A study of the GB transmission constraint envelope tells one story: every megawatt of new wind capacity north of the central belt costs the system in re-dispatch payments until a new boundary transfer is built. The Holistic Network Design answered with 21 GW of HVDC bootstrap capacity, the bulk of it subsea. The procurement implications for the converter-transformer industry are the substance of this article.
The UK's grid is being fundamentally re-plumbed. As the remaining Scottish coal and nuclear plants fade out, they are being replaced by a colossal build-out of offshore and onshore wind. The problem? The bulk of this clean generation is in Scotland, while the bulk of the demand remains stubbornly in the south of England. The existing 400 kV AC backbone, already creaking, is simply not fit to transport this much power over such distances. The solution is a trio of massive subsea high-voltage direct-current (HVDC) projects: the Eastern Green Links (EGL) 1, 2, and the newly scoped EGL3 and EGL4, which will collectively ferry more than 8 GW of power southwards, complementing the existing 2.2 GW Western HVDC Link. This is the biggest grid investment in generations. But while these "bootstraps" solve the bulk power transfer issue, they introduce subtle but profound changes to the physics of the grid itself.
The Physics: Why AC Just Won't Cut It
For a century, the alternating current (AC) system has been the undisputed king of transmission. Its primary advantage is the ease with which voltages can be stepped up and down using simple, robust transformers. However, AC transmission over very long distances runs into physical limits that even the beefiest conductors cannot overcome. The core issues are reactance and capacitance.
Every overhead line or cable has an inherent series inductance and shunt capacitance. Over hundreds of kilometres, the reactive power losses (I²X) become enormous. To keep the voltage stable at the receiving end, you need to install huge, expensive reactive compensation devices like STATCOMs or synchronous condensers. At a certain distance, known as the surge impedance loading limit, the line is consuming so much reactive power just to stay energised that it has little capacity left to transmit useful active power. For a 400 kV line, this practical limit is around 250-300 miles. The 600+ km journey from Aberdeenshire to Yorkshire is simply too far to be practical for new AC overhead lines, never mind the impossibility of gaining planning permission for them.
Subsea or underground AC cables are even more constrained. The proximity of the conductors in a buried cable creates very high capacitance. This capacitance generates a massive amount of reactive power. For a long 400 kV AC cable, this "capacitive charging current" can be so large that it uses up the entire thermal capacity of the cable after just 50-60 miles. You cannot simply build a 600 km AC cable; it would be physically impossible to operate.
This is where HVDC shines. By converting the power from AC to DC, you eliminate reactance and capacitance as system-wide issues. The power flow becomes a simple function of the voltage difference at either end, controlled with precision by the converter stations. Losses are dramatically lower, consisting mainly of I²R resistive losses in the conductors and conversion losses at the terminals. A modern voltage-source converter (VSC) HVDC system can transmit power over 1,000 km with losses of less than 3%—a feat impossible with AC. It’s the enabling technology for the Scottish bootstraps.
The Components: Inside an MMC Converter
The heart of the new Eastern Green Links is the Modular Multilevel Converter (MMC), a type of VSC. Unlike older Line-Commutated Converter (LCC) technology which relied on thyristors, MMCs use insulated-gate bipolar transistors (IGBTs) which can be switched on and off independently. This gives the operator exquisite control over the power flow.
An MMC station is a city of power electronics. Each "valve" is composed of hundreds of sub-modules stacked together. Each sub-module contains IGBTs, capacitors, and control electronics. By switching different combinations of these sub-modules in and out of the circuit in rapid succession, the converter can create a near-perfect AC sine wave on one side, while maintaining a steady DC voltage on the other. The key advantages are:
- Black Start Capability: MMCs can create their own AC voltage waveform without needing a live grid, a vital function for restoring power after a blackout.
- Reactive Power Control: They can independently control active (MW) and reactive (MVAr) power, acting like a giant, ultra-fast STATCOM to support grid voltage.
- Fault Ride Through (FRT): Modern MMCs have sophisticated controls to ride through faults on the adjoining AC network, a critical requirement under ENA EREC G99.
But the unsung hero of these stations is the converter transformer. These are not your standard distribution units. The transformers connecting the 525 kV DC side to the 400 kV AC grid are some of the most complex power system components ever built. Each one must handle immense electrical, thermal, and mechanical stresses. They are specified under demanding standards like IEC 60076 and IEEE C57.12, but the custom requirements for an MMC application go further. The valve-side windings, for instance, see a bizarre mix of AC and DC voltage stresses, plus extremely high-frequency switching harmonics from the IGBTs. This requires specialised insulation design and electrostatic shielding to prevent premature failure. Procuring one of these units is a multi-year, multi-million-pound endeavour involving a handful of specialist global manufacturers. Getting the specification right is a discipline unto itself. You can learn more about our work on large power transformers on our products page.
The System: Weaving DC Links into an AC Grid
Operating a hybrid AC-DC grid is a new frontier for system operators. While the HVDC links solve the bulk transfer problem, they introduce two significant challenges: a reduction in system inertia and a drop in fault levels.
System Inertia: Traditional power grids are dominated by large, spinning synchronous generators in thermal and nuclear power plants. The immense rotating mass of these machines (turbines and generator rotors) stores kinetic energy. When a fault occurs or a large generator trips, this stored energy is instantly released, slowing the rate of change of frequency (RoCoF) and giving the system operator precious seconds to deploy reserves. Wind turbines and solar PV, being connected via power electronics, have zero inherent inertia. The new HVDC links are the same—they are asynchronous, decoupling the Scottish wind farms from the English grid. Every gigawatt of wind power connected via HVDC displaces a gigawatt of synchronous generation, reducing the grid's natural resilience to disturbances. Too little inertia, and a single large plant trip could cause the frequency to collapse so quickly that protection systems disconnect loads, leading to cascading blackouts.
Fault Current: This might seem counter-intuitive, but a strong grid needs a high "fault level" or "short-circuit current." When a fault occurs, a large amount of current rushes to the fault location. This high current is what allows protection relays to detect, locate, and isolate the fault quickly. Synchronous generators are the primary source of this fault current. Power electronic converters, including MMCs, are current-limiting by design. They simply cannot provide the massive surge of current that a traditional generator can. As more and more generation becomes converter-based, the overall fault level of the grid decreases. Below a certain point, protection systems may fail to operate correctly, a condition that deeply worries protection engineers. It’s like trying to find a needle in a haystack if the needle is made of hay.
National Grid ESO is tackling these issues through several initiatives:
1. Inertia Pathfinder Projects: Procuring inertia as a service from synchronous condensers or even specially controlled wind turbines.
2. Short Circuit Level Pathfinder: Contracting with providers who can guarantee fault current injection when required.
3. Grid Forming Converters: Developing new control strategies (known as "Grid Forming" or "Virtual Synchronous Machine" mode) for MMCs and wind turbines that allow them to mimic the behaviour of traditional generators, providing both inertia and fault current. The Eastern Green Links are expected to be among the first to deploy this at scale.
This is a paradigm shift. For decades, grid services like inertia and fault level were simply a natural by-product of large power stations. Now, they are becoming explicit products that must be defined, procured, and managed. For specialists in grid studies and power system planning, it's an incredibly busy time. If you need help modelling these complex interactions, our team is always available via our contact page.
The Grid: A Tale of Two Systems
The long-term vision is a Great Britain grid that is really two systems in one: a heavily DC-interconnected backbone for bulk power transfer, overlaid with regional AC distribution networks. The Eastern and Western HVDC links are the first phase of this. But the future holds the potential for a multi-terminal DC (MTDC) grid, where multiple HVDC links are interconnected offshore, creating a true subsea transmission network.
This "meshed" offshore grid would allow power to be rerouted with incredible flexibility. For instance, if demand in the south-east is low but a new HVDC interconnector to Norway is importing cheap hydro power, that power could be routed directly from the North Sea to the Midlands via the offshore grid, without ever touching the onshore AC network. This reduces losses, improves reliability, and increases market efficiency. Projects like EGL3 and EGL4 are being designed with future MTDC connection points in mind.
However, the engineering challenge is immense. DC circuit breakers, a key enabling technology for MTDC grids, are only just moving from prototype to commercial reality. Co-ordinating the control and protection of a grid with multiple VSC converters from different vendors is a task of mind-boggling complexity, far exceeding that of the AC grid which has had a century to mature. Standards are still being written. But the direction of travel is clear. The future of UK transmission is offshore, and it is direct current.
Key Takeaways
- Long-distance AC transmission is physically limited by reactive power losses, making HVDC essential for linking Scottish renewables to English demand centres.
- While solving the bulk transfer problem, the shift to power electronics (wind turbines, HVDC) reduces critical grid properties like system inertia and fault current.
- The future UK grid will be a hybrid system, with a robust DC backbone for bulk transfers and regional AC networks, requiring new services and control philosophies to maintain stability.
The Engineer's Takeaway
For a generation, the core challenge of transmission planning was thermal: can the wires carry the load? Now, the game has changed. The new frontiers are the dynamic, system-level problems of inertia, fault levels, and harmonic stability introduced by a world of power electronics. We solved the simple problem of bulk power transfer only to reveal a far more complex and interesting set of challenges beneath.
