Synchronous Grid Forming STATCOMs With and Without Energy Storage

	Jyri Kivimäki WG B4.107 Convenor		Michael Schütt WG B4.107 Secretary

 

Introduction

 

The global energy transition is fundamentally reshaping the electrical characteristics of the AC power system. As fossil fueled synchronous generation is progressively displaced by power electronic interfaced renewable resources, one of the most consequential side effects is the steady erosion of system inertia. A conventional synchronous generator inherently opposes changes in AC system frequency through its rotating mass, a property historically taken for granted as a stabilizing feature of the grid. As synchronous machines are retired or displaced, alternative means of delivering this functionality are urgently needed.

This challenge is already visible in operating experience. Grid operators managing networks with high wind penetration have reported Rate of Change of Frequency (RoCoF) values well above levels considered normal a decade ago, and several systems pursuing ambitious renewable targets are approaching protection limits once seen as comfortable margins. This is a present-day operational reality for a growing number of transmission networks, not a distant risk.

 

In response, a new class of technology has emerged: the combination of a STATCOM, traditionally deployed for AC voltage support, with an integrated energy storage system controlled to actively oppose RoCoF events. Unlike purpose-built, long-duration transmission-level storage such as Battery Energy Storage Systems (BESS), the storage associated with these STATCOMs is comparatively modest in energy capacity. Because RoCoF events, angle jumps, and other inertia-related phenomena require relatively small energy throughput compared with the primary frequency reserve typically targeted by BESS, this storage is engineered for high-frequency, repeated cycling, providing dynamic, near-instantaneous support to the AC system throughout the asset's operational life.

 

To exploit this capability in real time, Synchronous Grid Forming (SGFM) controllers have been developed. These regulate the converter as a controlled voltage source rather than a current source, so the current exchange with the AC system is determined directly by the instantaneous voltage difference across the network impedance, enabling naturally coordinated interaction with other voltage sources and stable operation even on very weak grids. This is a fundamentally different control philosophy from conventional grid-following controllers, which regulate injected current with respect to the measured grid voltage. Notably, the value of SGFM control is not limited to applications with energy storage; even without storage, it can help meet demanding requirements such as operation on very weak grids. SGFM control is therefore now being proposed both for STATCOMs with energy storage and for STATCOMs without storage, including designs intended to accommodate a future storage upgrade, with early units of both types now gaining initial operating experience.

 

This growing operational base is a useful input to the Working Group's formation, giving the industry a timely opportunity to capture lessons learned, compare vendor approaches, and establish harmonized guidance before the technology becomes entrenched in divergent, vendor-specific practices.

 

Why This Work Matters Now

 

The introduction of SGFM control and integrated energy storage does more than address inertia and RoCoF concerns; it opens the door to additional ancillary services that STATCOMs could offer network operators, extending their value well beyond traditional reactive power support. This is consistent with a broader industry shift: rising penetration of variable renewable generation is challenging both frequency and voltage stability, requiring new categories of essential reliability service and faster-responding flexibility resources, whether from batteries, pumped hydro, demand-side participation, or power electronic devices such as SGFM STATCOMs.

 

This shift is not only technical but evaluative. Well-established criteria have long existed for assessing traditional grid-following converters, but high renewable penetration, the rapid growth of power-intensive loads such as data centers, and the transition towards grid-forming converters mean much of this evaluation framework no longer translates cleanly to SGFM technology. There is a clear need for updated, robust standardization to close this gap. The rapid growth of large, non-traditional loads such as green hydrogen electrolyzes and AI data centers reinforces this need: these loads are increasingly connected at points on the network not originally designed for such demand, and when sited on weak grid nodes can create voltage instability and destabilizing interactions with nearby inverter-based generation. This is exactly the condition where SGFM control has value even without energy storage, reinforcing why the Working Group's scope deliberately covers both storage and non-storage configurations.

 

This expanded functionality brings technical and commercial questions the industry has not yet answered consistently, and independent reference material remains limited, with specifiers currently relying largely on individual OEM documentation.

Chief among the open questions is sizing: what operating conditions should determine the amount of energy storage required, and consequently the peak power and discharge duration a specification should demand? Should the storage be bidirectional, that is, capable of both absorption and supply, or is unidirectional support sufficient for most applications, with the opposite direction instead handled through braking choppers rather than active storage? These are not academic questions; they directly affect capital cost, footprint, and the value delivered to the network. WG B4.107 has been formed specifically to address these questions and provide clear guidance to future specifiers.

 

Beyond storage sizing, practical project planning considerations remain inadequately covered by current guidance: what circuit arrangements are appropriate for a Synchronous Grid Forming STATCOM with storage, what might a representative site layout look like, and how much land does such an installation require? These are exactly the early-stage planning questions utilities, developers, and consultants need answered well before detailed engineering begins, and they form a core part of the Working Group's intended output.

 

The Harmonization Challenge

 

Perhaps the most strategically important aspect of this work concerns the harmonization of requirements across different SGFM STATCOM systems and vendors. As SGFM technology moves towards wider deployment, TSOs face a critical challenge: ensuring defined, homogeneous behavior from all SGFM STATCOMs under transient and fault conditions, regardless of OEM. While diversity in controller implementation can foster innovation, a common understanding of expected external behavior is essential for equipment from different manufacturers to operate together safely and predictably. Such consistency is not an objective in itself, nor does it require identical control algorithms; rather, it gives TSOs confidence that vendor equipment will interact predictably, reducing integration risk, simplifying network studies, and avoiding unforeseen interactions that could compromise system stability.

 

There are broadly two approaches to this problem, each with drawbacks. One is for each TSO to define bespoke requirements, burdening OEMs with adapting to a proliferation of differing specifications, a costly and potentially fragmenting outcome for the supply industry. The alternative is to mandate identical requirements across all systems and vendors, risking stifled innovation and failing to account for genuine differences in system needs. A third option, leaving behavior entirely unspecified, is not viable either: it would undermine the premise that homogeneous external behavior is essential, resulting instead in costly, project-specific integration studies and ongoing monitoring for every vendor combination connected to the network.

 

The absence of clear, standardized evaluation criteria also complicates procurement. Without a common reference framework, suppliers can struggle to determine precisely what a customer requires, while customers can struggle to understand the trade-offs and limitations on offer; where the customer lacks deep in-house technical expertise, this disconnect can lead to unintentionally unfair or inaccurate supplier comparisons during tender. Procurement requirements need to be standard, reusable, and unambiguous. WG B4.107 is not alone in recognizing this gap; as outlined below, several TSOs and standardization bodies are already active in adjacent areas, and this work is intended to complement, rather than duplicate, those parallel efforts.

 

WG B4.107 therefore proposes a balanced way forward: defining basic requirements universally applicable to all SGFM STATCOMs, while clearly identifying which additional requirements should be specified by individual network operators based on their own system needs. This gives TSOs confidence in predictable baseline behavior while preserving flexibility to tailor equipment to local grid conditions, an outcome that will require close engagement with multiple OEMs to ensure the final Technical Brochure is genuinely inclusive of the technologies and design philosophies currently in the market.

 

Objectives of the Working Group

 

The central objective of WG B4.107 is to produce a Technical Brochure giving the AC power system community a practical guide to the functionality, that is, the steady-state capability and dynamic behavior under normal and disturbed grid conditions, together with the rating, power circuit design, and layout of STATCOMs with energy storage. A key deliverable will be the definition of fundamental requirements applicable to all Synchronous Grid Forming STATCOMs, independent of underlying technology or OEM, alongside clear guidance on which requirements should instead be left to individual network operators to define.

 

Part of what makes this objective challenging is the inherent adaptability of GFM STATCOMs: because control behavior can be reconfigured through software, a single hardware platform can be adapted to different grid requirements, operating philosophies, and system conditions. While this flexibility is a genuine strength, it also increases the complexity of performance evaluation, since suppliers must demonstrate compliance across a growing range of operating scenarios and evolving national grid codes. This is why defining a stable, universally applicable set of basic requirements, while leaving room for case-by-case specification, is both necessary and non-trivial.

 

Scope of Work

 

To deliver on this objective, the Technical Brochure will address six core topics:

 

1. Functionality, that is, steady-state capability and dynamic behavior, of a Synchronous Grid Forming controlled STATCOM with energy storage, including comparison against grid-following controlled STATCOMs with storage and against rotating synchronous compensators.
2. Functionality of a Synchronous Grid Forming controlled STATCOM without energy storage, including comparison with grid-following controlled STATCOMs that likewise lack storage.
3. Energy storage options available for integration with STATCOMs.
4. Selection of the magnitude of energy storage, addressing the sizing questions outlined above.
5. Potential single line diagrams for STATCOMs with energy storage.
6. Potential layouts for STATCOMs with energy storage, supporting early-stage project planning.

 

Covering both the "with" and "without storage" cases, and comparing SGFM STATCOM performance against established alternatives such as rotating synchronous compensators, the Brochure will give planners and specifiers a genuinely comparative basis for technology selection.

 

Building on Existing CIGRE Work

 

WG B4.107 does not start from a blank page. The work will build directly on the definitions of Synchronous Grid Forming behavior established by WG B4.87, and on the energy storage options previously discussed by WG B4.84, with close coordination planned with WG B4.101 on Grid Forming Energy Storage Systems (GFM ESS) to ensure complementary rather than duplicative outputs.

 

Several TSOs, regulatory bodies, and industry initiatives have already begun defining requirements relevant to grid-forming behavior, representing valuable input to this task. In Europe, ENTSO-E has been progressing discussions on grid-forming capability within its network code framework, Germany's VDE FNN guideline has set out early national requirements, and National Grid ESO in Great Britain has published best-practice guidance alongside its Stability Pathfinder procurement rounds. In the United States, NERC has developed related guidance, complemented by the UNIFI initiative and IEEE's P2800 series of standards. Australia has also been an early mover, with AEMO defining specific grid-forming requirements for its procurement of grid-forming battery storage capacity. WG B4.107 intends to review and draw on these efforts, ensuring the requirements ultimately proposed are grounded in, and compatible with, the direction already being taken internationally.

Given the close relationship between converter-based control behavior and downstream protection and automation performance, the Working Group will also monitor and reference relevant outputs from Study Committee B5 work on the impact of Inverter Based Resources (IBR) on protection and automation systems, including B5/C4.61, B5.65, and B5/C4.79, with a liaison member from SC B5 supporting this coordination.

 

Conclusion

 

As power systems worldwide continue their transition away from synchronous generation, the ability of new technologies to replicate, or improve upon, the stabilizing properties of the machines they replace becomes increasingly critical. No single technology will solve the flexibility challenge alone; the future grid will rely on a diverse portfolio, from the millisecond stabilizing response of STATCOMs and synchronous condensers, to the load-shifting capability of battery storage, to the controllability of next-generation inverter-based transmission technologies, and increasingly, the intelligence of secure Synchronous Grid Forming inverters. Synchronous Grid Forming STATCOMs, whether equipped with energy storage or not, sit firmly within this portfolio, offering TSOs a practical means of managing RoCoF events, supporting weak grid conditions, and accessing new ancillary services from a single flexible asset.

This value can only be fully realized if the industry also solves the accompanying evaluation and procurement challenge. As grid-forming control becomes more software-configurable and more widely adopted, the ability to assess and compare vendor solutions consistently, and to specify requirements clearly and fairly during tender, becomes as important as the underlying hardware capability itself.

 

WG B4.107 has been established to bring clarity and consistency to this fast-moving technology space at a timely moment: providing practical guidance on storage sizing, circuit design, and layout, while tackling the harder question of harmonizing vendor requirements without stifling needed innovation. By engaging broadly across OEMs, coordinating with related CIGRE groups in SC B4 and SC B5, and working alongside parallel international standardization efforts, the Working Group aims to deliver a Technical Brochure that serves as a genuine reference point for the industry, helping ensure that as inertia leaves the system with retiring synchronous generators, reliable and well-understood alternatives are ready to take its place.