Risk of multiple cross-over of control characteristics in multi-terminal HVDC

Abstract

HVDC grids have been made practical with the introduction of the voltage source converter (VSC). The development of VSC technology has been, and continues to be rapid, with new converter topologies and new manufacturers entering the market. For the power transmission industry to fully exploit the benefits a `multi-vendor' supply chain is considered essential. However, unlike AC grids, there is at present no standardised grid code for HVDC grids and interoperability of equipment and systems, purchased from many different and competing suppliers, is a major concern. Compatibility of control between converters supplied by different manufacturers has been considered by several groups such as CENELEC TC8X - WG6. The initial findings were that there was little reason to suggest that restrictions should be placed on which control strategy should be employed. However recent work uncovered an adverse interaction between converters with differing operating modes. This study illustrates how multiple operating points in the control characteristics could occur and show why such operation is undesirable and how it can be avoided by careful scheme design.