BESS technology: Grid Forming vs Grid Following

Nowadays,the energy storage PCS control technologies are primarily divided into 2 categories: Grid Forming and Grid Following. Grid Following, which lacks abilities to autonomously provide voltage and frequency support as a current source. It strictly relies on the stability of the external grid's voltage and frequency, thus offering limited system support capabilities. On the other side, Grid Forming, voltage source as the core design, realizes the abilities through internal presetting reference signals of voltage and frequency. It enables not only grid-tied operation but also standalone off-grid functionality, which results in stronger gird support and enhanced stability.

Along with wind and solar power play an increasing dominate roles in the power system, the operation mode is undergoing a significant transition. Particularly in regions such as Africa, the Middle East,and South America, where with high proportion of renewable energy power generation, yet grid infrastructure remains relatively weak. In these areas, large-scale renewable energy bases- often located in vast desert or Gobi regions- are facing multiple challenges:low short-circuit ratios among multiple sites, frequent broadband oscillations, insufficient system inertia, and notable declines in voltage and power angle stability. In response to those complex issues, grid-forming energy storage technology has emerged as a prominent focus within the industry.

1. Grid Forming vs Grid Following

1.1 Grid Following Technology

The principle of grid-following energy storage system operates as a current source, and its operation relies on the grid's voltage and frequency. Under the control mode, the system's converter synchronizes with the gird by tracking grid phase information, using a phase-locked loop(PPL) to measure the phase at the point of common coupling(PCC). As this approach doesn't enable the system to independently provide voltage and frequency,it can only work when the grid is present. Grid-following energy storage systems are typically employed to supply instant power compensation, thereby enhancing grid's stability and repeatability.

1.2 Grid Forming Technology

The principle of grid-forming energy storage system operators as a voltage source, capable of internally setting voltage parameters to deliver stable voltage and frequency. Grid-forming converters employ a power synchronization strategy, similar to synchronous generators, realizing synchronization without PPL. This control method is particularly suitable for grids with system strength and low physical inertia.

Grid-forming converters can also provide virtual inertia and damping to the system, enabling independent operation even in the absence of external grid phase information. As the penetration of renewable energy and power electronic devices increases- leading to reduced system inertia and weakened grid strength- grid-forming control technology enhances converters' ability to support voltage and frequency, thereby improving the power system stability.

Due to its voltage source characteristic, grid-forming BESS efficiently mitigates issues of insufficient short-circuit capacity and lack of rotatinal inertia in new-type power systems.

2. Differences in Technology and Application

The primary difference between grid-following and grid-forming BESS is the source characteristics and control methods. Grid-following system essentially as a current source, unable to provide voltage and frequency supports, and must rely on the grid. Grid-forming system essentially as a voltage source, by internally setting voltage reference signals to output voltage and frequency, they can function both in grid-tied and off-grid modes, offering strong grid support capabilities.

Furthermore, grid-following converters face stability issues in weak grids, whereas grid-forming converters can offer stable frequency support, thereby improving gird's stability.

Grid-following BESS suits for those application scenarios with stable grid and without additional voltage and frequency supports. While grid-forming BESS suits for new-type power systems, particularly in the regions with high penetration of renewable energy and relatively weak grid stability.

3. PCS Comparisons

3.1 Grid-following PCS Structure

Grid-following PCS mainly controls the AC-side current. It tracks the voltage phase angle of the existing grid through a PPL, after which coordinate transformation and PWM modulation generate the control signals fed back to the switching devices.

The phase-locked loop technology used in the grid-following PCS control structures is relatively mature at this stage, enabling system operation under predetermined current and maximum power point conditions. However, even the PPL technology it relies on is mature, it still requires passively obtaining stable frequency and voltage reference from grid to function properly. Moreover, its control loop stability is weak than that of grid-forming energy storage systems,rendering it unable to provide active gird support.

3.2 Grid-forming PCS Structure

Unlike grid-following controls rely on phase-locked loops for grid synchronization, grid-forming PCS sets voltage reference signals internally,and synchronizes with the rest of the grid through a power calculation module and frequency droop control. This approach, resembling synchronous generator operation, requires no external voltage reference signals.

Grid-forming PCS can autonomously adjust its output in real-time without external generation references. By regulating power output to maintain voltage,it operates as a voltage source for gird connection while ensuring system's stability. It can establish an independent grid in weak networks lacking stiff voltage sources. However the surge current capability of grid-forming PCS has been enhanced from 1.5 to 3.0 times rated current, resulting in significantly higher costs than grid-following systems.

4. Grid-forming PCS Functions

4.1 Grid Voltage Formation
Grid-forming PCS can replicate the operational mechanisms and characteristics of synchronous generators through a virtual synchronous generator (VSG) model. The VSG mimics the frequency and voltage regulation processes of a physical synchronous generator, simulating the excitation control via reactive power-voltage regulation. This provides active inertial support, replicates the output behavior of synchronous generators, and enables grid-forming capabilities.

4.2 Frequency Regulation and Inertia Response
During power system disturbances, parameters such as the virtual rotational inertia (J) and damping coefficient (D) in the VSG control help slow down frequency variations, thereby enhancing frequency and power angle stability. Since the virtual inertia J is not a physical component, its value can be flexibly adjusted without hardware limitations. Additionally, grid-forming PCS offers 1.2 times instantaneous overload capability, providing robust support for inertia response.

4.3 Suppression of Broadband Oscillations
Grid-forming PCS can mitigate low-frequency oscillation risks through adaptable damping coefficients (D). It also employs techniques such as virtual impedance and active damping to reshape impedance characteristics at specific frequencies and suppress oscillations.

4.4 Voltage Fault Ride-Through
During low-voltage ride-through (LVRT) events, grid-forming PCS employs current limiting and overload virtual impedance control to maintain voltage source operation and support grid formation. In high-voltage ride-through (HVRT) scenarios, it incorporates real-time terminal voltage regulation into the control strategy, using rapid reactive power response to suppress overvoltage and contribute to grid stability.

4.5 Black Start Capability
Grid-forming PCS can initiate and support system restoration after a grid collapse. It autonomously establishes AC voltage, provides excitation for main generation units, and if necessary, forms an independent grid. However, due to high intrush currents during black start, a single unit cannot energize auxiliary loads alone. An Energy Management System (EMS) must coordinate multiple grid-forming PCS units in parallel to support transformer excitation and voltage establishment.

5. FAQ

Q1:What is the fundamental difference between grid-forming and grid-following energy storage?

A:Grid-following systems act as dependent current sources that require a stable grid to synchronize with, while grid-forming systems act as independent voltage sources that can create and sustain a grid's voltage and frequency on their own.

Q2: Why is grid-forming technology critical for modern power systems?

A: As renewables (like wind and solar) replace traditional generators, system inertia declines, leading to instability. Grid-forming storage provides essential virtual inertia and voltage support, making it vital for weak grids and high-renewable environments.

Q3:What are the key functional advantages of a grid-forming PCS?

A: Its key advantages include: establishing grid voltage (black start), providing virtual inertia for frequency stability, damping oscillations, and robustly riding through grid faults, all while operating in weak-grid or off-grid scenarios.

Q4:What is a main cost consideration for grid-forming PCS?

A: A primary cost driver is its enhanced surge current capability (3.0 times rated current vs. 1.5 for grid-following), which requires more robust and expensive power semiconductor components.