Electric power systems throughout the world are experiencing new challenges from an increasing share of intermittent renewable energy sources (RES) and in some grid areas the installed capacity already exceeds the load. The variable and unpredictable output of RES units leads to challenges for transmission and distribution system operators. These challenges can be summarized as large surpluses and deficits of RES infeeds into the power system together with large and often rapid ramps between the different system conditions.
Working Group C1.30, ‘Technical risks and solutions from periodic, large surpluses or deficits of available renewable generation’ has examined these issues and recently published a technical brochure (TB 666). (Free download to members. Non members €150 )
Donald Vaughan (Entura) was the CIGRE Australia representative on the working group supported by Dusan Nikolic (Entura) as a young expert.
The objective of the CIGRE Working Group was to develop a structural framework to deal with the upcoming risks and provide a set of possible solutions.
The working group identified the following 10 main risks:
1. Increased need for dispatchable generation to balance rapid changes in RES output.
2. Increased reserve capacity requirements for short term balancing as the forecasts of the residual load can be wrong.
3. Reduced system adequacy (e.g. insufficient generation available to meet load) when RES generation output is low for long periods of time.
4. Increased voltage variability due to rapidly changing power flows as RES output changes.
5. Reduced system security during maintenance outages as power flows may change very quickly.
6. Increased congestion of the transmission system due to large power flows from RES.
7. Reduced visibility of power system conditions as transmission and distribution system operators do not know what output is coming from RES.
8. Decreased reliability of distribution networks with large amounts of small RES.
9. Reduced short-circuit power when large numbers of converter based RES displace rotating plant. This can reduce system stability and can cause incorrect protection operations.
10. Reduced system inertia when large numbers of converter based RES displace rotating plant. This can result in larger system frequency variations.
A technical risk-solution matrix was defined. However, the Working Group focused only on technical solutions so the matrix does not cover the necessary market design and regulatory framework issues or the economic implications of the solutions. The risk-solution-matrix gives an overview of the solutions that can be implemented. It also states where particular solutions are of no use. Not one solution will fit all challenges.
There are single or combined solutions to all of the challenges. While not all solutions have been implemented today, they are technically available. There are also some solutions such as curtailment and demand side management which need regulatory or market driven support together with public acceptance and operational implementation. Energy markets may need to be changed to enable some of the technical solutions.
The issues examined in the working group are very relevant to Australia where some parts have high amounts of solar and/or wind generation installed.
A particular example is South Australia which has over 43% installed capacity from wind and solar. In 2015/16, wind generation exceeded demand from the grid for around 24 hours across 10 separate days with surplus generation being available for export to Victoria. The maximum instantaneous wind penetration (119%) occurred when there was no contribution from rooftop PV generation and operational demand was low. The maximum instantaneous rooftop PV penetration (ratio of rooftop PV generation to underlying demand) has reached 38%.