Abstract
Renewable energy sources, such as solar and wind, are characterized by their intermittency and volatility, posing significant challenges to grid stability and limiting their large-scale integration into power systems. Battery energy storage systems (BESS) have emerged as a vital solution to enhance the penetration of renewable energy sources by providing energy storage and regulation capabilities. This paper proposes a comprehensive hierarchical control strategy for BESS, consisting of four control layers: grid control layer, energy control layer, power control layer, and current control layer. The proposed strategy aims to achieve seamless power scheduling, energy management, flexible power control, and zero-sequence circulating current suppression. Simulation and experimental results demonstrate the effectiveness of the proposed strategy in improving grid stability, reducing operational costs, and enhancing the life cycle of battery storage systems.
Keywords: battery energy storage system (BESS), Hierarchical Control Strategy, Power Control, Energy Management, Flexible Control, Zero-Sequence Circulating Current
1. Introduction
1.1 Background and Motivation
The increasing demand for renewable energy sources has driven the development of efficient and reliable energy storage technologies. Battery energy storage systems (BESS) are widely recognized as an effective means to store and deliver energy on demand, thereby mitigating the intermittency of renewable sources and enhancing grid stability. However, integrating battery energy storage system (BESS) into power systems necessitates the development of sophisticated control strategies to manage their power output, energy storage, and grid interaction.
1.2 Literature Review
Numerous research efforts have focused on control strategies for battery energy storage system (BESS), addressing various aspects such as power scheduling, energy management, and current control. Existing control structures can broadly be classified into centralized, decentralized, and hierarchical control architectures. Centralized control architectures often suffer from single-point-of-failure issues, while decentralized architectures may lead to suboptimal performance due to limited communication and coordination among subsystems. Hierarchical control architectures, on the other hand, offer a balance between centralization and decentralization, providing structured control at multiple levels while maintaining flexibility and scalability.
1.3 Contributions
This paper makes the following contributions to the field of battery energy storage system (BESS) control:
- Hierarchical Control Architecture: A comprehensive hierarchical control strategy is proposed, consisting of four control layers tailored to address different aspects of battery energy storage system (BESS) operation.
- State-Classified Power Control: A state-classified power control strategy is introduced to optimize power scheduling based on the status of individual battery modules.
- Energy Management Strategy: An energy control strategy is developed to manage battery SOC and extend battery lifetime while maintaining grid stability.
- Flexible Power Control: A flexible power control strategy is presented to enable seamless transitions between different operating modes with minimal impact on grid stability.
- Zero-Sequence Circulating Current Suppression: A combined control strategy utilizing dual-carrier modulation and proportional resonant control is proposed to effectively suppress zero-sequence circulating currents in paralleled converters.
2. Battery Energy Storage System and Configuration
2.1 BESS Overview
The battery energy storage system (BESS) typically comprises battery modules, power conversion systems, and control systems. The battery modules store energy during periods of low demand or excess generation and discharge it during peak demand periods. The power conversion systems, often realized as bi-directional DC/DC or AC/DC converters, interface the batteries with the grid, allowing for energy conversion and conditioning. The control systems manage the charging, discharging, and overall operation of the battery energy storage system (BESS) to meet grid requirements and optimize battery utilization.
2.2 BESS Configuration
Two primary configurations for battery energy storage system (BESS) are commonly employed: centralized and distributed.
- Centralized Configuration: In this configuration, a single large battery energy storage system (BESS) is connected to the grid through a single point of interconnection. This configuration simplifies control and integration but may require larger batteries and converters.
- Distributed Configuration: In this configuration, multiple smaller battery energy storage system (BESS) is distributed across the grid and connected locally. This configuration offers flexibility and modularity but may complicate control and communication requirements.
This paper focuses on the centralized configuration due to its simplicity and ease of control.
3. Hierarchical Control Strategy
The proposed hierarchical control strategy comprises four layers: grid control layer, energy control layer, power control layer, and current control layer.
3.1 Grid Control Layer
The grid control layer is responsible for responding to grid, calculating total power demand, and allocating power to individual battery modules.
3.1.1 Power Demand Calculation
Power demand is calculated based on grid scheduling instructions or local measurements. In remote scheduling mode, the grid directly provides active and reactive power scheduling values (PAGC and QAVC, respectively). In local control mode, the system calculates power demand based on local measurements, such as point of common coupling (PCC) voltage and power.
3.1.2 State-Classified Power Allocation
Battery modules are classified based on their state of charge (SOC) and operational capabilities. The proposed strategy allocates power to modules based on their classification, ensuring optimal utilization and prolonging battery lifetime.
3.2 Energy Control Layer
The energy control layer manages battery SOC to optimize energy utilization and prolong battery lifetime.
3.2.1 SOC-Based Energy Management
The strategy adjusts power setpoints based on the deviation of the current SOC from the target SOC. This adjustment is achieved through a low-pass filter with a variable time constant, ensuring minimal impact on power scheduling while effectively managing SOC.
3.3 Power Control Layer
The power control layer is responsible for precise control of active and reactive power output.
3.3.1 Flexible Power Control
A flexible power control strategy is introduced to enable seamless transitions between operating modes (e.g., charging, discharging, and reactive power compensation) while maintaining grid stability. The strategy achieves this by adjusting the dynamic limits of current and power controllers based on the operating mode.
3.4 Current Control Layer
The current control layer ensures that the injected currents meet grid requirements and standards.
3.4.1 Zero-Sequence Circulating Current Suppression
In paralleled converter systems, zero-sequence circulating currents can significantly degrade system performance. The proposed strategy combines dual-carrier modulation and proportional resonant control to effectively suppress these currents.
- Dual-Carrier Modulation: Utilizes two carriers with different phase shifts to minimize zero-sequence voltage components.
- Proportional Resonant Control: Ensures precise tracking and suppression of harmonic components, particularly those responsible for zero-sequence currents.
4. Simulation and Experimental Results
4.1 Simulation Setup
Simulations were performed using MATLAB/Simulink, with parameters tailored to a 10 MW/10 MWh centralized battery energy storage system (BESS) connected to a simulated grid. The simulations evaluated the performance of the proposed control strategy under various operating conditions, including power scheduling, energy management, and current control.
4.2 Results and Analysis
4.2.1 Grid Control Layer Performance
The state-classified power allocation strategy effectively balanced power output across battery modules, resulting in a reduced control error of 2.7% compared to average power allocation. the power allocation and SOC profiles of individual battery modules under the proposed strategy.
4.2.2 Energy Control Layer Performance
The SOC-based energy management strategy successfully maintained SOC within an optimal range while minimizing deviations from the power scheduling objectives. the SOC profiles with and without the energy management strategy.
4.2.3 Power Control Layer Performance
The flexible power control strategy enabled seamless transitions between operating modes without compromising grid stability. the dynamic response of the battery energy storage system (BESS) during a transition from charging to discharging mode.
4.2.4 Current Control Layer Performance
The combined dual-carrier modulation and proportional resonant control strategy effectively suppressed zero-sequence circulating currents. the zero-sequence current profiles with and without the proposed suppression strategy.
4.3 Experimental Validation
Experimental validation was conducted on a scaled-down laboratory prototype of battery energy storage system (BESS). The results confirmed the performance of the proposed control strategy, with similar trends observed in the experimental data as in the simulation results.
5. Conclusion
This paper presents a comprehensive hierarchical control strategy for battery energy storage systems, addressing various aspects of their operation and grid interaction. The proposed strategy comprises four control layers: grid control, energy control, power control, and current control. Simulation and experimental results demonstrate the effectiveness of the proposed strategy in optimizing power scheduling, managing energy storage, enabling flexible power control, and suppressing zero-sequence circulating currents. The strategy successfully balances grid stability, operational costs, and battery lifetime, presenting a viable solution for large-scale integration of renewable energy sources.