PV Fed EV Charging System Based on Re-lift Luo Converter and DAB Converter

The insufficiency and soaring rate of fossil fuels, as well as growing concerns about global warming, have pushed electric vehicles (EVs) to become a significant aspect of the future transportation system. The growth in the adoption of EVs results in the need for broadly spread charging stations, hence a Photovoltaic (PV) based EV charging system is adopted since the generation of power from PV is possible everywhere. An EV charging system with two converters is constructed in this research work, one of which is a re-lift Luo converter and the other is a Dual Active Bridge (DAB) converter. A re-lift Luo converter is adopted for the conversion of the low PV voltage output to an enhanced voltage of a higher level. The DAB converter is effective in providing the required galvanic isolation and allowing bidirectional power flow. The PV system’s output voltage fluctuates with changing weather conditions and solar irradiation. Hence, the operation of the re-lift Luo converter is controlled by using the Fuzzy Logic Controller (FLC), which is highly capable of addressing the uncertainties associated with PV output. The operation of the DAB converter is controlled by implementing the Phase Shift (PS) control technique. MATLAB simulation is done to validate the proposed system's performance.


Introduction
Globally, two of the most promising and mushrooming technologies, which are projected to play a major part in the electricity business over the next decade, are EVs and PVs. The uncertainties that exist around the availability and affordability of fossil fuels along with increasing concerns regarding global warming have driven the evolution of EVs as a substantial part of the transportation system of the future. The booming adoption of EVs demands a huge amount of electricity, which is required to be obtained from renewable sources of energy for effective minimization of carbon footprint. Among various renewable energies, PVs are more suitable for EVs, since their generation is possible everywhere, even in urban areas. The rapid flourish seen recently in the adoption of PVs is owing to their capability in tackling the growing demand for energy in addition to lowering prices [1][2][3][4]. The power from the PV is not stable, it is intermittent and it fluctuates with the variation in climatic conditions. Hence there arises the need for the implementation of a suitable DC-DC converter that is capable of delivering regulated and controlled output from the PV system [5,6].
Some of the prevalent converters used for PV applications involve boost converters [7], buck-boost [8], Cuk [9], and SEPIC [10] converters. The boost converters are predominantly used in PV applications but they are only capable of boosting their input voltage level. The buck-boost converters are however able to achieve both step-down and step-up voltage conversion levels but the intermittent nature of their input current hinders their extensive applications. Both boost and buck-boost converters are fundamental converters and they are affected by a higher amount of voltage stress and conduction losses. The Cuk converter and SEPIC converter possesses the ability to provide both higher and lower voltage conversion ratio and unlike buck-boost converters, their input current is continuous but, on the downside, both these converters are also limited by the existence of a large amount of input current ripples [11,12]. The Luo converter has high gain with enhanced voltage regulation ability and better light load efficiency under rapid fluctuating line voltages, even so, the presence of voltage stress across the switches limits its overall efficiency [13]. The choice of a suitable controller is essential in improving the dynamic operation of the converter and obtaining unity power factor in addition to reduced THD [14]. The conventional approach of using the PI controller is simple and effective when operated in a fixed range but they are not suitable in non-linear working conditions. The response of the PI controller is rather slow and suffers from peak overshoot problems in case of disturbances and uncertainties [15]. This limitation is overcome by using FLC, which is an intelligent controller, highly capable of tacking issues based on non-linearity and disturbances. The problem of peak overshoot is minimized in FLC-based systems along with providing improved dynamic performance and accurate response [16][17][18]. When an EV battery is interfaced directly with a dc bus, it is vulnerable to various potential risks if there is a rapid change in load. To regulate the discharging current, a bidirectional DC-DC converter has to be installed between the DC bus and the EV battery [19]. These converters are broadly grouped into two major classifications: (i) isolated and (ii) non-isolated. The former has gained more attention owing to various benefits such as smaller size, lower cost, smooth power flow regulation, and ability to offer isolation between two ends [20]. Many types of isolated DC-DC converters are available, which are used to afford high voltage gain by raising the number of turns of the high-frequency transformer [21]. DAB converter is a type of isolated bidirectional DC-DC converter, popularly preferred for EV charging. The main benefit of the DAB converter is to provide galvanic isolation of high frequency, soft-switched turn-on functionality in addition to enhanced efficiency and minimized turn-on losses [22][23][24][25].
In this research work, a PV-fed EV charging system based on a re-lift Luo converter and DAB converter is designed. For transforming the low PV voltage output to a significantly enhanced voltage, a re-lift Luo converter is employed. The converter's unregulated DC output voltage is stabilized and made distortion-free with FLC. The DAB converter is capable of providing galvanic isolation as well as bidirectional power flow. The DAB converter's operation is examined using the simple averaged and small-signal models.
The motivation of the research work, based on the re-lift Luo converter & DAB converter, a PV-fed EV charging system is created. a re-lift The Luo converter is used to increase the low PV voltage output to a greater level. The necessary galvanic separation is effectively provided by the DAB converter, which also permits bidirectional power flow. The electrical industry is predicted to see tremendous growth over the next decade thanks to two of the most promising and quickly developing technologies: electric vehicles and photovoltaics.

Proposed Methodology
EVs and PVs are two of the most viable and fast-expanding technologies that are expected to contribute a significant part to the electrical industry over the next decade. The rising usage of EVs requires the dSevelopment of an effective charging system, which supports fast and efficient EV charging. Figure 1 illustrates the proposed EV charging system model comprising of the solar PV system, DAB converter, re-lift Luo converter with FLC, and EV battery/load. The relift Luo converter boosts the low-voltage output V PV obtained from the PV. The voltage output of the converter is fluctuating and is not stable owing to the solar power's intermittent nature. Hence the FLC, which is a closed-loop control approach is adopted to make the converter output stable and distortion-free.
The converter's actual output voltage V O is analogized to the reference voltage V ref for generating an error e , which is fed as an input for FLC. By comparing e and change in error ce using FLC, a control signal is generated, which is supplied to the FPGA controller for the generation of PWM pulses. The magnitude of these PWM pulses is increased to 15 V from 5 V using a driver circuit. The driver circuit provides a steady gate drive voltage and enhances the re-lift Luo converter's switching operation. The controlled output obtained from the re-lift Luo converter is then fed to the DAB converter, which offers galvanic isolation and a high conversion ratio. The DAB converter with PS control is used for achieving Zero VoltaSge Switching (ZVS) for all the switches present in this converter. While compared to other isolated bidirectional DC-DC converters, the DAB converter has received remarkable interest in applications that integrate EV with a PV system.

Solar Panel Modelling
The direct electric current is generated from a solar cell when it gets exposed to sunlight by utilizing the phenomenon of the photovoltaic effect as shown in Fig. 2. This effect is created by combining two layers of a semi-conducting substance.
The amount of electrical energy produced by the individual solar cells is generally very low. For the sake of getting high power generation, the solar cells are combined to form a solar module which is also termed a solar panel. The PV panel's output current is,

The PV panel's open circuit voltage is,
Here R sh stands for parallel resistance, R s stands for series resistance, I sh represents the current flowing through the parallel resistor, I ph represents the photocurrent, I D represents the diode saturation current, a denotes the diode ideality constant, N s stands for several series-connected solar cells, k denotes the Boltzmann constant, T represents the temperature on an absolute scale and q represents the electron charge. The Re-Lift Luo converter is adopted in (2) this research for enhancing the solar PV panel's output voltage owing to its high voltage gain.

Re-Lift Luo Converter Design and Analysis
Re-Lift Luo converter comprises three diodes D 1 , D 2 , D 3 ; three capacitors C 1 , C 2 ,C 3 ; three inductors L 1 , L 2 ,L 3 ; two power switches S 1 , S 2 and an output capacitor C o as shown in Fig. 3. Capacitors C 2 and C 3 possess voltage boosting characteristics which makes the capacitor voltage V C two times higher than the source voltage V PV . The inductor L 3 serves as a ladder joint for connecting the capacitors C 2 and C 3 to raise the capacitor voltage V C 1 . The source instantaneous current I PV = i L 1 + i L 2 + i L 3 + i C 2 + i C 3 flows when the power switches S 1 and S 2 are turned ON as illustrated in Fig. 4a. Meanwhile, the inductors L 1 and L 3 stores  The source current I PV becomes zero when the power switches S 1 and S 2 are in OFF condition as illustrated in Fig. 4b. The discharging of the inductor L 1 takes place and the inductor's current i L 1 flows via path 1 and results in the charging of the capacitor C 1 . Meanwhile the inductor L 2 discharges energy and the current i L 2 flows to the load R and output capacitor C O . The current i L 1 and i L 2 the decline at this stage. Figure 5 represents the waveforms of the re-lift Luo converter. By considering capacitors C 2 and C 3 value as very large, during steady state condition V PV = V C 2 = V C 3 . At the switch ON condition, V L 3 = V PV . The peak-to-peak current ripple of the inductor L 3 is expressed by the equation as follows: At the switch OFF condition, The voltage across the inductor L 3 is given as During the period kT, the switch is in ON condition, and the current i L 1 increases whereas during the period (1−k) T, the switch is in OFF condition, and the current i L 1 decreases. V PV And − V C 1 − 2V PV − V L 3 are the related voltages supplied to the inductor L 1 . Thus, Similarly, during the period kT, the switch is in ON condition, and the current i L 2 increases whereas during the period (1−k) T, the switch is in OFF condition, and the current i L 2 decreases. 3 are the related voltages supplied to the inductor L 2 . Thus, The value of the inductors L 1 and L 2 are obtained from the following equations, The value of the capacitors C 1 , C 2 , C 3 and C 0 are obtained from the following equations, The converter's voltage output is not steady and it is affected greatly by the non-linear nature of the PV output, hence, the adoption of an effective controller is crucial to make the converter output constant. In this work, FLC is used to control the operation of the re-lift Luo converter as it is capable of enhancing the operation of the converter.

Fuzzy Logic Controller (FLC)
The FLC is defined by IF-THEN rules that are developed based on experts' knowledge about the systems, performance, and so on. Here, the FLC functions as a voltage controller. Figure 6 illustrates the fuzzy control approach for the re-lift Luo converter. The FLC comprises five modules: fuzzifier, decision maker, rule base, database, and DE fuzzifier which are used to perform calculations. The actual DC voltage output of the re-lift Luo converter V O is analogized with a reference voltage V ref to obtain the error voltage e. The error voltage e and the change in error ce are provided to the FLC as inputs and are specified as follows: The value taken at the start of the kth switching cycle is denoted by the letter k . The duty cycle, which is considered the output of the FLC is specified as follows: Here, the gain factor of FLC is represented as , which is adjusted for the sake of getting an effective gain for the FLC. The term d k specifies the fuzzy controller's inferred duty cycle change at the kth sampling time. The fuzzy rules are represented by the following form, Here, A i and B i represent the fuzzy subsets in their universe of discourse and C i denotes the fuzzy singleton. Positive Big (PB), Positive Medium (PM), Positive Small (PS), Zero (ZE), Negative Small (NS), Negative Medium (NM), and Negative Big (NB) are seven fuzzy subsets present in each universe of discourse. The Fuzzy control rules are specified in Table 1.

Fuzzifier
In a fuzzifier, the fuzzification process takes place i.e., a linguistic variable (fuzzy number) is obtained from the transformation of a numerical variable (real number). Here, membership value is generated for a fuzzy variable with the assistance of the membership function. Figure 7 illustrates the input and output membership functions in addition to the overall operational flow chart of the FLC.

Rule Evaluator (Decision making)
Control laws or control gains represent the set of numerical values used in traditional controllers whereas, linguistic rules are used in FLC.

Defuzzifier
In defuzzifier, defuzzification process takes place i.e., the reverse of fuzzification. Here, the fuzzy output is converted into crisp output. The linguistic variable (fuzzy number) obtained as output from the rule evaluator has to be converted into crisp output (real number) as per the requirement of the real world. The change of duty cycle is obtained from the equation given below, Here, i represents the membership function value of the ith output set.

Database
The membership function definitions needed for both fuzzification and defuzzification processes are stored in a database.

Rule Base
The linguistic control rules that are essential for the decision-making logic (rule evaluator) are stored in the rule base.

Analysis of Dab
Converter DAB converter is an isolated bidirectional DC-DC converter that is preferred for high-power applications like charging electric vehicles. This converter possesses bidirectional power flow capacity, softswitching characteristics, high power density, and provides galvanic isolation. Multi-terminal configurations to incorporate loads, energy storage devices and DC sources are possible in the case of a DAB converter. This converter comprises two symmetrical active bridges interlinked by a high-frequency transformer. Galvanic isolation and high voltage conversion ratio are offered by the transformer. The power transfer process is highly influenced by the leakage inductor L s of the transformer and the High Frequency (HF) square waves in the high-frequency link. Thus, when the voltage amplitudes at both sides of the transformer become unequal, the circulating power increases and the power losses also get increased and the efficiency is considerably lowered. To overcome these issues, the PS control method is adopted. Figure 8 illustrates the equivalent circuit diagram of DAB with PS control (Fig. 9). This converter topology is easily controlled by turning ON both the bridges with complementary constant pulse width modulated signals at the duty cycle value of 0.5. At the transformer terminals ( ±V 1 , ±V 2 ) , a square wave voltage signal of high frequency is generated using the aforementioned modulation technique. The two square waves thus generated are phase shifted properly for controlling the power flow by consideration.
where, i L s refers to the leakage inductance current, V T 1 and V T 2 refers to the transformer's primary and secondary voltages respectively. The subsequent equations are attained by solving Eq. (22) in consideration of leakage inductance current representation from Fig. 10 for different converter states in a semi-period.
The operational waveforms for the adopted control approach are illustrated in Fig. 9a and the DAB's simplified schematic representation with PS control is illustrated  Fig. 9b. The leakage induction current in addition to the input current and output current is given in Fig. 10. When the leading and lagging full bridges are switched, the corresponding leakage inductance current values are given as I 1 and I 2 . By utilizing the subsequent i L s conditions, By solving Eqs. (23) and (24), The output average current is given as, The above equation is simplified and given as, The average input current is, The simplified equation of average input current is, From Eqs. (32) and (34), DAB's average equivalent circuit is obtained as shown in Fig. 11.
The analysis of DAB's small signal model is carried out for ensuring its stable operation through the design of a feedback loop. By perturbing and particularizing the Eqs. (32) and (34) of average input and output current, The above equations are used to calculate the g parameters, which are given as, Equations (35) and (36) are used to obtain an equivalent small-signal circuit as shown in Fig. 12.
The DAB converter's dynamic response is given as, It is concluded that the first-order system response has similar characteristics as the variation in output voltage due to the input voltage and phase shift variations.

Results and Discussions
An effective PV-fed EV charging system based on the combination of a re-lift Luo converter and a DAB converter is developed in this work. Using MATLAB simulation, the effectiveness of the proposed system is ascertained. The specifications of the solar panel, re-lift Luo converter, and DAB converter are mentioned in Table 2.
In above Table 2 mention the parameters rating of the Solar Panel, Re-lift Luo converter, and DAB converter. In this work, 8 PV panels are used each panel is 250 W. The rating of the peak power value is 2kW . PV panels have the Open circuit voltage V OC value is 22.6V , short circuit Voltage V SC is 12V and short circuit current I SC rating is 20.833A.
The waveforms of solar radiation and temperature are shown in Fig. 13. The waveforms of temperature and solar irradiance are shown in Fig. 13a and b, respectively. Irradiance wave form is calculated using time, temperature, and irradiance (W/sq.M). Temperature (oC) and time are used to calculate the waveform. To test how well the system works, the solar intensity is changed from 980 W/m2 to 1000 W/m2 in 0.1 s, while the temperature is changed from 25 to 35 C in the same amount of time.
The changes in temperature and solar irradiation have an impact on PV panel output voltage and power as illustrated in Fig. 14. The voltage output of the solar panel is 60V at the beginning, which alters to 80V at 0.1s with the variation in operating conditions. Similarly, the PV-generated power also varies at 0.1s from the value of 1360W to 1500W. Figure 15 presents the output voltage waveform of the Relift Luo converter. It is noted that the converter possesses the ability to provide a stable voltage output of 270V at 0.07 s with the assistance of FLC. Moreover, the obtained output voltage is also free from the peak overshoot problem.
The regulated output of the Re-lift Luo converter is supplied to the DAB converter, which provides galvanic isolation and a high conversion ratio. Figure 16 presents the waveforms that indicate the voltage output of the full bridge inverter and an isolation transformer.
A stable voltage of 290V is acquired from the full bridge inverter and voltage output of 270V is acquired from the isolation transformer. Figure 17 illustrates the waveforms of the output current and output voltage of the synchronous rectifier.  After experiencing peak overshoot issues at the initial stage, the output current and voltage produced from the synchronous rectifier become steady at 0.04s using the adopted control strategy.
As illustrated in Fig. 18a, the voltage gain ratio of the re-lift Luo converter is 1:14, which is comparatively higher than the voltage gains of the other three converters. The voltage gain ratio of boost, SEPIC, and Luo converter is 1:1.5, 1:8, and 1:12 respectively. From Fig. 18b, it is obvious that the efficiency of the re-lift Luo converter is 96% , which is comparatively greater than the efficiency of the other three converters.
The settling time comparison of the PI controller and FLC is illustrated in Fig. 19. The Fuzzy control approach helps the re-lift Luo converter to provide a stable regulated output voltage of 270V at 0.07s . However, in the case of the PI controller, the constant voltage is obtained only at 0.1s.

Conclusion
To tackle the growing energy crisis and reduce the overuse of fossil fuels, an efficient EV charging system based on renewable energy is designed in this research work. The output of the PV system is enhanced with the aid of a re-lift Luo converter and galvanic isolation is achieved by using a DAB converter. The performance of the DAB converter and the Zero Voltage Switching (ZVS) operation range is improved with the implementation of the PS control technique. The re-lift Luo converter delivers a regulated and stable output with 96% efficiency using the FLC method.

Future Work
The efficiency of the EV battery charger will increase as a result of the suggested improved converter's ability to reduce the number of components. Additionally, the EV battery's constant current charging process can be used to extend its lifespan.
Studying and Developing on-Road Dynamic High Burst-Power in Brief Bursts.
Design the system to continue transferring power as efficiently as possible even when there are nearby objects.
To design, create, and test a bi-directional current-fed charging system for electric vehicles that can participate in a smart grid & meet peak demand with a two-way power transfer capability between the grid and the car and achieve more than 90% efficiency over a gap of 25-30 cm.

Conflicts of interest
The authors declare that they have no conflict of interest.