Curtailment of wind and solar energy is seen as one of the most important issues in some countries, and will be an even bigger problem in future electricity systems that use large amounts of renewable energy. International cooperation under the scheme of the International Energy Agency's (IEA) TCP (Technical Cooperation Programme) Wind Task 25 (Design and operation of energy systems with large amounts of variable generation) and its continuation task, Task 63, has been investigating the global situation of wind and solar curtailment for more than a decade.

Curtailment is not necessarily a bad thing, as it may increase the potential for flexibility, such as upward reserve from wind and solar power, but excessive curtailment may have a negative impact on the business interests of power producers and the rapid implementation of renewable policies,

The meaning of curtailment has changed in the past and is likely to change in the future. The technical reasons for curtailment can be divided into three categories: local network constraints, system-level curtailment and energy surpluses, although few countries provide statistics that distinguish between these reasons. Furthermore, in some countries, negative prices in the spot market create ‘voluntary curtailment’ or ‘economic curtailment’ as a market behaviour. The statistical data published in some countries do not distinguish between forced curtailment by TSOs and the market-based ‘economic curtailment’.

This paper includes four topics;

1. rethinking and redefining curtailment

2. proposing for unified statistics on curtailment

3. updating curtailment statistics in various countries/areas

4. surveying and summarising future curtailment estimates in various country/areas.

Specifically, the paper reviews previous publications and discussions on curtailment and rethinks the definition of curtailment. It will also make recommendations to governments, regulators and system operators around the world for uniform statistics on curtailment. Although many papers and reports on estimating future suppression have already been published in various countries and areas, reviewing and summarising these methodologies and results will help to reduce the uncertainty caused by curtailment and help to ensure efficient operation and use of flexibility in future power systems and energy systems.