The first multi-terminal high voltage direct current connections are being installed in meshed networks, offering significant improvements in system reliability and efficiency, according to Niklas Persson, Managing Director of Hitachi ABB Power Grids' Grid Integration business unit.
HVDC's control and reliability characteristics were beginning to prove themselves in today's more volatile renewable energy markets, Persson said in a March 17 interview with S&P Global Platts.
"Nearly 30 years ago, when HVDC Light was first being developed [by ABB Power Grids], we saw that in future there would be a different generation mix with increased frequency and voltage distortion in the network," he said.
The first commercial HVDC Light link, on the Swedish island of Gotland, was used to integrate wind power into a fairly weak island grid. The aim was to beef up grid resilience through active and reactive power control. The power level was a modest 55 MW at 80 kV.
Today's HVDC Light converters, however, scale up to 800 kV and 4,600 MW, with rated DC currents up to 3,000 A.
The increase in size and power has enabled huge interconnectors to be delivered, notably the 580 km NorNed link, the 623 km Nord Link and the 730 km North Sea Link, Persson said.
Other benefits include reliability and safety, Persson said, noting Norway's seven shore-to-platform links in the oil and gas sector, and a recent tender in the UAE and Abu Dhabi for electrification of offshore gas field platforms.
"We put a lot of R&D into synchronizing IGBTs [insulated-gate bipolar transistors] with control and protection systems to decrease losses. As ratings increased, we began to see the capabilities of HVDC Light in frequency control, voltage control and in black start, allowing an operator to very easily restart a network," he said.
HVDC Light losses are now equal to or lower than for HVDC classic, and much lower than for AC.
"AC losses are typically about 10% from A to B, while for DC, losses are basically zero on transmission itself, and 1% per converter," Persson said.
Next steps
As noted, to date HVDC applications have overwhelmingly been 100 km-plus, point-to-point connections. The next step is to move HVDC into a grid setting, with multi-terminal applications such as the Caithness-Moray-Shetland link project in Scotland.
"To make a multi-terminal connection into a grid you need the functionality to break it, isolate a fault on one line, while maintaining flows on the system. We've now proved we can do that with our HVDC breaker [following independent tests concluded in 2020 by KEMA Laboratories in the Netherlands]," Persson said.
The next piece of the puzzle is to manage shared assets between different price zones.
"A solution that decides whether the energy from a wind farm goes to the Netherlands, the UK, Germany or Denmark, for instance, is of utmost importance," Persson said.
The Kreigers Flak Combined Grid Solution is demonstrating such a solution between the Danish region of Zealand and the German state of Mecklenburg-Western Pomerania, providing cross-border transmission capacity and integrating the Baltic 1 and Baltic 2 offshore wind farms.
The project, a joint venture between German transmission system operator 50Hertz and Danish counterpart Energinet, began trial operations in December 2020. It offers an additional 400 MW of capacity in both directions.
"We need the right regulatory framework and innovative business models if we are to scale-up these meshed networks to meet the EU's Green Deal targets," Persson said.
Dynamic supply chain
Examples of much larger projects were beginning to emerge, notably Denmark's North Sea energy island concept, serving as a hub for offshore wind serving Denmark, Germany and the Netherlands.
"In the beginning, platforms were 600 MW, now they are 1.2 GW with 2 GW to come -- there are only so many companies that have the financial strength to bid in an EPC process," Persson said.
The lump-sum, turnkey approach limited the sector's ability to create a dynamic supply chain, he said.
"Our view is each party should do what they are best at. The AC world is moving towards lots, creating a more dynamic supply chain. If you stick with the old EPC, lump-sum turnkey approach, it is harder to scale fast in your core businesses," he said.
SSE/Equinor's Dogger Bank project was proof a business model based on lots could deliver the speed needed to build 1.2 GW/year over three years, Persson said, noting the mix of Hitachi ABB Power Grids, Aibel and NKT delivering technology, converter platforms and cables for the UK's first HVDC-connected wind farm.
"This is a fantastic mix compared to the lump-sum, turnkey approach we saw in the past," he said.
