The HVDC Inter-Island link is a long, 1200 MW high-voltage direct current (HVDC) transmission system connecting the electricity networks of the North Island and South Island of New Zealand together. It is commonly referred to as the Cook Strait cable in the media and in press releases, although the link is much longer than its Cook Strait section. The link is owned and operated by state-owned transmission company Transpower New Zealand.
The HVDC link starts in the South Island at the Benmore Hydroelectric Power Station, on the Waitaki River in Canterbury and then it travels on an overhead transmission line through inland Canterbury and Marlborough to Fighting Bay in the Marlborough Sounds. From Fighting Bay, the link travels 40 km via submarine power cables underneath Cook Strait to Oteranga Bay, near Wellington, before travelling the final 37 km on overhead lines to Haywards transmission substation in Lower Hutt.
The HVDC link first became operational in April 1965 to primarily transport electricity from the generation-rich South Island to the more populous North Island. The link originally was a bipolar 600 MW link with mercury arc valves, until the original equipment was paralleled onto a single pole (Pole 1) in 1992, and a new thyristor-based pole (Pole 2) was constructed alongside it, increasing the link's capacity to 1040 MW. The ageing Pole 1 was fully decommissioned effective 1 August 2012, and a replacement thyristor-based pole, Pole 3, was commissioned on 29 May 2013, restoring the DC link to a bipolar 1200 MW configuration.
Rationale for the link
The HVDC link is an important component of the transmission system in New Zealand. It connects the transmission grids of the two islands, and is used as an energy-balancing system, helping to match energy availability and demand in the two islands.
The two islands are geographically different – the South Island is 33 percent larger than the North Island in land area (151,000 km2 vs 114,000 km2), but the North Island has over three times the population of the South Island ( million vs million). As a consequence, the North Island has a substantially larger energy demand. However, the South Island uses more electricity per capita due to its cooler climate and the presence of the Tiwai Point Aluminium Smelter, which at a peak demand of 640 MW is New Zealand's largest single electricity user. In 2011, around 37.1% of the total electricity generated was consumed in the South Island, while 62.9% was consumed in the North Island. South Island generation accounted for 40.9% of the nation's electricity in 2011, nearly all (97%) from hydroelectricity, while the North Island generated the remaining 59.1% from a mixture of mainly hydroelectric, natural gas, and geothermal generation, plus smaller amount of coal and wind generation.
If all currently commissioned generation is available, both islands have enough generating capacity at peak times, without the connection between the two islands. However, the HVDC link provides benefits for customers in both the South Island and North Island: The link provides the South Island consumers with access to the North Island's thermal generation resources that can support the South Island demand during times of low water storage levels and low inflows to South Island hydroelectric lakes. The link provides North Island consumers with access to the South Island's large hydro generation resources that can support the North Island demand at times of peak load.
The link plays an important role in the New Zealand electricity market, and allows North and South Island generators to compete with each other, therefore driving wholesale electricity prices down.
The inter-island transmission system was designed as a high-voltage direct current (HVDC) system, despite the cost of the conversion from alternating current (AC) to direct current (DC) and back again, to suit the requirements of a long transmission line and a sea crossing. The link crosses Cook Strait, between the two islands, using submarine power cables laid along the sea floor. HVDC is more suitable than AC for transmission over long distances, and particularly where submarine cable transmission is required, because it is typically more economic, and has lower energy losses, despite the high costs of the AC/DC conversion process.
Route
The HVDC Inter Island link starts at two converter stations located adjacent to Benmore Hydroelectric Power Station in the Waitaki Valley. Electricity is taken from the main Benmore switchyard, which interconnects the Benmore generators and rest of the South Island transmission grid, at 220 kV via tie-lines across the Benmore tailrace. The AC power is converted at the stations to ±350 kV HVDC for transmission
The HVDC transmission line crosses the Benmore power station tailrace and takes a route along the eastern side of the dam. The line continues north along the eastern shore of Lake Benmore, before turning north-east and then east to meet the Christchurch to Twizel HVAC line. Crossing State Highway 8 south of Fairlie, the line then turns northeast, passing between Fairlie and Geraldine. North of Geraldine to Oxford, the HVDC line broadly follows the Inland Scenic Route tourist highway through the inland Canterbury Plains, passing close to the towns of Methven, Sheffield and Oxford, before continuing northeast towards Waipara.
The HVDC line passes through Weka Pass into the Amuri district, travelling north through the region, west of Culverden, to Hanmer Springs. From here, the line turns north-east and travels through Molesworth Station into Marlborough and down the Awatere River valley, before turning north to meet State Highway 1 through the Dashwood and Weld Passes. The line travels east of Blenheim, meeting the eastern coast of the island at Cloudy Bay, and travelling up the coast into the Marlborough Sounds. The line turns east and then south-east around Port Underwood, before crossing to Fighting Bay on the coast, where the South Island cable terminal is located.
At this location, the lines connect to three undersea cables taking electricity underneath Cook Strait. Pole 2 uses two of the cables, and Pole 3 the third. The cables initially head south out of Fighting Bay, before turning east towards the North Island, and then turning northeast towards the North Island cable terminal at Oteranga Bay.
From Oteranga Bay, the land-based North Island transmission line travels northeast through Mākara just west of Johnsonville. West of Ngaio, the electrode line from the North Island shore electrode at Te Hikowhenua, north of Mākara Beach, merges with the main transmission line towers for the final connection to the North Island converter station. The line turns eastwards around Churton Park, crossing to Horokiwi before turning north-east and passing through Belmont Regional Park to Haywards in northern Lower Hutt, the site of the North Island static inverter plant.
At Haywards, two converter stations receive HVDC power at ±350 kV, and convert it to alternating current at 220 kV AC. From here, the power from the Inter Island link flows to the main Haywards HVAC substation, where it is distributed to the Wellington urban area, or is transmitted north to the rest of the North Island grid.
Technical description
The New Zealand Inter-Island HVDC link is a long distance bipolar HVDC "Classic" transmission scheme that uses overhead lines and submarine cables to connect between the South and North Islands. It uses thyristor-based line-commutated converters at each end of the link for rectifying and inverting between AC and DC. The link includes ground electrode stations that enable the use of earth return current. This permits operation with unbalanced current between the two poles, and monopolar operation when one pole is out of service.
Converter stations The converter stations for each pole, at each end of the link include: converter valve hall, cooling system and control building converter transformers 220 kV AC switchyard equipment and connections 220 kV AC harmonic filters 350 kV DC switchyard equipment, including DC smoothing reactor
The converter valves are twelve-pulse converters, arranged as three water-cooled quadrivalve assemblies. Both Pole 2 and Pole 3 use a design that suspends the quadrivalves from the roof of the valve hall. This provides superior seismic performance compared with a ground mounted arrangement, which is especially important in New Zealand's highly seismic environment. There are three single-phase converter transformers for each converter valve (plus one spare transformer), and each transformer has two secondary windings connected to the valve.
Each converter station requires power factor correction equipment to generate reactive power for the converters and provide voltage support to the surrounding AC grid. At the Benmore converter station, reactive power is provided by the generators at Benmore Dam. At the Haywards converter station, reactive power and power factor correction is provided by eight synchronous condensers, two shunt capacitors, two shunt reactors, and one static synchronous compensator (STATCOM).
Details of the converter station equipment and ratings are given in the table below:
Converter Station Pole 2 Pole 3 Notes Commissioned 1991 May 2013 Manufacturer ABB Siemens Operating voltage −350 kV +350 kV Converter nominal rating 560 MW 700 MW Converter continuous overload rating 700 MW 735 MW Short term overload rating 840 MW for 5 s 1000 MW for 30 min Thyristor type diameter, electrically triggered, water cooled diameter, light triggered, water cooled Valve maximum continuous current rating 2,000 A 2,860 A Thyristor peak reverse voltage 5.5 kV >7.5 kV Thyristors per valve 66 52 Thyristors per quadrivalve unit 264 208 Thyristors per station 792 624 Quadrivalve mass 20 tonnes 17 tonnes Number of converter transformers 8 total: 3 plus 1 spare at each converter station 8 total: 3 plus 1 spare at each converter station Converter transformer mass 324 tonnes, including oil 330 tonnes, including oil Oil volume per transformer
Submarine cables
The three submarine power cables installed in 1991 are each rated to carry 1430 A continuously at 350 kV operating voltage. They are constructed with a compacted multi-strand copper conductor as a central core, with mass-impregnated paper insulation surrounded by a lead sheath. Two layers of galvanised steel wire armour provides strength and mechanical protection. The outer layer of the cable is a serving made from polypropylene rope and the outer diameter is approximately 130 mm. The cables have a 30-minute overload capacity of 1600 A.
To ensure security of the link's submarine power cables, a seven-kilometre wide Cable Protection Zone (CPZ) is enforced where the cables cross Cook Strait. Vessels are not permitted to anchor or fish in this area, and the area is routinely patrolled by sea and air. Anyone found anchoring or fishing in the area is liable for fines up to $100,000 and forfeiture of their vessel – more if a cable is subsequently damaged.
HVDC transmission line
The transmission line was designed and built by the New Zealand Electricity Department, and was completed in January 1965. The original construction of the line included the erection of 1623 steel lattice towers. In some South Island sections, the line reaches an altitude of 1280 metres. The longest span is 1119 m, near Port Underwood, close to the Fighting Bay cable terminal station.