Bo Normark
ABB Power Technologies

The Cross Sound Cable Interconnector uses HVDC Light technology to provide a vital submarine cable connection between the transmission grids of New England and Long Island, New York.

Historically, the Long Island grid has only been connected to the US main grid through New York City and through a small interconnection in southwest Connecticut. But for at least 30 years there have been plans to build a submarine cable connection across the Long Island Sound to provide a direct connection with the New England transmission grid. Concerns about the potential environmental impact of such an interconnector, especially on the delicate aquatic ecosystem, have prevented previous schemes from gaining the required statutory approvals.


Figure 1. The Cross Sound Cable provides a high voltage subsea power interconnection linking the Long Island and New England power grids. The 330 MW link uses HVDC Light technology
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However, ABB together with TransÉnergie US has successfully developed a $120 million project utilizing environmentally friendly High Voltage Direct Current (HVDC) Light technology to provide a 330 MW, 40 km link of cable buried below the ocean floor, with short underground sections connecting to converter stations on each shore.

Improved reliability

The Cross Sound Cable is a critical tool for the Independent System Operators (ISO) in New York and New England since it will enable them to improve the reliability of their power supply systems by increasing their capability to share generation plant capacity. It is estimated that it could reduce the potential for blackouts in Connecticut by more than 50 per cent.

The Cross Sound Cable will provide economic benefits to the regions of Connecticut, New York and New England by facilitating electricity trading among power generators and customers and promoting market competition.


Figure 2. The SmartJet hydraulic plough buried the cable 2 m below the sandy seabed. The cable laying process was completed in four days
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The need for access to more power is particularly important to the Long Island Power Authority (LIPA), which serves nearly 1.1 million electric customers and has experienced major increases in demand in recent years. This is largely due to a 20 per cent rate reduction in 1998 and is compounded by a real estate boom, which has sparked the development of expansive homes on the northeast and southeast forks of the island.

During the months of June, July and August 2002, LIPA delivered 6 609 112 MWh of electricity, which is a ten per cent increase from last year’s record figures, and represents a 22 per cent increase in summer power demand since LIPA took ownership of the transmission system in 1998.

Merchant transmission link

The builder, owner and operator of the interconnector is Cross Sound Cable Company, owned by United Capital Investments, the unregulated subsidiary of United Illuminating Company and TransÉnergie US, a subsidiary of the transmission division of Hydro-Quebec, which conceived the project as a privately-owned merchant transmission link, rather than a conventional regulated transmission link.

So rather than being solicited or planned by an ISO or other regulatory authority it is driven by market-based opportunities. The investors have assumed 100 per cent of the financial risk of the project, foregoing any guaranteed recovery of their investment through mandatory charges to ratepayers. Instead it will charge those customers who use it.

An underwater cable routed across Long Island Sound was the natural choice to link the Long Island and New England power grids since the path for a traditional land-based transmission line would adversely impact considerably more people and property along its path. HVDC, rather than conventional HVAC, is now well established as the optimum transmission technology for this type of submarine cable link. Indeed, the world’s first commercial HVDC link – built by ABB nearly 50 years ago – was via a submarine cable providing a 20 MW interconnection between the island of Gotland and the mainland of Sweden. Today, the combined transmission capacity of all the sub-sea and underground HVDC cable links in the world is more than 8000 MW.

HVDC Light

ABB’s new HVDC Light technology was selected for the Cross Sound Cable. Whereas classical HVDC is most cost effective in the high power range, above approximately 250 MW, HVDC Light comes in unit sizes ranging from a few tens of MW up to 330 MW, and for DC voltages up to +/-150 kV.


Figure 3. The Shoreham converter station marks the cable’s southern terminal on Long Island
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HVDC Light consists of two elements: converter stations and a pair of advanced technology cables – the DC circuit is not connected to ground so two conductors are needed. The converter stations are voltage source converters employing state of the art turn-on/turn-off insulated gate bipolar transistor power semiconductors. The circuit is therefore quite different to classical HVDC, which employs converter stations based on thyristor valves.

The HVDC Light cable is of extruded construction and the selected materials provide high mechanical strength, high flexibility and low weight. They are protected by solid insulation and contain no insulating or cooling fluids, unlike some older cable systems still in operation, which makes them environmentally friendly. The cables will have an installed life of at least 40 years.

Unlike conventional HVDC, HVDC Light does not rely on the AC network’s ability to maintain a stable voltage and frequency. This means that less reinforcement is required for the local grid as well as providing extra flexibility regarding the location of the converters in the AC system.

The HVDC Light design is based on a modular concept with a number of standardized sizes. Most of the equipment is installed in enclosures at the factory, which makes the field installation and commissioning short and efficient (typically three to four weeks compared with the three or four months required for conventional HVDC).

HVDC Light stations are compact and need little space and can easily blend into the local surroundings. The stations are designed to be unmanned and are virtually maintenance free with operation either carried out remotely or even automated. No communication links are required between the converter stations. HVDC Light also comes equipped with measurement and control systems that enable power trading.

Prior to the Cross Sound Cable project, HVDC Light was already proven in projects such as Gotland (Sweden), Directlink (New South Wales to Queensland, Australia) and Eagle Pass (US to Mexico).

Cable path

The Cross Sound Cable is designed to provide a 330 MW connection between New England and Long Island, operating at +/-150 kV DC and 1175 A. The cable’s southern terminal on Long Island is a new substation at the site of the decommissioned Shoreham nuclear power station in Brookhaven, New York, where the DC is converted to 138 kV AC. A short underground land cable connects the station to the submarine cable.


Figure 4. Directional drilling was used for the cable landfall at New Haven in order to create a 580 m tunnel underneath the shellfish beds. The cable is housed in a plastic conduit
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The submarine cable heads north across Long Island Sound for 40 km, before coming to land on the eastern shore of New Haven Inner Harbour. The cable route was carefully chosen to minimize environmental impacts. The northern terminal is a new converter station, which converts the DC to 345 kV AC.

Prior to burial, the two 125 mm diameter DC cables were bundled and laid on the sandy seabed within a precisely defined corridor across Long Island Sound. It was deployed by the specially designed sea-spider vessel with a positioning system capable of maintaining its position to an accuracy of less than 1 m. A hydraulic jet plough was then used to bury the cables to a depth of up to 2 m. This environmentally sensitive technique uses a controlled jet of pressurized water to create a trench, around 1 m wide. The trench is not excavated and back-filled as in land-based construction. Instead during the installation process the trench remains filled with loose, fluidized sediments into which the cables settle immediately.

Jet ploughing is widely acknowledged to be a more controlled and less sediment-disturbing installation method than mechanical ploughing and dredging. It is also extremely fast and efficient, and the whole cable laying process was completed in just four days. The seabed can self-restore to its natural contours very quickly.

Environmental considerations

Respect for the local environmental conditions were paramount throughout the project, not least because an earlier project had been denied because it would have crossed a large portion of cultivated shellfish beds in New Haven Harbour. In response to concerns the project was able to avoid all but 230 m of the actively cultivated beds by locating the cable within the federal navigation channel (FNC) in the harbour, where no shellfish cultivation takes place.

For the 580 m section of the cable between the FNC and landfall at New Haven the cable installation used directional drilling to create a tunnel beneath the seabed so that the cable could be housed in a plastic conduit, and avoid disturbing the shoreline.

There will also be no adverse impact to temperatures in the seabed or surrounding waters. The temperature increase at the seabed directly above the cable will be less than 0.1°C, with a related increase in water temperature measured in millionths of a degree. These increases are negligible and well within annual temperature variations in New Haven Harbour and Long Island Sound.

The cable will not result in any major changes to magnetic fields affecting navigation or animal life. This is because the twin-cable design, with equal and opposite current in each cable, largely cancel the cable’s magnetic field. The design will produce only a slight magnetic field that will result in a negligible magnetic compass deviation of less than one degree in the narrow area above the cable. Since it is a DC system the cable will not produce any AC electromagnetic fields, which are frequently associated with health concerns.

Interconnection

The Cross Sound Cable was completed in mid-2002 and commissioned in just fifteen days. Beyond this rapid completion, its capability to transmit electricity over long distances with efficient operation at a lower investment level than conventional HVDC power transmission systems, demonstrates the applicability of HVDC Light technology in larger MW applications.

Cross-Sound Cable’s completion was followed closely by another HVDC Light installation called the Murraylink project in Australia. Murraylink, the world’s longest underground high voltage interconnection, can transfer 220 MW of power between the states of Victoria and South Australia (see PEi May 2001). The interconnection was developed by Murraylink Transmission Company, also a subsidiary of TransÉnergie, was put in operation in October 2002 to help relieve South Australia’s anticipated energy supply shortages.

Both the Murraylink and Cross Sound HVDC Light systems are equipped with fast response control systems that enable power trading over short dispatch periods, making them especially suitable for operation in deregulating and privatized power markets.

The Cross-Sound link is expected to improve the reliability of power supply in the Connecticut and New England power grids, while providing urgently needed electricity to Long Island. The HVDC Light connection is also designed to promote competition in the New York and New England electricity markets by enabling electricity to be traded among power generators and customers in both regions.

Table 1. HVDC cable transmission offers many technical advantages compared with HVAC:

•For cable links longer than around 40 km, HVDC provides lower investment costs. For Cross Sound the saving gained from installing only two DC cables bundled in a single trench instead of three AC cables, which have to be widely spaced in separate trenches, more than compensates for the cost of the AC/DC converter stations.

•Long AC cables produce high amounts of reactive power, requiring shunt reactors at both ends. In extreme cases the reactive current may seriously reduce the active power transmission capability.

•HVDC links can connect two asynchronous power grids and are ideal for cases where it is not feasible to establish a synchronous connection.

•In an AC system it is not possible to directly control the power flow, while an HVDC system allows rapid, direct control of both direction and quantity.