Degrees of freedom for primary control with batteries

With a higher share of renewable energies also the control of the grid must be organized by renewable energy generators. Storages are attractive for grid control and especially for primary reserve power. Theoretically, positive and negative frequency deviations should average to zero and charging and discharging of the battery should compensate each other. However, in reality, an offset may appear. Then, the state of charge of the battery quickly exceeds any limits. To avoid this, degrees of freedom are allowed while providing primary reserve power. This publication investigates their effect on the state of charge. It is shown that the compensation of a systematic frequency measurement error by applying a running average correction signal is most effective.