NESO sends GC0168 to Ofgem: EMT models from plant built before September 2022

Serguey Asael Shinder09 October 2026, 07:472 min read

Great Britain's system operator NESO submitted the final modification report for Grid Code change GC0168 to the regulator Ofgem on 8 October. The change would let NESO require electromagnetic transient (EMT) models from users whose plant was completed before 1 September 2022 — the fleet connected before such models were routinely demanded.

Why NESO wants them. The report's reasoning is the shift from large synchronous machines to many smaller electronic power converters — wind, solar, batteries, HVDC — whose power electronics interact with the transmission system differently. NESO lists the problems it needs to study: system oscillations, inverter instability, low fault levels and transient overvoltage. Slower RMS models do not capture these fast interactions; EMT models do, and without them for older plant the studies have gaps.

Two versions on Ofgem's desk.

• The original proposal: legacy users provide EMT models on NESO's request, under a new Electrical Standard (Annex 08 of the report) that sets the model requirements and the process for retrospective submission.
• WAGCM1, the workgroup alternative: the same obligation, but its effective date for pre-September-2022 plant is deferred until a cost-recovery mechanism exists in the CUSC, the charging code. Two CUSC changes, CMP456 and CMP466, have been raised for that.

The workgroup backed the alternative by majority and did not back the original; the Grid Code Review Panel also recommended WAGCM1 by majority. The consultation drew five non-confidential and one confidential response.

The cost argument. Workgroup members agreed that older plant should be compensated but said the bill is hard to quantify: it depends on unit size, age, type and whether the manufacturer still supports the equipment — for some legacy converters, building a validated EMT model means reverse-engineering controls the vendor no longer documents. NESO's representative noted cost recovery would apply only to historical sites; new connections are already expected to supply models.

What happens next. Implementation is set for 10 business days after Ofgem's decision. A parallel change for transmission owners (STC CM097) and a new code procedure (PM0147) run alongside. If Ofgem picks the alternative, the practical start depends on the CUSC cost-recovery changes — so owners of pre-2022 wind, solar and battery sites should follow CMP456 as closely as GC0168 itself.

Source: NESO, "GC0168 Final Modification Report", 8 October 2026 — https://www.neso.energy/gc0168-final-modification-report