Enabling the Existing Point-to-Point VSC-HVDC Control for Multi-Terminal Operation

Existing point-to-point (PtP) VSC-HVDC link's control is usually not prepared for a multi-terminal HVDC (MTDC) operation, e.g. the DC voltage droop control might not exist in the PtP link or active power control might not exist in the offshore wind farm (OWF) link. Therefore, an existing PtP link's converter control needs to be adjusted when this link is expanded into an MTDC system by interconnecting new converters into the existing system. If only one vendor is involved in the MTDC system, these MTDC specific controls can be applied directly since the vendor has the knowledge of the installed system. However, if several converter vendors are involved, they need to define a common requirement and adjust their converters to fulfil this requirement. Another option is to implement the MTDC specific control as an extension of the existing converter control. The converter control is operated in DC voltage control, which is commonly found in every PtP link, and the extension gives the appropriate DC voltage reference such that the converter operating point lies along a predefined characteristic. Simulation results show that both implementation approaches can be adopted when a PtP link is expanded into an MTDC system, although each of them has different advantages.