Strategies for the Provision of Dynamic Reactive Power Reserves

Within the energy transition ("Energiewende") the generation of electricity is gradually shifted from conventional power plants to decentralised generation units based on renewable energies. Due to the shutdown of conventional power plants, the reactive power capacities in the German transmission grid progressively decrease. The reactive power capacities of conventional power plants are mainly utilised for voltage control during normal grid operation, as well as during contingencies or disturbances. Therewith a reduction of the reactive power capacities can endanger long and short-term voltage stability. One option to ensure voltage stability is the installation of reactive power compensation devices. Generally, compensation devices are able to keep a high voltage profile without any voltage band violations during grid operation, as well as to provide a dynamic reactive power reserve. This reserve is required to control the voltage during dynamic conditions and therewith for example to reduce the voltage gradient during disturbances. Due to the different physical characteristics of the compensation devices, only fast controllable compensation devices can supply this dynamic reserve. Therefore, the question arises, whether the use of these compensation devices in the normal grid operation should be limited in order to reserve reactive power for unexpected contingencies or disturbances. Within this paper, two strategies for the consideration of a dynamic reactive power reserve within the steady state operation point are analysed. The conducted investigations demonstrate that during critical disturbances only a sufficient reserve can ensure a stabilisation of the voltage.