Operational Modes of Battery Energy Storage Systems: Grid Support and Stability Mechanisms

Modes of Operation in Battery Energy Storage Systems

Battery Energy Storage Systems (BESS) play a crucial role in modern electrical networks by supporting grid stability and enhancing renewable energy integration. The operational modes of BESS inverters significantly influence their ability to provide ancillary services, voltage control, and other essential grid functions. This article delves into the primary modes of operation for BESS, focusing on grid-following (GFL) and grid-forming (GFM) functionalities.

Grid-Following (GFL) Mode for Ancillary Services

The majority of BESS deployments operate in grid-following mode (GFL) when connected to the grid. In this mode, the BESS inverter synchronizes with the grid frequency and voltage using a phase-locked loop (PLL), ensuring that its output aligns with the existing power system.

Key Features of Grid-Following Mode:

1. Synchronization with Grid – The inverter relies on external voltage and frequency signals to determine its operation.

2. Power Injection and Absorption – The system can inject or absorb both active and reactive power based on grid demands.

3. Support for Frequency and Voltage Stability – Through precise control, BESS in GFL mode helps maintain stable frequency and voltage levels.

Control Mechanism in GFL Mode

A typical GFL control scheme consists of three primary components:

? Phase-Locked Loop (PLL): Measures grid frequency and phase to maintain synchronization.

? High-Level Control Loop: Determines the desired active (P) and reactive power (Q) setpoints.

? Inner Current Control Loop: Ensures the inverter’s output current matches the reference values derived from power setpoints.

By adjusting the active and reactive power output, BESS can provide essential ancillary services such as frequency regulation, demand response, and voltage stabilization.

See figure 1 for the control scheme of a BESS inverter operating in GFL mode:

Voltage Control in Grid-Following Mode

One of the essential services provided by BESS inverters is voltage control, which helps maintain grid stability, especially in distribution networks with high penetration of renewable energy. Several control strategies are implemented to optimize voltage support.

Common Voltage Control Strategies:

1. Fixed Power Factor (cos φ) Mode: The BESS maintains a constant power factor, either absorbing or supplying reactive power.

2. Reactive Power Setpoint (Qref) Mode: The inverter injects or absorbs a fixed amount of reactive power.

3. Power Factor as a Function of Active Power (cos φ(P)): The reactive power output is adjusted based on the active power level. See figure 2 below:

4. Voltage-Dependent Reactive Power (Q(U)) Characteristic: The inverter dynamically adjusts reactive power based on grid voltage variations. See figure 3 below:

Among these, the Q(U) characteristic control is considered more advanced, as it directly responds to voltage deviations, enhancing grid resilience. However, improper tuning can lead to instability issues, particularly in low-voltage networks with high resistance-to-reactance (R/X) ratios.

Transition to Grid-Forming (GFM) Mode

While most BESS installations operate in GFL mode under normal grid conditions, grid-forming mode (GFM) is essential for ensuring power stability during islanded operation or black-start scenarios. Unlike GFL mode, where the inverter follows grid conditions, GFM mode allows the BESS to actively regulate voltage and frequency, effectively mimicking the behavior of a traditional synchronous generator.

Key Features of Grid-Forming Mode:

1. Independent Voltage and Frequency Regulation: Essential for microgrids and isolated networks.

2. Black-Start Capability: Enables restoration of power in case of grid failures.

3. Enhanced System Stability: Supports islanded operation without external grid reference.

In systems where BESS operate alongside conventional generators, a seamless transition between GFL and GFM modes is necessary to prevent instability during grid disturbances or power outages.

Conclusion

The ability of BESS to operate in different modes—grid-following for normal operation and ancillary services, and grid-forming for islanded operation—makes them indispensable in modern power grids. With advanced control strategies such as Q(U) voltage regulation and dynamic reactive power management, BESS enhance grid resilience, reduce reliance on conventional generators, and support the integration of renewable energy.