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Is connecting at transmission level the solution to grid delays? 

13 March 2024

The UK now has three solar farms hooked up to the high-voltage transmission grid rather than the lower-voltage distribution grid. It’s an important milestone.  

The first was Cero Generation and Enso Energy’s Larks Green project in 2022, managed by RES and co-located with a 49.5MW/99MWh battery energy storage system (BESS) – connected at 132,000 volts to National Grid’s Iron Acton substation near Bristol, rather than the more commonly encountered 11-33kV.  

Later that year, the 49.9MW Sutton Bridge project followed. Delivered by Anesco, it was EDF’s first major solar project in the UK. The third, Burwell solar farm, was built by Agr Renewables, again for EDF, and commissioned last year.  

But why are there so few, compared to the hundreds of sites up and down the county connected to the distribution networks? 

Firstly, it’s simply a question of what and where the networks are. The lower-voltage distribution networks are necessarily very dense, so they are much more likely to be closer to a potential development site. In contrast, the transmission grid could be dozens of kilometres away, so connecting to it is much less likely to be feasible.  

Secondly, the transmission grid’s function is to carry large amounts of energy over long distances, from centralised power plants to where it is consumed. Solar and BESS sites may not supply enough to justify connecting to it. Doing so also involves “stringent regulatory, planning, and approval processes.  

The need for more significant infrastructure investment from a technical side, coupled with potential environmental and community impact, makes transmission connections more complex and time-consuming,” said Conor Murphy, Principal Engineer at NovoGrid. Such connections also require specialised transformers, switchgear, and protection systems, all operating to enhanced safety standards, pushing up costs further. So, greater levels of expertise are needed to design and install them, which again comes at a premium as there is a limited pool of contractors able to provide such services.  

Transmission Network Use of System (TNUos) charges are another part of the mix. They are higher in areas with excess generation capacity due to the costs of transporting electricity over long distances to where it is consumed, noted Murphy.  

So why connect to the transmission grid at all?  

Where it is practicable, there are clear advantages. First and foremost is the lesser congestion on the transmission network, which means the long waiting times that beset the distribution networks may be avoided. This is especially true when a project can bypass a bottlenecked transformer at a grid supply point. It’s not a guarantee of preventing all reinforcement works, which in some cases may be needed on the high-voltage grid, though it does make it more likely.  

Another advantage is plain for anyone who can remember doing A-level physics. Carrying the same amount of energy at a higher voltage means a lower current, which means losing less energy from Ohmic heating.  

“For NSIP-scale projects, a transmission grid connection is a logical choice due to their scale, especially where the projects overlap with transmission grid nodes requiring injection. Connecting to the transmission grid allows these projects to efficiently distribute electricity across wide areas, supporting national energy needs and contributing to grid stability,” said Murphy.