Minimising carbon footprint in terms of electricity generation has emerged as one of the global issues in tackling the existential dread of climate change. The UK aims to meet 40% electricity demand via electricity generation by 2030 while Malaysia is determined to include at least 20% of its generation mix with renewables by 2025. Seeing how wind energy is relatively abundant in the former and the tropical climate in the latter allows ample solar energy year-round, what is slowing down the decarbonising progress of both countries? The answer lies in the perspective of the grid operator in balancing supply and demand.
One of the biggest challenges to integrate both solar and wind into the national grid is the unpredictability of its supply. If electricity demand is higher than supply, fossil-fuel plants such as coal can ramp up production simply by adding more fuel. However, asking the wind to blow faster or the sun to shine brighter is obviously impossible. When electricity supply by the system is higher than demand, cutting down electricity generation to match demand may sound like obvious choice. It may be tempting to cut down fossil-fuel plants, but this will lead to problems due to a factor known as grid inertia.
Inertia is the resistance of any physical object to alter its velocity when an external force acts on it. In this context, grid inertia is the resistance of the system to change the amount of electricity provided. The higher the grid inertia, the less volatile electricity system is, and the more stable the overall system. Fossil-fuel plants provide power through steam or gas turbines, which provide grid inertia while solar and wind energy are connected through power electronics which do not provide grid inertia. The rotating mass from these steam and gas turbines provide inertia as they spin at the same frequencies as the electric grid.
One way to observe the amount of stability or grid inertia in the system is by looking at the electric frequency of the system. As more consumers turn on power-hungry devices in their households, demand exceeds supply, thus, frequency drops. If there is more supply than demand, for example, on a clear and windy day providing plenty of solar and wind energy, frequency rises. The rate of change of frequency, or how quick it falls or surges, measures grid inertia. Thus, if demand for power spikes, the frequency of the grid tends to decrease. Having a lot of rotating mass on the grid acts like a shock absorber and slows the rate of change.
On 9 August 2019, the limits of the electricity grid in the UK were tested. A freak lightning strike rendered an offshore windfarm and a steam turbine to reduce its output as a protective measure. There were major disruptions to parts of the rail network and critical facilities such as Ipswich hospital and Newcastle airport were disconnected from electricity. In the graph below, we see a steep fall in frequency as supply dropped but demand remained constant.
This sudden drop in frequency needed immediate attention as a dangerous domino effect could occur, where electricity in other areas would be disconnected from the system to save themselves from damage. When more generators are disconnected from the system and demand for electricity stays constant, this risks a sharper drop in frequency, and the cycle continues. Frequency started to stabilise again as electricity supply started to recover. Between 15 and 45 minutes, 1.1 million electricity customers were left without power while the electricity grid operator struggled to balance the grid frequency.
This case study prompted policy makers to pursue pre-emptive steps towards avoiding such unfavourable situations to happen again. Most importantly, solutions in providing grid inertia were studied more intently. As more solar and wind are connected to the grid, the system is bound to have less grid inertia.
One way to add artificial or manage grid inertia is to connect more capacitors, compensators and condensers – these are expensive and require further planning. Moreover, the planning of these capital equipment needs to cover a large time window as they are expected to operate for several decades. The utilisation of smart grid technologies to manage peak demand and allow a flatter demand profile throughout the day may also help ease the transition of green energy into the system.
Hence, it is simple to see how complex the managerial and technological issues impede the advancement of green energy.
Reference: National Grid ESO Technical Report on the events of 9 August 2019, 6 September 2019. Read the report here.