New HVDC concepts introduced in Moyle link

21 August 2000

An innovative coaxial integrated return conductor submarine HVDC cable design with built-in fibre optics communications cable will be used in the Moyle submarine interconnector between Northern Ireland and Scotland David Smith

Construction of the 500 MWe, 250 kV submarine high voltage direct current (HVDC) link between Northern Ireland and Scotland scheduled to be in operation in December 2001 is now well advanced. The new coaxial integrated return cables are being manufactured, construction is well under way on the converter stations, and the advanced light triggered thyristor valves are being produced.

The innovative technologies used in this unique project as well as the environmental impact considerations resulted in much of the first-of-a-kind solution content which could represent a turning point in high power HVDC technology. It will also be the first commercial application of Siemens’ light triggered thyristor converter valve technology, originally field tested at the Celilo converter station of the Western Intertie in the USA.

Restored links

The new HVDC link, in conjunction with the re-establishment of the 275 kV north–south interconnection between Northern Ireland and the Republic of Ireland and further 110 kV links are intended to create a strongly interconnected two-island electricity system. The North–South interconnector was restored in 1995 after being out of service for 20 years.

Although the original justification for the link was based on the needs of the Northern Irish system, a potential additional use emerged when it became evident that there would be continuing shortages of power in the Republic of Ireland reaching an initial peak during 2001/2002, following the rejection of plans to install two new power generating plants on environmental grounds owing to intervention by neighbourhood groups.

At six per cent per annum growth, electricity demand in the Republic of Ireland is increasing at a higher rate than in other western European countries, driven by accelerating economic and industrial expansion. There were no fewer than seven generating projects in the pipeline when the Republic of Ireland opened up 30 per cent of its power supply sector to competition in February 2000.

Major objections also had to be resolved before the new inter-grid HVDC link between Ballycronan More in Island Magee, County Antrim and Auchencrosh in Ayrshire could proceed, not only from potential competing generators but also from environmental, shipping, naval and sea fishing interests. In particular, interference with conventional magnetic marine navigational systems had to be eliminated.

In December 1999, however, objections to ESB’s planned joint venture 400 MWe combined cycle power plant project, with Statoil of Norway in Dublin’s docklands, were quashed and the development is now proceeding for operation in the year 2002. And in January 2000, objections to Viridian’s proposed joint venture 600 MWe combined cycle power plant with CRH plc of Ireland, at Huntstown, near Dublin, were also quashed and that development too is now proceeding.

In 1991, Northern Ireland Electricity, the electricity transmission and distribution company for Northern Ireland – now part of the Viridian Group plc – signed agreements with Scottish Power, its counterpart in the south of Scotland, for the construction of an HVDC link, to be known as the Moyle Interconnector, between the two transmission systems.

In 1999, Moyle Interconnector plc (Moyle), a member of the Viridian Group, was established to construct the link. Of the total investment in the project of some $234 million, 35 per cent is being funded by the European Regional Development Fund.

The cable route

Apart from the environmental considerations, key system requirements, integration with the reinstated north–south link and the routing of the 275 kV overhead transmission line connection to the Scottish grid influenced the route and configuration of the Moyle Interconnector. The 500 MWe capacity of the interconnector could, at times, represent as much as 40 per cent of Northern Ireland’s electricity demand when considered as an island system.

The restoration of the 275 kV north–south link brings the Northern Ireland system into a network approximately three times the size that it is when isolated. This means that the restrictions determining the maximum capacity of a single infeed into an isolated system are less critical. Future enhancement of the connections between Northern Ireland and the Republic of Ireland are expected to further relax these restrictions, but great emphasis is still placed on reliability and power quality.

Applications were made in December 1993 for the various consents in the relevant jurisdictions. Public inquires were called in Scotland and Northern Ireland with full consent granted in Scotland for the overhead line and outline consent for the converter station at Auchencrosh in October 1997. In Northern Ireland, the application for the first converter station site was refused and after an application was made for another site on Island Magee, outline consent was granted in June 1998. Detailed planning permission for both converter stations was not finally granted until March 2000.

Selection of the present route was a result of much reconsideration following many detailed surveys over a period of more than thirty years. The route had to avoid such geographic features as strong sea currents, shipwrecks, and the Beaufort Dyke – a deep undersea trench with steep sides descending over 300 m in places over which cable suspensions would be unavoidable. The area is also one in which post-war dumping of munitions had taken place. During installation of gas pipeline connections between Scotland and Northern Ireland, post-war dumps had apparently been disturbed on the sea bed further north than reported locations.

Consultation with local fishing interests resulted in a diversion of the route from an earlier location to one further to the north in order to avoid herring breeding grounds and frequently fished areas.

The two 250 MWe monopole submarine cables will be connected with the Northern Ireland converter station at Ballycronan More via a relatively short 3 km long underground dc cable. On the Scottish side there will be 5 km of underground dc cable from the submarine cable to the Scottish converter station at Auchencrosh, and some 64 km of new overhead line to join up with the existing Coylton substation to connect with the Scottish 275 kV transmission system. Since the converter stations will be unmanned, the link is designed for fully automatic remote operation, including automatic load scheduling operation, from NIE’s dispatch centre.

The cables

Configuration of the Moyle Interconnector cables entails two monopolar submarine HVDC cable links spaced about 1 km apart operating in parallel between the two ac systems. The converters will have a power rating of 2 x 250 MWe in either direction referenced at the inverter station. The operating voltage of the monopoles is 250 kV and the nominal direct current is 1000 A per monopole.

The submarine cables are of the well proven mass impregnated paper insulated, copper conductor type. It is, however, a development of the conventional design which includes an integral return conductor and six fibre optic communications channels contained in tubes embedded in the polyethylene sheath about half way through the circular cross section of cable (see diagram, p 29).

The integral return conductor (IRC) cable dispenses with the need for the large sea electrodes for return current through the water, which is sometimes associated with chlorine gas emissions, and largely eliminates external magnetic field effects.

This innovative design of cable, which has the copper coaxial return conductor layer integrated into the structure, is already being manufactured in the Halden, Norway, works of Alcatel Kabel Norge A/S, who will also manufacture the 8.5 km of underground cable connecting the converter stations to the landfalls at Currarie Port in Scotland and Portmuck South in Northern Ireland. The fibre optic cables are being manufactured by Alcatel in Rognan, Norway.

The IRC design has important attributes. The single cable has good transport and laying properties; it is torsion free and has higher tensile strength; and it creates no external magnetic fields The cable weighs 42 kg/m. The two undersea cables will be laid as continuous lengths without joints, which simplifies the problems of bypassing cable joints with the fibre optic systems.

The undersea cables will be laid by the ubiquitous Norwegian purpose built and frequently modified vessel – the c/s Skagerrak. The cables will then be buried into the seabed by Alacatel’s Capjet water jetting system (see picture, p 27), or otherwise protected by concrete mattresses or rock dumping, to a depth of around 1 m.

The underground land cables will be installed from a separate vessel before the submarine cables are laid. These will be pulled on shore in 1.5 km lengths to be jointed together and laid in a 1.2 m deep trench from the shore line to the converter stations.

The fibre optic cables in this case will not initially be used for temperature measurement, although they will retain their control function; single point thermocouples will monitor the conditions in the land cables.

Converter stations

The 110 million Euros contract to build the converter stations for the Moyle Interconnector went to Siemens plc in the United Kingdom. As previously stated, the same direct light triggered thyristor valves have been field tested in a single valve stack in the Celilo converter station in the Pacific Intertie in the USA. These advanced high voltage semiconductor technology direct-light-triggered thyristors have integrated overvoltage protection. The control and protection systems use the latest Siemens technology with communications by fibre optics.

The converter stations have been built to an identical design except for the ac switchyards and the ac filters. Each station consists of two valve halls – one for each monopole – with the control building in between as shown in the artists’ impression above.

The thyristor valves (see illustration, right) are arranged in three branches with a quadruple valve in each branch fed from a single-phase three-winding transformer. Each quadruple valve has four identical single valves connected in series. The quadruple valves will be suspended from the ceiling of the valve hall with the high voltage connections at the bottom.

Conventional thyristor valves for HVDC converter stations use high voltage semiconductor devices with a peak blocking voltage of 8 kV and a rated current of up to 4000 A ac for transmission voltages up to 500 kV dc. This requires the series connection of up to 80 thyristors to make a complete thyristor valve.

Each thyristor requires its gate pulse at the same time but at different electrical potential for correct operation of the complete valve. This has been achieved in the past by using a complex hybrid system, including light emitting diodes and fibre optics for transmitting triggering signals from ground to each thyristor.

An electronic board powered locally by an auxiliary energy circuit is used to generate the gate pulse and perform various monitoring and protection functions.

The new direct-light-triggered thyristor no longer requires the electronic board at thyristor potential. This thyristor only requires 40 mW of light power for reliable turn-on. Therefore, triggering is initiated by light pulses generated near ground potential and applied directly to the thyristor gate through a set of fibre optic cables.

The light pulses are generated by laser diodes which have a life expectancy in excess of 40 years. In addition, this new thyristor has an integrated forward overvoltage protection function, which makes the separate external circuits used so far for this purpose unnecessary But it’s not the first time this arrangement has been used. Bonneville Power Administration (BPA) of Portland, Oregon replaced a complete mercury arc valve at the Celilo Converter Station of the Pacific Northwest-Southwest HVDC Intertie and were so impressed by its performance in service that they opted to purchase the valve after one year of operation. Since then, the new technology has been used in the complete valve for more than two years.

Ireland no island

A recent paper by Moyle Interconnector project manager Clem Harvey points out that when the connector transmits power between the electricity systems in Ireland and Great Britain from the end of 2001, it will provide Northern Ireland customers with access to new sources of electricity generation, promoting competition in the emerging markets in Northern Ireland and the Republic of Ireland and enhancing security and the quality of supply.

Indeed, electricity could be imported from England, from France via the cross channel HVDC link, and other parts of Europe beyond. It will be interesting to see how the prices compare and how the import and export power flows will be affected.

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