Bi-Directional Single-Stage Grid-Connected Inverter for Battery Energy Storage System
Abstract
The objective of this paper is to propose a bidirectional single-stage grid-connected inverter (BSGinverter) for the battery energy storage system. The proposed BSG-inverter is composed of multiple bidirectional buck-boost type dc-dc converters (BBCs) and a dc-ac unfolder. Advantages of the proposed BSG-inverter include: single-stage power conversion, low battery and dc-bus voltages, pulsating charging/discharging currents, and individual power control for each battery module. Therefore, the equalization, lifetime extension, and capacity flexibility of the battery energy storage system can be achieved. Based on the developed equations, the power flow of the battery system can be controlled without the need of input current sensor. Also, with the interleaved operation between BBCs, the current ripple of the output inductor can be reduced too. The computer simulations and hardware experimental results are shown to verify the performance of the proposed BSG-inverter.
EXISTING SYSTEM:
Instead of using balancing circuit, Fig. 2 shows the different configurations of grid-tied battery energy storage system to balance the charge of all battery modules. A two-stages configuration to step-up the voltage by a dc-dc converter and transfer the DC power into AC power by a dc-ac inverter. Because of the parallel connection of each battery module, the equalization of battery modules can be naturally achieved. Also, the two-stage configuration implies a simpler system design with lower control complexity. However, the high current stress of the dc-dc converter and an inverter as the second-stage will reduce the overall conversion efficiency. Thus the power capacity of this type of configuration is limited due to the efficiency and the current stress considerations. An alternative configuration for the battery energy storage system is to adopt a dc-ac microinverter for each battery module. Compared to the two-stages configuration, the micro-inverter offers more flexibility and fault tolerance in battery energy storage system. Since each battery module has its own grid- connected micro-inverter, the output power of the battery module can be individually controlled in despite of other battery module mismatching. However, many challenges still remain in the way of achieving lower cost and higher conversion efficiency.
PROPOSED SYSTEM:
The proposed BSG-inverter can improve the power conversion efficiency, reduce the output inductor size, eliminate the input current sensor, and simplify the control circuit. Moreover, the proposed BSG-inverter can achieve individual power control of each battery module so that important features of battery equalization, capacity flexibility, and hot swapping can be accomplished. In this paper, the operation principle of the BSG-inverter will be introduced and the power flow control of each battery module without current sensor will be developed. Computer simulation and preliminarily experiments are shown to verify the performance of the proposed BSG-inverter. The circuit diagram of the proposed BSG-inverter, which is composed of m sets of distributed buck-boost type dc-dc converters (BBCs) and a dc-ac unfolder. Each BBC consists of two switches, two diodes, and one inductor. It can convert the dc current generated by the battery module into a high frequency pulsating dc current. This high frequency pulsating output current of the BBCs will be converted into sinusoidal one with utility line frequency by the dc-ac unfolder of 4 active switches operated at low switching frequency and a LC filter. The proposed BSG-inverter will comply with the power commands, which is coming from the central control unit of the BMS, to charge or discharge the battery modules. The power flow from each battery module is transferred to the ac mains by means of single-stage power conversion. Also, the BBCs can be operated with interleaving to reduce the current ripple
of the output inductor.
CONCLUSIONS
A novel BSG-inverter, which consists of multiple distributed BBCs and a dc-ac unfolder, for the battery energy storage system is proposed in this paper. The proposed BSGinverter has individual power control capability for each battery module while fulfill the functions of battery charging and discharging by using pulsating current. Eventually, the equalization, lifetime extension, and capacity flexibility of the battery energy storage system can be achieved. According to the developed mathematical equations, the power control capability of each individual battery module can be achieved without the need of input current sensor. Also, with the interleaved operation, the current ripple of the output inductor can be reduced significantly. A design guide line of the proposed BSG is presented. Finally, computer simulations and hardware measurements are shown to verify the validity of the proposed BSG-inverter.
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