Abstract
In this paper, the research status of topology and control strategy of energy storage grid-connected system is analyzed, and aiming at the working characteristics of the repurposed battery, a cascade power electronic transformer (CPET) with independent DC output is proposed. The working principle of current fed isolated bidirectional DC-DC converter (CF-IBDC) and cascaded H-bridge (CHB) is analyzed, and the decoupling control strategy is designed. In this paper, a hierarchical control strategy is designed for the repurposed battery energy storage (RBES) grid-connected system based on CPET, which consists of three layers: energy layer, power layer, and state of charge (SOC) layer. The energy layer responds to active and reactive power scheduling instructions, the power layer controls the grid-connected current and tracks the grid voltage, and the SOC layer equates the charged state of repurposed batteries. A 3 MVA/12 kV three-phase grid-connected simulation system was established, and a 1 kW single-phase system experiment platform was designed. The simulation and experimental results can verify the correctness of the theoretical analysis and the feasibility of the control strategy.
Highlights
In recent years, the proportion of renewable energy in the power system has gradually increased, but its output power is characterized by volatility and intermittency, which limits the capacity of renewable energy generation to connect to the grid on a large scale [1, 2]
Supporting battery energy storage system can effectively improve the ability of power grid to accept renewable energy [3,4,5,6]. e cost factors of large-capacity converters and energy storage batteries limit the promotion and application of the battery energy storage system, while repurposed lithium batteries and link topology provide a new idea to solve the cost problem [7]
Aiming at the inconsistency problem of repurposed batteries, this paper presents a cascade power electronic transformer with independent DC outputs. e repurposed battery energy storage (RBES) combines the control target and topology structure to design the three-layer control architecture including energy layer, power layer, and state of charge (SOC) layer. e energy layer is used to respond to scheduling instructions and suppress power fluctuations, peak load clipping, and reactive power compensation. e power layer distributes active power based on SOC and divides reactive power to make full use of system capacity
Summary
The proportion of renewable energy in the power system has gradually increased, but its output power is characterized by volatility and intermittency, which limits the capacity of renewable energy generation to connect to the grid on a large scale [1, 2]. PET integrates voltage transformation, high-frequency electrical isolation, and flexible power flow control, making it a research direction of topological structure of battery energy storage system. E control strategy based on the idea of a virtual synchronous generator is used on the way to balance the power and energy distributed among cells in the same phase and between phases. In [19], two SOC balancing techniques are proposed for an electrical vehicle charging station which is based on a grid-tied cascaded CHB multilevel converter. E first proposed technique uses the redundant states of the CHB converter to generate different AC voltages to balance the SOCs of the CHB cells. E CHB, as the energy control unit, can be used to balance the voltage/SOC of battery packages individually and significantly improve the modularity, stability, and safety of EV system. SOC layer equalizes the charged state of decommissioned lithium battery to avoid short plate effect and stabilize the voltage of high voltage bus. e security and reliability of the RBES are enhanced
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