In order to increase the transmission capacity of medium
and low-voltage distribution grids, in the AC2DC research
project it was investigated how existing alternating current
cable connections (AC-links) can be converted into
direct current cable connections (DC-links). First of all,
system concepts were developed and simulations were
used to evaluate how three-phase distribution networks
can be operated as DC grids with higher transmission
power. The required grid-forming DC-operation modes of
the grid-sided converters were defined, based on feasibility
and network benefit. The outcome of these activities are
aimed DC voltage levels of ±55 kV for medium voltage and
±750 V for low voltage. In both cases, the conventional
AC-components of the cable systems are loaded with significantly
higher voltages. In addition, the differing DCcurrent-
profiles and their distribution in the conductors
lead to changed thermal loads.
Therefore, this paper analyses to what extent the conventional
AC-components (cables and accessories) are suitable
under DC operation. Special attention is given to the
changed thermal stress that the components of an AC cable
system experience due to the changed load with DC.