To combat climate change, the UK needs clean energy. The UK is very well positioned to generate clean electricity because our coasts provide a large potential for offshore wind. The UK currently has an installed offshore wind capacity of 12GW and is targeting increasing the total capacity to 40GW by 2030. Given the scale of the developments proposed and their increasing distance from the onshore grid, the most efficient option is to connect these to the network using Direct Current (DC) cables. The electricity used by the consumer is alternating current (AC) and there is a need to convert the DC to AC at a convertor station, usually positioned on the coast and connected point-to-point to the wind farm via an offshore cable. The current method of connection is to connect each wind farm to an AC convertor station with an AC circuit breaker to protect the electricity grid from faults. However, as the number of wind farms increases, so the number of AC convertor stations increases in a point-to-point system. This has impacts on coastal communities through ever increasing number of convertor stations and cables. It is also costly to install and maintain many convertor stations, which will increase the cost of electricity to consumers.
The big idea is to create DC networks that can connect multiple wind farms into a DC substation, that then can connect to fewer convertor stations. This will reduce the impact on coastal communities, reduce costs and has the potential to deliver lower cost wind energy to consumers. It will also help us open new areas for developing windfarms. To do this we need to use DC circuit breakers (DCCB), which are an innovative technology that is untested in the UK and European market. DCCB will allow us to bring multiple windfarms into a DC system, containing the impact of any single failure safely and securely. We will need to develop and test these DCCBs before we can develop a DC network. This project will test and prove the use of DC breakers so that we can implement our big idea of DC networks that can deliver safe, reliable, and cost-effective energy to the consumer.
Problem Bring Solved
The problem that the Network DC Project is trying to solve is how to reduce the high cost of delivering tomorrows DC networks, whilst improving network resilience. It will do so by enabling integrated DC meshed networks of offshore wind farms that can be more efficiently connected to the onshore transmission network with a reduced asset base. A key component, and the ultimate deliverable of this project, will be the ability to install DCCBs, which can be used to isolate sections of network in the event of a fault and maintain network resilience.
The UK currently has an installed offshore wind capacity of 12GW and is targeting increasing that capacity to 40GW by 2030. Given the scale of the developments proposed and their increasing distance from the onshore grid, the most efficient option to connect these to the network is via a DC network.
Offshore wind farms traditionally have a point-to-point (PtP) connection via a DC cable with the onshore AC network. This has been successful to date as it is operationally straightforward to isolate the DC circuit at the onshore AC connection point using a proven low-risk AC circuit breaker (ACCB). The drawback of this network design is it results in stand-alone assets connected directly to the transmission grid PtP, increasing the total number of required AC convertor stations. The alternative is to connect multiple dispersed wind farms to the grid in a meshed network to a single AC convertor station.
An integrated meshed DC network has the advantages of:
- Reduced onshore infrastructure:
- Potential for a smaller footprint
- Reduced operational expenditure
- Reduced environmental impact on coastal communities
- The ability to expand offshore networks quicker and easier
However, in a DC network, where numerous windfarms are connected to the AC network using a single AC convertor station with an ACCB, there is the risk that if a fault develops in the network, multiple windfarms will be disconnected at the ACCB. This problem could be solved using DCCBs within the DC network that would simplify fault isolation and increase network reliability.
However, DCCBs are at a technology readiness level (TRL) of 5 and would present an unquantified risk to network stability without further testing and assessment. This project aims to raise the TRL of DCCBs to TRL 7 by the end of Beta phase, with a DCCB system prototype demonstration and implementation engineering in an operational environment.
Impacts and benefits
In all three use cases discussed in Section 1, we have compared ourselves with the performance, cost, and time-to-connect of a conventional Point to Point (PtP) solution proposed by the GB System Operator and transmission operators. The conventional solution would make best efforts to address not only a single projects' need but also subsequent projects foreseen in the same area, without DC Circuit Breakers.
The early Cost-Benefit Analysis (CBA) carried out during Discovery Phase indicates a positive benefit for GB customers compared to current market practices of PtP HVDC links for off-shore wind-farm connections. All three uses show relative improvements in NPV due to the avoided ancillary services costs enabled by the fast recovery enabled by DCCBs. In one use case, this is paired with additional benefits from reduced capital expenditure. Appendix 1 sets out the potential benefits of a HVDC breaker relevant to Ofgem goals
The key outputs from this analysis were the total expenditure and the NPV of expenditures and benefits in each of the use cases and the counterfactual. This enables us to compare the relative NPV of each use case compared to its counterfactual. Given that the DCCB Hub use case appears to be preferable to its counterfactual in terms of both total expenditure and NPV, it is recommended that this is considered the primary option for further study as it is likely present the most beneficial and robust use of DCCBs for a first implementation. The estimated use case benefit is estimated to be ~10 billion for the DCCB Hub
Although further work is required to develop specific alternative measurements of the avoided cost of increased needs for ancillary services, thus reducing uncertainty around the monetary value of this benefit, it appears that even if 10% of our modeled benefit can be realized, this will still support the use of DCCBs to reduce the overall cost to the consumer of developing an offshore HVDC network to connect the required capacity of off-shore wind for achieving Net Zero 2050.
In addition, early indications from the market suggest that the technology being proposed is deliverable and there is appetite in the market for refining solutions to suit the UK network’s specific requirements. Stimulating this supply chain and providing evidence for the benefits of the DCCBs in the UK network will be key outcomes of the Alpha and Beta phase.
