The aim of this TB is to serve as a practical guide for preparing models and performing studies necessary during the assessment of the technical performance of HV/EHV systems with a large share of (long) AC cables. The brochure follows all phases of planning and analysis of a typical underground transmission project. For each phase and type of study proper cable models are recommended. Cable models are analyzed from various perspectives including sensitivity to variation of input parameters. Accuracy of parameters provided in data sheets by manufacturers is analyzed. Results of comprehensive measurements performed on the 100 km long 150 kV AC cable connection of the Horns Rev 2 offshore wind farm are given.