Objective and Background
In recent years there has been a significant expansion in the capacity of both wind and solar power in Great Britain. As a result, the system operator is observing days where a high proportion of the electricity demand is provided by renewables. This has led to concerns regarding system stability due to reduced levels of inertia. However, for temperate climates like Great Britain with high capacities of both wind and solar, it is unclear as to when the high penetration events occur. This study has developed a framework method which allows the characteristics of the national demand and renewable generation to be examined for a range of scenarios (i.e. changes in wind and solar capacity and/or the ratio of wind and solar capacity).
Method
Studies investigating the integration of renewables typically simulate the power system for a range of penetrations of wind and solar power production and evaluate the impact on the system. One key requirement is a long term dataset in order to capture the wide range of meteorological conditions which can affect the system. This study uses reanalysis data to derive an hourly time series of GB-aggregated wind and solar power and electricity demand for the period 1980-2015. The hourly proportion of demand provided by renewables is then determined.
The derived data enables the characteristics of the power system to be determined including (1) the volume of curtailment for a range of System Non Synchronous Penetration (SNSP) thresholds (2) when curtailment is likely to occur (in terms of season and time of day) and (3) how the characteristics of curtailment will change as the capacity of wind and solar increases, with a particular focus on the impact of the ratio of wind to solar capacity.
Principal Findings
Based on the current capacity of renewables, the infrequent nature of high penetration events indicates that some level of curtailment would be the most effective remedy for maintaining system stability. Weekends can be particularly challenging, especially Sundays. Across 36 years investigated, the days with the highest penetration of renewables tend to be sunny, windy weekend days between May and September. This is when there is a significant contribution of both wind and solar generation and demand is suppressed due to human behaviour.
The required system interventions can vary by the time of day and season. The worst-case combination of high renewable generation from sun and wind coinciding with low demand can fall at any time of year but has a bimodal nature. For winter, supply surplus is likely to come overnight, whereas during the summer surplus is most likely to fall at mid-day. The magnitude of the excess energy is highly dependent on the ratio of wind and solar capacity. This analysis therefore reinforces the merits of a blend of renewables and points to a 'sweet spot' mix of wind and solar. For Great Britain, the level of excess energy is at a minimum value for a blend of renewables capacity of 48% solar and 52% wind.