Curtailment is when a power station, and that includes wind and solar farms, is temporarily asked to reduce its electricity output because there is transmission congestion, system security constraints, more electricity being produced than required, or due to operational or environmental requirements. This is often referred to as technical curtailment. A power station can also elect to reduce its output when the price signals from the spot market are negative, and it would cost more to generate than what is paid for the electricity. The industry refers to that as economic curtailment.
A simple way to think about technical curtailment is a household power circuit. You might have several appliances ready to run, but if using them all at once would exceed the capacity of the circuit, some need to be switched off or used later. The appliances haven’t stopped working – there is simply a limit to how much electricity the system can handle at that time.
The same principle can apply to renewable energy. There may be plenty of wind or sunshine available to generate electricity, but the network may not have enough capacity to transport all of that electricity to where it is needed at that time. Or coal fired power stations cannot ramp down enough or shut down temporarily as they want to sell their power during the more profitable peak demand periods and for a little while they elect to pay for their electricity to be used ahead of that of other power generators that can more easily reduce their output. They then recoup their losses during the typical morning and evening peak demand periods.
Curtailment is therefore not necessarily a sign that renewable energy is failing. It is a normal part of managing an electricity system that has significant demand fluctuations and the generation fleet supplying the network evolves to accommodate that. It is important to remember that fossil fuel generators are also required to curtail, not just renewables, and many gas fired power stations only operate for short periods of time and some only for 2 to 5% of the year.
