Risk of Multiple Cross-Over of Control Characteristics in Multiterminal HVDC

HVDC grids have been made practical with the introduction of Voltage Source Converters (VSC). The development of VSC technology has, 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 of this rapidly developing field of engineering a "multi-vendor" supply chain is considered essential. However, unlike AC grids, there is at present no standardized 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 different groups. 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 undertaken by Cardiff University, on behalf of Alstom Grid, uncovered an adverse interaction between converters with differing operating modes. Using results taken from the studies, this paper illustrates how multiple operating points in the control characteristics could occur and show why such operation is undesirable.