An EV Battery Charger Based on PFC Sheppard Taylor Converter

Transcription

1 An EV Battery Charger Based on PFC Sheppard Taylor Converter Radha Kushwaha, Member, IEEE Department of Electrical Engineering Indian Institute of Technology, New Delhi New Delhi, India Abstract- With the increasing global warming and pollution issues, the transport sector needs to be modified to a green energy based travelling system since this is the only sector which employs quarter of the whole world s energy. It also, is the main contributor to CO2 emissions. Therefore, here an EV (Electric Vehicle) could be the best alternative to solve above issues. To improve the energy utilization of an EV, an attention is needed by the battery charging phenomenon. When an EV battery is charged from a conventional AC-DC converter, it draws a peaky current from the supply and hence makes the whole power profile and indices worst. Therefore, this paper presents a solution to the poor power quality issues. A PFC (Power Factor Correction) converter based on Sheppard Taylor converter topology for a 1kW EV battery charger is presented with the proper design equations. The constant current/constant voltage (CC/CV) mode control is used to regulate the charging commands to the battery by generating the proper gate sequence to the switches used in proposed PFC converter. The proposed converter is designed and its performance is simulated using MATLAB/Simulink tool with a 48V, 100Ah lead acid battery at the output and the obtained results are shown for the EV battery charging process. The observed results verify that obtained input supply current meets the PQ (Power Quality) limits of an IEC standard. Keywords Electric Vehicle; Constant current/constant voltage (CC/CV) mode; AC-DC converter; Sheppard Taylor converter, power quality I. INTRODUCTION A battery powered electric vehicle (BEV) is a recent emerging technology which is being used these days for shifting the transportation globally to a greener and cheaper way. In order to power these vehicles, huge battery packs having finite energy capacity are employed which need to be recharged periodically, typically through an AC-DC converter based battery charger. These chargers are basically consisting of an AC-DC converter (DBR-Diode Bridge Rectifier) as a pivot component to communicate the charging signals to the battery packs employing constant voltage, constant current or constant voltage/constant current (CC/CV) control mode [1-2] to give a regulated DC at the load side. While communicating these charging signals, a careful attention is needed by the input current power quality profile and harmonics spectrum i.e. all the indices should be well within IEEE/IEC standard defined by [3] so that energy usage is maximized and the Bhim Singh, Fellow, IEEE Department of Electrical Engineering Indian Institute of Technology, New Delhi New Delhi, India bhimsinghiitd60@gmail.com battery may have a prolonged life. Since DBR, due to its nonlinear nature, has a bad impact on input power profile, so, for a low THD (Total Harmonic Distortion) and an improved PF (Power Factor), DPF (Displacement Power Factor)and DF (Distortion Factor), a PFC based isolated DC-DC converter is used. A high frequency isolation in PFC converter is beneficial for improved charging characteristics by avoiding any harmonics or unwanted signals enter the battery and hence, increasing its overall performance. Conventional AC-DC converters based EV battery chargers have inherent drawbacks of harmonic injection, voltage distortion, high THD (order of 50-55%) and a very poor PF ( ) of the input mains current. Therefore, to improve the real power utilization of the charging circuits, a number of AC-DC converters, with or without high frequency isolation, are described in the literature. PFC converters are developed with HF isolated transformer (HFT) either in single or two stages. In both types, several topologies like forward, full-bridge, push-pull and half-bridge are defined in the literature in buck and boost categories, while in buck-boost configurations, flyback, Cuk, Zeta and SEPIC converters, as summarized in [4-6], are remarkable topologies to overcome the PQ limitations of the input supply current of the EV battery charger. Where, single stage chargers have the limitations of a large amount of low frequency ripple in the load current, two stage architecture overcomes the situation as described in [6-8].However, with the low power applications, it shows some disadvantages like size, cost and difficulty due to two control loops [9-10]so there should be a topological based solution in single stage only and Sheppard Taylor converter meets the optimum requirement due to having high urge to be used as a PFC converter with improved output voltage regulation and other advantages like 1) shaping the input current when input inductor is operating in CCM (Continuous Conduction Mode) and,2) high PF when input inductor operates in DCM(Discontinuous Conduction Mode) [11]. A Sheppard Taylor PFC converter based topology has the most noticeable property of, inherently, having no controldetuning problems [12] i.e. the input current is pure sine wave even near zero crossings so it's ideal for the proper shaping of the input current and hence improving overall performance of the converter. The new modified arrangements of Sheppard /14/$ IEEE 1

2 Taylor converter have been identified in [13-14] along with the proposed design, modeling and control for the same. The detuning problem is also avoided by the proper switching arrangements for the converter while various improved switching methods have been developed in the literature [[13].Therefore, in this paper, a 1kW EV battery charger circuit based on Sheppard Taylor converter topology with the specifications of 220V AC to 65 V DC (open circuit voltage of the charger ) is designed to charge a lead acid EV battery with 48V, 100Ah rating. The charger with the battery is being simulated in MATLAB/Simulink environment to achieve a high PF and low THD sine wave current drawn from mains so that with proper PQ indices, the supply current follows the IEC standard. The paper is organized in the following way: Section II entails the EV battery charger configuration and different operating stages of Sheppard Taylor converter topology after which in section III, detailed design of the converter is presented with the help of necessary equations and selected values of the different components are displayed. This section also includes the modeling of storage battery used and control technique for the proposed converter. Section IV depicts the simulation results of the proposed Sheppard Taylor converter with the battery load, obtained by simulating the designed circuit in the MATLAB/Simulink environment followed by concluding the work presented, finally, in section V. II. SHEPPARD TAYLOR TOPOLOGY BASED PFC CONVERTER SETUP CONFIGURATION Active Power Factor Correction (PFC) is the process of shaping the input mains current, drawn by the charger, which has to be synchronized with the input supply voltage so that maximum real power is drawn from AC mains. Therefore, the proposed system configuration as well as operating principle of the proposed converter is described in this section to obtain a supply current free from any harmonics having a high PF, high DPF and low CF (Crest Factor): A. Topology Description Fig.1 shows the basic configuration of a Sheppard Taylor PFC converter for the EV battery charging purposes. The charger is designed for 1kW rating maintaining a constant 65V at an open circuit voltage across it throughout the converter operation so that when the battery is connected across the charger terminals, a continuous flow of current into the battery is maintained depending upon the control methodology used for charging. In order to maintain a high PF and sinusoidal nature of the supply current, input inductor L 1 is designed to operate in DCM while at the output side L O is having such a value which maintains the output current continuous throughout the operation. The capacitor C 1 is a capacitor which is main contributor to the current shaping feature. It controls the input current position during on and off time of switches with a particular switching arrangements and thus adding a DC component to the average input supply current to make it perfectly sinusoidal and avoiding any control detuning problem. Various operating stages of the proposed converter are elaborated in the following subsection. Fig.1 Sheppard Taylor Topology Based PFC Converter Setup Configuration B. Operating Stages In order to understand the phenomenon of maintaining high PF and improved harmonics profile at the input, the principle of the proposed PFC converter based battery charger has to be defined very well. The proposed system operates on the basis of the charging and discharging of the three components, an input inductor, a capacitor and an output inductor. The input inductor L 1 is being operated in DCM so a small value of it has been taken in the design while with a larger value, an output inductor is working in CCM. Therefore, three operating stages are defined for the proposed PFC. In stage-i, both the switches are ON and inductor L 1 is charging through the ON switches and diode D 3 so, according to the dot convention, diode D 4 becomes forward biased and (a) Fig.2 Three operating stages of the proposed PFC Sheppard Taylor Converter (a) Operating stage-i Operating stage-ii Operating stage-iii 2

3 energy of the capacitor C 1 is available to the output stage as depicted in Fig.2(a). During OFF time of the switches, reverse process happens which defines stage-ii as in Fig.2. An input inductor L 1 discharges through diodes D 1 and D 2 giving the favorable charging conditions to the storage capacitor C 1 due to restricting the HFT (High Frequency Transformer) from drawing any significant magnetizing current from the input. At this stage, D 5, the freewheeling diode, comes into the picture and provides the necessary energy to the load. The final stage is stage-iii, an inductor L 1 is fully discharged, current I L1 in inductor comes to zero and the output inductor supplies to the load through freewheeling diode and operation of the converter for one switching cycle is completed. III. DESIGN AND CONTROL OF PROPOSED PFC SHEPPARD TAYLOR CONVERTER BASED EV BATTERY CHARGER For perfect power factor correction in any circuit, the input current should follow the input voltage, in the same manner, as in a pure resistive circuit, free from any input current harmonics and the first key technique to achieve this goal is to design the components with the appropriate specifications and then control is to be implemented in such a way to maintain an input current within international IEC standard for a well modeled battery equivalent circuit. Therefore, in this section, the design process of a Sheppard Taylor based PFC converter is presented, followed by the modeling of the storage battery and the control technique employed for charging process, respectively, in the subsequent parts for maintaining a high PF and low THD current drawn from AC mains. A. Design Equations This subsection deals with the design of a front end PFC converter of 1kW supplied by a 220V AC at the input for maintaining DC link voltage constant at 65V, used for charging 48V,100Ah EV battery along with the improving power quality indices of the mains current. The necessary design formulas for different components have been presented as follows. 1) Estimation of Duty Cycle Since, this is a buck-boost type converter therefore, duty cycle is obtained as, Vo = ( N2 / N1)V 1D/(1 D),whereV1 = 2 2 Vs/ π Vo Hence, D = (1) ( N2 / N1) 2Vs + Vo N2 / N 1 =HFT turns ratio considered as /220=..656 and D, the duty cycle is calculated at the nominal peak value of supply voltage. 65 D = = * So, duty ratio is considered as 0.2 for operation of the proposed PFC converter. 2) Estimation of Input inductor L 1 An input inductor L 1 is selected such that it goes into discontinuous conduction mode in corresponding one switching cycle when selecting a switching frequency of 50 khz. The design value of L 1 is given as, DV ( c1+ V1) L1 = fs * I (2) L1 V1 198 wherevc1 = = = 330V = storage (1 2 D) 1 2*0.2 capacitor voltage, fs = 1/ Ts = witching frequency = 50kHz and duty ratio is calculated corresponding to rated peak source voltage(220 2V). 0.2( ) L1 = = 418.2μH 50000*(1000 / 198) To operate L 1 in DCM over wide AC voltage and load range, the selected value is taken very small i.e. 42μH approximately, which is one tenth of calculated one. 3)Estimation of Storage Capacitor,C 1 Like the other components in the proposed circuit, in order to transfer the HFT energy properly to the output side and to improve the control tuning at zero crossings of the input mains current, the storage capacitor is given by the equation, VD o C = 1 ( Δ V f R ) (3) c1 s o where Δ Vc1 = 10% ofvc1=ripple in the storage capacitor voltage and R o being the emulated load resistance depicting battery load= R 2 o = Vo / Po 65*.2 Now, C1 = = 1.864μF 2.1*330*50000*(65 / 1000) The value C 1 is also selected to operate in DCM therefore the selected value is.15μf less than the calculated one. 4) Estimation of Output inductor, L o This component is designed to operate in CCM at rated value of dc link voltage and given by the equation, 2 2 Vo (1 D) N1 Lo min = 2IDf o s N2 (4) where duty ratio has been calculated for rated source voltage (220 2V). 2 65*(1.2) Lo min = = μH 2*(1000 / 65)*50000*.2 L o is designed to operate in CCM so selected value is taken large enough to keep the current continuous for one switching cycle. Selection of output inductor depends on the inequality given as, 2 D 1 L1 Lo ' 2 2π f C s 1 which gives the calculated value of L o as.862μh so selected value is 5μH for CCM operation. 5) Estimation of DC Link Capacitor, C o The value of DC link Capacitor Δ V o is calculated for a given ripple, in the output side voltage, 3

4 Io Co = (2ωΔ V o ) (5) where Δ Vo =.1* Vo =ripple in DC link voltage. The value of DC link Capacitor is calculated for 10% ripple in the output side voltage as, Co = = 3.77mF (2*314*.1* 65) Therefore, a DC link capacitor of a 4mF is selected that maintains 65V DC as the optimum charging voltage for battery. 6)Estimation of Input Filter Capacitor, C f An input EMI filter (a low pass L-C filter) is used to prevent any higher order harmonics to enter into the supply. The design expression is given as [12], Cf << Cf max, I pk (P 2 / V ) (6) s Cfmax = tanθ = tanθ ωvm ( ω 2V s ) Here, θ defines the displacement of the two fundamental values of supply voltage and mains current. This capacitor value is estimated as, ( / 220) fmax tan1 C = = 1149 nf (314*220 2) Selected value of the input filter capacitor is taken as 330nF, very less than the maximum value of capacitor. 7) Estimation of Supply Inductance, L s L s is the source inductance that is considered as 2% of the base impedance and ω =line frequency (rad/s). 2 2 s 1 V Ls =.02( ) mH ω P = = Selected value of the input side source inductance is taken as 5mH. All the necessary design specifications are tabulated in table-i. TABLE-I DESIGN SPECIFICATIONS FOR SHEPPARD TAYLOR CONVERTER Output Power, P o 1kW Output dc voltage, V o 65V Output DC current, I o 15.38A Switching frequency, f s 50KHz Input inductor,l 1 42μH Intermediate Capacitor, C1.15μF Turns Ratio, N1/N Output Capacitor, Co 4mF Input Filter Capacitor, Cf 330nF B. Modelling of Lead Acid Storage Battery for EV The battery is a key element for well-functioning of an electric vehicle so the proper modeling is necessary to show the charging characteristics of the proposed PFC converter based battery charger. In the proposed work, a 48V, 100Ah (4 packs of 12V, 100 Ah) lead acid battery, a generalized rating for EV in function these days, is modeled and implemented using MATLAB/Simulnk tool. Fig.3 Model of storage Lead acid battery The thevenin s equivalent model of the storage battery is presented in Fig.3, where symbolizing, R s as the equivalent series resistance of the battery, V oc as the open circuit voltage of the battery (nominal battery voltage i.e. 48V, in this case), R b as the self-discharge resistance and C bb as the equivalent capacitor to show the energy storage capacity of the battery. The equivalent storage capacitor of the battery, C bb is calculated as, kwh*3600*1000 C bb = (7) (Voc,max V oc,min ) where, kwh is the battery energy storage capacity and V oc,max and V oc,min are the battery terminal voltages at fully charged and discharged condition i.e. 56V and 42V for the proposed work. Therefore, the value of C bb is calculated as 22.5*10 6 F for required purpose. R s is taken as 0.01Ω, while value of R b is taken very high (10kΩ, in this case) so that the battery may not discharge very rapidly while kept idle. Therefore, while simulating the converter circuit with MATLAB, abovementioned parameters are used in the equivalent circuit of storage lead acid battery to acquire the charging characteristics of the charger within specified PQ limits. C. Control used for Charging EV Battery To control the signals for issuing the charging commands to the battery, two PI (Proportional and Integral) controllers are used, as shown in Fig.4. These controllers take care of the timing and sequence of the switches responsible for regulating output DC voltage and in accordance the current going into the battery. Based on the battery charge and voltage level, the battery charging operates either in constant current (CC) mode or constant voltage (CV) mode. Therefore, having the two charging modes, either voltage PI or current PI controller generates reference voltage or reference current to control the battery charging voltage and current for both the modes by adjusting the duty cycle, in effect, with PWM generator used for the purpose. The control employed here known as constant voltage/constant current mode control in which initially the battery is charged at constant current value but the battery voltage increases depending upon SOC (State Of Charge) level (CC Mode)and after reaching a certain battery voltage, the control shifts to CV mode, keeping the battery voltage fixed at the constant reference value. For the voltage control, DC link voltage is sensed and after comparing with the 4

5 reference given, an error signal is generated that is symbolized by V e. This error V e at k th instant of sampling is given as, * V ( k) = V ( k) V ( k) (8) e o o Fig.4 Block Diagram of Proposed Control Strategy The abovementioned error is the input to outer PI controller. The output of PI controller which generates the reference for the inner loop PI controller, at any k th instant of sampling is defined as, Cv( k) = Cv( k 1) + Kp{V(k) e V(k e 1)} + Ki V(k) e (9) This loop is responsible for controlling the current, accordingly for switching sequence generation of both the switches in synchronism. The current going into the battery is kept within desired limits using another PI controller. The battery current at the output is observed and after being compared to the reference i.e. the output of voltage PI controller, an error signal I e is generated which is integrated by the inner PI controller to decide the current handling capability of the proposed charger. Therefore, like voltage PI controller, the output of an inner PI controller is also decided at k th instant of sampling as, C ( k) = C ( k 1) + K {I (k) I (k 1)} + K I (k) I I p e e i e (10) After the inner PI controller, a PWM generator is used along with the saw-tooth generator and a comparator to create gating pulses for the converter in synchronism with the other signals for both the switches, well shown in Fig.1. Thus, the duty cycle is varied according to the change in output voltage and current of the proposed battery charger. IV. PERFORMANCE OF PFC SHEPPARD TAYLOR CONVERTER BASED EV BATTERY CHARGER The proposed PFC converter based battery charger for EV is tested and verified by simulating it with the help of MATLAB/Simulink tool which clearly proves that the converter behavior is following the international PQ standard based on the results shown not only for the rated battery load and supply conditions but it also for the wide changes in the supply voltages (170V-270V). Simulated results are shown in Fig.5 (a)-, Fig.6 (a)-, and Fig.7 (a)-, respectively for rated load and supply conditions in constant current mode charging with initial SOC of 60%, charging in CC mode while there is a reduction in supply voltage(170v) and charging in CC mode while there is an increment in the supply voltage (270V). Figs.5 (a)- show the satisfactory performance of the (a) Fig.5 performance evaluation of PFC Sheppard Taylor converter based battery charger at rated load and supply conditions(a) Input and Output side waveforms Source current waveform Source current harmonics spectrum (a) Fig.6 Performance evaluation of PFC Sheppard Taylor converter for reduction in supply voltage from 220V to 170V at.4sec (a) Input and Output side waveforms at 170V Source current waveform Source current harmonics spectrum 5

6 (a) Fig.7 Performance evaluation of PFC Sheppard Taylor converter for reduction in supply voltage from 220V to 270V at.4sec (a) Input and Output side waveforms at 270V Source current waveform Source current harmonics spectrum proposed PFC converter with 1.69% THD of the mains current along with a constant and low ripple in DC link voltage at rated supply voltage. In Fig.6 (a),when the supply voltage is decreased from its nominal value of 220V to 170 V at 0.4 s, the supply current still depicts a low THD in the FFT analysis tool(figs.6 b-c) that remains within standard (1.21%) along with maintaining a high PF and perfect sine wave shape. Fig.7 (a) characterizes the increment in the input voltage up to 270V from its nominal value of 220V at 0.4 s. The converter is drawing sinusoidal mains current and harmonics spectrum shows a THD less than 5% (3.93%) as depicted by Figs.7 and so PQ recommendations are not violated, for this case also. The battery voltage increases from 48 V in CC mode that is nominal value of the battery voltage and charging current is maintained at 10A for a 48V, 100 Ah lead acid battery. The battery is charged at constant current mode initially and it switches to the constant voltage mode after achieving a particular SOC level depending upon the charging conditions. Thus, the above evaluation proves the suitability of the PFC based charger for EV application with enhanced qualities of good output regulation and low ripple in the output to improve the charging profile of the electric vehicle battery. V. CONCLUSION The proposed methodology has been found suitable for charging an EV battery with constant current/constant voltage mode control using two switches operating in synchronism in Sheppard Taylor converter topology with inherent advantages of reduced components, high PF with low THD, switching at constant frequency, low switch current stress and hence low conduction losses, simple control due to use of one signal for gating, low ripple at the output and no detuning in control. An active power factor corrected technique employing Sheppard Taylor topology has been used with necessary design equations and simulation results based on MATLAB/Simulink tool has been used to verify the converter's operation for rated specifications as well as for certain disturbances in rated supply. The overall performance of the proposed Sheppard Taylor converter is found satisfactory with high PF and low THD according to the international IEC PQ standard. REFERENCES [1] Marian K. 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