As the proportion of wind power on the Danish electricity system grew, there were increasing numbers of hours when thermal generators were required to run in order to support network stability. These “must-run” periods were expensive and counter to the country’s climate ambitions.

In 2013 and 2104, Danish transmission system operator, Energinet, installed synchronous condensers at the Bjæverskov, Fraugde, and Herslev substations to help stabilise the transmission system and enable the deployment of more wind power generation.

Each unit included a synchronous generator with brushless excitation, a generator step-up transformer and electrical auxiliary systems, such as control and safety systems, voltage regulators and start-up systems, able to each deliver more than 900 MVA of short-circuit power and +215/-150 MVAr of reactive power.

The start-up time is designed so that the generators can reach up to 3,000 rpm within 10 minutes and be synchronised with the grid. They were designed for continuous operation and the provision of short-circuit currents when voltage dips occur, and have a minimum availability of 98%. If required, additional rotating mass could be fitted to an external flange.

At the time, inertia was not a particular concern on the Danish grid, and these three units provide approximately one third of the inertia support of thermal generators. However, since then, the share of wind generation has increased significantly, to 40% of the generation capacity, and 57% of electricity production in 2020.

By 2018, levels of inertia became so low that the frequency containment reserves could not guarantee frequency stability after an incident. This was most often the case when there was low output from synchronous generators in combination with a high proportion of wind power and import. Low inertia in Denmark is generally a problem during summer and in hydrological dry years. This has led to the development of a Fast Frequency Response service which sees its highest requirement overnight on summer weekends.