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The project manager demonstrates the Kombikraftwerk 2 project in the control centre.
© Fraunhofer-Institut für Windenergie und Energiesystemtechnik IWES, Kassel
Renewable power grids
Projektinfo 06/2015

Illustration of the topological distribution of grid nodes in Germany with the corresponding ratio of offered to demanded energy.
© Fraunhofer-Institut für Windenergie und Energiesystemtechnik IWES, Kassel
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Stable grid with 100 per cent green electricity

In a power grid, the energy provided must always be equal to the power demanded. Only then can frequency and voltage remain stable. Ensuring this is the task of grid operators. But when volatile — meaning not arbitrarily adjustable — energy sources are the main sources of electrical energy, grid operators face a challenge. In the combined power plant (Kombikraftwerk 2) project, researchers modelled a power supply system based on 100 per cent renewable energy, and demonstrated that the grid can function reliably even with an ample supply of photovoltaic and wind energy.

In order to balance consumption and generation, transmission system operators rely on so-called ancillary services. An important issue on the way towards a predominantly green power supply is therefore: to what extent can renewable energy support the grid and reliably provide balancing power? Project manager Kaspar Knorr from the Fraunhofer Institute for Wind Energy and Energy System Technology IWES explains that, “In the Kombikraftwerk 1 project, we already demonstrated that renewable energy sources combined with storage systems can cover the electricity demand in Germany at any time. The Kombikraftwerk 2 project now shows: they can even offer ancillary services. A scenario with one hundred percent renewable energy is therefore conceivable.”


Currently, ancillary services are provided by conventional power plants almost exclusively. Energy generation based on coal and gas fired power plants is weatherindependent and predictable, and the conditions for participation in the balancing power market, for example, were tailored to them in the past. In addition, the rotating masses of large synchronous generators are important for supporting the grid in the first fractions of a second after a sudden load increase or decrease, owing to their inertia. Even rapidly switchable balancing power takes some time to fully ramp up.

Ancillary services facing change

New framework conditions are required for renewable energy sources to contribute to frequency stability control: wind and solar power have little or no rotating masses, and the grids therefore lose a part of their inertia. However, they react far more quickly to changing conditions. It is therefore necessary to adjust the market for it to meet the new demands. Currently, primary balancing power must be fully available after 30 seconds. But what does that mean?

If the grid load suddenly increases, then the frequency must initially remain stable before primary balancing power can fully support the grid. Rotating masses currently perform this task. If they were not there to support the grid, it would collapse. “In our scenario, we need new requirements to primary balancing power. The power electronics of PV systems can provide primary balancing power within milliseconds, while wind turbines need approximately five seconds,” Kaspar Knorr explains. “In this scenario, there still are rotating masses in the biomass power plants that stabilise the grid until the onset of PV systems. Technically, renewables are so fast that they are able to compensate for the lower inertia.” Currently, power plant operators must offer their primary balancing power one week in advance. In this context, the project manager adds that, “This system is not sustainable indefinitely. If renewables are to provide ancillary services, then daily tendering will be necessary.” This means: every day, wind turbine and solar power plant operators can react to current weather conditions and then define the grid-relevant contributions they can make the following day.

Accurate modelling of energy generators

According to the researchers, the scenario of the research project Kombikraftwerk 2 offers a realistic outlook for the year 2050. The assumptions are based on detailed weather information, today‘s plant locations and grid expansion plans. The Figure offers an overview of various grid nodes and their performance as recorded during noon one day in February. Wind energy forms the bulk of the mix with 60 per cent. The researchers assume that the designated offshore areas are fully utilised and integrated into the grid, and that additional potential is tapped onshore. For photovoltaic systems, they expect four times the current installed capacity. Roughly speaking, wind turbines are mostly located in the north and offshore, while the sunnier south increasingly employs photovoltaic systems. A large number of decentralised, inflexible and variable individual systems can be combined to form a virtual power plant, and be considered as one unit. For grid operators, this has the advantage of having to work with a low number of generators only, similar to large-scale power plants. In virtual power plants, each individual electricity producer complements the other, which helps offset fluctuations.

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