Japanese Researchers Develop DC Transmission System for Offshore Wind

May 12, 2023 by Shannon Cuthrell

Researchers in Japan are developing a DC power transmission system to transmit offshore wind power over long distances without interruptions from fault chains. 

Researchers at the Tokyo Institute of Technology are working on a high-voltage transmission system that uses direct current (DC) to transport offshore wind power to land long distances without interruption. 


Noshiro Port Offshore Wind Farm

The Noshiro Port Offshore Wind Farm began operating in December 2022 as Japan’s first commercial offshore wind site. Image used courtesy of the Akita Offshore Wind Corporation


If commercialized, such a design would prove valuable in the offshore wind market, as trends indicate development is moving further out at sea. 


Why DC Transmission Instead of AC?

Most land-based power lines use alternating current (AC) transmission systems. Many offshore wind farms also utilize AC electricity, but only when those sites are relatively close to shore. For example, many developers have used AC power in projects near the mainland in the United States. 

Over longer distances, offshore wind projects require submarine power cables that can’t use AC transmission because current leaks cause losses in the amount of power being transmitted back to the mainland. As such, AC cable lines are limited to about 30 miles offshore, according to a 2022 study from the U.S. Bureau of Ocean Energy Management (BOEM). 

DC systems address those challenges. Converting high-voltage electricity from AC to DC minimizes losses and becomes cost-effective when placed 37 to 60 miles from shore, according to the BOEM report. 

This formed the basis for the DC transmission system developed recently by researchers at the Tokyo Institute of Technology’s Power Electronics Laboratory.


Video used courtesy of the Tokyo Institute of Technology

While DC systems avoid current losses, fault lines around subsea power cables present new gaps to overcome. This is of particular concern to Japan, one of many stops along the so-called “Ring of Fire” containing Earth’s largest exposed fault. Featuring over 450 volcanoes, this strip of the Pacific Ocean stretches nearly 25,000 miles from New Zealand, through the Philippines and Japan, all the way down to South America’s southern tip surrounding Chile. The region is home to 90% of the world’s earthquakes. 

Connecting multiple power plants means a fault in one plant or cable can cause a massive blackout across several sites. The researchers are studying control algorithms to detect faults and redirect transmission routes. Combining conventional power transmission equipment with controllable power electronics devices using semiconductors, they can conduct high-speed switching with a single device. 


experimental facility was used to test the researchers’ DC transmission system

This experimental facility was used to test the researchers’ DC transmission system. Image used courtesy of the Tokyo Institute of Technology 

The researchers tested their DC-AC power supply system using a 1/100,000-scale experimental facility to improve their transmission design for higher reliability. Their goal is to build energy infrastructure that connects multiple offshore wind farms with high-voltage DC transmissions, linking them to onshore AC power grids that transfer power over long distances. 


Offshore Wind and Renewables in Japan 

The Tokyo Institute of Technology’s transmission research comes as the Japanese government aims to reduce its greenhouse gas emissions by 46% by 2030 (compared to 2013 levels) and reach carbon-neutral status by 2050. To do that, it plans to boost its renewables deployment to make up 36% to 38% of the electricity mix. 

That would be a significant change from today. According to the latest figures from Japan’s Ministry of Economy, Trade, and Industry (METI), renewables account for 20.3% of the country’s electric power generation, while nuclear’s share is 6.9%. Fossil fuels still dominate the electricity mix, with thermal power (excluding biomass) claiming 72.9%. 

Japan’s supply of coal and oil grew by 8.8% and 2.9%, respectively, in 2021—the last available reporting year in METI’s statistics. Renewables (excluding hydroelectric) jumped by 11.7%, driven by solar power and biomass generation. Also, non-fossil fuels increased their share by 16.8%, surpassing the increase in fossil fuels and marking the highest level since 2011. 

The island country is almost entirely reliant on imports for its energy needs, sourcing fossil fuels like natural gas and coal from overseas. Still, it has ambitious plans in the offshore wind market, targeting 10 gigawatts (GW) of capacity by 2030 and at least 30 GW by 2040. It has launched two offshore wind tender rounds so far to attract new projects. The second round, which opened in December 2022, covers four promotion zones. 

But offshore wind development is still a nascent industry in Japan, home to several demonstration sites but few commercial projects. Its first large-scale offshore wind project began operations in late-2022: The Noshiro Port Offshore Wind Farm (part of a 140-megawatt project across two locations) became the country’s first commercial site of its kind.