Preparing the expansion of a point-to-point VSC link into a multi-terminal HVDC transmission system:a COBRAcable research project

In an area where there are already several HVDC links in operation, it is more likely that a DC grid emerges as a gradual process, i.e. started by interconnecting two existing point-to-point (PtP) HVDC links or adding a new converter into an existing link. COBRAcable project has the same vision, i.e. it is currently being built as an ordinary PtP HVDC link and in the future more terminals will be added into its cables. A parallel research project is currently commenced to prepare COBRAcable for this future expansion.This paper aims to disseminate the results of the COBRAcable research project. At first, expandable plans of an existing PtP link into a multi-terminal HVDC (MTDC) in the world are listed and compared with COBRAcable expansion plan. Two main factors are discussed, i.e. DC voltage operating range of the expanded system and the implementation of DC grid control concept. An analytic approach to redesign the existing link’s operating range to reflect the one for MTDC system is explained. Furthermore, an additional DC grid control layer is proposed in order interface between the coordinated MTDC control and converter control. By using this interface, the replacement of the existing PtP link’s converter control system when this link is expanded into an MTDC system can be avoided.Apart from mitigating the expansion challenges, additional studies related with the AC-side impact of the converter control are also discussed. These studies are useful for both PtP and MTDC operations, because the same converter is connected with the same AC network. It is found that the converter can contribute to the long-term frequency stability of the AC system by changing the active power level through this converter. However, an active power gradient (APG) control should be carefully considered to avoid risking the AC system in the other side of the link. Furthermore, a new power factor control method is proposed. The proposed power factor control mitigate the challenge of the conventional control approach during active power reverse direction event.