DC Grid Energy Storage with Battery SOC Balance: A Reliable Operational Test Solution for Modular Cascaded Systems

As renewable energy (wind, solar) integration accelerates, high-voltage, large-capacity DC grid energy storage systems have become critical for grid stability and flexibility. Modular cascaded energy storage devices—composed of series-connected sub-modules—offer advantages like low power loss, high reliability, and scalable design. However, a key challenge persists: State of Charge (SOC) imbalance between sub-modules (caused by self-discharge or initial capacity differences) can lead to overcharging/overdischarging, reduced battery lifespan, and even system failures.

To address this, we propose a closed-loop current control strategy integrated with SOC balance management for modular cascaded energy storage systems. Validated via MATLAB-Simulink simulations, our solution ensures SOC differences between sub-modules in the same valve tower are within 1%, meeting long-term high-current operation requirements. This research provides actionable guidance for engineering applications in renewable energy integration, offshore wind power, and solar-PV-storage hybrid systems.

Key Background & Challenges

Traditional Voltage Source Converter (VSC) stations rely on power module reliability, but modular cascaded energy storage systems depend on both power modules and batteries. Operational tests are essential to verify sub-module performance under real-world voltage/current stress, but existing solutions have limitations:

• Fail to simulate battery module operating conditions or verify SOC balance.

• Cause excessive current harmonics when applied to systems with limited sub-modules.

• Lack DC-to-DC traction control or SOC balance mechanisms.

With the growing demand for high-voltage energy storage in DC grids, there is an urgent need for a reliable operational test platform that addresses SOC imbalance while validating system reliability.

System Topology & Communication Architecture

1. Modular Cascaded Energy Storage Topology

The energy storage system consists of n identical sub-modules (SMs) connected in series, forming a valve tower. Each sub-module integrates:

• Half-bridge power module (DC capacitor, IGBTs, diodes, bypass switch, voltage equalizing resistor).

• Battery module (battery cluster, Battery Management System (BMS), Sub-module Battery Control Unit (SBMU)).

Sub-modules operate in three states:

• Inserted: Upper IGBT on, lower IGBT off (capacitor and battery connected to the main circuit).

• Bypassed: Lower IGBT on, upper IGBT off (sub-module disconnected from the main circuit).

• Blocked: Both IGBTs off (for pre-charging or fault protection).

2. Dual-Channel Communication Architecture

To ensure system reliability, we adopt a redundant communication structure:

• Core controllers: Sub-module Controller (SMC) and BMS.

• Data flow: BMS → SMC → Valve Base Controller (VBC) (for real-time battery monitoring and control commands).

• Redundancy: Backup link between BMS and VBC (avoids system downtime if SMC communication fails).

Future upgrades will integrate BMS functions into VBC for simplified operation and comprehensive control.

Operational Test Platform Design

1. Platform Topology & Logic Control

The test platform uses two valve towers (sample tower + auxiliary tower), each with 6 sub-modules. Key components include:

• VBC (master controller): Monitors cooling/ fire protection systems and issues control commands.

• Cooling system: Maintains sub-module operating temperature.

• Fire protection system: Triggers emergency shutdowns for faults.

Test logic:

1. VBC sends power-on commands to SMC; BMS activates battery circuits.

2. Capacitors pre-charge; sub-modules enable fault detection.

3. System unlocks and operates at target current; VBC locks modules and shuts down if anomalies occur.

2. Closed-Loop Current Control with SOC Balance

To mitigate SOC imbalance, we add a SOC balance adjustment term (Sₙ) to each sub-module’s modulation wave. Sₙ is calculated via Proportional-Integral (PI) control, based on the difference between a sub-module’s SOC and the average SOC of the valve tower:

• Sₙ = Kp×(SOCₙ - SOC_avg) + Ki×∫(SOCₙ - SOC_avg)dt

• Output limit: Sₙ ≤ 0.1 (avoids duty cycle overflow or battery damage).

Key control features:

• Closed-loop current control: Stabilizes DC current at the rated value (2000A in simulations).

• Carrier Phase-Shifted PWM: Reduces current ripple.

• Adaptive phase-shift adjustment: Compensates for bypassed sub-modules (phase-shift angle = 360°/(n-1) for n-1 active sub-modules).

3. Critical Test Items

The platform validates core performance metrics:

Test ItemRequirementsCommunication TestNo data loss or abnormal alarms.Control TestCurrent control accuracy < design threshold; no sub-module bypass/fault.Battery System TestCell temperature rise < safety limit; SOC difference between clusters < design threshold; cell voltage difference < design threshold; total power loss < design threshold.

Simulation Validation with MATLAB-Simulink

1. Simulation Parameters

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2. Key Results

• Current Stability: Inductance current stabilizes at 2000A (rated value) with minimal ripple.

• SOC Balance Performance:

  • Proportional control: SOC values converge gradually.

  • PI control: Eliminates steady-state errors; SOC difference between sub-modules drops to <1% within 50 seconds.

PI control outperforms proportional control by ensuring higher precision and faster convergence, making it ideal for long-term high-current operation.

Practical Applications & Value

Our solution addresses critical needs in DC grid energy storage:

• Grid Support: Enhances flexibility of flexible DC transmission systems.

• Renewable Integration: Ideal for offshore wind and solar-PV-storage hybrid projects.

• Cost-Efficiency: Reduces system losses and extends battery lifespan via SOC balance.

• Reliability: Redundant communication and fault protection ensure safe operation.

Conclusion

This research presents a robust operational test platform and SOC balance control strategy for modular cascaded DC grid energy storage systems. Simulation results confirm that PI-based closed-loop control maintains SOC differences within 1%—meeting long-term high-current operation requirements. Future work will focus on optimizing the SOC balance rate and quantifying the optimal Sₙ value to further improve current control precision.

For engineering inquiries, collaboration opportunities, or technical details, contact our research team.

#DCGrid #EnergyStorage #SOCBalance #ModularTopology #PowerElectronics #RenewableEnergy #MATLABSimulation #PIControl

Published in Battery Bimonthly (Vol. 54, No. 6, 2024) | DOI: 10.19535/j.1001-1579.2024.06.005

Authors: Zeng Fanli, Wang Weilun | Institutions: Longyuan (Beijing) New Energy Engineering & Technology Co., Ltd., Nanjing University of Posts and Telecommunications