Inertia Estimate

About

Generators at power stations rotate at the same frequency as the electricity grid, but when there is a rapid change in frequency this rotation is resistant to the change slowing the rate of change of the electricity grid's frequency. This means that having a lot of rotating mass on the grid acts like a shock absorber and slows the rate of frequency change and this rotating mass can be quantified as 'Inertia', measured in GVA.s.

Solar farms are connected to the grid without this rotating mass and wind farms similarly do not act in the same way as a frequency converter commonly sits between the wind turbines and the electricity grid, preventing these wind farms from providing inertia.

The shift in the market away from thermal generation and towards renewable generation has decreased this inertia, but with this rarely being an issue until March 2020 when reduction in demand due to the covid-19 virus increased the proportion of electricity coming from renewable sources.

This Inertia Estimate chart uses analysis of inertia data released by National Grid to provide a means of tracking this activity and analysing the corrections carried out by National Grid by estimating Inertia levels pre- and post- balancing.

Included Data

The data provided is based upon a replication of information and data provided by National Grid that calculates inertia by unit - with a close correlation being found for the single week period in which this data was published by National Grid. With inertia levels being higher at two power stations running at 50% load than one station running at 100% load, National Grid can increase Inertia by turning down power stations or by turning off renewables and replacing it with thermal generation.

Detailed analysis of the data suggests that National Grid aim to keep Inertia at a minimum of 120GVA.s. Although the corrections from pre- to post- balancing inertia will often exceed this level as other factors come into play such as increasing levels of margin for the evening peak.

When using this data it is worth being in mind that the reduction in output at wind farms in Scotland and increase in generation of thermal plants in England/Wales naturally increases levels of inertia and then for any further required corrections 1GW of CCGTs brought on at part load to replace renewables providing zero Inertia can be considered to approximately provide 10GVA.s of Inertia.

By the end of 2021 National Grid will have 12.5GVA.s of synthetic inertia being created by units that produce no or limited active power and this has been accounted for in the figures provided.

On a windy overnight period where 90GVA.s of Inertia is expected from planned generation and where 2GW of wind must be turned off in Scotland, this should see 2GW of CCGTs come on at part load to replace the wind providing approximately 20GVA.S of inertia and then an additional 1GW of CCGT would be expected to come on to increase the inertia to 120GVA.s. Around the evening peak further increases might be required to boost levels of operating margin (ability to turn plants up in the market), but this is different to managing inertia requirements on the system.

Periods of low Inertia will almost certainly result in periods of high BSUoS costs.

Data Service

The data for the Inertia Estimate chart can be extracted from the API. The parameters are as follows: