UK Electricity Interconnectors: Role and Operation

1. What are Electricity Interconnectors?

At its simplest, an interconnector is a high-voltage cable that connects the electricity grids of two different countries. It acts like a massive extension lead, allowing power to be traded in either direction.

To get a formal definition and understand their purpose, let's start with a short reading from National Grid.

As the article explains, interconnectors play several crucial roles:

• Energy Trading: They allow the UK to import electricity when it's cheaper elsewhere and export it when we have a surplus, particularly from renewable sources.
• Security of Supply: They provide an extra source of power, acting as a backup if there are issues with generation within the UK.
• Integrating Renewables: They are vital for managing the variability of wind and solar power. When it's very windy in the UK, we can export the excess electricity. When it's calm and dark, we can import power from countries with different generation mixes, like hydropower from Norway.

The UK has a growing network of these connections. The map below shows the operational and planned interconnectors linking Great Britain to its neighbours.

2. The Physical Operation: How They Work

You might wonder why we can't just use a standard underwater version of the overhead lines we see on land. The physical operation of these long-distance subsea links relies on specific and fascinating technology.

Why High-Voltage Direct Current (HVDC)?

The electricity grid in the UK (and in every country) operates on Alternating Current (AC). However, for transmitting power over long distances underwater, AC is very inefficient. Instead, interconnectors use High-Voltage Direct Current (HVDC).

To understand why DC is better for this job, let's watch a video that breaks down the technical differences.

To summarise the key point from the video: for long subsea cables, the energy losses associated with AC become too high to be practical. HVDC technology minimizes these losses, making it the only viable choice for connecting the UK to mainland Europe.

Because the main grid uses AC, this means that at each end of the interconnector, a special facility is needed to handle the conversion. This brings us to the two main physical components of an interconnector.

Component 1: The Converter Station

A converter station is a large industrial facility that takes high-voltage AC power from the grid and converts it to HVDC for transmission. At the other end of the link, a similar station converts the HVDC power back to AC to feed into the receiving country's grid.

The diagram below shows this process for the EGL2 interconnector project connecting Scotland and England. Although this is a domestic link, it uses the exact same technology as the international interconnectors.

Component 2: The Subsea Cable

The HVDC cable itself is a marvel of engineering, consisting of multiple layers designed to conduct electricity efficiently and withstand the harsh undersea environment.

Let's take a look inside a real interconnector project to see what these components look like in practice.

The key takeaways from the video are:

• The cable has a central copper or aluminium core, wrapped in layers of insulation, waterproofing, and steel armour for protection.
• The converter station functions like a giant, powerful version of the adapter you use to charge your phone, changing the type of electricity to suit the device (or in this case, the transmission cable).

3. Ancillary Services: A Hidden Role

Besides trading bulk energy, modern HVDC interconnectors have another important operational role: they can help keep the grid stable.

To properly understand this, let's briefly look at the technology choices for HVDC.

The key takeaway is that the newer VSC technology is more advanced. VSC-based interconnectors are like a Swiss Army knife for grid operators. Because their power electronics are so fast and controllable, they can provide a range of "ancillary services" that traditionally came from power stations. These include:

• Fast Frequency Response: Rapidly adjusting power flow to help manage the generation-demand balance and keep the grid's frequency stable at 50Hz.
• Voltage Control: Injecting or absorbing reactive power (which we covered in Module 1) to help maintain stable voltage levels on the local grid.
• Black Start Capability: Some interconnectors can even provide power to a dead section of the grid to help restart it after a blackout.

These services are becoming increasingly important as we transition away from traditional power plants, making modern interconnectors a critical piece of operational infrastructure, not just a tool for trade.

Conclusion

Today we've seen that interconnectors are far more than just extension cords. They are sophisticated HVDC systems that play a vital and expanding role in the UK electricity system.

Key Takeaways:

• Interconnectors are high-voltage cables (mostly subsea) that connect the UK grid to neighbouring countries, allowing for the two-way trade of electricity.
• Their primary roles are to enhance security of supply, lower costs through energy trading, and help integrate variable renewables like wind and solar.
• They use HVDC technology for long-distance subsea transmission because it is much more efficient than standard AC.
• The physical operation relies on two key components: converter stations to change power from AC to DC and back again, and the multi-layered subsea cable.
• Modern interconnectors using VSC technology can also provide crucial ancillary services to help manage grid stability.

Preview of the next lesson:
Now that we understand the physical infrastructure that allows electricity to flow between countries, the next logical question is: how is that flow decided and paid for? In the next lesson, we will look at how cross-border electricity flows are scheduled via market coupling and settled within the GB market.