Multi-Terminal Multi-Vendor HVDC systems

The future electricity network will be characterized by a growing role of Multi-Terminal Multi-Vendor HVDC systems (or grids) within the current AC transmission networks, both onshore and offshore. Such innovative HVDC grids exploit the feature of VSC HVDC transmission, which reverses power through reversal of the current direction, without the need for voltage polarity reversal. This enables [1]:

1. an efficient and safe use of innovative extruded cables – cheaper and more eco-compatible than traditional oil-paper insulated cables – on long-distance VSC HVDC sea/land cable lines;
2. power reversal at intermediate taps, independently of the main power flow direction.

This latter is the reason why, aiming at reducing the costs and improving the reliability of VSC-HVDC links, VSC Multi-Terminal HVDC (MTDC) grids have been introduced (Figure 1). Compared to the traditional “two-terminal” – or point-to-point – HVDC transmission, MTDC grids consist of more than two VSC stations connected through a DC link, allowing the connection of all renewables (e.g. wind farms) and grids within reach of a transmission network and in turn increasing the flexibility and reliability of transmission.