Interconnectors are thick high voltage cables that link the electricity grids between countries overland and under the sea. This exchange of power helps to balance supply and demand making the grid more reliable and stable1. But how exactly does electricity travel hundreds of kilometres under the sea?
Figure 1: Interconnector (8)
From AC to DC
Our homes and national grids mainly use alternating current or AC electricity. This means that it switches direction 50 times per second (50 Hz in Europe). Its efficient and easy to transform between voltage levels for short distances. However, a problem occurs when we send AC electricity across long distances, and especially under water. The AC electricity flowing through the cable generates an electric field around it and saltwater conducts electricity quite well. This causes a strange side effect. The saltwater and cable act like two sides of a capacitor!
Lets understand this effect in detail. A capacitor is a device made of two conductive plates seperated by an insulating material (dielectric). When you apply a voltage across the plates, positive and negative charges build up on each side, creating an electric field between them. Capacitors store and release energy through this field9. So in the interconnected, the central metal wire that carries the electricity acts as one plate of the capacitor, the saltwater around the cable acts as the other plate and the insulating material acts as the dielectric.
Figure 2: Capacitor Diagram (4)
In AC, the voltage and direction of the current changes constantly which means that the electric field between the plates is also constantly reversing. Every time the field changes, the cable must charge up that capacitor by pushing charges into the field and then half a cycle later it ‘discharges’ it and starts building the opposite field. This repeated charging and discharging consumes or wastes energy that could otherwise be delivered to the destination. In physics terms this is called capacitive reactance3. Capacitive reactance is the resistance to the flow of alternating current caused by a capacitor. The longer the cable the worse this effect becomes.2
The solution? Direct current (DC). The voltage is steady and does not oscillate. This means no constantly changing electric field, no charging and discharging of the ‘capacitor’ and much lower energy losses over long distances. This is why modern interconnectors are based on High Voltage Direct Current or HVDC. Even over land DC is still used as it has lower losses than AC over long distances.
The Full Picture
Now that we know the physics behind efficient long distance transmission, let’s consider interconnection from start to finish. Electricity is almost always generated as AC at a power station and enters the national AC grid. This AC power is converted to DC using a converter station which involves high power electronics and transformers. This conversion process loses about 1 – 1.15% of energy as heat. The DC power then travels a long distance over which it loses roughly 2-3% per 1,000 km of transmission6,7 which is significantly lower than the losses which AC would suffer. Once electricity reaches the other side, another converter station converts it back to AC (with the same loss) ready to be used by homes and industries on the receiving grid. In total the entire HDVC system loses only 3-5% of electricity from source to destination.
Figure 3: Interconnection Transmission Diagram (5)
Interconnectors may be hidden beneath the sea or strung across the landscape, but they’re doing some of the most important work in our modern energy systems as they quietly and efficiently keep the lights on. Thanks to the physics of electricity and the smart application of HVDC technology, we can move clean energy across vast distances with minimal loss. These systems don’t just help balance supply and demand between countries they’re also important for integrating renewable energy, improving energy security, and reducing our reliance on fossil fuels thanks to physics and clever engineering to help power our world.
References:
- “Interconnectors | National Grid Group.”nationalgrid.com, www.nationalgrid.com/national-grid-ventures/interconnectors-connecting-cleaner-future.
- Liu G, Fan M, Wang P, Zheng M. Study on Reactive Power Compensation Strategies for Long Distance Submarine Cables Considering Electrothermal Coordination. Journal of Marine Science and Engineering . 2021;9(1):90. doi:https://doi.org/10.3390/jmse9010090
- Capacitive Reactance – an overview | ScienceDirect Topics. sciencedirect.com.https://www.sciencedirect.com/topics/engineering/capacitive-reactance
- Donald F. Capacitor tutorial : Working and How to use in Circuits. Gadgetronicx. Published August 3, 2019. https://www.gadgetronicx.com/capacitor-working-tutorial-applications-circuits/
- Blocked Page. com. Published 2025. Accessed May 5, 2025. https://allumiax.medium.com/hvdc-transmission-3d447d29df16
- Electricity Transmission Systems – World Nuclear Association. World-nuclear.org. Published 2020. https://world-nuclear.org/information-library/current-and-future-generation/electricity-transmission-grids
- Drummond J. System Efficiency and the New Era of Grid Operations. com. Published July 2, 2024. Accessed May 5, 2025. https://www.tdworld.com/overhead-transmission/article/55092197/system-efficiency-and-the-new-era-of-grid-operations
- Colthorpe, Andy. “UK Loses 1.4GW of Power in Interconnector Trip, Battery Storage Keeps Lights On.”Energy-Storage.News, 10 Oct. 2024, energy-storage.news/uk-loses-1-4gw-of-power-in-interconnector-trip-battery-storage-keeps-lights-on/
9. Wikipedia Contributors. Submarine power cable. Wikipedia.
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