Moyle interconnector approaches start-up

The Moyle Interconnector between Northern Ireland and Scotland is due to start commercial operation at the end of the year using direct-light-triggered thyristor technology from Siemens Power Transmission and Distribution Group (PTD).

The high voltage direct current (HVDC) link will strengthen the link between the supply networks of both regions and allow Northern Ireland to gain access to the European interconnected network, add competition to its market and enhance the security and quality of supply.

Siemens PTD is managing the project and is building and commissioning the converter stations for the HVDC link and supplying the electrochemical equipment for the stations.

Connecting the grid

The two converter stations – at Ayrshire in Scotland and Ballycronan More Island Magee, Northern Ireland – will be connected to one another via two monopolar submarine HVDC cables. Nexans Norge AS supplied the 55 km of submarine cables and 8.5 km of underground cables that connect the converter stations to the land falls at Currarie Port in Scotland and Portmuck South in Northern Ireland.

The cables are rated at 250 MW, use the mass-impregnated paper insulated design and include an integrated return conductor (IRC). This has a metallic coaxial layer integrated in the HVDC cable to form the return path for the current. This design has three main advantages:

  • A single cable with concentric design, good transport and laying properties
  • Good mechanical properties, torsion free and high tensile strength
  • No external magnetic fields.

A fibre optics cable for control and communications has been integrated into the cables. The two undersea cables were laid as continuous lengths, separated by 1 km from each other.

The converter stations

The two stations are identical in design except for the AC switchyards and the AC filters. Each converter station consists of two valve halls – one for each monopole – with the control building in between.

The DC hall, which is located next to each valve hall, contains the DC switchyard and measuring equipment, the smoothing reactor and the cable seaking ends. The Ballycronan More station has a double busbar arrangement in the AC switchyard which is fed by the existing 275 kV transmission system.

Siemens has supplied the converter valves at the stations with direct-light-triggered thyristors which include integrated overvoltage protection.

New technology

The thyristor valves are arranged in three branches with a quadruple valve in each branch fed from a single-phase three winding transformer. Four identical single valves connected in series make up these quadruple valves which are suspended from the ceiling of the valve hall with the high voltage connections at the bottom.

The direct-light-triggered thyristors only require 40 milliwatts of light power for reliable turn-on. Triggering is initiated by light pulses that are generated at ground potential by laser diodes and are applied directly into the thyristor gate through a set of fibre optic cables.

The design of these thyristors allow for an 80 per cent reduction in the number of electrical components in the HVDC converter valves. In addition, the fibre optics simplify the wiring in the valve, and the number of fibre optic cables required to trigger the thyristors is also reduced by about 85 per cent. This improves reliability, and reduces the problems with electromagnetic compatibility.

Six single-phase three-winding transformers are used at the stations which are located next to the valve hall with their dry-type valve side bushings penetrating the valve hall walls. It was not necessary to install separate DC wall bushings with this arrangement.

The stations’ control systems combine all of the functions for control, supervision and protection of the link. Delta power control, emergency power control, direct current control, frequency limit control, actability control functions and automatic reactive power control is also available.


The HDVC link between Northern Ireland and Scotland will begin commercial operation at the end of this year.
Click here to enlarge image

The control link will be fully automatic as the stations will be unmanned, and will go directly to Northern Ireland Electricity’s dispatch centre.

Supply to Ireland

Northern Ireland Electricity (NIE), the company responsible for power transmission and distribution in Northern Ireland, plans to improve the north-south link between the six counties in Northern Ireland and the Republic of Ireland.

When the Moyle link starts commercial operation at the end of the year, it will have cost in the region of 3234 million. The project was sponsored by the European Regional Development fund which contributed 383 million.

Siemens is currently the only manufacture in the world that is producing direct-light-triggered thyristors.


Siemens supplied the direct-light-triggered thyristors at both plants
Click here to enlarge image

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