Here is the technical English translation:
Grid-Forming and Grid-Following in the BESS Universe — What They Really Are, How Complex They Are in Practice, and Where VSG Fits In
Within the BESS universe, the topics of grid-forming and grid-following are always included — but what exactly are they, how complex are they in practical application, and where does VSG fit in?
In the vast majority of cases, a BESS will connect to a grid that is already dynamically established, and will simply — after readings performed by the PCS/EMS ensemble — follow it, adapting to its contours and characteristics. In certain cases, however, when the grid is disconnected, the BESS must form the grid itself and adjust all of its electrical characteristics in preparation for future grid reconnection. These are the Grid-Following and Grid-Forming control paradigms.
Grid-Forming
In Grid-Forming mode, the BESS converter acts as a “voltage source,” establishing the voltage amplitude and angle at the bus without relying on a pre-existing network. Technically, this is achieved through an outer voltage control loop that regulates the output voltage, combined with a frequency self-sustaining/synchronization mechanism based on power laws. The most common approaches include:
(a) P–f and Q–V droop control, in which active power error dynamically adjusts frequency/angle, and reactive power error adjusts the voltage reference, enabling load sharing among multiple units;
(b) VSG / “Virtual Synchronous Machine”, in which virtual inertia and damping variables improve stability and power sharing across the network, frequently employing virtual impedance (primarily inductive) and current control with limiting and anti-windup logic, so that during faults the unit prioritizes reactive current injection and maintains stability even under current saturation.
VSG — Virtual Synchronous Generator: A Closer Look
The VSG (Virtual Synchronous Generator) is a family of grid-forming controls in which the converter is commanded to equivalently reproduce the dynamic behavior of a synchronous generator. In practice, the controller computes frequency/angle from a “swing equation” with virtual inertia (J) and damping (D) parameters, and generates the converter’s voltage reference; in parallel, a virtual excitation law (analogous to an AVR) regulates voltage and/or reactive power (Q) with a Q–V characteristic. The result is the provision of synthetic inertia and fast frequency response.
To ensure stability and load sharing among multiple units, VSG is typically combined with droop control, virtual impedance, and current-limiting logic during faults — maintaining the “virtual angle” stable even under saturation. From a design perspective, the tuning of J and D defines the trade-off between robustness (higher J reduces ROCOF but slows the response) and damping (higher D reduces oscillations). This tuning must account for grid strength (Short-Circuit Ratio — SCR), measurement/filtering delays, and coordination with higher-level controls (EMS/AGC) as well as grid code requirements (LVRT/HVRT, reactive power support during voltage dips).
Grid-Following
In Grid-Following mode, the BESS typically operates as a controlled “current source”: it measures the grid voltage and synchronizes to the phase angle via a PLL (Phase-Locked Loop). The classical structure uses an inner current control loop in the dq reference frame (with decoupling and grid voltage feedforward compensation) and an outer loop that converts active and reactive power references (P* and Q*) into current references (Id* and Iq*). Active power is therefore primarily commanded by Id (the in-phase component) and reactive power by Iq (the quadrature component), with converter limits enforced so that the combined P and Q operating point remains within equipment capability.
In weak grids (low SCR) and under harmonic distortion conditions, the PLL can introduce couplings and instabilities — such as control-network interaction — which is why techniques including robust PLL design, impedance-based control, filtered feedforward, coordinated current limiting with LVRT compliance, and even migration to grid-forming strategies are considered to increase stability margins.
A Critical Practical Note
Less sophisticated BESS systems rarely feature Grid-Forming and VSG capabilities. It is therefore of the utmost importance to verify interconnection requirements and obtain supplier guarantees that, in networks with specific characteristics, the BESS will form the grid and remain connected upon grid re-energization. Situations where the batteries are fully charged on one side and the grid is present on the other — but synchronization fails to occur — must be avoided at all costs.
Flavio Marqueti — Engineer and Executive Director, EVENERGY