Winding down: NGESO proposes virtual inertia specifications

By this time next year, intermittent generation may be able to provide virtual synchronous services to the industry under a new Grid Code modification proposal.

Raised by National Grid Electricity System Operator (NGESO), GC0137 Minimum Specification Required for Provision of Virtual Synchronous Machine (VSM) Capability seeks to create requirements for Virtual Synchronous Machines (VSMs). These allow generators which do not operate at the same frequency as the rest of grid to function as if they were, reducing the effect they have on system inertia.

System inertia is essentially how rapidly the system frequency responds to change. It has steadily reduced over recent years as more and more intermittent generation comes online and traditional synchronous generation moves off the system. Traditional generators like coal plants, combined cycle gas turbines and hydropower stations have their turbines spin at 3,000rpm so they generate electricity at 50Hz, matching the frequency of the system. If they suffer a fault, inertia means they do not stop spinning immediately so frequency drops more slowly. However, most renewables are asynchronous as they are connected via an inverter, so do not work this way, which[VT1]  gives NGESO[VT2]  less time to respond to system issues and increases the cost of managing the system.

VSM technology provides a solution, in which case the asynchronous generator uses its power electronics to mimic a synchronous one. This would mean a fault will not cause frequency to drop so sharply, giving NGESO more time to take other actions. GC0137 envisions specifications for what VSM capability should look like so that it can operate in a market-based service. NGESO has not yet published the appendix which sets out the specifics of this, but indicated an updated draft could be circulated in January. Commercial details are to be discussed and agreed at a later date.

The modification will be presented to the Grid Code Panel on 19 December and NGESO said it intended implementation would be in Q320. However, timescales will depend on how quickly a VSM specification can be agreed. NGESO expects that enabling VSM should lead to improvements in its own ability to balance the system and revenue opportunities for generators.

We provide regular coverage of regulatory issues relating to flexibility in our Flexibility Matters service, helping you understand what changes mean for you and your assets. For more information, contact Steven Britton at or on 01603 542126.

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