Electric power transmission through interconnectors using high-voltage direct-current (HVDC) involves usually two converter stations and a transmission line. Generally overhead lines are used, but an important class of HVDC projects use submarine power cables. A back-to-back station has no transmission line and joins two separate AC grids at a single point. Historical HVDC systems used the Thury system of motor-generators but these have all been made obsolete by later developments such as mercury-arc valves (now also obsolete), thyristors, and IGBT power transistors.
Converter technology:
Line status:
<sup>2</sup> As of 9 December 2015, LitPol Link started trial operations.<br /> <sup>3</sup> Caithness Moray HVDC serves a number of renewable energy sources in the north of Scotland - see map <br /> <sup>4</sup> Contracts due to be placed in 2016. <br /> <sup>5</sup> Elia says construction expected to start in late 2017. <br /> <sup>6</sup> Date is that quoted to support the Shetland Wind Farm that SSE is a part investor in.<br /> <sup>7</sup> Decision on project expected in 2017 following consultation - see SSE PD reference.