Curtailment means the action of restricting or reducing something. In power systems, it refers to the restriction of certain power generation sources even in situations when they are available to generate. Although there have been incidents of restricting solar power generation (even when the sun is out), curtailment has most commonly applied to wind power generation, meaning that wind turbines are turned off even when the wind is blowing. Given that wind energy represents free fuel, while those wind turbines have been paid for and so need to recoup their costs by selling power whenever they can, curtailment is obviously not an ideal economic situation!

Common reasons for curtailment include:

  • Lack of local transmission capacity, such that when all the wind (or other) power capacity attached to a segment of the power system is operating close to peak output, there simply isn’t enough “space” in the available power lines to export all that power. Unless the economic losses due to curtailment outweigh the costs of upgrading transmission capacity, it can be cheaper to curtail (and compensate the curtailed generators) than to build more grid.
  • Oversupply in the system as a whole, for example when large amounts of renewable power capacity (required to offset the low capacity factor/energy generation of some of those sources) and/or inflexible other capacity (e.g. nuclear) are together operating close enough to full power to exceed the required demand; plus any surplus that can be exported into another system.
  • Operational constraints, such as those related to the requirement to ramp up (or down) power plant outputs to meet rising (or falling) demand. Since this ramping up and down can take time for certain generating types (particularly nuclear and coal), it may have to start early and cause temporary displacement of other available sources.

There are a variety of possible solutions to curtailment, including:

  • Increased grid capacity (to avoid congestion in specific localities within a system)
  • More storage (to store excess supply until it can be used later)
  • More interconnectors (to import/export excess supply into other systems)
  • Greater flexibility in the generating mix (so that one source can quickly ramp down to make space for another one)