Abstract
The application of retired power batteries in energy storage stations represents a crucial solution for power battery recycling. Modular Multilevel Converter based Battery Energy Storage System (MMC-BESS) boasts advantages such as flexible control and high modularity. However, retired power batteries exhibit lower consistency and safety. To address these issues, this paper delves into the research of MMC control strategies.
Keywords: energy storage; retired power batteries; MMC-BESS; control strategy; battery consistency
1. Introduction
With the rapid development of the electric vehicle industry, power batteries have begun to enter a stage of large-scale retirement. By 2020, approximately 25GWh of power batteries had been retired nationwide, and this number is projected to grow significantly by 2025. The utilization of retired power batteries in energy storage stations not only alleviates resource waste but also poses new challenges, particularly in terms of battery consistency and safety.
Table 1: Overview of Retired Power Battery Utilization
| Year | Retired Battery Capacity (GWh) | Projected Growth by 2025 (GWh) |
|---|---|---|
| 2020 | 25 | – |
| 2025 (Projected) | – | Significant Increase |
2. Literature Review and Research Status
Research on Modular Multilevel Converter based Battery Energy Storage System (MMC-BESS) has been relatively mature, with a focus on distributed systems due to their numerous advantages over centralized systems. Distributed Modular Multilevel Converter based Battery Energy Storage System (MMC-BESS) primarily investigates scenarios where the common DC bus is connected to a DC power source, centering on battery state of charge (SOC) balancing, DC port voltage control, and AC port grid-connected control.
Table 2: Comparison of Centralized and Distributed Modular Multilevel Converter based Battery Energy Storage System
| Feature | Centralized Modular Multilevel Converter based Battery Energy Storage System | Distributed Modular Multilevel Converter based Battery Energy Storage System |
|---|---|---|
| Advantages | – | Higher modularity, flexibility |
| Research Focus | Capacitor voltage balancing, AC/DC port power control | SOC balancing, DC/AC port control |
3. Fundamentals of Modular Multilevel Converter based Battery Energy Storage System for Retired Battery Energy Storage
Modular Multilevel Converter based Battery Energy Storage System integrates batteries with MMCs to form a novel battery energy storage system. This paper focuses on the application of Modular Multilevel Converter based Battery Energy Storage System in AC scenarios, where the system lacks a common DC bus voltage support, relying instead on the superposition of submodule output voltages to provide bus voltage.
3.1 Submodule Topology and Operating Principles
The submodule topology and operating principles of Modular Multilevel Converter based Battery Energy Storage System are crucial for understanding its control strategies. The submodule has two main operating modes: insertion and bypass.
Table 3: Submodule Operating Modes
| Mode | VT1 Status | VT2 Status | Current Flow | Battery Status |
|---|---|---|---|---|
| Insertion | On | Off | Charging/Discharging | Active |
| Bypass | Off | On | Bypassed | Inactive |
3.2 Mathematical Model and Operating Characteristics
The mathematical model of Modular Multilevel Converter based Battery Energy Storage System is established based on its working principles and modulation strategies. The system’s output voltage and current are related to the submodule battery voltage and the modulation index.
Equation (1): Relationship Between Output Voltage and Modulation Index
Udc=m⋅UGB
Where Udc is the DC bus voltage, m is the modulation index, and UGB is the battery voltage of the submodule.
4. Grid-Connected Control and SOC Balancing of Modular Multilevel Converter based Battery Energy Storage System
Grid-connected control and SOC balancing are fundamental control strategies ensuring the stable operation of Modular Multilevel Converter based Battery Energy Storage System. Grid-connected control primarily involves grid current control and system output power control based on current control.
4.1 Grid-Connected Current Control
Traditional dq-axis current feedforward decoupling control is analyzed, with further research into MPIDNN-based control for improved performance.
Table 4: Comparison of Control Strategies
| Control Strategy | Parameter Dependence | Overshoot Suppression |
|---|---|---|
| Traditional PI Feedforward Decoupling | High | Limited |
| MPIDNN | Low | Enhanced |
4.2 SOC Balancing
SOC balancing is essential to prevent overcharging and over-discharging of batteries. This can be achieved through submodule, arm, and phase power control.
5. System Output Power and Circulating Current Control Considering Battery Operating Current
To mitigate capacity decay and enhance safety, the operating current of retired batteries needs to be limited. However, this poses challenges due to variations in battery aging degrees and system operating conditions.
5.1 Circulating Current Control
Circulating current control is crucial for balancing the SOC of batteries in different submodules. By adjusting the circulating current, the SOC of batteries can be balanced across phases and arms.
Equation (2): Circulating Current Adjustment
Ix_dc=Ix_out+Ix_cir
Where Ix_dc is the DC current of phase x, Ix_out is the output current, and Ix_cir is the circulating current.
5.2 Power Limit Adjustment
To prevent long-term battery operating current violations, a real-time control strategy is implemented to adjust the power limit based on the maximum battery operating current deviation.
Table 5: Power Limit Adjustment Strategy
| Condition | Action |
|---|---|
| Current Violation | Adjust Power Limit |
| No Violation | Maintain Current Power Limit |
6. Fault Diagnosis and Fault-Tolerant Operation Control of Modular Multilevel Converter based Battery Energy Storage System Submodules
IGBTs and batteries are more prone to faults than diodes in Modular Multilevel Converter based Battery Energy Storage System submodules. Rapid fault diagnosis and fault-tolerant control are crucial for ensuring system safety.
6.1 Fault Diagnosis
Fault diagnosis involves monitoring key parameters such as current, voltage, and temperature to detect anomalies.
Table 6: Fault Detection Parameters
| Parameter | Fault Type | Detection Method |
|---|---|---|
| Current | IGBT Fault, Battery Fault | Threshold Comparison |
| Voltage | Battery Fault | Threshold Comparison |
| Temperature | Battery Fault | Threshold Comparison |
6.2 Fault-Tolerant Control
Fault-tolerant control strategies, such as adjusting the nearest level approximation modulation and injecting third harmonics, are implemented to maintain system operation after faults are detected.
7. Conclusion
Modular Multilevel Converter based Battery Energy Storage System, as a novel battery energy storage system, exhibits excellent output waveform quality, high modularity, and flexible control. By integrating retired power batteries into submodules, the number of batteries required for series-parallel connection is reduced, alleviating the demand on battery management systems.
Table 7: Key Findings
| Finding | Description |
|---|---|
| Control Strategy | MPIDNN and Model Predictive Control enhance performance |
| SOC Balancing | Achieved through circulating current control and sorting algorithm |
| Fault Diagnosis | Rapid detection through monitoring key parameters |
| Fault-Tolerant Control | Maintains system operation after faults |
Future work will focus on further optimizing control strategies, enhancing fault diagnosis accuracy, and exploring more applications of retired power batteries in energy storage stations.