Research Paper MULTIPLE INPUT BIDIRECTIONAL DC-DC CONVERTER Gomathi.S 1, Ragavendiran T.A. S 2 Address for Correspondence M.E.,(Ph.D).,Assistant Professor, St. Joseph s institute of Technology, Chennai M.E., Ph.D., Principal, Anand Institute of Higher Technology, Chennai India. ABSTRACT In this paper two batteries of different voltage level are interfaced and also the power is transferred in both the directions i.e., bidirectional power flow capability. By using more than one dc sources the system is supplied more power during peak demands and by using bidirectional converter the charging and discharging of batteries takes place. KEYWORDS Batteries as sources, bidirectional power flow, dc-dc converter, multiple input. I. INTRODUCTION In future, there will be more applications of using more than one dc sources so that more power is supplied during peak demand. Multiple-input converters (MIC) have been proposed as a cost-effective and flexible way to interface various sources and, in some cases, energy-storage devices. Multi input bidirectional DC-DC converter is used to interconnect the multiple sources with different voltage levels. It reduces the system size, cost and power losses due to the less number of components used in the system. The purpose of multi input bidirectional converter is increase or decrease the voltage level of the system with bidirectional power flow capability. Multi input bidirectional converter can transfers the power between any two sources with different voltage levels. In this paper the two sources used here are batteries of different voltage level. Using of battery as a source because it is simple and also stores energy well. So we are using batteries as source for that reason. And coming to load, dc motor is used as a load. Converter used here is bidirectional converter which is used for power flow in both the directions i.e., forward and reverse direction. In forward direction, the supply from batteries makes to run dc motor and in reverse direction, power flows from load to both the sources so that charging of batteries takes place. MICs are being used in aerospace, electric and hybrid vehicles, sustainable energy sources and micro grid applications. Fig 1. Shows the block diagram of multiple input bidirectional dc-dc converter. As discussed already here two sources used are battery1 and battery2 of different voltage level. Bidirectional converter is used for power flow in both directions i.e., sources to load as well as load to sources. Gate pulses are given to each switch for triggering purpose. And load connected here is dc motor as mentioned before. This is the explanation of block diagram which is given as follows. Fig1. Block diagram of multiple input bidirectional dc-dc converter. Bidirectional converter used here consists of 6 switches and for each switches the gate pulse is given for triggering purpose. Circuit diagram for multiple input bidirectional dc-dc converter explains the two sources used here and mainly the converter portion and dc link. Fig.2. Circuit diagram of multiple input bidirectional dc-dc converter. Here in the above circuit, in bidirectional converter the switches used here is mosfet because of its low limiting current and also used for high power applications. Sources Bt1 and Bt2 represent the battery and ultra capacitor, which are interfaced with the dc link of inverter Vdc. Two legs of switch modules are connected to these dc voltage sources instead of dc bus. Another leg of the converter is connected to the dc link of the inverter, which is also fed by an FC through a dc dc converter. In this project, a multi-input nonisolated bidirectional power converter with the flexibility of interfacing multiple sources is proposed. This converter has the minimum number of devices with independent transfer of power between any two sources with wide variation in voltage levels. II. MODES OF OPERATION There are 5 modes of operation of multiple input bidirectional dc-dc converter. It is based on the power transfer between the two sources and the load. Mode A explains the power transfer between battery unit and dc link of the output (i.e. the power is supplied from one of the source to the load). Mode B explains the power transfer between another battery unit and dc link (i.e. power is supplied from one of the source to the load).mode C explains the power transfer between both the batteries. It means between the two sources. Mode D explains the power transfer between two batteries to DC link (i.e. power is supplied from both sources to the load). Mode E is based on power transfer from DC link capacitor to both the batteries. These 5 modes are explained as below, A. Mode-A (power transfer between battery and dc link) In this mode of operation, the energy stored in the battery is transferred to the dc link to supply power to the load. The switching sequence of the power devices in this mode of operation is given in Table I. Mode a is further divided into two parts, mode A(i) and mode
A(ii).In mode A(i),the devices S2 and S3 are first turned on to store the energy during the interval T1. During time interval T2, both the switches are turned off. Inductor current il1 flows to the dc link capacitor through the diodes D1 and D4. Energy stored in L1 during time T1 is being discharged to Cdc in this interval. In the next time interval T3, switches S1 and S4 are turned on. Current continues to flow from L1 to the dc link through the switches as opposed to the diodes in T2.Switches S1 and S4 are made to operate as rectifiers. In mode A(ii),the switches S1 and S4 is made to on at time interval T1. In time interval T2, the current flows through diode D2 and D3.At time interval T3, the switches S2 and S3 are made to on. Table I. conduction of devices for mode A,B and C Mode-B (power transfer between another battery and dc link) For energy transfer from the ultra capacitor to the dc link, the switching sequence for three distinct time intervals of operation is given in mode B(i) in Table II. The operation in this mode is similar to that of A(i). similar devices are switched as given in mode B(ii) in Table I. The direction of current flow in inductor L2 is opposite to that of mode B(i), as the direction of power flow is now reversed. In this mode, it is important to observe the changes in conducting devices in three different time intervals. B. Mode-C (power transfer between two batteries) Whenever energy stored in the battery needs to be transferred over to the ultra capacitor or vice versa, the switching sequence given in Table I is followed. There are various combinations possible for switching the four devices S3 to S6, depending on the voltage level soft the two energy sources. While charging the ultra capacitor from the battery in mode C(i), the converter can be operated in boost mode, buck mode, or buck boost mode. The boost mode of operation is used when the ripple current in the battery is lower as compared to the other two modes of operation. This is chosen to improve the life of the battery by reducing the peak value of charging or discharging current. If the ultra capacitor voltage is lower than the battery voltage, it is operated in buck mode, and if its voltage increases from less than VBt to above VBt, then the controller needs to seamlessly maneuver from the buck mode to the boost mode of operation. On the other hand, buck boost mode can be implemented. C. Mode-D (power transfer between two batteries and dc link) Mode D is based on power transfer between battery1 and battery2 to DC link (i.e. power is supplied from both sources to the load). When there is peak power needed, both the sources supply power to the load. In this mode of operation, more than two sources transfers energy to dc-link. Identical gate signals are given to switches S1 and S6 with a duty ratio of d2, and the complementary of this signal is being provided to switches S2 and S5. The gate signal for the switch S3 is synchronized with that of switch S1 having a duty ratio of d1. S4 is switched complementary to switch S3 with a dead time. During interval T1, switches S2, S3, and S5 are gated on, charging both the inductors L1 and L2 by corresponding sources Bt1 and Bt2. During peak power demand from the propulsion drive, the battery units provide the peak power demand due to its faster dynamic response as compared with the FC system. In FCV, the auxiliary sources have to deliver rated power during the process of cold start-up of the FC system. Switching states during this mode of operation is given in Table II, dividing the switching cycle TS into five time intervals. Identical gate signals are given to switches S1 and S6 with a duty ratio of d2, and the complementary of this signal is being provided to switches S2 and S5 as demonstrated in Fig. 3. The gate signal for the switch S3 is synchronized with that of switch S1 having a duty ratio of d1. S4 is switched complementary to switch S3 with a dead time. During interval T1, switches S2, S3, and S5 are gated on, charging both the inductors L1 and L2 by corresponding sources. At the end of this interval, switch S3 is turned off, providing a freewheeling path for il1 through D4. In interval T3, switch S4 is turned on in order to avoid voltage drop across the diode D4 while inductor L2 continues to charge from the battery2. Switches S2 and S5 are turned off at the end of interval T3, forcing current to flow through antiparallel diodes of their corresponding complementary switches. Energy stored in the inductors L1 and L2 is transferred to the dc link through diode D1. The voltage drop across the switch is reduced by gating on switch S1 to act as the synchronous rectifier. At the end of this interval, gating signals for each switching leg are complemented to start charging the inductors L1 and L2 going back to interval T1. Fig 3. Gate pulses for mode D
Table II. Conduction of devices for mode D and E Fig 4. Current waveform for IBT, IL1, IL2, IDC Fig. 5. DC output voltage for mode D A. Mode-E(power transfer between dc link to two batteries ) Mode E is based on power transfer from DC link to both battery and ultra capacitor. When there is excess power in the load, it is fed back to both the Fig 6. Output current for mode D sources during regenerative braking. In this mode of operation, L2 is always connected to the ultra capacitor by keeping S5 and S6 in ON and OFF states, respectively, throughout the cycle. The switch pairs
S1 S2 and S3 S4 are operated in complementary fashion with duty ratios of d1 and d2. The switching sequence during this mode of operation is given in mode E of Table II. In this mode of operation, L2 is always connected to the battery 2 by keeping S5 and S6 in ON and OFF states, respectively, throughout the cycle. The switch pairs S1 S2 and S3 S4 are operated in complementary fashion with duty ratios of d1 and d2 as shown in Fig. 5. Switches S1, S3, and S5 are turned on in the time interval T1, transferring energy from the dc link to charge both the auxiliary sources. Inductor currents il1 and il2 increase as Vdc appears at point A. At the end of T1, switch S1 is turned off, providing a freewheeling path for the inductor currents through diode D2. In order to reduce the voltage drop across the diode, switch S2 is gated onto effectively function as asynchronous rectifier in T3. In order to control the charging of two sources independently, switch S3 is turned off, and S4 is turned on, maintaining the current through inductor L1 as shown in Fig. 5. During this time interval, the inductor current il2 continues to charge the ultra capacitor. Switches S2 and S4 are turned off at the end of the interval T4. Current in inductors flow from the dc link to the battery and ultra capacitor through switches S1 and S5 and diode D3.One cycle of operation completes when the switch S3 is turned on, providing a path for il1 by reducing the voltage drop across it. Fig 7. Gate pulses for mode E Fig 8. Dc link voltage for mode E III.RESULTS The proposed multiple-input dc dc converter has been tested in various modes of operation discussed in the previous section. The unit has been designed for Vdc = 300 V, output power Po =5kW, and switching frequency of 20 khz. The other details are as follows. Battery 1: 144-V 17-Ah lithium-ion battery bank. Voltage varies from 120 to 144 V. Battery 2: 177-V 17-Ah lithium-ion battery bank. Voltage varies from 150 to 177 V. DC-link capacitor: 3 mf. Inductors are 100 microh each. The switches are controlled from DSP320F2808. Fig.9 charging of batteries bt1 and bt2 for mode E
Fig.10 Battery1 input voltage for mode D Fig.11 Battery2 input voltage for mode D Fig.12 DC link output voltage for mode D Fig.13 DC link input voltage for mode E Fig.14 Charging voltage of battery1 for mode E IV. CONCLUSION Thus, the batteries of different voltage level is interfaced i.e., multiple input interfacing is achieved and can be used where more than two sources are needed. And by using bidirectional converter, the excess energy can be stored and used when needed. i.e., bidirectional power flow capability is achieved. REFERENCES Fig.15 Charging voltage of battery2 for mode E [1] A Emadi and S. S. Williamson, Fuel cell vehicles: Opportunities and challenges, in Proc. IEEE PES Meet., 2004, pp. 1640 1645. [2] S. Aso, M. Kizaki, and Y. Nonobe, Development of hybrid fuel cell vehicles in Toyota, in Proc. IEEE PCC, 2007, pp. 1606 1611. [3] K. Rajashekara, Present status and future trends in electric vehicle propulsion technologies, IEEE Trans. Emerging Sel. Topics Power Electron., vol. 1, no. 1, pp. 3 10, Mar. 2013. [4] B. Bilgin, A. Emadi, and M. Krishnamurthy, Design considerations for a universal input battery charger circuit for
PHEV applications, in Proc.IEEE ISIE, 2010, pp. 3407 3412. [5] K. Rajashekhara, Power conversion and control strategies for fuel cell vehicles, in Proc. IEEE IECON, 2003, pp. 2865 2870. [6] A. Emadi, S. S.Williamson, and A. Khaligh, Power electronics intensive solutions for advanced electric, hybrid electric, fuel cell vehicular power systems, IEEE Trans. Power Electron., vol. 21, no. 3, pp. 567 577,May 2006. [7] A. Emadi, K. Rajashekara, S. S. Williamson, and S. M. Lukic, Topological overview of hybrid electric and fuel cell vehicular power system architectures and configurations, IEEE Trans. Veh. Technol., vol. 54,no. 3, pp. 763 770, May 2005. [8] A. Khaligh and Z. Li, Battery, ultracapacitor, fuel cell, hybrid energy storage systems for electric, hybrid electric, fuel cell, plug-in hybrid electric vehicles: State of the art, IEEE Trans. Veh. Technol., vol. 59, no. 6pp. 2806 2814, Jul. 2010. [9] J. M. Miller, Power electronics in hybrid electric vehicle applications, in Proc. IEEE Appl. Power Electron. Conf., Miami Beach, FL, USA,Feb. 2003, vol. 1, pp. 23 29. [10] J.-S. Kim et al., Optimal battery design of FCEV using a fuel cell dynamics model, in Proc. Telecommun. Energy Conf., 2009, pp. 1 4. [11] E. Schaltz, A. Khaligh, and P. O. Rasmussen, Influence of battery/ultracapacitor energy-storage sizing on battery lifetime in a fuel cell hybrid electric vehicle, IEEE Trans. Veh. Technol., vol. 58, no. 8, pp. 3882 3891, Oct. 2009. [12] U. R. Prasanna, P. Xuewei, A. K. Rathore, and K. Rajashekara, Propulsionsystem architecture and power conditioning topologies for fuelcell vehicles, IEEE Trans. Ind. Appl., vol. 51, no. 1, pp. 640 650,Jan./Feb. 2015.