A grid-forming STATCOM is designed to establish a voltage waveform reference, while a grid-following STATCOM synchronizes to an existing waveform and injects current to meet a reactive-power or voltage-control command. That difference matters in weak grids, renewable-energy zones and systems with fewer synchronous machines. Both architectures can provide fast dynamic reactive current, but their behavior during low short-circuit strength, frequency disturbances and severe voltage events is not interchangeable.
Quick answer: grid-following control is mature and effective when a sufficiently strong voltage reference exists. Grid-forming control can improve stability where that reference is weak, but only if the converter, protection, current limiting, energy source and plant-level controls are engineered as one system. The label “grid-forming” does not by itself guarantee inertia, fault current, black start or stable operation for every network condition.
| Design question | Grid-following STATCOM | Grid-forming STATCOM |
|---|---|---|
| Primary reference | Tracks an existing grid voltage, commonly through a PLL | Creates an internal voltage angle and magnitude reference |
| Typical controlled output | Injected current, reactive power or bus voltage | Converter voltage/frequency behavior with controlled power exchange |
| Strong-grid performance | Well established and predictable | Can perform well but may add complexity without a defined need |
| Weak-grid challenge | Synchronization and control interaction | Current limiting and stable transition through faults |
| Energy requirement | DC-link energy for converter operation and losses | Depends on required active-power duration; some functions need storage |
A conventional voltage-source-converter STATCOM measures three-phase voltage and current, estimates the grid angle and transforms the signals into a rotating control frame. The controller regulates reactive current, DC-link voltage and, where specified, the AC-bus voltage. A phase-locked loop (PLL) is commonly used to synchronize the converter with the network. In a strong grid, the measured voltage provides a clear reference, so current commands can be applied quickly and accurately.
As the grid becomes weaker, the converter’s injected current has a larger influence on the voltage used by its own synchronization loop. Several inverters may also interact through their PLLs, filters and plant controllers. The result can be poorly damped oscillation, slower recovery or a loss of synchronization during a fault. These outcomes are not inevitable; properly tuned grid-following systems can operate at low short-circuit ratio (SCR). However, the acceptable SCR must be demonstrated for the actual network model and operating combinations.
A grid-forming controller behaves more like a controlled voltage source behind an impedance. Instead of waiting for a PLL to identify the external phase angle before producing a current command, it maintains an internal angle and voltage magnitude. Droop control, virtual synchronous machine control and other algorithms relate active and reactive power to frequency and voltage behavior. This can give inverter-dominated networks a stronger local reference and improve damping.
The converter is still limited by semiconductor current. A synchronous generator can deliver several multiples of rated current for a short fault period; a STATCOM normally cannot. When the current limit is reached, the controller must prioritize functions and change its behavior without losing stability. The design therefore needs explicit rules for positive- and negative-sequence current, reactive-current priority, DC-link protection, fault recovery and interaction with relay settings.
SCR is often used as an initial indicator of grid strength, but a single number does not capture every risk. Nearby inverter plants, series compensation, frequency-dependent network impedance, cable capacitance and operating topology can all influence stability. In a multi-infeed area, the effective strength seen by one plant depends on what the other converters are doing. Engineers should therefore use SCR for screening and electromagnetic-transient studies for the critical cases.
Reactive-power exchange does not require sustained net active energy in the same way as frequency support. A STATCOM without a battery can maintain its DC link and provide voltage-forming behavior within the limits of its converter and stored DC-link energy. However, active-power functions such as meaningful inertial response, frequency containment, islanded load pickup or black start require an energy source for the specified duration. That source may be a battery, supercapacitor, renewable DC system or another coordinated asset.
For this reason, specifications should separate voltage waveform formation, reactive-current response, active-power response and restoration capability. The 2026 CIGRE discussion of synchronous grid-forming STATCOMs highlights configurations both with and without energy storage. The correct architecture follows from the required grid service, not from a marketing category.
A requirement such as “STATCOM response below 5 ms” is incomplete unless it names the measured quantity and test method. Current rise time, bus-voltage settling and plant-level reactive response are different metrics. Communication delay, transformer leakage, filters and measurement processing may also affect the point-of-connection result. Acceptance criteria should be measurable at defined terminals under defined system strength.
Begin with the network problem: insufficient voltage recovery, poorly damped oscillation, unstable PLL behavior, renewable curtailment or a future loss of synchronous generation. Develop the minimum and maximum network equivalents, including planned projects and credible outages. Run load-flow and fault studies to define Mvar range, then dynamic and EMT studies to compare control options. Only after the required behavior is established should the project choose converter rating, overload envelope, cooling, coupling transformer and redundancy.
For medium-voltage applications, review our 3–35 kV MV SVG/STATCOM platform and the MV STATCOM flicker-mitigation application. These pages describe hardware capabilities, but a grid-forming project still requires project-specific control validation and network studies.
Consider a 220 kV renewable collection area whose minimum SCR falls after a synchronous unit retires. The existing grid-following STATCOM meets steady-state voltage and Mvar requirements, yet EMT studies show an oscillatory recovery after a remote fault when several solar plants operate at high output. Replacing the controller without reviewing current limits could move the problem rather than solve it. The study should compare retuning, plant-level coordination and grid-forming control across all credible dispatch and outage cases.
If grid-forming control is selected, the acceptance test should include weak-grid operation, balanced and unbalanced faults, transitions into current limiting and recovery after clearing. Where frequency support is required, the available DC energy and its state-of-charge constraints must be modeled. The result is a performance envelope: the combinations of SCR, active power, voltage and contingency for which stable behavior is demonstrated.
No. A grid-following STATCOM can regulate bus voltage while still using an external voltage reference for synchronization. Grid-forming refers to how the converter establishes its internal voltage angle and interacts with the network, especially during disturbances.
Only if the entire system is designed for that function. It needs an energy source, auxiliary-power strategy, transformer energization plan, protection and controls capable of building and maintaining an islanded voltage while loads are connected.
Provide the single-line diagram, voltage levels, grid equivalents or fault levels, operating scenarios, renewable and inverter models, required Mvar range, voltage and frequency objectives, fault-ride-through rules, ambient conditions, available footprint and the required model format. For a project review, contact Elumotive with the study assumptions and target performance.
The move toward converter-dominated networks makes grid-forming control increasingly relevant. The engineering value comes from a verified response under the site’s weakest and most disturbed conditions. Define the service first, model the interactions and then select the STATCOM architecture that can prove the required behavior.
