New Zealand’s earthquake-resilient transmission
In spring 2013, the upgrade for New Zealand’s HVDC inter-island link went live – designed, installed and commissioned for the national grid operator Transpower to strictest seismic requirements by Siemens. How did the new converter station cope with the unusual, devastating Kaikoura earthquake of 2016?
New Zealanders know about earthquakes. They endure thousands a year, some, a few, among the most damaging on record. In Christchurch, the South Island’s major city, two major quakes in 2010 and 2011 caused widespread damage; 185 people died in the rubble as hundreds of city buildings collapsed. Thousands were injured.
So, when Transpower, the nation’s electricity authority, was seeking for a new high-voltage thyristor-based converter and interconnector system for its HVDC link, they called on Siemens with a jaw-dropping specification: an installation capable of withstanding a one-in-2500-year seismic event. No engineering company, anywhere in the world, had ever built to such specifications, but Siemens saw it as a challenge and seized it with enthusiasm. Complex computer models were setup to calculate stresses in theory and massive shake table tests were conducted to verify actual equipment qualification.
Unprecedented seismic challenge
And so, Pole 3 in Transpower’s big substation at Haywards, north of Wellington, the nation’s capital, was designed, built and installed on shock resistant mountings of a resilience that set new world standards. It began service on May 29, 2013. Everything had been tested and re-tested. Transpower, the Government funding the considerable investment, and Siemens’ engineers were all pleased. A vital link in the 611-kilometer cable – carrying power from hydro generators in the south of the South Island to 77 percent of New Zealanders living in the North Island – had been made as earthquake-proof as world-class engineering could make it.
And then, two minutes after midnight on November 14, 2016, came its first, great real-life test. A quake of magnitude 7.8 hit the Haywards facility. Vital equipment in the central valve hall could be seen swinging substantially from side to side, but nothing broke and service continued as usual.
Ensuring security of supply – also in case of emergency
The quake was centered near Kaikoura, a small tourist town on the Pacific Coast on South Island’s northern tip, which suffered severe damage and some loss of life. The shock waves surged on across Cook Strait and into Wellington. Then, 40 km on to Haywards which, if lost, could cause a major blackout on the North Island, where three-quarters of New Zealand’s population lives.
“During the earthquake there was movement of the valves but no damage,” said Transpower’s HVDC and Power Electronics Manager, Ricky Smith. “Cracks appeared in some concrete structures around the site, but nothing of moment. One synchronous condenser a (large motor that provides reactive power for HVDC transfer) tripped because of the high vibration but was undamaged. The Kaikoura quake damaged Wellington, but Haywards was unscathed. The protections that we specified and Siemens delivered were very good indeed.”
New Zealand’s energy minister, Judith Collins, noting that Haywards had survived and that electricity supply had continued unbroken, said: “This gives me great confidence in the integrity of this asset, which remains a key part of ensuring security of supply in the national transmission grid.” The HVDC performed very well during the earthquake and the base isolators protected the Pole 3 building effectively, she said.
Construction under live working conditions
Günther Wanninger, the project director who headed the Siemens team in New Zealand, conceded Haywards was a huge task. “But Siemens is prepared to take up any challenge and we never give up,” he said. “When we commit to a project we will finish it, whatever the challenges.”
Building Pole 3, the 1,400 megawatt thyristor-based high-voltage direct current (HVDC) converter and interconnector system, took more than four years of meticulous engineering with exhaustive testing both at the Siemens factory at Erlangen, Germany, and on-site.
“We had to design new equipment, new damper solutions and rigorously test them on a shake table. But the transformers (300 tons each) were too big for that, so we made computer models and ran them with several ‘design earthquakes’ to show the equipment could withstand such forces,” Wanninger said.
The engineering was a great challenge, but there was a human triumph, too. While 350,000 volts surged constantly overhead, workers dug foundations and poured concrete from wheelbarrows, since there was no room for concrete pumpers, and on-site testing continued. Yet there was not a single serious injury.
New Zealand’s energy backbone
The HVDC system runs 611 kilometers as a single backbone from the hydro turbines in a series of dams centered on Benmore in the South Island, source of 50 percent of the country’s total power output, to Haywards, from where it is dispatched onwards to Auckland, the country’s biggest city. Nearly 75 percent of New Zealand’s 4.5 million people live in the North Island, a third of those in Auckland. As well as replacing Pole 1, the obsolete 49-year old mercury arc system, with thyristor-based Pole 3, the adjacent Pole 2, installed in 1992, was modernized as part of the Pole 3 Project by introducing a new state-of-the-art control system.
Matching equipment was also installed at the substation at Benmore where hydro output is collected and converted to 350kV DC current for transmission 534 kilometers north to the three submarine cables that carry it across Cook Strait to Haywards. There it is converted to 220kV AC for the 700-kilometer journey to Auckland and beyond.
Highest seismic standard successfully proven
The project took the engineers from Siemens, Transpower, and expert contractors such as Aurecon, a New Zealand company with a worldwide reputation in seismic engineering, into largely unexplored territory. Aurecon designed new 600-mm lead-rubber bearings and sliders to handle violent vertical and horizontal movement quakes could cause in the deep pilings on which everything is mounted. “Aurecon says no other building in the world has such high seismic standards as we built for Transpower,” said Wanninger. They can move 700 mm horizontally in an earthquake without damage.
On November 14, 2016, that revolutionary system came through with flying colors.