Module 4 — Technologies in the electricity system
Lesson 6 of 9
Interconnectors
Learning objectives
By the end of this lesson you should be able to:
- Understand what electricity interconnectors are.
- Explain how interconnectors transfer electricity between regions and countries.
- Recognise the benefits interconnectors provide to electricity systems.
- Understand the limitations and risks associated with interconnection.
- Appreciate how interconnectors contribute to reliability, flexibility and market efficiency.
- Recognise why the value of an interconnector depends on when and where it transfers electricity, not simply how much energy it carries.
Introduction
Electricity systems have traditionally been thought of as national infrastructure.
In reality, many modern electricity systems are increasingly interconnected across regions and even across countries.
High-voltage cables now connect electricity networks beneath seas, across mountains and between neighbouring nations.
These connections, known as interconnectors, allow electricity to flow from one electricity system to another.
Interconnectors increase the resources available to system operators, improve reliability and often reduce the overall cost of supplying electricity.
Rather than each country relying solely on its own generation, neighbouring systems can increasingly support one another.
What is an interconnector?
An interconnector is a transmission line that connects two separate electricity systems.
It allows electricity to flow in either direction depending upon system conditions.
For example:
- Country A may export electricity to Country B during one hour.
- Later the same day, electricity may flow in the opposite direction.
Modern interconnectors therefore allow electricity systems to share generation resources rather than operating entirely independently.
Why interconnectors are useful
Electricity demand is constantly changing.
Weather conditions also vary across different geographical regions.
Suppose:
- One country experiences strong winds.
- A neighbouring country experiences very little wind.
Without an interconnector:
The surplus renewable generation cannot be shared.
With an interconnector:
Electricity can flow to where it is needed.
Both systems benefit.
This ability to share resources is one of the greatest advantages of interconnected electricity systems.
High Voltage Direct Current (HVDC)
Many modern interconnectors use High Voltage Direct Current (HVDC) technology.
Unlike conventional alternating current (AC) transmission, HVDC offers several advantages for long-distance transmission.
These include:
- Lower transmission losses over very long distances.
- Easier control of power flows.
- Ability to connect asynchronous electricity systems.
- Submarine cable operation.
At each end of the interconnector, large power electronic converters convert alternating current into direct current and back again.
This allows two independent AC systems to exchange electricity without requiring their frequencies to remain synchronised.
Sharing generation resources
Interconnectors effectively enlarge the pool of available generation.
Imagine two neighbouring countries.
Country A experiences peak demand during the evening.
Country B experiences peak demand slightly later.
Because their demand patterns differ, each country may be able to rely partly upon the other's spare generation capacity.
This can reduce the amount of generation that each country needs to build individually.
Supporting renewable generation
Interconnectors become particularly valuable when renewable generation is widespread.
For example:
A windy day in Scotland may coincide with relatively calm conditions elsewhere.
Solar generation may peak earlier in southern Europe than in northern Europe.
By transferring electricity between regions, interconnectors allow renewable resources to be shared over much larger geographical areas.
This reduces the variability experienced by individual electricity systems.
Improving reliability
Interconnectors improve reliability in several ways.
They can:
- Provide additional generation during shortages.
- Allow excess electricity to be exported.
- Support recovery following disturbances.
- Reduce renewable curtailment.
- Increase competition between generators.
In effect, an interconnector provides access not only to another country's electricity but also to its generation portfolio.
An example
Imagine two countries.
Country A has:
- High electricity demand.
- Very little wind generation today.
Country B has:
- Low demand.
- Strong offshore winds.
If no interconnector exists:
Country A may need to start expensive gas generators.
Some wind generation in Country B may even be curtailed.
With an interconnector:
Country B exports surplus renewable electricity.
Country A imports that electricity instead of using more expensive generation.
Both countries benefit.
Operational flexibility
Interconnectors also increase flexibility.
Power transfers can often be adjusted rapidly in response to changing system conditions.
This makes interconnectors valuable for:
- Balancing supply and demand.
- Managing renewable variability.
- Responding to unexpected generator outages.
- Reducing price volatility.
Although they are not generators themselves, interconnectors often behave like highly flexible system resources.
Congestion
Interconnectors are transmission assets.
Like every transmission line, they have finite capacity.
During periods of heavy transfers, they may become congested.
When this happens:
- Not all desired electricity transfers can occur.
- Prices may differ between connected regions.
- Additional local generation may be required.
An interconnector therefore increases transfer capability but does not eliminate network constraints entirely.
Interdependence
Although interconnection improves reliability overall, it also creates interdependence.
Countries become increasingly connected.
This means disturbances in one electricity system may affect neighbouring systems.
Examples include:
- Generator failures.
- Major transmission outages.
- Frequency disturbances.
For this reason, electricity system operators coordinate closely across national borders.
Interconnection therefore requires cooperation as well as infrastructure.
Energy security
Interconnectors can improve energy security by providing access to a broader range of generation resources.
However, they should not be viewed as a substitute for maintaining an adequately resourced domestic electricity system.
Consider a winter evening affecting several neighbouring countries simultaneously.
Electricity demand is high everywhere.
Wind output is low across the region.
Under these conditions, every country may wish to import electricity at the same time.
Clearly, not everyone can be a net importer simultaneously.
Interconnectors allow countries to share available resources, but they do not create additional generation.
Ultimately, the total electricity available remains limited by the generation capacity across all interconnected systems.
Interconnectors as system resources
An interconnector should not be thought of simply as a cable.
It provides several valuable services.
These include:
- Energy transfers.
- Capacity sharing.
- Renewable integration.
- Operational flexibility.
- Competition between electricity markets.
- Emergency support during disturbances.
Like storage, the value of an interconnector depends upon how it is used.
Why timing matters
Imagine an interconnector capable of transferring:
1 GW.
If this transfer occurs:
- During a period of surplus renewable generation,
- It may prevent renewable curtailment.
If the same transfer occurs:
- During peak winter demand,
- It may prevent expensive peaking generators from operating.
The physical quantity of electricity transferred is identical.
Its value to the electricity system is very different.
Timing therefore matters just as much as energy volume.
Why location matters
The value of an interconnector also depends on where it connects.
An interconnector linking:
- Two regions with very similar generation patterns
may provide relatively modest benefits.
An interconnector linking:
- Regions with complementary renewable resources,
- Different demand patterns,
- Different generation technologies,
may provide substantially greater value.
The location of the connection is therefore as important as its capacity.
Why £/MWh is not enough
Suppose two interconnectors each transfer exactly:
10 TWh per year.
One:
- Relieves severe network congestion.
- Supports renewable integration.
- Provides emergency imports during shortages.
- Connects complementary electricity systems.
The other:
- Operates mostly during periods of low demand.
- Provides relatively little operational flexibility.
Although both transfer identical amounts of energy, their contributions to the electricity system differ significantly.
The value of an interconnector therefore depends on:
- When electricity is transferred.
- Where electricity is transferred.
- Which system services it enables.
- How it contributes to reliability.
Not simply on the annual volume of electricity exchanged.
A key insight
Interconnectors do not generate electricity.
Instead, they increase geographical flexibility.
They allow electricity produced in one place to be used somewhere else.
Together with storage—which shifts electricity through time—interconnectors help transform electricity systems from isolated national networks into highly connected regional energy systems.
Both increase flexibility.
One operates across time.
The other operates across space.
Key takeaways
- Interconnectors connect separate electricity systems and allow electricity to flow between them.
- Most modern long-distance interconnectors use High Voltage Direct Current (HVDC) technology.
- Interconnectors improve reliability by allowing neighbouring systems to share generation resources.
- They support renewable integration by transferring electricity from regions with surplus generation to regions with higher demand.
- Interconnectors increase flexibility but do not create additional electricity.
- Their benefits depend upon the availability of generation across the wider interconnected system.
- Congestion and simultaneous shortages can limit the value of interconnection.
- The value of an interconnector depends on when, where and why electricity is transferred, not simply on the amount of energy exchanged.
Looking ahead
Interconnectors increase flexibility by moving electricity across geography.
The next lesson examines demand response and flexible consumption, which increase flexibility from the opposite direction by allowing electricity demand itself to adapt to changing system conditions.