design and performance evaluation of bidirectional dc-dc

IPASJ International Journal of Electrical Engineering (IIJEE) Web Site: http://www.ipasj.org/IIJEE/IIJEE.htm

A Publisher for Research Motivation ........ Email:[email protected] Volume 8, Issue 9, September 2020 ISSN 2321-600X

Volume 8, Issue 9, September 2020

ABSTRACT Nowadays technology is growing very fast and our conventional sources of energy are limited and hence they are depleting. If these sources are used continuously, there will be no conventional sources in future. Solar and wind technology are most promising technology with battery storage system. With this we need a very robust interfacing device that can connect the battery storage system with the grid. These energy systems are not reliable to feed the power as a standalone system because of the large fluctuations in output and hence these energy system are always connected with energy storage devices such as batteries and super capacitors. These energy storage devices store the surplus energy during low demand and provide backup in case of system failure. A conventional dc-dc converter can be converted into a bidirectional converter using bidirectional switch by using diode in anti-parallel with MOSFET or IGBT allowing current flow in both the direction using controlled switching operation. Charging and discharging is based on the state of charge of the battery and direction of the current. Algorithm for charging and discharging of battery is proposed and given in upcoming sections. The constancy of the both charging voltage, discharging voltage, charging current, discharging current and the automatic switching of the system is tested via simulation. Hence there is no chance of overcharging of battery that leads to reduction in the life of battery. Keywords: Bidirectional, half-bridge, efficiency, switching, PMDC motor, Converter.

1. INTRODUCTION Basic buck and boost converters and their derivatives do not have bidirectional power flow capability. This limitation is due to the presence of diodes in their structure which prevents reverse current flow. In general, a unidirectional dc-dc converter can be turned into a bidirectional converter by replacing the diodes with a controllable switch in its structure. The bidirectional dc-dc converter along with energy storage has become a promising option for many power related systems, including hybrid vehicle, fuel cell vehicle, renewable energy system and so forth. It not only reduces the cost also it improves the performance of the system. In the electric vehicle application, an auxiliary energy storage battery absorbs the regenerated energy fed back by the electric machine. In addition, bidirectional dc-dc converter is also required to draw power from the auxiliary battery to boost the high-voltage bus during vehicle starting, acceleration and hill climbing. With its ability to reverse the direction of the current flow and power, the bidirectional dc-dc converters are being increasingly used to achieve power transfer between two dc source in either direction. In renewable energy applications, the multiple-input bidirectional dc-dc converter can be used to combine different types of energy sources. This bidirectional dc-dc converter provide galvanic isolation between the load and the fuel cell, bidirectional power flow, capability to match different voltage levels and fast response to the transient load demand.

Design and Performance Evaluation of Bidirectional DC-DC Converter

Chandrika S. Kukanur1, Shwetha S. Baligar2, Sushma Kashigoudra3, Uday Kumbar4, and

Manjunath B. Ranadev5

1Chandrika S. Kukanur Student of Electrical and Electronics Engineering Dept, KLE Institute of Technology Hubballi-

580027, Karnataka, India

2Shwetha S. Baligar Student of Electrical and Electronics Engineering Dept, KLE Institute of Technology Hubballi-580027, Karnataka, India

3Sushma Kashigoudra Student of Electrical and Electronics Engineering Dept, KLE Institute of Technology Hubballi-

580027, Karnataka, India

4Uday Kumbar Student of Electrical and Electronics Engineering Dept, KLE Institute of Technology Hubballi-580027, Karnataka, India

5Manjunath B. Ranadev Asst Professor of Electrical and Electronics Engineering Dept, KLE Institute of Technology

Hubballi-580027, Karnataka, India




Nowadays clean energy resources such as photovoltaic arrays and wind turbines have been exploited for developing renewable electric power generation systems. The bidirectional dc-dc converter is often used to transfer the solar energy to the capacitive energy source during the day time while to deliver energy to the load when the dc bus voltage is low. The bidirectional dc-dc converter illustrated in Fig.1 is characterized by voltage fed sources on both sides. Based on the placement of the auxiliary energy storage, the bidirectional dc-dc converter can be categorized into buck and boost type. The buck type is to have energy storage placed on the high voltage side and the boost type is to have energy storage placed on the low voltage side. To realize the double sided power flow in bidirectional dc-dc converter, the switch cell should carry the current on both direction.

Figure 1 Illustration of bidirectional power flow.

It is usually implemented with a unidirectional semiconductor power switch such as power Metal-Oxide-Semiconductor-Field-Effect Transistor (MOSFET) or Insulated Gate Bipolar Transistor (IGBT) in parallel with a diode. For the buck and boost dc-dc type converters, the bidirectional power flow is realized by replacing the switch and diode with the double sided current switch cell. A DC to DC converter is used for storage and retrieval of electrical power across the Electric Vehicle (EV), the grid targeting Grid-to-Vehicle (G2V) and Vehicle-to-Grid (V2G) technologies. A bidirectional DC to DC converter is used to implement the charging and discharging of the electric vehicle battery. In electric vehicle battery charging process Grid-to-Vehicle (G2V) must be regulated to preserve the power quality in the power grids. Nevertheless, with the proliferation of electric vehicle (EV) a considerable amount of energy will be stored in their batteries, arising the opportunity of the energy flow in opposite sense (V2G). Since many converter topologies were available such as DC to DC converter connected by a capacitor that shares a DC link voltage.

2. MOTIVATION AND PROBLEM STATEMENT At present there is a lack of broad knowledge and experience with systems using multiple batteries. Therefore the motivation of this report is to increase the knowledge about converters and energy control strategies specifically by studying, designing and simulation. In present systems the bidirectional dc-dc converter along with energy storage has become a promising option for many power related systems, including hybrid vehicle, fuel cell vehicle, renewable energy system, industries and so forth. The proposed converter is designed involving an open loop control. By using the modern controller high output voltage and high gain can be obtained by controlling the duty cycle of the switches. Heat loss can be reduced which in turn increases the life span of the switch. It not only reduces the cost and improves efficiency, but also improves the overall performance of the system.

3. METHODOLOGY Numerous studies have been conducted on bidirectional DC-DC converters. Bidirectional DC-DC converter is a sort of circuit which enables the electric energy to be transferred bidirectionally with the invariable polarity of input and output voltage and the reversible polarity of input and output current. Since it is controlled by variable control signals such a converter achieves bidirectional power flow transfer and a dual purpose only with sole device.

Figure 2 Schematic diagram of bidirectional DC-DC converter




Bidirectional dc-dc converters serve the purpose of power flow in both the direction. Traditionally they were used for the motor drives, speed control and regenerative braking. Fig. 2 shows the block diagram of bidirectional dc-dc

converter. During charging mode the power flows from high voltage level battery B1 to low voltage level battery B2 through bidirectional dc-dc converter and charges the battery B2. During discharging mode battery B2 discharges and power flows from low voltage level battery B2 to high voltage level battery B1 through converter and charges the battery B1. The operation of switching devices in the converter is controlled by converter controller. Power flow between battery B1 and battery B2 is bidirectional. Therefore bidirectional dc-dc converter is required for the efficient power flow.

4. CIRCUIT DIAGRAM The converter is operated in continuous conduction mode for charging and discharging. The MOSFET is used for switching in such a way that the converter operates in steady state with four sub intervals namely interval (t0-t1), (t1-t2), (t2-t3), and (t3-t4). It should be noted that the high voltage battery side is taken as VH and low voltage battery side is taken as VL. The circuit diagram of bidirectional dc-dc converter is shown in fig 3.

Figure 3 Bidirectional DC-DC Converter Figure 4 charging mode operation (a) Operation during Interval 1 (b) Operation during Interval 2 4.1 CIRCUIT OPERATION 4.1.1Mode 1(Charge Mode): In this mode switch Q1 and diode D2 will conduct depending on the duty cycle. While switch Q2 and diode D1 remains in complete off state. This mode can further be divided into two intervals and is shown in Fig 4. (a)Interval 1 (t0-t1): In this interval switch Q1 conducts. The battery voltage VL is less than battery voltage VH. So the power flows from higher voltage level battery VH to lower voltage level battery VL. During this time period higher voltage level battery VH charges the inductor L, capacitor C2 and supply power to lower voltage level battery VL through MOSFET Q1. Fig 4 (a) shows interval 1 operation. (b)Interval 2 (t1-t2): In this interval diode D2 conducts. Inductor L releases its stored energy to capacitor C2 and the lower voltage level battery VL. Hence current freewheels through diode D2. Fig 4 (b) shows interval 2 operation.

Figure 5 Discharging Mode operation (a) Operation during Interval 3 (b) Operation during Interval 4.

4.1.2 Mode 2 (Discharge Mode): In this mode switch Q2 and diode D1 conducts based on the duty cycle. while switch Q1 and diode D2 remains in complete off state. This mode can further be divided into two intervals. Fig 5 shows mode 2 operation. (a)Interval 3 (t2-t3): In this interval switch Q2 conducts. Lower voltage level battery VL and inductor voltage adds up and becomes greater than the higher voltage level battery VH. Therefore the current starts flowing from lower voltage




level battery VL and charges the inductor with opposite polarity through Q2. Hence current freewheels through Q2. Fig 5 (a) shows interval 3 operation. (b)Interval 4 (t3-t4): In this interval Diode D1 conducts. Hence current flows from lower voltage level battery VL to higher voltage level battery VH. Fig 5 (b) shows interval 4 operation. 4.2 CONVERTER CONTROLLER Pulse-width modulation (PWM) is used to control the output voltage of dc-dc converters by controlling the on and off state of the switches. As the name implies the width of a pulse is controlled by varying the time the pulse is on in relation to the switching period. The ratio of on period ton to the switching period Ts called the duty cycle. The duty cycle is expressed in eqn 1.

(1) =ܦ

Figure 6(a) shows a basic switch-mode dc-dc converter and Figure 6(b) illustrates PWM implementation to control a switch with a certain duty cycle that determines the average output voltage Vo even though the input voltage and output load may fluctuate.

Figure 6 Switch mode DC-DC conversion Figure 7 PWM (Pulse Width Modulation)

(a) Basic switch mode DC-DC converter (b) PWM implementation By comparing a control voltage with a repetitive signal with a constant peak, a switch control signal with a duty cycle that will achieve the desired output voltage can be generated from a PWM. Generally, the control signal is the difference between the actual and desired output voltage. The repetitive signal is a saw tooth signal where its frequency determines the switching frequency. The relation between the duty cycle, control voltage and peak of the saw tooth signal is expressed in eqn 2.

(2) =ܦ Where Vcontrol is the control voltage and Vst is the peak of the saw tooth waveform which is shown in Figure7.

5. BIDIRECTIONAL DC-DC CONVERTER (BDC) DESIGN

A dc-dc converter generates a pulsating ripple current having a high di/dt at the input. This can cause serious electromagnetic interference (EMI) problem. Moreover charging current with high ripple content raises the operating temperature of the battery and degrades its service life. So capacitors become necessary for filtering the voltage ripple so that a constant dc voltage is available at the output. The calculation of inductance and capacitance is discussed below. When the circuit tends to stable-state, the average current through the capacitor should be zero, so the variation of the electrical quantity in the half cycle is expressed as in the eqn 3.

(a) (b)




= (3)

Where is the variation of the charge, is the ripple current, T is the working time. Thereby the peak voltage of the capacitor can be computed using eqn 4.

= = (4) Where is the variation of the peak voltage of the capacitance C. Ripple current can be obtained using the eqn 5.

(5)

Where is the conduction angle. So the ripple current can be computed using the eqn 6.

= (6) By dividing equation 5 and equation 6, we get:

= (7) In eqn 7, let = 0.1%. The value of capacitors C1=C2=C=1000µF and switching frequency = 20KHz is assumed. So the working period T = 0.05ms. Hence the inductance L can be computed using the equation 7 = 156.25

The value of inductance calculated from the above design at conduction angle 0.5 is 156.25micro Hendry.

6. CONVERTER CONTROLLER FLOWCHART

Figure 8 shows the system simulation flowchart. Initially the status of M1 is verified. If it is on, power flows from Battery B1 to battery B2. This power flow continues until M2 ON condition is satisfied. If M2 turns on then the condition B2 greater than B1 (B2>B1). If it is satisfied then the current freewheels through Q2.

Figure 8 System Simulation Flowchart




If B2>B1 condition is not satisfied then current freewheels through D2 until the condition B2>B1 gets satisfied. After freewheeling through Q2 it checks for M2 OFF condition. If M2 is off then power flows from B2 to B1. If M2 is still on then current freewheels through Q2 until M2 OFF condition is achieved.

7. SIMULATION AND IT’s RESULTS

Simulation for battery charging and discharging is carried out. Bidirectional dc-dc converter was simulated in Matlab Simulink. The data is recorded when the batteries supply each other via bidirectional DC-DC converter. The simulation results were found satisfactory. Fig 9 shows the simulation model of Bidirectional DC-DC Converter. The components used in the simulation model are nickel metal hydride battery, MOSFET, series RLC circuit and pulse generator. The details pertaining to each component is discussed below.

Figure 9 Simulation model of Bidirectional DC-DC Converter

7.1 Simulation Results for 10% Duty Cycle

The following results are obtained when MOSFET 1 is operated for 10% duty cycle. MOSFET M1 is ON for 10% duty cycle (i.e., 0.1 second) and MOSFET M2 is OFF for 10%. For this 10% of ON time of M1, the power flows from battery B1 to battery B2. The SOC of battery B1 reduces and SOC of battery B2 increases. These both conditions can be observed in Fig 10(a) and 10(d) respectively. The maximum charging current observed is 0.32A. Fig 10(b) and Fig 10(e) shows the maximum charging current at 0.1 second of the pulse period. M1 turns OFF for 90% (i.e., 0.9 second) and M turns ON for 90%. The energy stored in inductor L is transferred to capacitor C and battery B2. So the current freewheels through diode D2. During this freewheeling, the voltage across the inductor increases. The battery B2 voltage and inductor voltage adds up and becomes greater than battery B1 voltage. Hence Battery B2 discharges. The polarity across the inductor reverses and current freewheels through M2. The freewheeling continues till M1 turns on again. The regeneration process occurs for shorter duration before next cycle starts charging and hence current flows from battery B2 to battery B1. The maximum discharging current observed is 37.76A. Fig 10(b) and Fig 10(e) shows the maximum discharging current at one second of the pulse period. At this time SOC of battery B2 reduces and SOC of battery increases. Fig 10(a) shows the SOC of battery B1 in which the SOC has increased after one second. Fig 10(b) shows the SOC of battery B2 in which the SOC has decreased after 1 second.




Figure 10 Waveforms for 10% duty cycle (a) SOC of Battery B1 (b) Battery B1 Current IB1 (c) Battery B1 Voltage VB1 (d) SOC of Battery B2 (e) Battery B2 Current IB2 (f) Battery B2 Voltage VB2.

7.2 Performance analyses of the converter

(a)

(b)

(c)

(d)

(e)

(f)




As duty cycle increases the time of freewheeling and discharging current decreases with increase in charging current. This can be observed in Table 1.

Table 1: Variation of current with change in duty cycle

Initial voltage of battery B1 is 51.72 and battery B2 48.49. With increase in duty cycle of M1 voltage of battery B1 decreases during charging and voltage of battery B2 increases during discharging. While the voltage across inductor decreases during discharging. This can be verified in Table 2.

Table 2: Battery voltages with change in duty cycle

During charging battery B1 delivers power to battery B2. The input power and the output power of the converter increases with increase in duty cycle of M1. While discharging battery B2 delivers power to battery B1.

Table 3: Efficiency during charging and discharging

The input power and the output power of the converter decreases with increase in duty cycle of M1. Efficiency of the converter is given by ratio of output power to input power. Efficiency of the converter increases with increase in duty cycle of M1 for both charging and discharging. This can be verified through Table 3.

7.3 Performance characteristics of the converter without delay

Figure 10 shows the variation of voltage with respect to duty cycle during charging. X-axis represents duty cycle and Y-axis represents voltage. During charging as duty cycle of M1 increases voltage across B1 decreases and voltage across B2 increases which is shown in Fig 11(a) and Fig 11(b) respectively.




(a) (b)

Figure 11 Variation of voltage with duty cycle during charging (a) Duty cycle of M1 v/s voltage across B1 (b) Duty cycle of M1 v/s voltage across B2 Figure 12 shows the variation of voltage with respect to duty cycle during discharging. X-axis represents duty cycle and Y-axis represents voltage. During discharging as duty cycle of M1 increases voltage across B1 decreases and voltage across B2 increases as shown in Fig 12(a) and 12(b) respectively.

(a) (b)

Figure 12 Variation of voltage with duty cycle during discharging (a) Duty cycle of M1 v/s voltage across B1 (b) Duty cycle of M1 v/s voltage across B2 Figure 13 shows the variation of current with respect to duty cycle. X-axis depicts duty cycle and Y-axis depicts current. As duty cycle of M1 increases charging current increases and discharging current decreases as shown in Fig 13(a) and 13(b) respectively.

(a) (b)

Figure 13 Variation of current with duty cycle (a) Duty cycle of M1 v/s maximum charge current (b) Duty cycle of M1 v/s maximum discharge current Figure 14 shows the variation of efficiency with duty cycle of M1. The efficiency of the converter increases with increase in duty cycle during charging.

(a) (b)

Figure 14 Variation of efficiency with duty cycle (a) Duty cycle of M1 v/s efficiency during charging (b) Duty cycle of M1 v/s efficiency during discharging




The rate of increase of efficiency for each duty cycle is about 1% to 2% as shown in Fig 14(a). Similarly the efficiency of the converter increases with increase in duty cycle during discharging. But the rate of increase of efficiency for each duty cycle is 0.06% to 0.08% as shown in Fig 14(b). The efficiency of the converter is relatively high during discharging when compared to efficiency of the converter during charging. All the cases considered in simulation are ideal and amount of losses are less. Hence higher efficiency is achieved. When the hardware model is implemented similar results with improved performance can be obtained.

8. CONCLUSION This paper presents a battery operated dc-dc converter based on bidirectional half bridge and performance of the system has been verified by simulation using MATLAB/SIMULINK environment. The use of the zero voltage switching technique makes the system operation more efficient. The design analysis of bidirectional converter is discussed, which reaches conversion efficiency of 94% and achieves charging and discharging. In charging mode the charging current is approximately constant regardless of the alteration in the input voltage. And the charging conversion efficiency is increased up to 94%. Besides in discharging mode, the output voltage is around 50V and conversion efficiency is up to 99%. Proposed algorithm is based on the state of charge and direction of the current so the problem related to battery overcharging gets eliminated which increases the battery life. References [1] L.Jian, H.Xue, G.Xu, X.Zhu, D.Zhao, Z.Y.Shao, “Regulated Charging of Plug-in Hybrid Electric Vehicles for

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[12] S.G. Archana Priyadharsini1, Dr.K. Yasoda “Design and Control of Bidirectional DC - DC Converter for Electric Vehicle Application,” IJSRD - International Journal for Scientific Research & Development| Vol. 7, Issue 03, 2019 | ISSN (online): 2321-0613.

AUTHOR

Mr. Manjunath B. Ranadev born in Dharwad district, Karnataka, India. He obtained Under Graduate (BE) degree in Electrical and Electronics Engineering from S.D.M College of Engineering and Technology, Dharwad affiliated to V.T.U Belgaum, Karnataka (India) during the year 2007. He also obtained Post Graduate (M.Tech) degree in Industrial Electronics from SJCE Mysore affiliated to V.T.U Belgaum, Karnataka (India) during the year 2009. He has teaching experience of 11 years and is presently working as Assistant Professor in K.L.E. Institute

of Technology, Hubballi, Karnataka, India. He has published 3 papers in international journals. His research interests are in the field of Electric Machines, Power Electronics and Industrial Drives. He is member of Institution of Engineers (India) and also a member of International Association of Engineers (IAENG).

Miss. Chandrika Shankar Kukanur born in Hubballi, Dharwad district, Karnataka, India. She obtained under graduate (BE) degree in Electrical and Electronics Engineering from K.L.E Institute of Technology, Hubballi, Karnataka, (India) during the year 2020. She has published 1 paper in international journals.

Miss. Shwetha Shanmukhappa Baligar born in Hubballi, Dharwad district, Karnataka, India. She obtained under graduate (BE) degree in Electrical and Electronics Engineering from K.L.E Institute of Technology, Hubballi, Karnataka, (India) during the year 2020.She has published 3 papers in international journals

Miss. Sushma Kashigoudra born in Hulkoti, Gadag district, Karnataka, India. She obtained under graduate (BE) degree in Electrical and Electronics Engineering from K.L.E Institute of Technology, Hubballi, Karnataka, (India) during the year 2020.

Mr. Uday Kumbar born in Lingasugur, Raichur district, Karnataka, India. He obtained under graduate (BE) degree in Electrical and Electronics Engineering from K.L.E Institute of Technology, Hubballi, Karnataka, (India) during the year 2020.

design and performance evaluation of bidirectional dc-dc

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