View source for HVDC Inter-Island

{{Short description|Electrical transmission system in New Zealand}}
{{Use New Zealand English|date=April 2024}}
{{Use dmy dates|date=August 2020}}
{{Infobox power transmission line
| name                = HVDC Inter-Island
| photo               = 
| caption             = 
| map                 = {{maplink|frame=yes|plain=yes|frame-align=center|frame-lat=-42.5|frame-long=172.5|zoom=6|frame-height=300|type=line}}
| map_caption         = Route of the HVDC Inter-Island
| country             = New Zealand
| coordinates         = 
| lat                 = 
| long                = 
| direction           = South-North
| start               = [[Benmore Power Station|Benmore Hydroelectric Power Station]], near [[Otematata]], [[Canterbury, New Zealand|Canterbury]]
| through             = 
| finish              = [[Haywards]] transmission substation, [[Lower Hutt]]
| owner               = [[Transpower New Zealand]]
| operator            = [[Transpower New Zealand]]
| cable_manufacturer  = 
| cable_installer     = 
| cable_layer         = 
| substation_manufacturer= [[ABB]] / [[Siemens]]
| substation_installer= 
| contractors         = 
| construction        = 1961
| est                 = April 1965
| type                = Bipole HVDC powerline with [[Overhead power line|overhead transmission line]] and [[submarine power cable]]s underneath the [[Cook Strait]]
| current_type        = HVDC
| length_km           = 610
| capacity            = 1200&nbsp;MW
| DC_voltage          = ±350&nbsp;kV
| poles_no            = Two
}}
{{GeoGroup}}

The '''HVDC Inter-Island''' link is a {{convert|610|km|abbr=on}} long, 1200 [[MegaWatt|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,<ref>{{cite web |url=http://www.beehive.govt.nz/release/cook-strait-power-cable-failure |title=Cook Strait Cable Power Failure |publisher=[[Government of New Zealand]]|date=28 April 2009 |accessdate=28 September 2011}}</ref> although the link is much longer than its [[Cook Strait]] section. The link is owned and operated by [[state-owned enterprises of New Zealand|state-owned]] transmission company [[Transpower New Zealand]].

The HVDC link starts in the South Island at the [[Benmore Power Station|Benmore Hydroelectric Power Station]], on the [[Waitaki River]] in [[Canterbury, New Zealand|Canterbury]] and then it travels {{convert|534|km}} on an overhead transmission line through inland Canterbury and [[Marlborough District|Marlborough]] to Fighting Bay in the [[Marlborough Sounds]]. From Fighting Bay, the link travels 40&nbsp;km via [[submarine power cable]]s underneath Cook Strait to [[Cape Terawhiti|Oteranga Bay]], near [[Wellington]], before travelling the final 37&nbsp;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&nbsp;MW link with [[mercury arc valve]]s, 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&nbsp;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,<ref>{{cite news |url= https://www.transpower.co.nz/news/new-hvdc-pole-3-commissioned |title= New HVDC Pole 3 Commissioned |publisher= Transpower New Zealand |date= 29 May 2013 |accessdate= 1 June 2013 |archive-date= 16 July 2021 |archive-url= https://web.archive.org/web/20210716045100/https://www.transpower.co.nz/news/new-hvdc-pole-3-commissioned |url-status= dead }}</ref> restoring the DC link to a bipolar 1200&nbsp;MW configuration.

==Rationale for the link==
[[File:New Zealand transmission grid.svg|thumb|right|A map of the major power transmission lines in New Zealand, with the HVDC Inter-Island link being marked with a dashed black line.]]
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&nbsp;km<sup>2</sup> vs 114,000&nbsp;km<sup>2</sup>), but the North Island has over three times the population of the South Island ({{Rnd|{{formatnum:{{NZ population data 2018|North Island regions|y}}|R}}/1000000|2}} million vs {{Rnd|{{formatnum:{{NZ population data 2018|South Island regions|y}}|R}}/1000000|2}} million).{{NZ population data 2018|4=y}} 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&nbsp;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.<ref name="EDF2010">{{cite web |url= http://www.med.govt.nz/sectors-industries/energy/pdf-docs-library/energy-data-and-modelling/publications/energy-data-file/energydatafile-2011.pdf |title= New Zealand Energy Data File 2012 |publisher=[[Ministry of Economic Development (New Zealand)|Ministry of Economic Development]] |date= June 2012 |accessdate=6 July 2012}}</ref>

If all currently commissioned generation is available, both islands have enough generating capacity at peak times, without the connection between the two islands.<ref>{{cite web|url=https://www.transpower.co.nz/sites/default/files/plain-page/attachments/hvdc-gup-vol-I-may-2008.pdf|title=HVDC Grid Upgrade Plan Volume 1, p10|date=May 2008|accessdate=2 September 2012}}{{Dead link|date=July 2025 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> 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.<ref name="APR">{{cite web|url=https://www.transpower.co.nz/sites/default/files/publications/resources/Annual%20planning%20report%202012%20complete.pdf|title=Annual Planning Report 2012|date=April 2012|publisher=[[Transpower New Zealand|Transpower]]|accessdate=30 August 2012|archive-date=14 February 2013|archive-url=https://web.archive.org/web/20130214005020/https://www.transpower.co.nz/sites/default/files/publications/resources/Annual%20planning%20report%202012%20complete.pdf|url-status=dead}}</ref>

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 cable]]s laid along the sea floor. HVDC is more suitable than AC for transmission over long distances, and particularly where [[Submarine power cable|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.<ref name="HVDCeconomic">{{cite web|url=http://www.usea.org/Archive/Climatechange/Chapter4/index.html |publisher=United States Energy Association |title=High Voltage Direct Current |access-date=11 March 2012 |url-status=dead |archive-url=https://web.archive.org/web/20100813152933/http://www.usea.org/Archive/Climatechange/Chapter4/index.html |archive-date=13 August 2010 }}</ref>

==Route==
[[File:HVDC Inter-Island pylon with great view - panoramio.jpg|thumb|left|HVDC transmission line tower in coastal Marlborough region]]
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&nbsp;kV via tie-lines across the Benmore tailrace. The AC power is converted at the stations to ±350&nbsp;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 [[New Zealand State Highway 8|State Highway 8]] south of [[Fairlie, New Zealand|Fairlie]], the line then turns northeast, passing between Fairlie and [[Geraldine, New Zealand|Geraldine]]. North of Geraldine to [[Oxford, New Zealand|Oxford]], the HVDC line broadly follows the [[Inland Scenic Route]] tourist highway through the inland [[Canterbury Plains]], passing close to the towns of [[Methven, New Zealand|Methven]], [[Sheffield, New Zealand|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 [[New Zealand State Highway 1|State Highway 1]] through the Dashwood and Weld Passes. The line travels east of [[Blenheim, New Zealand|Blenheim]], meeting the eastern coast of the island at [[Te Koko-o-Kupe / Cloudy Bay|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 physical location, the lines connects to three undersea cables taking electricity underneath [[Cook Strait]]. {{As of|August 2012}}, Pole 2 uses two of these cables, with the third cable unused waiting the commissioning of Pole 3. 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, New Zealand|Johnsonville]]. West of [[Ngaio, New Zealand|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&nbsp;kV, and convert it to alternating current at 220&nbsp;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==
[[Image:New Zealand HVDC link schematic.svg|thumb|300px|right|Simplified schematic of New Zealand HVDC scheme]]

The New Zealand Inter-Island HVDC link is a long distance bipolar HVDC "Classic" transmission scheme that uses overhead lines and [[submarine cable]]s 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&nbsp;kV AC switchyard equipment and connections
* 220&nbsp;kV AC harmonic filters
* 350&nbsp;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.<ref name=features>{{cite book |title= Principal Features of the New Zealand DC Hybrid Link |publisher=CIGRE |first1=M T |last1=O'Brien |first2= D E| last2=Fletcher | first3=J C |last3=Gleadow |at=International Colloquium on High Voltage Direct Current and Flexible AC Power Transmission Systems|location= Wellington |date= 29 September 1993}}</ref> 
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 [[AC power|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 condenser]]s, two shunt capacitors, two shunt reactors, and one [[static synchronous compensator]] (STATCOM).<ref>{{Cite web|title=Bunnythorpe–Haywards Conductor Replacement – Power System Analysis Report|url=https://comcom.govt.nz/__data/assets/pdf_file/0026/63872/1624479-Bunnythorpe-Haywards-Conductor-Replacement-MCP-Attachment-D-Power-systems-analysis-report-November-2013.pdf|access-date=21 January 2022|publisher=Transpower New Zealand}}</ref>

Details of the converter station equipment and ratings are given in the table below:<ref>{{cite web |url= http://www.gridnewzealand.co.nz/f3682,31735618/hvdc-pole-3-project-eea-paper-jun10.pdf |title= NZ Inter Island HVDC Pole 3 Project Update |first= Peter |last= Griffiths |author2= Zavahir, Mohamed |at= EEA Conference & Exhibition 2010 |location= Christchurch |date= 27 May 2010 |accessdate= 27 May 2012 }}{{Dead link|date=August 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

{| class="wikitable" border="1"
|-
! Converter Station !! Pole 2 !! Pole 3 !! Notes
|-
| Commissioned || 1991 || May 2013 || 
|-
| Manufacturer || [[ABB]] || [[Siemens]]
|-
| Operating voltage || −350&nbsp;kV || +350&nbsp;kV ||
|-
| Converter nominal rating || 560&nbsp;MW || 700&nbsp;MW || 
|-
| Converter continuous overload rating || 700&nbsp;MW || 735&nbsp;MW ||
|-
| Short term overload rating || 840&nbsp;MW for 5&nbsp;s || 1000&nbsp;MW for 30&nbsp;min || 
|-
| Thyristor type || {{convert|4|in|mm|spell=in|adj=on}} diameter, electrically triggered, water cooled || {{convert|5|in|mm|abbr=on}} diameter, light triggered, water cooled ||  
|-
| Valve maximum continuous current rating|| 2,000&nbsp;A || 2,860&nbsp;A || 
|-
| Thyristor peak reverse voltage|| 5.5&nbsp;kV || >7.5&nbsp;kV || 
|-
| Thyristors per valve || 66 || 52 || 
|-
| Thyristors per quadrivalve unit || 264 || 208 || 
|-
| Thyristors per station || 792 || 624 || 
|-
| Quadrivalve mass || 20&nbsp;tonnes|| 17&nbsp;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|| {{convert|85000|L}} || {{convert|91000|L}} ||
|-
|}

===Submarine cables===
[[File:HVDC Submarine Cable Cross Section - from New Zealand Inter-island scheme.jpg|thumb|HVDC 350 kV [[submarine cable]] cross section. Overall diameter 13&nbsp;cm/5&nbsp;in.]]
The three [[submarine power cable]]s installed in 1991 are each rated to carry 1430&nbsp;A continuously at 350&nbsp;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&nbsp;mm. The cables have a 30-minute overload capacity of 1600&nbsp;A.<ref name=features/>

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.<ref>{{cite book |url= https://www.transpower.co.nz/sites/default/files/publications/resources/cook-strait-booklet-2011.pdf |title= Cook Strait Submarine Cable Protection Zone |date= February 2011 |publisher= Transpower New Zealand and Maritime New Zealand |accessdate= 20 May 2012 |archive-date= 10 February 2013 |archive-url= https://web.archive.org/web/20130210113334/https://www.transpower.co.nz/sites/default/files/publications/resources/cook-strait-booklet-2011.pdf |url-status= dead }}</ref>

===HVDC transmission line===

[[File:Sample cross-section of high tension power (pylon) line.jpg|thumb|Sample cross-section of HVDC line conductor]]
[[File:Conductor 500kv DC sample Caption.jpg|thumb|Label on line conductor sample (original rating ±250&nbsp;kV)]]

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&nbsp;metres. The longest span is 1119&nbsp;m, near Port Underwood, close to the Fighting Bay cable terminal station.

The line was originally designed for operation at ±250&nbsp;kV. During the DC Hybrid link project of 1989 to 1992, the transmission line was reinsulated with DC fog type porcelain insulator units, to enable operation at 350&nbsp;kV. There are 15 units per insulator string in the inland parts of the route, and 33 units per insulator string in the coastal parts of the route that are exposed to salt condensation. The insulator strings in the coastal portions are around 5&nbsp;m long.<ref name=features/>

The transmission line insulators support a pair of [[ACSR]] conductors on each side of the towers. The conductors are each 39.4&nbsp;mm in diameter, and are spaced 432&nbsp;mm apart.<ref name="Taylor"/>

The HVDC line has a continuous overhead earthwire for lightning protection, except for a 21&nbsp;km section at the Haywards end, where the line is shielded by the electrode line conductors. A 13&nbsp;km section of the North Island HVDC line uses an overhead earthwire containing a fibre optic core (OPGW), and a further 169&nbsp;km section of OPGW is installed the South Island line.<ref name=features/>

About 20 new towers were constructed in 1992 to reroute the HVDC line north of Johnsonville to make way for new residential development. This was known as the Churton Park deviation.<ref>{{cite web|url= http://www.gridnewzealand.co.nz/f5634,69954053/otb-hay-recon-pptx-oct-2010_.pdf|title= OTB-HAY A Re-conductoring Project|publisher= Transpower|date= 26 October 2010|accessdate= 2 June 2012}}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

Around 92.5 percent (1503) of the towers on the line in 2010 were identified as being original, with the remaining towers having been replaced due to line deviations, collapse, or corrosion.

Following the DC Hybrid Link project, the line was rated to carry 2000&nbsp;amperes continuously on each pole, at an operating voltage of 350&nbsp;kV HVDC.

===Earth electrode stations===
The connection between the North Island converter station and earth uses a shore electrode station located at Te Hikowhenua, approximately 25&nbsp;km from Haywards. Following upgrades carried out during the DC Hybrid Link project, the [[electrode]] station is capable of carrying 2400&nbsp;A continuously. Forty electrode cells are buried along an 800&nbsp;m length of a stony beach. Each electrode cell consists of a high silicon-chromium iron electrode, suspended in a vertical porous concrete cylinder. The cells are surrounded by selected and graded stones and geotextile layers to allow seawater ingress, but prevent the buildup of silt. The electrode to ground resistance is 0.122&nbsp;Ω.<ref name=features/>

The South Island ground electrode station is located at Bog Roy, 7.6&nbsp;km from Benmore. It comprises buried electrode arms arranged in a star configuration over a site of approximately 1&nbsp;km<sup>2</sup>. Each electrode arm is a 40&nbsp;mm mild steel rod buried in a coke bed of around 0.26&nbsp;m<sup>2</sup> cross sectional area, in a 1.5&nbsp;m deep trench. The electrode to ground resistance is 0.35&nbsp;Ω.<ref name=features/>
A small transmission line carries a twin conductor electrode circuit from the Benmore converter station site to the South Island land electrode at Bog Roy, which in conjunction with the shore electrode in the North Island, allows one pole to operate using earth return when the other pole is out of service.

==Transmission faults and outages==
Like all transmission systems, the HVDC Inter-Island link is not immune to failures. The importance of the link means that an unplanned outage can have major implications for the entire New Zealand electricity system, potentially causing nationwide frequency deviation (underfrequency in the receiving island, overfrequency in the other island), electricity shortages in the receiving island, and a spike in wholesale electricity prices. The most catastrophic situation is a simultaneous bipole outage at high transfer when there is low to medium generation in the receiving island—instantaneous reserve generation and load shedding systems in the receiving island would not be able to come online fast enough to prevent the frequency dropping, resulting in [[cascading failure]] and outage of the entire receiving island.<ref>{{cite web |url= http://www.systemoperator.co.nz/f1688,36650695/AUFLS_Technical_Report.pdf |title= Automatic Under-Frequency Load Shedding (AUFLS) Technical Report |publisher= Transpower New Zealand |date= August 2010 |access-date= 7 June 2012 |url-status= dead |archive-url= https://web.archive.org/web/20130207234514/http://systemoperator.co.nz/f1688,36650695/AUFLS_Technical_Report.pdf |archive-date= 7 February 2013}}</ref>

Planned outages of the link are required occasionally to carry out maintenance that is not possible while the system is live. Maintenance outages are planned well in advance to minimise the effects – they are usually carried out in summer when national electricity demand is at its lowest, and on only one pole at a time, with the other pole remaining in operation providing half of the full two-pole capacity, using the earth electrodes providing a path for return current through the ground.

Notable faults and outages on the HVDC Inter-Island link:
* 1973 – an electrical fault occurred in the shore joint of Cable 1 at Fighting Bay.<ref name=Reilly/>
* August 1975 – A strong wind storm caused a string of seven [[transmission tower]]s to collapse and damage the line. The link took five days to repair.<ref name=Reilly/>
* 1976 – A fault occurred at the Cable 1 undersea joint, 15.5&nbsp;km from the South Island end at a depth of 120&nbsp;metres. The joint was repaired in 1977.<ref name=Reilly/>
* 1980 – Cable 3 failed at the Fighting Bay shore joint.<ref name=Reilly/>
* 1981 – A gas leak on Cable 1 occurred at Oteranga Bay. It was repaired in the 1982/83 summer.<ref name=Reilly/>
* 1988 – Cable 2's Oteranga Bay end joint exploded, spilling insulating oil into the switchyard.<ref name= Reilly/>
* 2004 – In January, three HVDC towers collapsed as a result of extreme winds, and in August the line voltage had to be reduced for long periods because of insulation flashovers caused by severe salt pollution at the cable station at Oteranga Bay. In October, a fault occurred in one of three Cook Strait cables that reduced the Pole 1 capacity from 540&nbsp;MW to 386&nbsp;MW. Repairs took almost six months.<ref>{{citation | last=Transpower |title=Quality Performance Report 2004-05| year= 2005}}</ref>
* 19 June 2006 – The link experienced an unplanned outage just before the evening peak period on one of the coldest days of the year. With four North Island power stations out for service and an outage of Tauranga's ripple load control equipment, even with the reserve Whirinaki Power Station called upon, the North Island experienced electricity shortages and Transpower subsequently declared a nationwide Grid Emergency at 5:34&nbsp;pm. The link was restored shortly after the emergency was declared.<ref name=Reilly/>
* 28 August 2008 – A [[transmission tower]] in the Marlborough Sounds was found buckled after its foundations slipped. The tower was reinforced with steel guy ropes until it could be replaced, as the link couldn't be shut down without causing widespread power shortages in the South Island.<ref>{{cite web |url= http://www.nzherald.co.nz/nz/news/article.cfm?c_id=1&objectid=10529788 |title= Power supply safe, unless it's windy |work=[[New Zealand Herald]]|date= 30 August 2008 |accessdate= 23 July 2011}}</ref>
* 12 November 2013 – During the commissioning of the new two-pole control systems, a test to assess the control's response to a trip on a 220&nbsp;kV line out of Haywards during high north flow caused three filter banks at Benmore to trip off the grid. The HVDC controls automatically cut northbound transfer from 1000&nbsp;MW to 140&nbsp;MW, causing automatic underfrequency load shedding (AUFLS) systems to deploy in the North Island, and blacking out thousands of customers. A software bug was found to be the cause of the filter bank trips.
*17 August 2021 – The HVDC transmission line failed in the [[Weka Pass]] region, leading to conductors of the Pole 2 circuit falling across [[State Highway 7 (New Zealand)|State Highway 7]]. Transpower issued a Grid Emergency Notice later in the day, advising that there was insufficient generation and transmission capacity to meet the forecast evening peak load in the North Island. Severe weather conditions delayed the repair and the return to service until 26 August.<ref>{{Cite news|date=17 August 2021|title=Transpower issues emergency notice for national grid|work=[[The New Zealand Herald]] |url=https://www.nzherald.co.nz/business/transpower-issues-emergency-notice-for-national-grid/XC37LGOJTLCS6HKID5XVUMTXUM/|access-date=24 August 2021}}</ref><ref>{{Cite news|last=Pullar-Strecker|first=Tom|date=18 August 2021|title=Transpower expects fallen power line won't be fixed until Sunday|work=[[Stuff (website)|Stuff]] |url=https://www.stuff.co.nz/business/126107341/transpower-expects-fallen-power-line-wont-be-fixed-until-sunday|access-date=24 August 2021}}</ref><ref>{{Cite web|date=27 August 2021|title=26 August Update Regarding Conductor Repair|url=https://transpower.co.nz/news/26-august-update-regarding-conductor-repair|url-status=live|archive-url=https://web.archive.org/web/20210909071818/https://transpower.co.nz/news/26-august-update-regarding-conductor-repair|archive-date=9 September 2021|access-date=9 September 2021|website=Transpower New Zealand}}</ref>

== History ==

=== The original link ===
[[Image:Pole 1 mercury arc valves.jpg|thumb|250px|right|Mercury arc valves in valve hall at Haywards.]]

==== Planning ====
The initial vision for electricity transmission between the South and North Islands was developed by Bill Latta, the Chief Engineer of the State Hydroelectric Department. In 1950, he prepared a paper on the future of the North Island's electric power supply and he drew attention to the projected growth of the load and the limited potential for more [[hydroelectric]] generation development on the North Island. Latta's vision was to build more hydroelectric generating capacity in the South Island, where there was still significant opportunities for new schemes, and to transmit the power to the southern half of the North Island to meet the increasing demand.<ref name=Taylor>{{cite book |author=Taylor, Peter|title= White Diamonds North: 25 Years' Operation of the Cook Strait Cable 1965–1990|location=Wellington|publisher=Transpower|year=1990|isbn=0-908893-00-0 |pages=109 pages}}</ref>

In 1951, the cable manufacturing company [[British Insulated Callender's Cables]] (BICC) advised the State Hydroelectric Department that a cable crossing of Cook Strait was possible, but difficult, since there was no precedent for installing power cables in such difficult marine conditions.<ref name=Martin>{{cite book |editor = Martin, John E |title = People, Politics and Power Stations: Electric Power Generation in New Zealand 1880–1998 |location = Wellington |publisher=Bridget Williams Books Ltd and [[Electricity Corporation of New Zealand]] |year = 1998 |edition = Second |isbn = 0-908912-98-6 |pages = 356}}</ref>

The development of high power [[HVDC#Mercury arc valves|mercury arc valve]] converters in the 1950s led to the development of several HVDC transmission schemes in other countries. This demonstrated that a long distance, high power HVDC transmission scheme was feasible in principle.

In 1956, the Government appointed BICC to undertake detailed investigations of the practicality and cost of a Cook Strait cable crossing. In December of that year, BICC reported that the project was "thoroughly practicable".<ref name=Reilly>{{cite book |author=Reilly, Helen|title= Connecting the Country: New Zealand's National Grid 1886–2007| location=Wellington|publisher=Steele Roberts|year=2008|isbn=978-1-877448-40-9|pages = 376 pages}}</ref>

In parallel with the technical investigations for cables underneath Cook Strait, the Minister responsible for the State Hydroelectric Department appointed a committee of key stakeholders to report on the options for power supply to New Zealand as a whole, not just the North Island. In 1957, the committee recommended that work commence on a large hydroelectric power station on the Waitaki River at Benmore, and that approval in principle should be given for linking the North and South Island's power systems.

Recommendations were also received from the Swedish company [[ASEA]] (today part of the [[ABB]]), about the technical aspects of the HVDC converter stations.

The unique planning considerations for the overall proposal included:<ref name=Taylor/>
* The hydroelectric generators at Benmore would need to be capable of absorbing the [[harmonic]] currents that would be created by the operation of the mercury arc converters.
* The Benmore generators were proposed to have an operating voltage of 16 kV, which was a new high for New Zealand hydroelectric generators at the time.
* The 16 kV circuit breakers required at Benmore would be state of the art.
* The mercury arc valves would be larger than any previously constructed, and would require water-cooled cathodes.
* The overhead HVDC transmission line would be one of the longest and most difficult built in New Zealand up to that time.
* The Cook Strait submarine cables would have to be specially designed for the seabed and tidal conditions, and require special armouring at the Oteranga Bay end, of a kind that had not been used before.

In 1958, BICC laid two 0.8&nbsp;km trial lengths of cable off Oteranga Bay in Cook Strait to demonstrate their ability to resist the abrasion, bending and vibration caused by conditions on the seabed. These trial lengths were recovered and inspected in 1960, and by October that year, BICC reported that the trial had been successful and that the prototype cable would provide good service underneath Cook Strait.<ref name=Taylor/>

In the period 1958 to 1960, some differing views were offered to Government about the most appropriate power developments for the country as a whole, and there were reservations about the risks involved in the planned Cook Strait cable crossing.<ref name=Martin/>

However, in March 1961, against a background of increasing urgency in meeting the forecast demand, the Government approved the project. A [[New Zealand pound|NZ£]]6.5 million contract was placed with ASEA for the design, manufacture, installation and commissioning of the converter plant at Benmore and Haywards, and a NZ£2.75 million contract was placed with BICC for the manufacture, delivery, laying and testing of the Cook Strait submarine cables.<ref name=Taylor/>

==== Construction ====
The HVDC inter-island link was designed and built between 1961 and 1965 for the New Zealand Electricity Department. The major equipment suppliers were ASEA and [[British Insulated Callender's Cables]].<ref name=Taylor/> The original Cook Strait cables were installed in 1964, from the cable laying ship ''Photinia''.<ref>{{cite web|url=http://www.teara.govt.nz/en/energy-supply-and-use/6/2/3|title= The original Cook Strait cable is hauled ashore at Ōteranga Bay on Wellington's south-west coast in 1964|publisher=[[Te Ara: The Encyclopedia of New Zealand]]|accessdate=20 September 2011}}</ref>

When it was completed, the New Zealand HVDC link was the world's longest HVDC transmission scheme, with the highest power rating, and the largest undersea power cables.<ref name="To1990">''[http://www.ipenz.org.nz/heritage/Engineering-to-1990.pdf Engineering to 1990] {{Webarchive|url=https://web.archive.org/web/20081018050527/http://www.ipenz.org.nz/heritage/Engineering-to-1990.pdf |date=18 October 2008 }}'' – [[Institution of Professional Engineers New Zealand|IPENZ]], Engineering Publications Co Ltd, Page 38</ref> 
The terminal stations at each end of HVDC link used large [[mercury arc valve|mercury-arc]] [[rectifier]]s and [[inverter]]s – 1960s technology – to convert between AC and DC. The South Island converter station was established at the Benmore hydroelectric power station in the Waitaki Valley. The North Island converter station was built at [[Haywards]] in the Hutt Valley near Wellington.

The HVDC transmission line that connects Benmore and Haywards converter stations has an overall length of 610 kilometres. The overhead transmission line is supported by 1649 [[transmission tower]]s and has a total route length of 570&nbsp;km. The submarine cables underneath [[Cook Strait]] are 40&nbsp;km long.<ref name="AMP2010">{{cite web|url=http://www.transpower.co.nz/f3655,31396927/asset-management-plan-apr-2010.pdf|title=Asset Management Plan|publisher=[[Transpower New Zealand|Transpower]]|date=April 2010|url-status=dead|archive-url=https://web.archive.org/web/20120319150954/http://www.transpower.co.nz/f3655,31396927/asset-management-plan-apr-2010.pdf|archive-date=19 March 2012}}</ref>

Until it was upgraded in 1993, the HVDC Inter-Island link had normal operating voltages of ±250&nbsp;[[kilovolt|kV]], and a maximum power transmission capacity of about 600&nbsp;[[megawatt|MW]].

The HVDC link was originally designed to transfer power northwards from Benmore to Haywards. In 1976, the control system of the original scheme was modified to allow power to be sent in the reverse direction, from Haywards to Benmore.<ref name=Taylor/>

==== Engineering heritage status ====
The original HVDC link was recognised as a significant part of New Zealand's engineering heritage by the Institution of Professional Engineers New Zealand, (now [[Engineering New Zealand]]), during the "Engineering to 1990" project, which helped to celebrate the country's sesquicentenary in 1990.<ref>{{cite web |url= https://www.engineeringnz.org/our-work/heritage/heritage-records/hvdc-link-benmore-haywards-electricity-cab/ |title=HVDC Link - Benmore to Haywards Electricity Cable| publisher= Engineering New Zealand| accessdate= 26 March 2020}}</ref>
{{Clear}}

=== The Hybrid Upgrade Project ===
[[Image:Pole 2 Thyristor Valve.jpg|thumb|250px|right|Haywards Pole 2 thyristor valve, during maintenance shutdown]]
In 1987, the [[Electricity Corporation of New Zealand]] began investigations to find the best means of upgrading the inter-island link. A hybrid upgrade was chosen over total replacement, for economic reasons. The term "hybrid" was adopted because the increase in capacity was to be obtained through a combination of voltage and current upgrades. The upgrade project involved continued use of the existing mercury arc valve converter equipment alongside new solid-state [[thyristor]] converter stations. The scope of work included:<ref name=Reilly/>
*  Providing three new HVDC submarine cables underneath Cook Strait, to supplement and ultimately replace the original cables. Each new cable was rated at 350&nbsp;kV, 1430&nbsp;A, giving a maximum power capacity of 500&nbsp;MW per cable. The three new power cables were installed in 1991 by the cable laying vessel ''Skagerrak''.<ref>{{cite web |url= http://www.ship-technology.com/features/feature91186/ |title= Extending the Skagerrak |date= 27 July 2010 |publisher=Ship-Technology.com |accessdate= 28 September 2011}}</ref>
* New cable terminal stations at Fighting Bay and Oteranga Bay
* The existing mercury arc valve converters at each end of the link were reconfigured to operate in parallel at each station (they had previously operated with opposite [[electrical polarity]]). They were redesignated as Pole 1.
* The operating voltage of the mercury arc valve converters was increased from the original 250&nbsp;kV to 270&nbsp;kV
* New HVDC thyristor converter stations were added at each end of the link. These had an operating voltage of 350&nbsp;kV, and were designated as Pole 2.
* The reinsulating of the entire HVDC overhead transmission line to increase its rating to 350&nbsp;kV. Work on transmission structures and conductors was also carried out to ensure that the line conductors could operate at up to 2000&nbsp;A on each Pole.

The Pole 2 converter stations and new submarine cables were commissioned in March 1991.

The upgrade brought the total converter station capacity to 1348&nbsp;MW (648+700&nbsp;MW), however the link was restricted to 1240&nbsp;MW due to the overhead transmission line rating's restricting Pole 1's operating capacity to 540&nbsp;MW. After the retirement of the last of the original submarine cables, the overall HVDC link transfer capability was restricted further to 1040&nbsp;MW due to the single Pole 2 cable underneath Cook Strait.<ref name=Taylor/>

In its Asset Management Plan 2018, Transpower indicated that in the regulatory period 2020-2025 it planned significant expenditure to extend the life or replace ageing equipment in the Pole 2 converter stations that is near the end of its original 30-year design life.<ref>{{cite web |url= https://www.transpower.co.nz/resources/asset-management-plan-2018 |title=Asset Management Plan 2018| publisher= Transpower New Zealand| accessdate= 26 March 2020}}</ref>

=== Decommissioning of Pole 1 ===
On 21 September 2007, the original Pole 1 mercury-arc converter stations were shut down "indefinitely". However, in December 2007, Transpower announced that one-half of the capacity of Pole 1 would be returned to "warm standby" service before the winter of 2008 in order to meet the demand for power in the North Island if needed. The remaining half-pole equipment of Pole 1 was to be decommissioned.<ref>{{cite web|url=  http://www.scoop.co.nz/stories/BU0712/S00345/transpower-decommissions-half-of-pole-1.htm |title=Transpower Decommissions Half of Pole 1 |publisher= [[Scoop (website)|Scoop]]| date=19 December 2007}}</ref>

Transpower also announced in November 2007 that by December 2007, it would increase the south to north power transmission capacity of Pole 2 from 500&nbsp;MW to 700&nbsp;MW. This was done by reconfiguring the three operational submarine cables. One of the two cables previously connected to Pole 1 was transferred to Pole 2.<ref name="NZ_Herald_10477123">{{cite web |url=http://www.nzherald.co.nz/topic/story.cfm?c_id=187&objectid=10477123 |title=Geothermal plant in Contact's Taupo plan |date=20 November 2007 |work=[[New Zealand Herald]] |accessdate=3 October 2011}}</ref>

On 13 March 2008, Transpower announced that work had been completed to restore 50% of the capacity of Pole 1 to service at times when the demand for power on the North Island peaked.<ref>{{cite web| url= http://www.nzherald.co.nz/transpower/news/article.cfm?o_id=367&objectid=10497786 |title= Transpower gets green light to restore inter-island link |work= [[New Zealand Herald]]| date=13 March 2008}}</ref> Several mercury arc rectifiers were cannibalized from the [[Konti-Skan]] link between Denmark and Sweden for this restoration. The energy transfer on Pole 1 was strictly limited to the northbound direction, to reduce the stress and strain on the aging converter system.

In May 2009, Transpower placed the remaining capacity of Pole 1 back into service for a short period, at a limited capacity of 200&nbsp;MW, in response to a temporary loss of capacity on Pole 2.

The decommissioning of half of Pole 1 and the operational restrictions placed on the remaining Pole 1 capacity led to the HVDC link operating mostly in monopolar mode, using Pole 2 alone. In 2010, Transpower reported that continuous operation in monopolar mode has caused the HVDC link to act as a [[galvanic cell]] with the earth, causing Benmore's Bog Roy earth electrodes to erode as they acted as an [[anode]], and causing the buildup of [[magnesium]] and [[calcium hydroxide]] deposits on Hayward's Te Hikowhenua shore electrodes as they acted as a [[cathode]]. Additional replacement and maintenance work was required.<ref name="AMP2010" />

On 1 August 2012, Transpower decommissioned the remaining half of the Pole 1 mercury arc valve converter stations at Benmore and Haywards, after 47 years in service. The Inter Island link at the time was the last HVDC system in the world with mercury arc valve converters in operational service.<ref>{{cite web| url= https://www.transpower.co.nz/news/pole-1-decommissioned |title= Pole 1 decommissioned |publisher= Transpower| date=31 August 2012|accessdate=3 September 2012|archive-url=https://web.archive.org/web/20211003053902/https://www.transpower.co.nz/news/pole-1-decommissioned|archive-date=3 October 2021}}</ref>

=== The Pole 3 Project ===
In May 2008, Transpower submitted an investment proposal to the [[Electricity Commission (NZ)|Electricity Commission]] for the replacement of the old mercury arc valve Pole 1 converter stations with new thyristor converter stations. In July 2008, the Electricity Commission announced its intention to approve the project.<ref>{{cite web|url=http://www.ea.govt.nz/industry/ec-archive/grid-investment-archive/gup/2007-gup/hvdc-grid-upgrade/|title=HVDC Upgrade Proposal|publisher=[[Electricity Commission (NZ)|Electricity Commission]]|date=31 July 2008|url-status=dead|archive-url=https://web.archive.org/web/20120401020521/http://www.ea.govt.nz/industry/ec-archive/grid-investment-archive/gup/2007-gup/hvdc-grid-upgrade/|archive-date=1 April 2012}}</ref>

[[Image:Pole 3 Benmore Valve Hall roof lift.jpg|thumb|right|Lifting the roof of the Pole 3 valve hall into position at Benmore]]
This project involved the construction of new converter stations designated as Pole 3, to operate at +350 kV 700&nbsp;MW, matching the existing Pole 2 (−350 kV, 700&nbsp;MW). Site works on the $672&nbsp;million project were formally commenced on 19 April 2010, when Minister of Energy [[Gerry Brownlee]] turned the first sod. The new converter stations were to be commissioned by April 2012,<ref>{{cite web |url=https://www.transpower.co.nz/news/ceremony-marks-start-electrical-construction-pole-3-project |title=Ceremony marks start of electrical construction on Pole 3 project |publisher=[[Transpower New Zealand|Transpower]] |date=19 April 2010 |access-date=30 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140224051258/https://www.transpower.co.nz/news/ceremony-marks-start-electrical-construction-pole-3-project |archive-date=24 February 2014}}</ref> but in May 2011, Transpower announced that commissioning was delayed until December 2012 because of difficulties being experienced by the manufacturer.
<ref>{{cite web |url=https://www.transpower.co.nz/news/transpower-notice-commissioning-hvdc-pole-3 |title=Transpower Notice - Commissioning of HVDC Pole 3 |publisher=[[Transpower New Zealand|Transpower]] |date=11 May 2011 |access-date=30 August 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140224051325/https://www.transpower.co.nz/news/transpower-notice-commissioning-hvdc-pole-3 |archive-date=24 February 2014}}</ref>

Work involved in replacing Pole 1 with the new Pole 3 converter stations included:<ref name="APR" />
* New valve halls adjacent to the Pole 2 valve halls at both Benmore and Haywards, each containing the thyristors converters
* New transformers connecting the valve halls to the 220&nbsp;kV buses at both Benmore and Haywards
* Connecting the Pole 3 thyristors to the existing Pole 1 lines at both Benmore and Haywards
* Connecting the Pole 3 thyristors to the existing electrode lines at both Benmore and Haywards
* Switching the number 5 Cook Strait cable from Pole 2 back to the Pole 1/3.
* New 220&nbsp;kV [[electronic filter|filters]] on the 220&nbsp;kV buses at both Benmore and Haywards
* New transformers connecting the four [[synchronous condenser]]s C7 to C10 to the 110&nbsp;kV bus at Haywards
* New 5th and 7th harmonic filters connecting to the 110&nbsp;kV bus at Haywards.
* Removal of the existing converter transformers connecting the Pole 1 mercury arc valves and two of the synchronous condensers to the 110&nbsp;kV bus at Haywards.
* Removal of all remaining mercury arc valve Pole 1 equipment at both Benmore and Haywards.
[[Image:Pole 3 building seismic base isolator.jpg|thumb|250px|right| Pole 3 building seismic [[base isolation]] at Haywards]]

The decommissioning of Pole 1 was scheduled for July 2012, allowing works to switch the existing lines over Pole 3 to occur, and to allow testing of the new pole to occur during the summer months where electricity demand and therefore inter-island electricity transfer is low. The new Pole 3 was able to operate at 700&nbsp;MW from commissioning, but due to inadequate voltage support at the Haywards end of the link, Pole 2 and 3 combined transfer was limited to 1000&nbsp;MW. After the commissioning of a new [[STATCOM|static synchronous compensator]] (STATCOM) at Haywards in January 2014, Pole 3 was able to operate at its full capacity with Pole 2 in operation (1200&nbsp;MW total transfer).<ref name="pole3project">{{cite web |url= https://www.transpower.co.nz/projects/hvdc-inter-island-link-project |title= HVDC inter-Island link project – Grid New Zealand |work= Transpower New Zealand |access-date= 30 August 2012 |url-status= dead |archive-url= https://web.archive.org/web/20130210075035/https://www.transpower.co.nz/projects/hvdc-inter-island-link-project |archive-date= 10 February 2013}}</ref>

==== Pole 2 control system replacement ====
Pole 2 was commissioned in 1992 with HVDC control systems using late 1980s technology. After 20 years in service, the control systems are nearing the end of their useful life, are [[technologically obsolete]], and are incompatible with the new Pole 3 control systems, making bipole control impossible.

In late 2013, Transpower took Pole 2 out of service for four weeks to allow the control systems to be replaced with new systems identical to those used in Pole 3, and to install a new bipole control system to control both poles. This was followed by three months of testing the new control systems. Pole 3 continued to operate during the outage and most of the testing in a monopolar configuration with the earth electrodes.

==== Other associated works ====

===== Line maintenance =====
During the time that Pole 1 was removed from service for replacement with Pole 3, maintenance and remedial work was undertaken on some sections of the transmission line. Work included:<ref name="pole3project" />
* Replacing around 100 [[transmission tower]]s in the South Island to fix clearance issues
* Replacing some conductor lengths in the North Island as they approach the end of their useful life
* Reinforcing some North Island transmission towers.

===== Benmore generator transformers =====
The original design of the inter-island link at Benmore was integrated with the design of the 540&nbsp;MW Benmore hydroelectric power station. The 16&nbsp;kV generator busbars in the power station were the point of connection between the HVDC link and the South Island grid. The power from the six Benmore generators could flow directly from the 16&nbsp;kV busbars to the HVDC link via converter transformers, with the interconnecting transformers connecting to the Benmore 220&nbsp;kV busbar to export or import electricity from the rest of the South Island. The design of the power station was optimised with the HVDC link, and the interconnecting transformers were designed with a significantly lower rating than the maximum output of the Benmore generators, because so much of the generator output power would normally flow to the HVDC link.

Following Transpower's decommissioning of the original Pole 1 equipment, there was no longer any direct connection between the generator 16&nbsp;kV busbars and the HVDC link, and the limited capacity of the Benmore interconnecting transformers would have constrained the maximum output of the station. In co-ordination with the Transpower programme for decommissioning of the Pole 1 equipment, Benmore owner [[Meridian Energy]] replaced the interconnecting transformers with new generator transformers. The six generators were reconnected to the 220&nbsp;kV national grid via six new generator circuit breakers and three 220/16/16&nbsp;kV three winding transformers. The new transformers each connect two generators, via two 16&nbsp;kV secondary windings.<ref>{{cite web |url= http://pbanz.com/Documents/PBA%20News%20Issue%204%208.pdf |title= An Iconic Reconfiguration (Benmore Power Station) |publisher= PBA Electrical Contractors |date= 3 May 2011 |access-date= 2 July 2012 |url-status= dead |archive-url= https://web.archive.org/web/20130208044604/http://pbanz.com/Documents/PBA%20News%20Issue%204%208.pdf |archive-date= 8 February 2013}}</ref><ref>{{cite web |url= http://www.odt.co.nz/the-regions/north-otago/29502/benmore-gets-more-with-first-full-rebuild?page=0%2C0 |title= Benmore gets more with first full rebuild|work=[[Otago Daily Times]]|date= 30 October 2008 |accessdate= 2 July 2012}}</ref>

==Planned upgrades==

===Cook Strait cables===
In 2024, the [[Commerce Commission]] approved a proposal from Transpower to upgrade the capacity of the HVDC link by adding a fourth Cook Strait cable and additional reactive support at Haywards. The new investment would enable an increase in maximum HVDC capacity to 1,400{{Nbsp}}MW. The upgrade was approved as a contingency, where the timing would depend upon triggers such as the exit of the aluminium smelter from [[Tiwai Point Aluminium Smelter|Tiwai Point]], developments in the load and generation forecasts, and Transpower demonstrating a net positive market benefit from the investment.<ref name=":0">{{Cite web |date=2024-03-07 |title=Transpower to consult on future-proofing North and South Island power link |url=https://www.rnz.co.nz/news/business/511079/transpower-to-consult-on-future-proofing-north-and-south-island-power-link |access-date=2024-03-23 |website=[[RNZ]] |language=en-nz}}</ref><ref>{{Cite web |date=28 February 2024 |title=Transpower's Net Zero Grid Pathways stage one major capex proposal - Final decision and reasons paper |url=https://comcom.govt.nz/__data/assets/pdf_file/0027/344583/Transpower-Net-Zero-Grid-Pathways-stage-one-Final-decision-and-reasons-paper-28-February-2024.pdf |access-date=24 March 2024 |website=Commerce Commission |page=}}</ref> Transpower stated that the existing submarine cables would reach end-of-life in the early 2030s.<ref name=":0" />

In May 2025, Transpower announced that it was planning for replacement of all three existing Cook Strait HVDC cables laid in 1991. The forecast cost of replacement was $NZD 1.4 billion and the project was scheduled for completion by 2031. The replacement plan included adding a fourth cable to increase inter-island transmission capacity and resilience, together with enhancements to the cable terminal stations on either side of Cook Strait. Regulatory approval from the Commerce Commission would be sought via a Major Capex Proposal.<ref>{{Cite news |last=Franks |first=Raphael |date=7 May 2025 |title=Transpower proposes $1.4 billion replacement for ageing Cook Strait electricity cables linking North, South Islands |url=https://www.nzherald.co.nz/nz/transpower-proposes-14-billion-replacement-for-ageing-cook-strait-electricity-cables-linking-north-south-islands/JXXXBSJGKVGMPNVOAUI7TALW6I/ |access-date=7 May 2025 |work=[[The New Zealand Herald]]}}</ref>

== Site locations ==
* Haywards HVDC Converter Station: {{coord|-41.151446|174.981691|region:NZ_type:landmark|name=Haywards HVDC Converter Station}}
* Te Hikowhenua deviation line take-off point: {{coord|-41.234167|174.758611|type:landmark|name=Te Hikowhenua deviation line take-off point}}
* Te Hikowhenua Shore Electrode Station: {{coord|41|12|28|S|174|43|11|E|type:landmark|name=Te Hikowhenua Shore Electrode Station}}
* Oteranga Bay Cable Terminal Station: {{coord|41|17|37|S|174|37|48|E|type:landmark|name=Oteranga Bay Cable Terminal Station}}
* Fighting Bay Cable Terminal Station: {{coord|-41.3098|174.202|region:NZ_type:landmark|name=Fighting Bay Cable Terminal Station}}
* Bog Roy Land Electrode Station: {{coord|-44.574|170.099|region:NZ_type:landmark|name=Bog Roy Land Electrode Station}}
* Benmore HVDC Converter Station: {{coord|44|33|55|S|170|11|24|E|type:landmark|name=Benmore HVDC Converter Station}}

==See also==
* [[National Grid (New Zealand)]]
* [[High-voltage direct current]]
* [[List of HVDC projects]]
* [[Electricity sector in New Zealand]]

== References ==
{{Reflist|33em}}

== External links ==
{{Commons category}}
* [http://www.transpower.co.nz/projects/hvdc-inter-island-link-project New Zealand's HVDC Link]
* [http://www.abb.com/industries/ap/db0003db004333/d7276aa1bceaece4c125774b00407449.aspx New Zealand HVDC – the interisland link] (from the [[ABB]] website)
* https://web.archive.org/web/20050526185217/http://www.transmission.bpa.gov/cigresc14/Compendium/NEW-ZEAL.htm
* https://web.archive.org/web/20050526185217/http://www.transmission.bpa.gov/cigresc14/Compendium/New-zeal%20Pictures.pdf

{{DEFAULTSORT:Hvdc Inter-Island}}
[[Category:Energy infrastructure completed in 1965]]
[[Category:HVDC transmission lines]]
[[Category:Submarine power cables|Inter-Island]]
[[Category:Electric power in New Zealand]]
[[Category:Electric power transmission systems in New Zealand]]
[[Category:Buildings and structures in the Wellington Region]]
[[Category:Cook Strait]]
[[Category:Buildings and structures in the Marlborough District]]
[[Category:Buildings and structures in the Canterbury Region]]
[[Category:Buildings and structures in Otago]]
[[Category:1965 establishments in New Zealand]]