The construction of a 500 MW interconnector between Scotland and Northern Ireland is underway. The Moyle Interconnector will use state-of-the-art technology to achieve low losses, a high availability and reliability, and a low maintenance requirement.

Figure 1. The Moyle Interconnector is connecting the island of Ireland to the British grid and, through it, to the central European grid (UCPTE)
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In 1990, Northern Ireland Electricity (NIE), the electricity transmission and distribution company for Northern Ireland, signed agreements with ScottishPower, its counterpart in the south of Scotland, for the construction of a High Voltage Direct Current (HVDC) interconnector between the companies’ transmission systems. Ten years on, after a protracted period in which all the necessary consents were obtained, construction of the Moyle Interconnector has now started.

NIE is a member of Viridian Group plc, Northern Ireland’s largest company by turnover. In 1999, another member of the Viridian Group – Moyle Interconnector plc – was established to construct the link. The Moyle Interconnector will transmit power between the electricity systems in Ireland and Great Britain from the end of 2001. It will provide Northern Ireland with an important new source of electricity supply, promoting competition in the emerging markets in Northern Ireland and the Republic of Ireland and enhancing security and quality of supply. The overall project costs are approximately £150 million (a250 million). The project is sponsored with a contribution of £52.5 million by the European Regional Development Fund.

Figure 2. The Moyle Interconnector, configured as dual monopole with a capacity of 2 x 250 MW
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In 1999 a turnkey contract was awarded to Siemens for the design, installation and commissioning of the HVDC converter stations, linking the interconnector to the transmission systems in Northern Ireland and Scotland. The stations will be located in Ballycronan More in Islandmagee, Co. Antrim, Northern Ireland and in Auchencrosh, Ayrshire, Scotland. Both converter stations will be connected through a submarine and underground cable system, which will be supplied by Alcatel Kabel of Norway.

The configuration of the Moyle Interconnector consists of two monopolar submarine HVDC cable links operating in parallel on the ac systems. The converters will have a power rating of 2 x 250 MW in either direction, referenced at the inverter station. The dc operating voltage of each monopole is 250 kV, the ac sides of the converters are connected to the 275 kV networks of Northern Ireland Electricity and ScottishPower (SP) respectively. The nominal direct current is 1000 A per monopole. The dc cable system, which connects the two converter stations, consists of 55 km of undersea cable and 8.5 km of underground cable. The cable system is of the Integrated Return Conductor type (IRC), where the return cable is integrated into the HVDC cable, i.e. a metallic coaxial layer integrated in the cable forms the return path for the current.

Converter stations

Figure 3. The thyristor valve is the heart of the HVDC system
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Both converter stations have identical designs 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. Adjacent to each valve hall is a dc hall, in which the dc switchyard including the measuring equipment, the smoothing reactor and the cable sealing end are located. The ac switchyard in Ballycronan More converter station is designed as a double busbar arrangement fed by the existing 275 kV transmission system. Auchencrosh converter station will be connected to the ScottishPower grid by a new 65 km overhead transmission line. Both outdoor switchyards are designed for a short circuit current of 31.5 kA and a specific insulator creepage distance of 31 mm/kV.

Latest technology

The arrangement of the thyristor valves will be three branches with a quadruple valve in each branch fed from a single-phase three-winding transformer. Each quadruple valve consists of 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.

The converter stations will be equipped with the latest high voltage semiconductor technology: direct-light-triggered thyristors with integrated overvoltage protection.

Conventional thyristor valves for HVDC converter stations are made from high voltage semiconductor devices with a peak blocking voltage of 8 kV and a rated current of up to 4000 A dc for transmission voltages up to 500 kV dc. This requires 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 a different electrical potential for the correct operation of the complete valve. So far, this has been achieved by using a rather 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 electric gate pulse and perform various monitoring and protection functions.

The new direct-light-triggered thyristor, developed by Siemens, 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 at ground potential and applied directly into 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. As a result, by introducing direct-light-triggered thyristors, the electrical parts required for the HVDC thyristor valve is reduced to approximately 20 per cent, resulting in better reliability with longer maintenance intervals.

The new technology has been used in the commercial operation of a complete valve for a period of two years. Bonneville Power Administration (BPA) of Portland, Oregon installed the valve in its Celilo Converter Station of the Pacific Northwest-Southwest HVDC Intertie, replacing a mercury arc valve. Due to its excellent performance in service, BPA has opted to purchase the valve after one year of operation.

The converter transformers will be designed and manufactured by Siemens’ transformer division. The transformers are of the single-phase three-winding type rated 96/48/48 MVA each. A tap changer with ±8 x 1.25 per cent steps is used to keep the valve side voltage at the ideal value. The transformers will be located directly adjacent to the valve hall with their dry-type valve side bushings penetrating the valve hall walls. It is not necessary to install separate dc wall bushings with this arrangement.

Three triple tuned ac filters rated at 59 MVAr each and tuned to the 3/12/24 harmonics in combination with one 59 MVAr shunt capacitor cover the similar demands on reactive power and harmonic currents filtering of both stations. To compensate for the reactive power demand of the 65 km transmission line, two 59 MVAr filters tuned to the 12 harmonic will be installed additionally in Auchencrosh. Each filter branch can be switched individually by means of Siemens SF6-type circuit breakers with two interrupter units. This design satisfies both the maximum limit of switching overvoltage and the requirements on the harmonic currents suppression. Since both converters are directly connected to the dc cable system, there is no need for a dc filter.

A reliable system

A smoothing reactor rated 200 mH will be installed in each dc hall to smooth the direct current and to limit the overcurrent in the cable system during faults. The smoothing reactor will be of the air insulated type. In combination with the dry type bushings of the converter transformers and the carefully selected fire-resistant materials in the converter valves this reduces the risk of a catastrophic damage by fire.

A reliable HVDC transmission system requires an adequate measurement of the direct current at several locations in each station. The measured direct current is used in the fast converter firing angle control loop and for protection functions. Siemens has developed its own direct current measuring system, which will be installed in the Moyle project. The hybrid-optical system uses an ohmic shunt, which is integrated into the dc circuit. The voltage drop across the shunt is proportional to the direct current. This signal is measured and digitized by an electronic circuit located at high voltage potential. The measured signal is transmitted as a serial telegram via a fibre optic link to ground potential. The main advantages are no electromagnetic interference to the measuring signal due to the optical system and an excellent dynamic performance.

Control system

The control system is fully redundant in order to meet the very high demands on availability and reliability of the installation. The control system, which combines the functions for control, supervision and protection of the link, is based on Siemens standard automation systems Simatic and Simadyn. The normal operating mode of the link is absolute power control. In addition, other control functions which are typical for modern HVDC transmissions are available, including delta power control, emergency power control, direct current control, frequency limit control, stability control functions and automatic reactive power control. Since the converter stations will be unmanned, the link will be designed for fully automatic remote operation from NIE’s dispatch centre including automatic load scheduling operation.

The main design objective of the Moyle Interconnector is to establish an electricity interconnection with low losses and a very high availability and reliability combined with a low maintenance requirement throughout the expected life time of more than 30 years. This is reflected in a state-of-the-art design using a high degree of redundancy and a combination of the latest HVDC technology and components with a long time record of operation experience.

The high performance of the converter stations is reflected in the guaranteed losses of less than 1.35 per cent and a guaranteed value for the energy availability of more than 99.6 per cent. The converters are designed for a biannual scheduled maintenance.