Analysis of DC/DC converters in multiterminal HVDC systems for large offshore wind farms

The development of far-offshore wind farms and other large-scale renewable energy sources at long distances from load centers will rely on HVDC transmission. Due to the capability for operation in isolated AC grids and integration into multi-terminal DC grids (MTDC), Voltage Source Converters (VSCs) are becoming the preferred technology for HVDC systems. However, most HVDC transmission schemes are currently constructed as point-to point connections, and there is not yet any clear standardization of voltage levels. Thus, DC/DC converters will become necessary if existing or emerging HVDC links should later be interconnected into MTDC configurations. DC/DC converters might also be needed for power flow control in meshed MTDC grids. In this paper, the general requirements for DC/DC converters in different MTDC grid configurations are briefly analyzed before particular attention is dedicated to converters without galvanic separation for power flow control in meshed MTDC grids. The control and operation of a 4-quadrant DC/DC converter for power flow control with voltage ratio close to unity is then analyzed and demonstrated by simulations in a 4-terminal configuration based on the Cigré DC grid test system.