This chapter introduces the topic of this PhD thesis. First, the background of the research is described and then the Danish power system is introduced. The technical challenges posed by underground cables, including their impact on harmonic distortion in high-voltage grids, are examined using a part of the Danish meshed transmission grid as a case study. Issues such as harmonic distortion and resonance phenomena are highlighted. Real-life examples, like the Anholt offshore wind power plant, demonstrate the complexities of integrating UGCs into existing grids. Several such examples are presented. This is followed by the project objectives and delimitation of the research project. Finally, the tools used, and the thesis outline are presented.