Methods of controlling a combined plant including at least one generator and an energy store

The present invention provides a method of controlling a combined plant to provide frequency support to a power grid operating at a variable grid frequency. The combined plant includes at least one generator (e.g. a plurality of wind turbine generators) and an energy store (12). The combined plant is adapted to supply power to the power grid. The method includes the steps of charging the energy store (12) at less than its rated power when frequency support is not needed. When frequency support is needed the combined plant is controlled to increase or decrease the overall power that it supplies to the power grid to provide frequency support. The control of the combined plant can be designed to maximise frequency support revenues.