Secondary control reserve supplied successfully by onshore wind farms

An important milestone in the continuing realisation of the energy transition in Germany has been achieved by the partner companies Engelhart, Gridfuse, ENERCON and nttb. Since early December 2025, Engelhart has been marketing the first onshore wind farms with ENERCON wind turbines on the secondary control reserve (automatic frequency restoration reserve, aFRR) market in the control zone of the transmission network operator TenneT Germany. This represents the first ever approval for an onshore wind farm pool for aFRR in the largest German control zone.

Automatic frequency restoration reserve or secondary control (aFRR) is an automatically activated control reserve used for grid stabilisation. It is employed to restore the grid frequency to its setpoint of 50 Hz following short-term fluctuations. The reserve is supplied from power plants or storage facilities and responds within a few minutes, once the primary control (frequency containment reserve, FCR) has bridged the first seconds following a frequency deviation. For the longer-term stabilisation of the grid frequency, the aFRR is then replaced by manual frequency restoration reserve (mFRR) or tertiary control. FCR, aFRR and mFRR are all elementary system services for grid control and are activated by transmission network operators.

In the past, these system services were primarily provided from fossil fuel-based control power plants. Thanks to innovative forecasting and control technology from ENERCON and Gridfuse, however, it has been possible to demonstrate that groups of wind farms can be controlled just as precisely as conventional facilities.

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Since February 2025 Engelhart has offered flexibility in the minute control reserve (mFRR) market with roughly 100 MW from 20 wind farms. It has now been possible to transfer the first 3 wind farms of the operator Looft-Schmidt, located at Huje in Schleswig-Holstein with approximately 28 MW of flexibility to aFRR marketing.

Challenges were encountered in the pre-qualification process and in obtaining a precise live forecast (potential grid feed), plus in the development of a higher-level pool controller by Gridfuse. With an additional control function for the control reserve on the ENERCON wind farm controller, the companies have been able to overcome the technological obstacles and also to enable older wind farms to participate in the control reserve market in future.

Thanks to the agile, partner-like collaboration with the wind farm company, ENERCON Service, the wind farm operator nttb and the transmission network operator TenneT Germany, it was possible to implement the project within just a few months, including the necessary testing procedures. Wind farm operators benefit from concrete added value, while grid operators benefit from grid-friendly wind farm control.

‘Onshore wind farms demonstrably have the potential to provide a majority of the critical grid stabilisation volume within aFRR in a system-secure manner and to replace conventional gas-fired power plants within most time slices. With the planned changeover to quarter-hour time slices in aFRR marketing, we can see the possibility here of marketing both positive and negative secondary control reserves for our customers. Gridfuse has been developing the necessary technology for this for 4 years. We are delighted to be able to take a significant step, along with Engelhart, nttb, ENERCON and TenneT Germany, towards the scaling of this business field for onshore wind farms,’ says Konrad Wolf of Gridfuse.

‘As a provider of solutions for the interconnected green energy ecosystem, our primary concern is to enable our customers to obtain significant added value from their project,’ says Karsten Tonn, Head of the Service Business Unit at ENERCON. ‘Provision of aFRR functionality for legacy wind farms is a perfect example of the “smart optimisation” area of our portfolio. Thanks to clever solutions for wind farms in the operational phase we secure continuous value growth, future-readiness and growth for our customers and together we make a further important contribution towards the energy transition.’

Sevastos Kavanozis, Global Head of Renewables Sales & Origination at Engelhart, comments: ‘The energy transition depends on renewables taking an active role in system balancing. At Engelhart, our ambition is to make that possible, and commercially viable, for every client and asset we work with. This approval opens a new chapter for onshore wind, demonstrating that renewable assets can be a pillar of grid stability while unlocking new revenue opportunities for owners.’

Engelhart, Gridfuse and ENERCON are currently testing further ENERCON wind turbine types in order to expand the portfolio and so to enable more customers to participate in the control reserve market.