The growing level of wind and solar energies is bringing new challenges to grid operators. One of these is maintaining system stability with their intermittency. A second, applicable to solar, is maintaining stability during an eclipse when large amounts of generation over a large area can be removed from and reinserted in the system in a matter of seconds. Fortunately, with the eclipse track calculable to a high degree of accuracy the generation changes can also be predicted – subject of course to the ever variable weather – to enable planning. (There is growing evidence that the wind can also be affected during an eclipse, slowing down and changing direction.)
With the 20 March 2015 solar eclipse the first major one over Europe since the emergence of high levels of PV, European transmission system operators (TSOs) planned for and successfully managed the grid during the event. [Europe’s Solar Eclipse – Outcomes On The Transmission Grid] So what have they learned from it, and what lessons are there for the 2021 and 2026 eclipses when the solar PV capacity is expected to have respectively doubled and tripled?
Lessons from March 2015 eclipse
During the eclipse much of continental Europe was cloudier than expected, limiting the impacts. In Germany and Italy, where the PV concentration is greatest in the region, the conditions were sunny and the eclipse impact was strong. In the event, and with continuous communication between the TSO control rooms, it was not necessary for TSOs to provide any assistance to each other.
Among the lessons learned was that a clear description of the installed PV capacity and their capabilities is needed for the accuracy of forecast studies. Real-time measurement of the dispersed PV generation is key for adapting the operational strategy in real-time.
Italian TSO Terna showed that it is possible to disconnect in advance part of the installed PV production. This shows promising results for the future, but to use such practice on a large scale needs issues such as the exact amount of PV feed in that will be switched off and the timing of switching PV off from the grid and switching PV back on to the grid to be taken into consideration.
In Germany the TSOs had instructed the power plants to stay in continuous operation, which had a positive impact on the available control power within a quarter of an hour.
Regarding the markets, the hourly day-ahead market was mainly unaffected by the eclipse. German TSOs successfully marketed the PV in a first step at the hourly market and in a second step at the quarter-hour market. In case of high demand or supply, there is a de facto quarter hour market in Germany, Austria and Switzerland, which can provide significant contributions for intra-quarter-hourly compensation. This solution is a fine-tune balancing done by the TSO. The quarter-hour market showed big spreads but this should be reduced with a European coupling of quarter-hour markets.
Next solar eclipses
By the time of the next major solar eclipses over Europe in 2021 and 2026, the installed PV capacity is expected to reach 170GW and 250GW respectively, up from about 90GW currently. ENTSO-E and the European solar association SolarPower Europe say that change and rapid action is needed in five key areas:
• Upgraded market and system rules, with the speedy adoption of the new network codes that are in the process of being implemented
• Reinforced regional cooperation between TSOs through the Regional Security Coordination Initiatives
• Active customers who are able to enter markets and participate in demand response activities
• New system services provided notably by PV, such as voltage support and balancing power
• Enhanced cooperation between the TSOs and DSOs.
“The key insight is that those developing new generation technologies and those operating the grid at distribution and at transmission level have to work hand in hand,” say ENTSO-E and SolarPower Europe in their report.