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.

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Converter technology: * Thury = Series-connected generators as designed by

René Thury René Thury (7 August 1860 – 23 April 1938) was a Switzerland, Swiss pioneer in electrical engineering. He was known for his work with high voltage direct current electricity transmission and was known in the professional world as the "Kin ...

* Merc = Mercury-arc valve rectifier and inverter * Thyr = Thyristor rectifier and inverter * IGBT = Insulated gate bipolar transistor * BIGT = Bi-mode insulated gate transistor * IEGT = Injection enhanced gate transistor Line status:

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² As of 9 December 2015, LitPol Link started trial operations.
³ Caithness Moray HVDC serves a number of renewable energy sources in the north of Scotland - see map
⁴ Contracts due to be placed in 2016.
⁵ Elia says construction expected to start in late 2017.
⁶ Date is that quoted to support the Shetland Wind Farm that SSE is a part investor in.
⁷ Decision on project expected in 2017 following consultation - see SSE PD reference.

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