Modeling and Simulation of Bidirectional DC-DC Converter for Battery Charging and Discharging Applications

Manoj Rana11 and Ranvir Kaur2

1Research scholar, Electrical Engineering Department 2Assistant Professor, Electrical Engineering Department

1&2Baba Banda Singh Bahadur Engineering College, Fatehgarh Sahib, Punjab, India

Abstract – As renewable energy sources penetration is continuously increasing in grids the need of battery energy storage system become essential for achieving stability and reliability. The bidirectional converter is able to supply bidirectional power flow to energy charging and discharging of battery devices. In this paper, a bidirectional DC-DC converter for charging and discharging applications by using a PI controller is illustrated. The simulation is carried out in MATLAB/Simulink environment.

Keywords – Energy storage system, battery, constant current, constant voltage, control techniques.

I. Introduction

The need for renewable energy storage is important due to the continual climate change and the fickle nature of the weather upon which renewable energy sources depend [1]. Energy storage is growing rapidly globally. Decreasing costs and new organization incentives are stimulating record investments in energy storage devices. Depending on the application, there is a 74% decline in costs since 2013 and it is predictable to continue to decay at a steady 8% per year through the mid-2020s [2].

Figure 1: New installed capacity of energy storage for ancillary services by region

Figure 1 represents a region-wise graph for new installed capacity of energy storage for ancillary services in a decade (2013-2023) [3]. According to [3], the global market for energy storage system (ESS) for ancillary services applications will see strong growth in the coming decade, with revenues in the sector rising from $412 million in 2011 to more than $3.2 billion by 2021. Figure 2 represents the total installed revenue by ESS for ancillary services by technology in a decade (2011-2021).

1 Corresponding Author

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Figure 2: Installed revenue by ESS for ancillary services by technology

The output power obtained from renewable energy systems is intermittent and unreliable as these energy systems highly depend on weather conditions. As a result, renewable energy systems are planted with energy storage devices to cope with the demands. The primary objective of this paper is to simulate and implement a system, and is to harness energy in an efficient way. The conventional methods to harness energy consist of DC-DC converter, and charge controller. There are numerous stages of conversion [4]. Thus it increases the complexity, size and cost of the system. The presented system consists of bidirectional DC-DC converter and control circuits [5]. The converter reduces the component losses and increases the performance of the overall system. The complete system is implemented in MATLAB/Simulink environment.

The remaining paper is organised as: section II explains the preliminary concepts of bidirectional DC-DC converter. The system for charging and discharging applications is modelled in section III which is followed by simulation results in section IV. In the section V, concluding remarks have been reported.

II. Bidirectional DC-DC Converter

The power in conventional converter like buck-boost can flow only in one direction but bidirectional converters can manage power flow in both the directions. Bidirectional DC-DC converters can step-up or step-down the voltage level as well as can flow power in both the direction either forward or backward [6]. Around the globe, the penetration of renewable energy sources is kept increasing in the grids and these sources are highly dependable on different environmental conditions. The output power fluctuates with the change in environment conditions. A standalone system cannot rely on these kinds of energy sources because output power fluctuates and hence the energy storage devices like batteries and super capacitors are connected along with these energy sources. The excess power generated from the renewable sources are stored in these energy storage devices and utilized when demand is high or low power generation due to bad weather conditions. Here, a bidirectional DC-DC converter is needed to manage power flow in both directions; forward and backward [7]–[9]. A conventional dc-dc converter can be improved to a bidirectional converter by using diode in anti-parallel with MOSFET or IGBT, which allow current flow in either directions using controlled switching operation. Figure 3 represents the conventional buck-boost converter and this has transformed into bidirectional converter by switching diodes.

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Figure 3: Bidirectional buck-boost converter circuit

Bidirectional buck-boost converter shown in figure 3 operates in two modes, i.e. boost mode (diode D1 and switch Q2 conduct) and buck mode (diode D2 and switch Q1 conduct). Further, these two modes consist of two intervals each that depend on the conduction of diodes and switches. Typically on the basis of galvanic isolation provided between input and output; bidirectional DC-DC converter are classified as in table 1, researchers might refer [10] for more details.

Table 1: Types of bidirectional DC-DC converter

Types of Bidirectional DC-DC Converters

Non-Isolated Bidirectional DC-DC Converter (NIBDC)

Isolated Bidirectional DC-DC Converter (IBDC)

• Bidirectional Buck-boost Converter • Cascaded Bidirectional Buck-boost

Converter • Bidirectional CUK Converter • Bidirectional SEPIC-ZETA DC-DC

Converter • Switched Capacitor Bidirectional DC-DC

Converter • Interleaved Non-isolated Bidirectional DC-

DC Converter

• Dual Half Bridge (DHB)-Isolated Bidirectional DC-DC Converter

• Dual Active Full Bridge (DAFB)-Isolated Bidirectional DC-DC Converter

Non-Isolated bidirectional DC-DC converter (NIBDC) is suitable for low power application as there is no high frequency transformer to isolate electrically between source and load. In opposite, isolated bidirectional DC-DC converters (IBDC) use high frequency transformer to provide galvanic isolation.

III. Modelling of Bidirectional DC-DC Converter

The electrical circuit of the studied bidirectional DC-DC power converter associated with the battery is illustrated by simulation model in figure 4. The two MOSFETs are switched in such a way that the converter operates in steady state. An ideal switch is connected to connect or disconnect the main supply during simulation. The battery

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utilized in model is Lithium-ion type and its nominal voltage is 24 V and rated capacity is assumed to 50 Ah. Discharging parameters are determined through the nominal parameters of the battery.

Figure 4: Simulation model

Figure 5: Control schemes: (a) current control (b) voltage control

It is known that the batteries are charging in two modes: (a) constant current and (b) constant voltage. Figure 5 represents the current and voltage control strategies for charging and discharging.

IV. Simulation Results

The results are simulated in MATLAB/Simulink environment. For this, a common load is connected across DC source and battery. Initially it is assume that, battery is charged to 43% and main supply is ON. Therefore the load is fed from the main supply as well as the battery. After a period, the main supply gets OFF and now the load is started to feed from battery. From the results, it is found that maximum allowable current in both control schemes (current and voltage control) is 22 A. The open circuit voltage at the 80% state of charge (SOC) of battery is 25.98 V. The PI controller is used for determining the reference battery current. PI controller minimizes the error between reference and

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measured value. When the DC source is supplying power, battery will be in charging mode. The current through battery is 17.6 A, and voltage is maximum (25.98 V). The voltage across the load is 48 V. But in case of DC source is disconnected battery start supplying power to load. The current is 16.2 A and voltage decline to 25.6 V. Figure 6-9 shows the simulated results during charging and discharging of battery. Figure 6 denotes the state of charge of battery during charging and discharging mode of operation. Figure 7 shows the current flow through the battery and reference current. Battery voltage characteristics are presented in figure 8. Finally, voltage across load is portrayed in figure 9.

Figure 6: State of charge of battery (Lithium-ion)

Figure 7: Battery current characteristics actual and reference

Figure 8: Battery voltage characteristics during charging and discharging

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Figure 9: Load voltage characteristics during charging and discharging

V. Conclusion

The design analysis of bidirectional converter is discussed and the simulated bidirectional converter is able to share the power with the supply and with the battery. The proposed algorithm can be used in many applications like uninterrupted power supplies, integration of renewable energy with the grid and dc motor drives. The simulation is carried out in MATLAB/Simulink.

References

[1] E. O. Ogunniyi and H. Pienaar, “Overview of Battery Energy Storage System Advancement for Renewable ( Photovoltaic ) Energy Applications,” in 2017 International Conference on the Domestic Use of Energy (DUE), 2017.

[2] W. E. Council, “Energy Storage Monitor Latest trends in energy storage: 2019,” 2019.

[3] P. Reasearch, “How Energy Storage Will Grow,” 2013. [4] B. Bryant and M. K. Kazimierczuk, “Small-signal duty cycle to inductor current

transfer function for boost PWM DC-DC converter in continuous conduction mode,” in 2004 IEEE International Symposium on Circuits and Systems, 2004, pp. 7–10.

[5] K. Suresh and R. Arulmozhiyal, “Design and Implementation of Bi-Directional DC-DC Converter for Wind Energy System,” Circuits Syst., vol. 7, no. 11, pp. 3705–3722, 2016.

[6] D. Ravi, B. M. Reddy, S. L. Shimi, and P. Samuel, “Bidirectional dc to dc Converters : An Overview of Various Topologies , Switching Schemes and Control Techniques,” Int. J. Eng. Technol., vol. 7, pp. 360–365, 2018.

[7] X. I. E. Jun, Z. Xing, and Z. Chongwei, “Research on Bi-Directional DC-DC Converter For a Stand-Alone Photovoltaic Hybrid Energy Storage System,” in 2010 Asia-Pacific Power and Energy Engineering Conference, 2010, pp. 1–5.

[8] J. Li-jun, Y. Guang-yao, and J. Miao-Miao, “Study of Bi-directional DC-DC Converter of Micro-grid Hybrid Energy Storage System,” in 2015 IEEE 10th Conference on Industrial Electronics and Applications, 2015, pp. 1166–1169.

[9] A. Aroliya, S. Gautam, A. Kumar, and V. Shrivastva, “Analysis of Energy Storage System for Wind Power Generation with application of Bidirectional Converter,” in 2016 Second International Conference on Computational Intelligence & Communication Technology Analysis, 2016, pp. 419–423.

[10] H. R. Karshenas, H. Daneshpajooh, A. Safaee, P. Jain, and A. Bakhshai, “Bidirectional DC-DC Converters for Energy Storage Systems,” in Energy Storage in the Emerging Era of Smart Grids, 2011, pp. 161–178.

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