About 1.2 million km of cables at the bottom of the ocean carry most of the internet traffic across continents. Fiber optics is a crucial enabler for this monumental achievement in the telecommunications world. 

On the flip side, electric power transmission under the sea is prevalent using AC and DC-based cable systems. Yet, due to the fundamental benefits of using DC-based power transmission, high-voltage direct current (HVDC) is the go-to choice for long distances (>50 km). 

 

An internal diagram of an HVDC cable.

An internal diagram of an HVDC cable. Image used courtesy of Castellon et al

 

Undersea HVDC power transmission can be considered a key enabler in scientific and industrial exploration and sharing renewable energy between countries or regions separated by water bodies. 

 

Why HVDC?

HVDC has become quite popular today and is used throughout the globe in overhead electrical power transmission. Compared to AC, HVDC has no voltage drop due to parasitic inductance and capacitance and no need to provide reactive power compensation. This attribute results in lower losses, grid stability, efficient use of copper, and is more economically friendly. However, one differentiating aspect is that it requires conversion from/to AC at the terminal ends. 

It may be surprising for some to learn that HVDC isn’t that recent, especially in undersea use. Even before the silicon MOSFET was developed in 1954, HVDC was used to transmit power of about 20 MW at a distance of 98 km near Sweden––called the Gotland 1 project.

While the primitive Gotland 1 utilized 110 kV transmitting over 100 km distance, the current state-of-art development is in the Viking Link project using 450 kV, which would transmit 1400 MW over a distance of 760 km.

 

Diagram of the Viking Link project.

Diagram of the Viking Link project. Image used courtesy of Viking Link

 

Once running, the Viking link interconnector would become the longest undersea HVDC power cable.  With projects like this happening, it has started to raise the question of creating an undersea power grid. 

 

Developing Undersea Power Transmission

Setting up cables for undersea power transmission is far from simple and much more complex than overhead transmission on land. It’s also a niche market as the high reliability required for such projects demands expertise in various aspects such as cable design, protection, reliability testing, assessment of environmental and geological conditions at the seafloor. 

The mission life can be at least 30 years and is expected to have no or minimal maintenance. However, compared to transoceanic optical fiber cables, which require repeaters and associated circuits every 50-70 km, it is more straightforward with power cable installation and maintenance. Additionally, these power cables are not used at transoceanic lengths, and their use is seen to be typically limited to 1000 km. 

With increasing amounts of offshore renewable energy, the prospect of undersea power transmission might start to sound like a suitable investment, especially when the benefits are pitted against the challenges.

 

Integrating HVDC with Undersea Power Distribution Networks

From a scientific point of view, having access to real-time data from the seabed creates several benefits, though it would require a suite of sensors at the bottom of the ocean, which in turn would require power distribution networks. The benefits could outweigh the challenges by helping with ocean exploration, marine resource development, earthquake, and tsunami monitoring. 

Integrating such information in weather forecasting and public awareness could create a difference in numerous lives saved. Researchers at Utah State University Power Electronics Laboratory (UPEL) have developed a prototype and received a patent for a 1 kW DC current-to-voltage converter with this advantage in mind. 

 

System-level block diagram of undersea DC constant current distribution network.

System-level block diagram of undersea DC constant current distribution network. Image used courtesy of UPEL

 

The researchers claim that their converter is suitable for different sensors on the seabed, whose voltage and power requirements could vary over a range. It uses a constant current source as input, employed zero voltage switching (ZVS), resulting in high efficiency, high power density, lower EMI, and robustness against voltage drop over cable length and cable faults. 

 

Test setup of the series resonant converter with active ZVS-based soft-switching enabling wide load operation.

Test setup of the series resonant converter with active ZVS-based soft-switching enabling wide load operation. Image used courtesy of Tarak Saha

 

Future Prospect for Undersea Power Distribution Networks 

With industry players such as ABB with their subsea power distribution and conversion system and Siemen’s subsea power grid have already laid out their subsea power distribution station plans, it appears to within the realm of reality to eventually establish long-distance power transmission undersea, but only after tackling several reliability challenges, as demonstrated by numerous projects presently running. 

Direct access to power from HVDC transmission lines would require the use of DC-DC converters, inverters, transformers, and switchgear, along with implementing systems for fault detection and isolation. 

Ultimately, this has applications in extracting raw materials for the oil and gas industry, seafloor observatory/monitoring facilities, and potentially areas yet to be decided.

 


 

Interested in an underwater power grid? What are your thoughts about this concept? Share your comments down below.


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