National Grid Electricity System Operator (ESO) has proposed a modification to the Grid Code adding minimum specifications for the provision of synthetic inertia by asynchronous sources of power such as wind, solar and interconnectors.
It comes shortly after the ESO launched its first ever tender for inertia.
Inertia is provided by synchronous generators, such as coal or gas plants, which feature large spinning masses rotating in harmony with the frequency of the electricity system. They act as shock absorbers, limiting the rate of change of frequency when there is a loss of load to prevent the problem from snowballing.
As fossil fuel generation is gradually phased out and replaced by asynchronous renewable generation, the inertia of the power grid is declining, leaving it more susceptible to disturbances.
But asynchronous generators can mimic its effects by using their power electronics to rapidly raise or lower their output in responses to changes in frequency. This fact-acting frequency response is often called artificial or synthetic inertia.
The modification GC0137 would add a non-mandatory technical specification Grid Code for this new type of service, which the ESO refers to as “virtual synchronous machine capability” (VSM).
“While traditional mechanically connected power generation is synchronous with the wider grid, most renewable sources are asynchronous as they are controlled via other means, such as power electronic converters,” the proposal document explains.
“Consequently, the inherent inertia of traditional power generation is effectively absent, and thus any generation which trips off in response to a fault on the grid such as a significant weather event, could result in a rapid fall in system frequency.”
“In a high-inertia system the fall in system frequency would be slower, allowing ESO greater opportunity to restore balance through the provision of slower-acting frequency response,” it adds. “VSM capability will contribute to mitigating this risk.”
The proposal will be presented to the Grid Code panel on Thursday (19 December).
Speaking to Utility Week in August, former Ofgem technical director John Scott said the development of synthetic inertia would key be to avoiding repeats of the recent blackout as the switch from fossil fuels to renewables continues.
Synthetic inertia is already widely used in the Canadian province of Quebec, where the local public utility, Hydro-Quebec, has required all new wind turbines to be capable of providing the service since 2005. The first machines with this functionality went up in 2011 and they now account for around two thirds of the turbines in the region.