Project Summary

Meeting SIF Innovation Challenge aims 

The whole system innovation challenge requires the coordination of design, to reduce the duplication and complexity of networks. This will help deliver an integrated system capable of providing net-zero electricity generation. Network-DC will support the coordination of offshore and onshore networks by connecting multiple wind farms into higher-capacity DC substations. 

How the energy network innovation evolved 

Offshore wind developers rely on the GB transmission system to reach energy markets. Without the careful design of the system, there can be an impact on the ability to utilise all the power generated by wind farms and system reliability can be compromised. Projects seek to make landfall close to offshore wind areas and establish substations to access the GB electricity network. This situation will create network congestion and environmental and social impacts on nearby coastal areas. 

One solution is adopting HVDC technology to export power to shore and move power from remote locations, over large distances, to nodes of energy demand. To minimise landfalls and onshore substations, the recent Holistic Network Design (HND) created by the NGESO recommends the use of a Direct Current Switching Station (DCSS) to act as a terminal for gathering and distributing DC power. This requirement presents some limitations: while more wind farms and interconnectors could be added to the same DCSS, the connections at the DCSS become a single point of failure and could increase the total amount of potential power infeed losses. 

By adding DC Circuit Breakers (DCCBs) to the DCSS it would be possible to connect more offshore wind at a DCSS without increasing the risk of infeed losses. Therefore, combining a DCCB with a DCSS could reduce point-to-point connections and prevent new infrastructure from being built. While the installation at a DCSS is not the only use case for DCCBs, it is the most tangible use case available and makes a good case for bringing DCCBs forward as a viable option for network design.

Alpha Phase solution 

In Alpha Phase, we developed the use case of a DCSS constructed with two modifications: a) provisioning space for DCCBs and b) including switches allowing connection of DCCBs. 

The Alpha Phase highlighted the best arrangement and number of DCCBs optimising electrical circuit selectivity: should a major fault occur, the fault can be isolated immediately by the DCCBs, preventing a shock to the system. As a result, the network users are unaffected and wind farms can continue operating while repairs are made.

Once proven, DCSSs with DCCBs could operate up to an increased loss of infeed limit agreed with National Grid ESO, potentially allowing exploitation of unused capacity in onshore connection points and simplifying and accelerating new customer connections, as well as reducing grid balancing service requirements. 

Into Beta Phase

In the Alpha Phase, responses from Original Equipment Manufacturers (OEMs) and NGESO have emphasised uncertainty. While the OEMs agree that the task is achievable with appropriate support, they highlight a contingency problem: a market cannot develop without manufactured and specified devices, but a manufacturer cannot invest in DCCBs without the certainty of market requirements. 

Our Beta Phase project will address confidence issues by demonstrating the performance of DCCBs through detailed testing of a DCCB Replica as part of a UK network model. 

Innovation Justification

State of the art

The current strategy for accommodating increased offshore power generation is to construct more high voltage Direct Current Switching Stations (DCSS) reliant on conventional AC-side circuit breakers and conventional High Voltage Direct Current (HVDC) technology. Direct Current Circuit Breakers (DCCBs) increase the flexibility of these offshore or onshore HVDC networks. So far, DCCBs have been implemented only in China and are the cutting edge of HVDC transmission technology. 

This Project will pave the way for a new market, using a commercially risky technology that may have significant benefits but is unlikely to come into use through normal competitive markets without significant support from SIF funding. 

Innovation statement

Introducing DCCBs into the GB market is an innovative approach to improve system security and connect more energy generation with a more efficient network design. Alternatives to DCCB use have been considered: (1) building DCSSs at additional coastal locations, or (2) offsetting reliability issues stemming from increased use of existing DCSSs by expanded ancillary service provision. We have found that these options are costly, incremental, and uncertain in the case of ancillary services. The cost-benefit analysis highlighted that DCCBs are more cost-effective, except under the most conservative and restrictive assumptions. DCCBs allow better use of existing infrastructure and stimulate a new market.

DCCB technology is novel in the UK. Despite DCCB use in China, the Alpha Phase project confirmed that DCCB deployment is not a simple "lift and shift" exercise.

This Project will spur innovation by increasing confidence in the technical, commercial, and regulatory aspects of DCCB use for manufacturers, the operator, and owners. The Beta Phase will increase confidence in the following areas: 

Confidence to specify 

NGESO emphasises that its existing Holistic Network Design could be re-visited and use DCCBs, delivering flexibility and reduced substation construction. However, ESO lacks sufficient evidence to classify DCCBs as proven and is reluctant to include DCCBs in the long-term network plan. 

Confidence to customers 

The Carbon Trust's stakeholder engagement highlighted that offshore wind developers are unaware of the advantages of DCCB. Exploring the commercial and regulatory standards will increase confidence to specify and fund DCCBs. 

Confidence to design 

Modelling, simulation, and technical review carried out between The National HVDC Centre, SuperGrid Institute, and the University of Edinburgh highlighted that before introducing DCCBs to offshore GB networks, there is a need to address a wide range of performance and integration challenges. The experience of installing DCCBs in China is based on overhead DC links. The use case of subsea cables is more applicable to GB due to the increase in demand and the need to connect more offshore wind. Subsea connections bring different challenges, and therefore we cannot directly translate the experience in China to the UK market. 

Confidence to own and operate 

Whilst the Alpha Phase has demonstrated positive benefits, the levels of reliability and suitable network designs are not yet agreed by the Transmission Owners' (TOs') engineering teams. 

IRL and CRL 

We currently assess DCCB Integration Readiness Level (IRL) as 3, and Commercial Readiness Level (CRL) as 5. Beta Phase intends to raise IRL to 6 (Integration of Technologies Verified and Validated) and CRL to 7 (Financial Model Validation).

Technological maturity levels differ significantly among manufacturers from China, Japan, and Europe. Although China has made more progress regarding technology readiness, the integration and commercial implementation of the technology in GB's de-verticalised and privatised energy system is lagging. The OEM responses to a set of questions posed by SSEN-T illustrate disparities and areas lacking confidence. The five OEMs contacted are interested in participating but had varying reactions to the project scope, timelines, and costs.

This wide variation in responses and readiness and the relative lack of confidence implies that the use of DCCBs is unlikely to happen in GB without SIF funding. The novel use of DCCBs as an effective solution to grid design requires funding and support.

Project scale

The technical barriers and risks can be reduced by demonstrating the control and protection equipment in a system replica at the HVDC Centre. This approach does not require the construction of a full-scale DCCB (including its high-voltage equipment). Testing a replica at the HVDC centre avoids disrupting energy transmission or creating the risk of system failure. This approach enables physical testing that will give more confidence than continued simulation and represent a cost-efficient and lower-risk approach to demonstrating system reliability.

Market creation

This Project can stimulate a new competitive supplier market for DCCBs in GB and Europe. The budget has been set to allow several manufacturers to participate in the Beta Phase Project. We will complete the contracting strategy at the start of the Beta Phase, leading to the selection of preferred suppliers.

Benefits

The benefit of implementing Direct Current Circuit Breakers (DCCB) is that more offshore wind can be connected at lower costs and with a reduced environmental impact. This approach addresses the Government's net-zero targets by enabling the connection of more renewable technologies and reducing energy transmission costs, which could lead to savings by end consumers.

The benefits of this Project and the long-term adoption of DCCBs into the energy grid are understood by comparison with counterfactual design cases. Alternative to DCCBs, the expansion of offshore wind can be accommodated by:

1. Increasing the number of converter stations or Direct Current Switching Stations (DCSS) built around the coastline (necessitating correspondingly greater quantities of transmission infrastructure),

2. Allowing more connections to existing DCSSs and offsetting the resulting risk of grid instability with increased contracting of ancillary service providers, or

Compared to the preferred use case of: 

·          Using DCCB to connect more generation capacity to existing DCSS (or other connection nodes), managing the risks, and increasing operational flexibility.

Compared with (1), using DCCBs can save valuable space by reducing the number of transmission assets, thus reducing impacts on local coastal communities and those who would otherwise be disrupted by expanded transmission infrastructure. It also reduces costs by avoiding the need to build additional infrastructure. This approach increases the Direct Current (DC) network's flexibility, allowing wind power to be routed more efficiently to centres of demand with reduced constraints and likely reduced curtailment on wind generation. Cost savings can be passed on to consumers.

Compared with (2), DCCBs can reduce expenditure on ancillary services. Given some of these services are provided by high inertia fossil-fuel powered turbines, there is also the potential to save on greenhouse emissions. 

We have quantified the benefits relating to differences in infrastructure expenditure and ancillary service provision (see the Cost-Benefit Analysis (CBA) worksheet in the Project Management Book). This CBA is based on a use case study of the Peterhead DCSS and compared against a counterfactual case of increased infrastructure and a case where more ancillary services are purchased to provide system protection and stability.

This analysis of asset costs and ancillary services indicates that using DCCBs can unlock benefits of several hundred million pounds in the central scenario over a 50-year period.

We performed a sensitivity analysis of the CBA using various scenarios, some favourable for adopting DCCBs and others unfavourable. Favourable cases involved reduced DCCB costs and increased switching station and ancillary service costs, while unfavourable cases featured the opposite. The results of this analysis are set out in the CBA analysis in the project management workbook. 

Qualitative benefits of operational flexibility reduced curtailment and network constraints, and reduced impacts on coastal communities will be quantified in Beta Phase and are expected to bolster the quantitative argument for DCCBs. 

By opening up DCCBs as an option for network designers via further work, benefits can be realised in other locations and situations. Enabling such a first-of- a-kind project somewhere like Peterhead will scale up across other sites bringing much larger benefits. 

We have taken a conservative approach to calculating the benefits based on a single use case at the Peterhead switching station and the approach to cost estimation (high-cost estimate). We have not included additional qualitative benefits listed in the CBA summary in the project management workbook. The quantified benefits are: 

1. Avoided costs for building point-to-point links and 

2. Avoided losses in the event of a system fault 

The included CBA analysis (see project management workbook) shows a combined positive benefit of NPV(3.5%) ~£3.5 million over the first ten years of operation and NPV(3%) ~£350 million in the expected 35-year lifetime of operation. The alternative case is to accommodate more connections at a DCSS through the provision of ancillary services to back up any losses due to faults. However, this is will incur a cost of negative ~£1200 million compared to the base case, and therefore is not a realistic choice compared to either (1) an increased number of point-to-point connections or (2) our preferred case of using DCCB's.

In Beta Phase, additional benefits will be tracked using the following metrics for key stakeholders: 

·          Reduced network and consumer costs: the unit cost estimate for a DCCB provided by OEMs participating in the Beta Phase, compared to our estimate and sensitivity ranges in the Alpha Phase.

·          Reduced network operator costs: the equipment redundancy requirements evidenced by an OEM in their Failure Mode Effects Analysis, compared with our conservative redundancy assumptions in Alpha Phase.

·          Benefits expansion across implementations: the continued evolution of the Holistic Network Design and the number of coastal DCSSs foreseen under a "Business as Usual" scenario without DCCBs.

Reduced constraints for network operators and curtailment for developers: assessing more rigorously the under-utilisation of converter capacity of existing DCSSs.