DC/DC Converters for interconnecting independent HVDC systems into multiterminal DC grids

Future multi-terminal HVDC (MTDC) grids
will be based on Voltage Source Converter (VSC)
technology. However, existing and planned VSC HVDC
connections for offshore wind farms and cross-border
interconnections are mainly point-to-point links which
are operated at different voltages and can have different
grounding schemes. Thus, DC/DC converters will become
necessary for interconnection of independent HVDC
transmission schemes into future meshed MTDC grids.
This paper starts with a brief review of topologies for
DC/DC converters in HVDC applications with particular
attention directed towards interconnection of HVDC
systems with different grounding schemes. The control
and operation of a front-to-front transformer-coupled
AC/DC/AC solution is analyzed as an example of a
topology that will be suitable when galvanic separation is
required. The investigated topology is demonstrated by
time-domain simulations in Matlab/Simulink when
interconnecting a monopolar HVDC link with a meshed
bipolar MTDC configuration defined in the Cigré DC
grid test system.