Grid-Forming Is Now a Tender Requirement, Not a Feature

In 2025, grid-forming capability was something storage vendors described as coming. Functional specifications discussed it in future tense, pilots demonstrated it, and procurement documents largely did not ask for it.

That framing no longer holds. Storage tenders now in circulation carry grid-forming performance as numbered clauses with numeric thresholds — the kind that a bid is scored against rather than credited for. We are currently answering one of them, and the shift in how these documents are written is worth describing, because it changes what a bidder has to hold before submission rather than after award.

We will describe the requirement families generically. The clause numbering, the issuing authority and the geography are specific to a live procurement and are not ours to publish.

What the clauses now say

Four families of requirement recur, and they are stated as thresholds rather than capabilities.

Synthetic inertia. An inertia constant of 20 seconds or more, with response activation inside 5 milliseconds, and the constant flexibly adjustable by the system operator. This is the clause that separates grid-forming from grid-following in procurement terms. A grid-following inverter measures the grid and synchronises to it; a grid-forming one establishes a voltage waveform and contributes to the system's effective inertia. Twenty seconds is a substantial figure — it is stated against the plant's own rating, and it has to survive a system operator's verification.

Primary frequency response. Response inside 0.2 seconds with active power adjustment deviation of 2% or less, and droop configurable across a 1–9% range with a defined default, operating independently of external signals. The independence requirement matters: it means the plant must respond to local frequency measurement, not wait for a dispatch instruction.

Converter overload. 110% continuous, 120% for at least two minutes, and preferably 150% for 10 seconds. This one is frequently misread as a battery specification. It is not.

Ride-through and reactive behaviour. Fault ride-through configurable in ten or more bands between 0 and 1.3 per unit, reactive current injection of 2% per 1% voltage drop as a default and settable across a 1–6% range, and reactive capability of ±0.3 × rated active power delivered continuously at any active power level, four-quadrant, across a 0.9–1.1 per-unit voltage window.

This is a converter question, not a cell question

The single most useful thing a prospective bidder can internalise is which half of the system these clauses address.

Inertia constant, activation time, frequency response speed, droop range and overload behaviour are properties of the power conversion system and its control firmware. They are not properties of lithium iron phosphate cells. A battery that is well within its thermal and C-rate envelope is a necessary condition for meeting them — it must be able to source the power — but it is nowhere near a sufficient one.

The battery side of the overload clause is worth separating explicitly, because it is the half a manufacturer can actually assert. Take a 10 MW plant built from twelve of our containers. Each unit has a maximum continuous DC power of 2,458 kW, so the DC capability is roughly 29.5 MW against a 10 MW AC requirement. A 12 MVA converter block at 150% is 18 MVA, and the DC side supports that with headroom. That is arithmetic from the configuration, and it is checkable.

What the same arithmetic does not establish is that the converter will deliver 150% for ten seconds, or that its inertia constant reaches twenty seconds, or that it activates in under five milliseconds. Those require type-test evidence from the converter, and until a specific SKU is selected and its evidence is in hand, the honest answer in a compliance matrix is that the requirement is pending that evidence — not that it is met.

We take that position in our own bidding. A compliance statement made to a system operator that later proves unbacked is a worse outcome than an amber row, because it is discovered during commissioning witness testing with the connection agreement already signed.

The duty-margin argument that does help

There is a real technical advantage available here, and it comes from sizing rather than from claims.

A storage plant specified for energy rather than power runs at a fraction of its rated C-rate. In the twelve-container example, the operating rate against a 10 MW requirement works out near 0.18 C against a 0.5 C rating. The consequences are worth stating because they bear directly on the grid-forming duty:

• Measured round-trip efficiency sits above the rated figure, because rated efficiency is quoted at rated power.
• Thermal load on the cooling system runs well below design, which matters for a plant expected to hold performance across a long operating period.
• Cycle life at that duty sits inside the ≥6,000 cycles at 90% DoD (25 °C) basis rather than at its edge.
• Short overload events are served from a large margin rather than from the top of the envelope.

None of that meets a clause by itself. All of it makes the clause easier to meet, and it is the kind of statement a technical evaluator can verify from a configuration table.

Four things a bidder now has to produce

Beyond the performance numbers, these documents carry a deliverables load that is routinely under-priced.

Simulation models. Typically a PSS®E model at a named version, a PSCAD/EMTDC model at a named version with a specified compiler, and PowerFactory models in both RMS and EMT formats — plus releasable user guides carrying site-specific settings, and model fidelity sufficient for the operator to change parameters themselves. Three toolchains, one of them in two formats, is a specialist scope item. It belongs on the price schedule as its own line, not folded into engineering.

Technical limits submission. Proposed operating limits with supporting data, with the system operator holding a defined window — often 60 days — to object before they are deemed accepted.

Telemetry. Real-time signals to the control centre over a specified protocol and physical path, with a separate remote-control level activated at the plant, and an explicit prohibition on simulated signals at commissioning.

Commissioning test plan. Submitted months ahead of testing, approved by the operator, with hold points and harmonic measurement obligations, and settings that must remain reconfigurable at the operator's request across the whole supply period — a multi-year obligation, not a commissioning task.

Why most deployed Indian BESS would not clear these

The majority of battery storage commissioned in India to date was procured for energy arbitrage, peak shaving and renewable firming. Those applications are well served by grid-following converters, and the tenders that procured them did not ask for inertia constants.

That is not a criticism of the installed base; it is a description of what it was specified to do. But it does mean that a track record of delivered MWh is not evidence against these clauses, and a bidder who assumes their existing converter package carries over may find the gap at the evidence stage. The Indian regulatory direction is visible in the connectivity and grid-code work we covered in CERC's grid code for storage, and the technical background to grid-forming operation is in our earlier post on grid-forming BESS in India, of which this is the procurement-side sequel.

What to do before the next one lands

If you expect to bid into a grid-forming specification, three steps are worth taking ahead of the document rather than after it.

Get converter type-test evidence in hand for the SKU you intend to offer, covering inertia constant, activation time, frequency response and the overload profile. Price the model and study package as a named line item. And read the environmental and telemetry clauses as carefully as the performance ones — degraded-rating declarations, humidity and temperature envelopes, and fibre routes to an adjacent substation are scope items that do not appear in a performance table.

The architecture we build to is described on our technology page. For a configuration sized against a specific grid-forming duty, talk to our team.