Balancing act
Once electricity has been generated, it is fed into the extra-high-voltage grid operated by Swissgrid. To ensure the safe and stable operation of the grid, the generation and consumption of electricity must always be balanced. If fluctuations occur, Swissgrid uses control energy. This represents a reserve for feeding electricity into the grid or removing it, depending on the situation.
Primary, secondary and tertiary energy
Swissgrid purchases control power on the control power market. In other words, Swissgrid commissions domestic power plants to provide control energy. The required capacity is put out to tender on internet platforms where power plants can place their bids. If they are awarded a contract, the relevant power plant is obliged to maintain the capacity they have offered for a certain period of time. In return, they receive financial remuneration from Swissgrid.
A distinction is made between primary, secondary, and tertiary control energy. All European transmission system operators adopt a three-stage approach to the use of control energy: a few seconds after an event that leads to fluctuations in the grid, use of the primary control reserves is automatically made. These reserves are replaced by the secondary control reserves after a few minutes – again, automatically. If the imbalance between electricity generation and consumption has still not been resolved after 15 minutes, the grid control room can manually activate the tertiary control reserves.
Time and again, the limits of the available tertiary control resources are reached when significant imbalances occur. That is why it is important for the control power and control energy markets to be constantly developed and for new resources to be acquired.
Photovoltaics for the tertiary control market
Photovoltaics (PV) offers largely untapped potential for obtaining control energy. Photovoltaic plants have an installed production capacity of around 6 gigawatt (the average electricity consumption per day is around 7 gigawatt). Therefore, photovoltaic plants represent a considerable proportion of Switzerland’s electricity generation fleet. According to the Swiss Federal Office of Energy, this figure is set to rise to as much as 30 gigawatt by 2040. This means that photovoltaic plants could be an important source of control energy and would be particularly suitable for stabilising the grid in the event of large fluctuations.
Pilot project for the use of photovoltaics
Not only is electricity from photovoltaics generated by large solar power plants, but also by many smaller systems. Because these systems do not have the necessary minimum capacity, they cannot currently participate in the control power market, for example. In order to be able to harness the potential of as many photovoltaic plants as possible, the idea is that «poolers» could offer tertiary control reserves tailored to photovoltaics in the control power market. «Poolers» are companies that bundle the generation and consumption capacities of several smaller participants. The result is a virtual power plant that can participate in the control power market like a large power plant.
To evaluate the potential of this idea, Swissgrid is working with interested bidders on the «PV4Balancing» pilot project. The aim is to develop a corresponding tertiary power service offering and to test its suitability for the control power market. To do so, a limited number of photovoltaic plants will be connected to the ancillary services market (which includes the control power market) and integrated into the operational control process. The pilot scheme is expected to start in the summer of 2025 and will last for one year.
PV4Balancing in detail
Several photovoltaic plants are aggregated, and their total nominal capacity is offered as negative tertiary control energy. Negative control energy is used when there is too much electricity in the grid. Unlike conventional control power, this form of control power does not require a constant output to be maintained. If necessary, the actual system production is reduced in real time, i.e. less electricity is generated.
The pooler continuously transmits the current or predicted generation of the PV pool to Swissgrid in the form of control energy bids. If Swissgrid accepts a bid, the PV production of the systems must be reduced within 12.5 minutes.
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