Since last Friday’s (9 August’s) power blackout there has been considerable interest in how National Grid Electricity System Operator (ESO) balances power demand and supply. Terms such as frequency, rate of change of frequency (RoCoF) and inertia have all gained in currency.
Today’s Chart of the Week shows the distribution of frequency on the British electricity transmission system from 2014 to 2018. It aims to show the spread of frequency distribution away from the standard 50Hz and towards the allowed operational boundaries as the years go on, based on second-by-second data (see Figure 1).
Frequency is a measure of the stability of an electrical power system and deviates because of changes in demand and supply. A surplus of generation increases frequency, while excess demand yields a fall in frequency.
During 2014-18 the penetration of renewables has risen significantly. In 2014, 15% of generation was from renewables, with this increasing to 33% by 2018. Wind power contributed to 65% of that increase. Over the period the distribution of frequency has widened (see Figure 1), but until 9 August remained within its allowed limits.
Renewables generation changes as the weather changes. Wind power especially can experience several variations within a small period. In addition, solar and wind do not provide inertia – the free energy stored in the rotating mass of turbines – which helps maintain stability and limit RoCoF.
As the UK pushes towards a net zero target, the use of renewables will rise. It is also widely expected that the number of power stations with spinning turbines will reduce as older coal, nuclear and gas plants close. While power flows from solar and wind can be held back if required, having fewer turbines on the system will reduce inertia, increase RoCoF and thus the challenge of frequency management.
9 August’s blackout saw two generators fail near simultaneously. System frequency dropped to
~48.8Hz, below the statutory minimum of 49.5Hz. National Grid ESO, the government’s Energy Emergencies Committee and Ofgem are all investigating the exact circumstances and what lessons can be learned.