Implementation of Battery Charging and Discharging System in Photovoltaic System
Hu Bo, Che Yanbo, Teng Wen, Ge Leijiao
Key Laboratory of Smart Grid of Ministry of Education, Tianji
n University, Tianjin, China Email:firstname.lastname@example.org Email:email@example.com Email:firstname.lastname@example.org Email:email@example.com Abstract-Nowadays, the photovoltaic system is widely used, but the charging and discharging controllers only provide protection to avoid overcharge and overdischarge. The poor charging ability leads the battery to have poor lifetime. The battery charging and discharging system in this paper can realize the three-stage charging, achieve the rapid charging of the battery, and make full use of the battery capacity. At the same time, the system can choose two modes of discharging to avoid overdischarge, also it can make the bus voltage stable to meet other applications. Keywords-Photovoltaic System, Lead-acid Battery, DC Bus, DC/DC Convertor, Charging and Discharging.
charging and discharging is small and the charging time is limited. Charging only happens in the sunshine, so that the life of the battery is shorted and easily damaged in the photovoltaic system because of overcharge and over discharge.
Along with the improving awareness of environment protection and the rising of the price in society, the development of new energy vehicles is gradually concerned .Electric vehicles have become the main direction for modern vehicles. At the same time, the popularity of electric vehicles must put the construction of the charging station in the first place. In the charging station which combines the new energy with electric vehicles, electric vehicles need to absorb energy from the power grid, clean energy such as wind energy and solar energy. Most of the new energy is affected by the natural condition, which is intermittent and uncontrollable, so the battery is necessary in energy storage system [1, 2]. Now charging and discharging controllers only protect batteries from overcharge and overdischarge, but it cannot make batteries realize maximum utilization ratio. This paper studies the battery charging and discharging system through BUCK/BOOST transformation. The battery stores electrical energy of the DC bus through three-state charging and can be taken advantage of. When the DC bus voltage is not enough, the energy of the battery will be released so as to ensure the stability of the charging station. II.CHARGING AND DISCHARGING OF THE BATTERY The charging and discharging of the battery is carried out according to the chemical reaction, and the structure and chemical composition of the battery changes continuously. So compared with the general electronic components, the battery is more sensitive to the change of the temperature. In addition, the charging current or discharging current affects the performance of the battery . The working condition of the battery in photovoltaic system is different from other work places, where rate of
Fig. 1: The block diagram of the PV system
Fig. 2: The relation between the discharging time and the terminal voltage
The charging and discharging reaction of lead acid battery is that 2PbS04+2H20 ^.Pb+Pb02+2H2S04
Overcharge is that the single cell voltage exceeds a certain level and the battery cannot make the oxygen react sufficiently. When the charging voltage is too high the hydrogen produced in the anode is difficult to be absorbed in the battery, which causes pressure and the loss of moisture. Overdischarge is that the battery discharging voltage exceeds the prescribed discharging termination voltage, as shown in Figure 2. In lead-acid batteries, two electrodes are sensitive to discharging. The change of the electrode material will cause danger to the battery. The design of the battery charging and discharging system is necessary, which prevents the battery from overcharging
and discharging and makes the DC bus voltage produced in photovoltaic system stable. III.THE DESIGN OF THE CHARGING AND DISCHARGING SYSTEM The paper shows the design and test result of batteries energy storage. Battery energy storage unit, whose main function is to store photovoltaic energy and discharge to supply energy when the DC bus-bar voltage is not stable. Therefore, the charging and discharging circuit of batteries can charge and discharge fast, convert the working state fast, and have high precision of stabilizing voltage/current and advanced charging and discharging control mode. Block diagram of the battery charging and discharging system is shown in Figure 3. The battery terminal voltage is obtained through the voltage converter unit, and the charging and discharging current and voltage adjustable and constant by sampling and the controlling of DSP.
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Fig. 5 : The drive circuit of the IGBT
In figure 4 , Boost circuit is composed of IGBT-V2, freewheeling diode D9 1 and inductance L5 . L5 stores electric energy on V2 conduction. The positive voltage of L5 is in the left after the closing of V2.Energy is transported to the side of DC bus-bar after the voltage of L5 integrated with the battery voltage, so energy in the battery is released through BOOST circuit. Meanwhile V 1 and V2 are always state-off[4,5]. In the circuit, V 1 and V2 mustn’t be on conduction at the same time in order to avoid a short circuit. IV. CONTROL ALGORITHM In conventional cases, the charging mode is three-stage: constant current, constant voltage and floating. However, the constant charging current will increase if the DC bus voltage rises fast due to the large amount of electric energy generated by solar power. The discharging mode of the battery is the selective constant voltage or the selective constant current. The charging mode is selected by users or system, and the system can select the discharging mode when the condition is OK. The control block diagram is shown in figure 6. The sampling voltage and current which are processed by PI algorithm adjust the PWM duty cycle to keep the current or voltage constant  . The control of the whole system is realized in DSP, whose main program flow diagram is shown in figure 7 . The charging or discharging process is respectively entered though the judgment of the mode.
Fig. 3: The diagram of the charging and discharging system
Figure 4 shows the main circuit of battery charging and discharging system, whose switching signals named G1 and G2 are controlled by drive circuit shown in Figure 5 and signal named Drive1 is controlled by DSP.
Fig. 4: Buck/Boost schematic diagram of a battery bi? directional charging and discharging In figure 4, Buck circuit is composed of IGBT-V1, freewheeling diode D92 and inductance L5.Power supply in the DC bus-bar side is transported to the battery side through L5 on V1 conduction. DC output voltage is different according to the different duty cycle of V1; therefore the battery is charged by the output voltage which is controlled by V1. Meanwhile V2 and D91 are always state-off.
Fig. 6 : Diagram of the constant voltage and the constant current control
81.3%. There will be current interruption causing the spikes because the floating current is too small. The phenomenon appears when the current decreases gradually in the second stage
Fig. 9: Collector-emitter voltage waveforms in first stage
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Fig. 10: Collector-emitter voltage waveforms in second stage
Fig. 7: Flow chart of the main program V. EXPERIMENTAL RESULTS AND ANALYSIS 1. Waveform of the drive circuit Figure 8 shows the final gate signal waveform of IGBT, which is obtained through the drive circuit. The one above is the gate waveform of the IGBT, and another is the output waveform of DSP.
Fig. 11: Collector-emitter voltage waveforms in third stage We set 10A to charge the battery in the constant current mode, and the waveform of the inductive current is highfrequency saw-tooth and the effective value is 10A, shown in Figure 12. Figure 13 shows the relationship between the current and the collector-emitter voltage of IGBT. When IGBT is on conduction, the inductor is charged and the current rises. On the contrary, the inductor is discharged and the current decreases. The rising rate is lower than the descending rate because the turn-on time is long.
Fig. 8: Gate waveform of IGBT and output waveform of DSP 2. Waveform for Charging Figure 9 shows the collector-emitter voltage of V1 in the first stage of the charging. The Buck circuit outputs the constant current whose value is 10A and duty cycle D gradually stabilizes at 67.2%. Figure 10 shows the collector-emitter voltage of V1 in the second stage of the charging. The output voltage is 145V which is the set value, and the duty cycle is 72.8%. Figure 11 shows the collector-emitter voltage of V1 in the third stage of the floating. The output current is 0.5A, and the duty cycle is
Fig. 12: The charging current and voltage of the battery
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it is necessary to design a zero-current detection circuit. The waveform is shown in figure 16.When the inductive current reaches zero, the detection circuit outputs the falling edge and IGBT is controlled to open by DSP. The function of the critical conduction mode is to eliminate the reverse recovery of the freewheeling diode by turning on the active switch when the inductor current drops to zero.
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3. Waveform for Discharging Through the experiment we can get the waveform when the battery is discharged in the two kinds of mode. Figure 14 shows the waveform of discharging voltage and current in constant voltage discharging mode. The discharging voltage is set to be 350V, and the current will decrease gradually with the discharging time increasing, as well as the voltage of battery. For maintaining a constant output voltage, duty cycle will increase gradually. Figure 15 shows the waveform of output current and the CE-voltage of IGBT in constant current discharging mode. Discharging current is constant at 7A and the pulse duty cycle is 17.8%.
Fig. 16: Zero-current detection wave of the inductor
VI. CONCLUSION The paper researches the charging and discharging system of the lead-acid battery in photovoltaic system, and achieves the bidirectional Boost/Buck power circuit which combines charging with discharging. The system achieves three-stage charging and selective discharging of the battery. The paper shows that the system can achieve the control of charging and discharging of the lead-acid battery and increase the utility ratio and the life time of the battery in the photovoltaic system. ACKNOWLEDGEMENT Thanks to the work of Sun Yue and Teacher Wang Yifeng who make me have some experience foundation for my research. I also give thanks to my lab friends because they give me some help when I'm in trouble.
Fig .14: Output current and voltage in constant voltage discharging mode
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Fig .15: Current and collector-emitter voltage of IGBT in constant current discharging mode Zero-Current B U C K / B O O S T Bi-directional D C / D C
In order to eliminate the current oscillation, we make the circuit operate in critical conduction mode [7,8]. Therefore,
Convertor. Electric Power and Electronic Technology. 2010, 44(7).  Ned Mohan, Tore M . Undeland, and William P . Robbins, Power Electronics Converters, Applications
and Design, 2nd ed., John Wiley& Sonsnc, 1995.  Chenghua Zhu, Fanghua Zhang, Yangguang Yan. Development of Dual Voltage Control Soft-Switching Bi-Directional Buck/Boost Convertor. Nanjing Aeronautics and Astronautics University Transactions. 2004,36(2)  XIaojie Xu, Zhenyi Hou. BOOST Power Factor Correction in High-Power Occasions. Chinese Electric Power and Electronic Technology Institute Proceedings.  Jong-Hu Park and B.H.Cho. The Zero Voltage Switching (ZVS) Critical Conduction Mode (CRM) Buck Converter with Tapped-inductor.
Hu Bo obtained his B . S . degree from Harbin Institute of Technology in 2012. He is a master student in Tianjin University now.