Load frequency control in an interconnected power system with an embedded HVDC Grid

To reduce emission of carbon dioxide the proportion of renewable energies is rapidly growing in German and European grids in general. Additionally, the bulk of them are generated in only a few spots in European transmission grid, whereas conventional power plants will shut down and the load center distribution will still be decentralized. This results in the need of a new and powerful transmission grid, which is able to transport bulk power over long distances. To overcome this challenge one of the discussed solutions for Europe is an Overlay-Network based on HVDC technology connecting spots of bulk renewable energy plants and important load centers. Because of different performance compared to an ac grid, it is necessary for an HVDC-Overlay-Network to actively sustain system services such as load frequency control to ensure system stability and service security as well as the ac grid does. A novel method for realizing participation of an HVDC-Overlay-Network in load frequency control is presented. It hasn´t only an advantage for load frequency control caused power flows, but also for contracted balancing power flows, which are also feed via the dc grid pro rata. Numerical case studies are made to show some benefits of this method in case of various disturbances. It is shown that an HVDC-Overlay-Network with the developed operating control releases the ac grid by an adjustable amount of power flows and improves the load frequency control for example in terms of promptness and smaller frequency deviations.