HVDC underground transmission is of high interest for future grid extension. Generally, directly buried DC cables and DC GIL are possible solutions, and both of them represent new technologies. This paper takes a first step in comparing their current carrying capabilities and investment costs. In order to do so, the ampacity ratings are first calculated for homogenous bedding properties with the help of analytical formulas and a numerical model. This serves as a validation of the model and gives important insights of the differences between the two technologies. In a next step, the ampacity ratings are derived under the influence of realistic constraints, such as the inclusion of drying out of the soil, variations of the backfill material or environmental demands into the calculations. Based on the derived ampacity ratings, the investment costs are assessed, considering a large variation of major cost factors. For two different scenarios, i.e. a transmission capacity of 2.5 GW and 5 GW, the relative project costs are compared.

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