Recent auction success for photovoltaics (PV) in the UK and Ireland will deliver a growing industry; however, this will not be without its challenges. A breakthrough transmission-connected solar project marks a new stage for UK renewables development. But for the sector to truly thrive, understanding the complexities and challenges of grid integration and compliance will be essential.

By Grant McCormick, senior power systems consultant, PSC Consulting, and Chris Smith, technical director HVDC, PSC Consulting.

The last few months have seen exciting news for solar farm developments in the UK and Ireland. In September 2023 Ofgem, the UK energy regulator, awarded Contracts for Difference (CfD), totalling nearly 2GW to solar farms under its Round 5 allocation. That represents a substantial majority of the 3.7GW of CfDs awarded in this latest round. More recently, in October of 2023, EirGrid’s Renewable Electricity Support Scheme (RESS) Round 3 awarded support to nearly 500MW of solar farms.

Looking in more detail at the UK auctions, a total of 56 Solar projects have been awarded support with a strike price of £47/MWh. Trade group Solar Energy UK received these awards warmly as they secured more contract volume than was expected.

In the Irish auction, a total of 20 projects were awarded support with an average strike price, across all technologies, of €100.47/MWh. The Irish Solar Energy Association has indicated that whilst positive about the amount of solar awarded contracts, they believe that opportunities have been missed.

Until now, connecting utility-scale solar projects to the distribution grid at the lower voltages found in those networks has been typical. However, two factors are driving the emergence of transmission-connected solar. Firstly, as the installed capacity of projects has increased, there has also been a steady progression in the voltage levels associated. This trend towards larger capacity projects is broadly anticipated to increase to meet the UK’s net zero commitments.

Secondly, as distribution networks are becoming steadily more saturated with solar capacity, projects are looking to be connected directly to the high-voltage transmission system so that the power can be more easily exported to more distant load centres. To date, 38 projects listed in the Great Britain Transmission Entry Capacity Register are detailed as photovoltaic or photovoltaic combined with energy storage.

The total potential GW installation is a huge 6.8GW over the next ten years or so. And, for projects that are listed as PV with onshore wind, the numbers increase by an order of magnitude. It’s clear that not all of this generation will be constructed for various reasons, however, the move towards transmission-connected PV in Great Britain is a clear trend.

In Ireland, 349MW of utility-scale solar projects (>5MW) are connected to the transmission system, including some very large projects. For example, the Ballymacarney Solar Project at 200MWp will connect to EirGrid’s 110kV system.

Transmission grid-connected solar projects mark ‘new era’

The transmission grid-connected solar project is, in fact, already a reality. The UK’s first transmission grid-connected solar farm has begun commercial operations, marking a new era of renewable energy development and establishing this as an emerging trend. At nearly 50MW, the solar farm, which is owned and operated by Cero Generation and Enso Energy, is the first in the country to feed electricity directly into the high-voltage transmission network.

The Larks Green solar farm connects to the transmission system at the 132kV Iron Acton substation located near Bristol. The connection will allow the 152,400 PV module solar farm to supply some 73GWh to the national transmission system annually. It is also set to be co-located with a 49.5MW and 99MWh battery energy storage system, which adds an extra dimension to the project’s flexibility of operation and, hence, value to consumers.

The connection of this first solar farm to the transmission system has highlighted some interesting learnings for the solar industry. One of these learnings concerns the Grid Code compliance requirements when connecting to the UK transmission system.

The Grid Code provides a publicly available set of requirements for connecting to the system and comprises of three main sections: The European Connection Conditions, the Planning Code, and the European Compliance Process. There is additional complexity in these codes as different technologies and the size of the connection needed, which depends on the size of the project, may have slightly different performance requirements.

The Grid Code is under regular revision, requiring the connectee to be agile and understand which changes are relevant. For example, the Grid Code modification GC141 has provided a greater focus on ongoing compliance during operation and modelling requirements. Ensuring that the solar farm is compliant is a major requirement in obtaining the Operational Notifications required from the System Operator, which allows for the export of electricity.

The compliance process required can provide challenges throughout the design, construction and commissioning phases for developers connecting to the transmission system for the first time. Close collaboration between the system operator, developer and equipment supplier is required to ensure compliance doesn’t hinder timely operation.

Key learnings to ensure projects are delivered efficiently

There are a number of key learnings that need to be brought to the PV industry to ensure that projects are delivered in as efficient a manner as is practicable. Firstly, as a developer, it is important to bake the compliance requirements into the procurement processes.

The differences between connecting assets to the distribution system and transmission system mean it is new to the solar supply chain. It requires effort to successfully deliver all the requirements to satisfy the compliance process. Several companies throughout the supply chain will be required to provide detailed information over and above what would be considered normal for a solar installation. To avoid potential delays and cost risk, requirements and expectations must be made clear during the development phase.

Secondly, a compliance manager with experience in Grid Code compliance in the operating territory should be appointed. Each system operator will have their own performance requirements, and familiarity with this complex regulatory framework is crucial. A glance at the GB Codes will quickly reveal they are dense and highly technical documents, not light bedtime reading.

Ideally, the compliance manager would be employed during the development phase to ensure the specification covers all the compliance process needs. The compliance manager should be engaged in the project development process right through to the final handover. At that point, the compliance manager can deliver the final operational notifications needed to move the project into commercial operation.

Thirdly, developers must ensure that the Engineering, Procurement, and Construction (EPC) contractor has the skill sets and is sufficiently resourced to create and supply all the information required as part of this process.

The entire grid compliance process starts during the development phase. Agility is required during the EPC phase, especially since the grid code constantly evolves. Under this scenario, the grid code requirements in place when the grid connection agreement is signed may not necessarily be the requirements that are in place when the project moves into commercial operation.

Fourthly, developers should engage with key elements of the supply chain. For example, the supplier of the inverter and power park controller may be required to provide detailed simulation models in a specific software application – in the UK system, these applications are DigSilent and PSCAD, for example.

They will also be required to provide evidence that the model is validated via factory acceptance tests or third-party testing. This is not a trivial issue as this will generally be a new requirement for suppliers, which may supply to different territories with different performance requirements in their grid code.

Finally, more power system simulation modelling is needed. The components of the solar farm – the inverter, power park controller, transformer, and cabling, for instance – need to be modelled as a system against the performance requirements in the respective grid codes. In the UK system, whilst compliance is via self-certification, the Electricity System Operator (ESO) must agree that enough evidence has been provided for that certification. There may even be requirements to revalidate models against any compliance testing undertaken at the site.

It’s clear that connecting to the transmission system for the first time brings new challenges with compliance. For timely project delivery, developers must ensure compliance management is established as a key workstream starting from the earliest development phases and carrying on through to project delivery. Working with the right partner in the field can minimise costly delays to the start of operations.

It’s also important to note that the UK’s Grid Code operates on an open governance model. Any changes are generated by workgroups comprising key stakeholders. These stakeholders include the System Operator, project developers, asset owners, equipment suppliers and more. Therefore, the solar industry must engage with these efforts to shape the Grid Code of the future in which solar will be key.