gaps, this paper proposes a novel technique of designing and developing an EV fast charger with integrated internal model controller (IMC), where the proposed design provides high dynamic operation and low THD of the drawn currents/ voltages. Besides, the developed IMC has superior control performance compared with an existing current control method. In this respect, the developed IMC controller is structurally simple and can be executed by proportional integral (PI) and proportional integral derivative (PID) controllers. The isolation mechanism is provided between the grid and vehicle through a three-phase transformer integrated with the charger system. The proposed controller demonstrates a low THD of the input current with almost unity power factor. Moreover, this EV charger is capable to perform two types of EV charging, where it provides three-phase AC and DC fast charging, as shown in
Figure 1. In this paper, IMC with decoupled controllers are integrated to control three-stage converters of the EV fast charger.
The core contributions of this research are as follows:
This work is designed to provide two approaches of fast charging, as it can deliver DC and AC fast charging.
The proposed EV fast charger is designed in a significant method that reduces the overall weight and size of the charger.
The proposed EV fast charger is more adaptable in different scenarios, specifically where there are restrictions in resources or space.
This manuscript is organized as follows. In
Section 2, the method of IMC with synchronized decoupled controller is briefly described.
Section 3 presents the simulation structure of the proposed EV fast charger by using MATLAB/Simulink 2018a software, and the simulation results are discussed as well.
Section 4 describes the prototype setup of the proposed EV fast charger with its experimental results. Outcomes verification and comparison are presented in
Section 5. Finally, in
Section 6, the conclusion highlights the significant contributions of this research.
2. IMC with Synchronized Decoupled Controller
This section introduces the novel design of EV fast charging (DC and AC fast charging) based on IMC with decoupling between active and reactive components, where feedforward decoupled controller has been modeled and combined with pulse width modulation (PWM) scheme to generate six pulse signals of each converter. The DC-link voltage controller and current controllers have been modeled by applying IMC technique, which requires coordinate transformation (Clarke and Park transformation), followed by decoupling between active and reactive components.
In the first step, the input line voltage
of the charger needs to be supplied to the input of the phase-locked loop (PLL). Secondly, the voltage angle (θ) is calculated in the PLL scheme to be used in three-phase
(abc) to
dq-coordinate transformation of line current and voltage. In the third step, the
dq-coordinate values (i.e.,
) with the DC-link voltage value (
are supplied into the decoupled controller based on three synchronized PI controllers. Finally, the reference voltages
which are transformed to three-phase
abc coordinate, are sent to the PWM scheme to generate the switching patterns
, as shown in
Figure 2.
2.1. Implementation of IMC with Three-Stages Converters
This stage is considered the most significant part of the charger, where it is linked to the main functions of IMC with decoupled controller process. The overall structure of multiple mode fast charger consists of three stages of power conversion controlled by IMC with decoupling between active and reactive components, as shown in
Figure 3. Three-phase bridge rectifier (PWM rectifier) is placed in the first stage of the power conversion followed by the DC-link capacitor. The current and voltage controllers are utilized by getting the feedback voltage from the EV battery as a part of closed loop operation control.
The output of DC-Link capacitor is connected to the three-phase inverter (SPWM inverter), which is controlled by SPWM switching technique, followed by a three-phase transformer, which is used between the AC fast charger (first mode charging) and EV connectors. Besides the isolation between the charger and the EVs, the main purpose of this transformer is to step-up or step-down the voltage based on the technologies of EVs. Passive filter (LC filter), is connected to the output of the SPWM in order to meet the standard charging distortion limits. Finally, a diode bridge rectifier is connected to the primary side of the transformer to provide DC fast charger (second mode charging).
2.2. Integration of PLL into Clarke and Park Transformation
In this EV fast charging system, the PLL is considered as a critical and important part, where it is designed to detect the phase angle (θ) of the utility voltages (
. This phase angle is then used for all
dq-transformations in the charging system, as shown in
Figure 4. The process of IMC is primarily executed in the synchronous reference frame (
dq0) and alpha–beta transformation (
αβ0), using Clarke and Park transformation matrices using Equations (1) to (4). These transformation matrices are applied to the model charging system as shown in Equations (5) to (6):
where
,
and
are input voltages in three-phase
frame,
,
,
,
, and
are the source voltages in
and
frames, as shown in
Figure 5.
2.3. Synchronized PI Controllers of IMC
Clarke and Park transformation matrices are applied to convert AC signals to DC signals in the controlled part. In this case, steady-state errors are minimized by the PI controllers which are based on the following equations:
where
and
are the reference signals of
and
in units of Amps., respectively, which are obtained from the measured DC voltage
of PWM rectifier, while
and
are the gains of PI controller, and
and
are the current source in
frame. The reference voltages
and
in unit of Volt are transformed using inverse Clarke and Park transformation to derive gate switching pulses
which controls the operation of PWM rectifier, as shown in Equation (9).
The DC-link voltage control is used to regulate the active current component ( in unit of Amp to obtain active power that flows during the charging process. The value of reactive components () is set to zero in unit of Amp., to achieve unity power factor and to mitigate the THD.
The proposed approach uses three PI controllers (one voltage controller and two current controllers). The output loop of DC-link voltage (
in unit of Volt is regulated by PI voltage controller, the measured
(V) with its pre-determined reference value
are compared to estimate the reference current signal
. The first current controller is used to regulate the inner loop of
current component by minimizing the error between
with
in order to estimate the reference voltage signal
. Likewise, the second PI current controller is used to regulate the second inner loop of
current component, where its
is set to zero to estimate the reference voltage signal
. As illustrated in
Figure 6, the characteristic of three PI controllers are given as follow:
where in PI voltage controller,
and
represent the gain values of the proportional and integral gains, respectively, while in the first PI current controller,
and
in represent the gain values of the proportional and integral gains, respectively, finally in the second PI controller,
and
represent the gain values of the proportional and integral gains, respectively. The bandwidth
is chosen to be smaller than a decade below the switching frequency (
) in inner current control loops in Hz [
17], as follows:
where
(rad/s) is the bandwidth.
The DC link capacitor, which is located in the voltage control loop, is used to tune the PI controller as follow [
17]:
where
is the angular frequency in
(rad/s), and
ς is the damping factor, which is fixed at 0.707.
2.4. Modulation Process of PWM Rectifier and SPWM Inverter
The modulation technique is operated by comparing two signals; sinusoidal modulating signal which is considered as a low-frequency signal and triangular carrier signal which is considered as a high-frequency signal. The intersection of these signals generates pulses with varying duty cycles. In this work, three balanced-sinusoidal controlled voltages are compared with their respective triangular voltage waveforms, where the output of these intersections is applied to control the switching function of the switching components in each leg of the converter. The switching frequency applied for the SPWM technique is set to 12 kHz. The value of switching frequency is set to 12 kHz. The switching devices are functioning in a complementary manner in each phase: When the upper leg is in the open position, the lower leg will be in the closed position, and vice versa.
2.5. Configuration Parameters
The main configuration parameters of the developed controller include the input source parameters, PWM rectifier, PI controllers, and SPWM inverter. In this regard, the input source parameters refer to the three-phase input voltage supply ( and three-phase input voltage supply (, where the frequency of these inputs is 50 Hz. In addition, inductors of 5 mH and resistors ( of 5 Ω are considered as the input filter of the charger, which is connected to the input of the next part of the charger (PWM rectifier). This rectifier consists of six switches of insulated-gate bipolar transistor (IGBT) transistors connected to and DC-link capacitor, where the output of this capacitor connected to the three-phase inverter (SWPM inverter). This inverter consists of six switches of IGBT transistors, where the output of each phase is connected at the middle of each inverter leg. The output of the comparators provides the gating pulses required for controlling the switching operation of all three legs in the inverter. The switching frequency applied for the PWM is set at 12 kHz. The charging process is performed by three PI controllers to control the input current and output voltage.
5. Verification and Comparison of Simulation and Experimental Results
This section describes the verification of the simulated results with experimental results. From both simulation and experimental works, by using the proposed IMC decoupled controller algorithms, the fast charging system is able to provide constant charging voltage and current charging. Besides, it can be clearly seen that the output DC fast charging voltage has a low ripple. Moreover, from both simulation and experimental results, the input current waveform is observed to be sinusoidal with minimum distortion (low THD value), and is working in phase with the input voltage, thus achieving almost unity power factor.
However, THD value of the input current to the charger from the experimental work (1.75%) is found to be higher than the simulated value (1.55%). One of the reasons could be due to the power losses in the experimental work that were not considered in the simulation. In addition, in experimental work, due to resource limitations, the maximum input supply voltage for the battery charger was set to 100 Vrms (line to line). Whereas in simulation setup, the reference voltage is set to 650 V. Further, in the experimental work, the charging system is operated at a much lower current value compared to the simulation work. In the experimental work, the maximum input supply voltage for the battery charger is set to be 200 V, while in simulation work the maximum input supply voltage for the battery charger is set to be 480 V. Therefore, the results that were captured from the experimental work are found to be different than the simulated value. Since the magnitude of the fundamental current is low, thus a higher THD value is recorded from the experimental work.
6. Conclusions
In this work, the proposed EV fast charger designed based on the integrated IMC with synchronized decoupled control for DC and AC fast charging is fabricated and tested in lab environment. This paper brings out the methodology of this research. First, the principles of the proposed control method of the converter which is IMC with synchronized decoupled controller algorithm is explained. Secondly, the simulation is carried out using MATLAB/Simulink software. Finally, the implementation of the experimental setup in laboratory is carried out to verify the proposed design. From both simulation and experimental work, the proposed IMC with synchronized decoupled controller algorithms used in the fast charging system is able to provide constant voltage and current charging. Besides, it also can be clearly seen that the output DC charging voltage has a low ripple. Moreover, from both simulation and experimental results, the input current waveform is observed to be sinusoidal with minimum distortion (low THD value), and is working in phase with the input voltage, thus achieving almost unity power factor, where the reactive component is set to zero. In addition, IMC with synchronized decoupled controller accurately regulates the output DC voltage. It is clear that the control algorithm accurately regulates the output DC voltage. Besides, it ensures a sinusoidal input current with minimum switching ripples and distortions. The power factor of the system is almost unity, and THD for the input current is less than 1.55%.