The world’s first half million volt DC transmission line gets an upgrade

19 September 2018

The historic New Zealand HVDC link, now rated at more than 1200 MW, is to be upgraded with new components and a new control system.

ABB has received an order from Transpower New Zealand Ltd to upgrade its high-voltage direct current link which connects the transmission grids of the North and South islands. The link is a primary element of the country’s transmission system and balances energy distribution between the country’s two main land masses.

The North Island has a population more than three times that of the South Island, which besides its picturesque landscape, offers huge amounts of hydro power. As a consequence, demand for power on the North Island is substantially higher and relies on power generated on the South Island. The 610 km long North-South HVDC interconnection enables efficient transmission of clean power from the south to areas of high demand in the north. The link also plays an important part in the New Zealand electricity market by allowing power trading between the two islands.

On average 80 % of New Zealand’s electrical energy production is from hydroelectric sources, mostly from the South Island, while the North Island accounts for almost two thirds of total electricity demand, with a peak load nearly double that of the South Island. After the HVDC transmission upgrade, up to 25 % of the North Island’s electricity demand will be met by the South Island hydro capacity.

The upgrade is intended to encourage the further development of renewable generation like wind and hydro, often located far from load centres. It gives the South Island access to the North Island’s gas and coal generation, which is important for the South Island during dry winter and summer periods. It provides the North Island with access to the South Island’s plentiful hydro generation, important for the North Island during peak winter periods.

The scope of the project includes a valve upgrade of pole 2 consisting of capacitors, fibre optics and valve control units based on the ABB Ability MACH control system. The new control system, which acts as the management centre of the entire link, will be a key component of the upgrade, which is scheduled to be finalised in 2020, and will be ‘carried out in a manner that minimises impact on the grid and the power-trading market’ says ABB.

“This upgrade will enhance grid reliability and availability thereby increasing power security and bringing clean power to consumers.” said Claudio Facchin, president of ABB’s Power Grids division. “The project reiterates our strategic focus on service, our commitment to integrating renewables, and the role of digital technologies based on our ABB Ability based technologies and reinforces our HVDC technology leadership, as a partner of choice for enabling a stronger, smarter and greener grid.”

ABB pioneered HVDC technology 60 years ago and has been awarded approximately 120 HVDC projects representing a total installed capacity of more than 130 000 MW, accounting for about half of the global installed base.

First link

ABB has an historic involvement in the link. The first New Zealand link was commissioned by ABB, (then ASEA) in 1965 as one of the first HVDC transmission systems in the world. It was originally a bipolar 600 MW link with mercury arc valves, until the original equipment was paralleled onto a single pole in 1992, and a new thyristor-based pole was commissioned by ABB alongside it, increasing capacity to 1040 MW. The 1965 installation was decommissioned in 2012 after 47 years in operation.

History of the link, 1965 to 1992

Connection of the AC systems on the South and North Islands of New Zealand was first achieved with the 600 MW ±250 kV HVDC inter-island link. In 1992 the grid owner Transpower upgraded the link to 1240 MW.

The already existing link, with its mercury arc valves, was modified to operate in a bipolar ‘hybrid’ scheme together with a new thyristor converter. The first stage of the upgrade was to add the 700 MW thyristor converter, and the second step was to operate the old and new equipment as a hybrid bipole rated 1240 MW. The two mercury arc valve poles were connected in parallel to form an upgraded pole 1.

The link is one of the two oldest DC links still running with original installed equipment from 1965. It was upgraded and modernised in 1992 when a new thyristor pole was added, the line voltage was raised, new cables added and the original mercury arc based poles were equipped with new control equipment and rearranged to operate in parallel on one pole.

The link consists of three operational HVDC power cables. It starts at two converter stations located adjacent to Benmore hydroelectric PS in the Waitaki Valley. The main Benmore switchyard connects the Benmore generators to the rest of the South Island transmission grid, at 220 kV via tie-lines across the Benmore tailrace. The AC power is converted at the substations to ±350 kV HVDC.

It then it travels 534 km on an overhead transmission line through inland Canterbury and Marlborough to Fighting Bay then via a 40 km via submarine cable beneath Cook Strait to Oteranga Bay, near Wellington, before travelling the final 37 km on overhead lines to the Haywards transmission substation.

The link first became operational in April 1965, primarily to transport electricity from the generation-rich South Island to the more populous North Island. It was originally a bipolar link with mercury arc valves (until the original equipment was paralleled to a single pole (Pole 1) in 1992) and with the new thyristor-based pole (Pole 2) turned on alongside it, this increased the link’s capacity to 1040 MW. Because of recurrent technical malfunctions, Pole 1 was fully turned off on 1 August 2012, and the replacement thyristor-based Pole 3 was turned on beginning 29 May 2013, restoring the DC link to a bipolar 1200 MW configuration. 

Location of the interconnector
Construction of the 1965 link involved working with ABB’s precursor ASEA New Zealand (1960 photo)
The converter stations and switchyard at Benmore hydroelectric power station in the Waitaki Valley on South Island

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