Offering Benefits Compared with Conventional HVDC Copper Cables in Offshore Grids
Subsea superconducting transmission cables, a new innovative cable technology, were shown to offer greater economic benefits to the energy system than conventional high-voltage, direct current (HVDC) copper technology in a 2050 offshore grid. The University of Strathclyde, Glasgow, Offshore Renewable Energy Catapult, UK, and SuperNode, Ireland, have conducted technical and techno-economic studies analysing the potential benefits of high-temperature superconductor subsea transmission cables compared with conventional copper 525kV HVDC cable technology.By Maria O’Neill, Technology Analyst, SuperNode, Ireland
The first study was a cost–benefit analysis of offshore wind transmission systems comparing a high-temperature superconductor (HTS) point-to-point connection scheme with a conventional 525kV high-voltage, direct current (HVDC) connection scheme. The second study advanced this work by comparing two offshore DC network designs, one based on HTS and one based on 525kV HVDC technology.
Background of Superconductor CablesSuperconducting transmission cables are an innovative cable technology harnessing the characteristics of superconductive materials. Superconductivity is a phenomenon that occurs in certain materials when they are cooled below their critical temperature (−200°C for an HTS). The unique characteristics of superconductors include zero electrical resistance, leading to zero electrical losses, which enables very high power density.
Superconducting cables are capable of transferring very large amounts of power efficiently over long distances in a much smaller surface area than conventional cables. Superconducting cables can also operate at higher currents and therefore lower voltage levels than conventional cable technology, meaning they require significantly less infrastructure, materials and space and have a smaller environmental footprint.
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Superconductors have the potential to revolutionise energy grids for a world powered by renewables.
Study 1: Superconductor Cable Shown to be Cost-Competitive with HVDC
This study implemented a cost–benefit analysis approach to assess the feasibility of HTS cable systems at medium voltage for offshore connection schemes compared with conventional 525kV HVDC systems. It shows that medium-voltage, direct current (MVDC) transmission cables based on superconductors have life-cycle costs up to 31% lower than a conventional grid based on HVDC technology.
This study implemented a cost–benefit analysis approach to assess the feasibility of HTS cable systems at medium voltage for offshore connection schemes compared with conventional 525kV HVDC systems. It shows that medium-voltage, direct current (MVDC) transmission cables based on superconductors have life-cycle costs up to 31% lower than a conventional grid based on HVDC technology.
Model Assumptions
The hypothesis is that increased offshore wind generation capacity will be optimally integrated into the energy system of beyond 2030 by higher current, medium-voltage connections due to three factors:
The hypothesis is that increased offshore wind generation capacity will be optimally integrated into the energy system of beyond 2030 by higher current, medium-voltage connections due to three factors:
- The ability to reduce AC-DC conversion steps and associated equipment and controls by aligning voltages more closely from turbine generator to onshore grid.
* DC wind turbines and DC array cable topologies can provide upstream DC power to collector stations (although this was not considered in detail in this analysis). - The ability to reduce costly and suboptimal HVDC and transformer equipment offshore, and hence reduce large offshore infrastructure costs, due to lower-voltage DC transmission.
* Optimised platforms for 2GW+ collector stations versus bespoke, costly, supply-constrained platforms for HVDC applications. - The ability to reduce environmental impact, improve public acceptance, and lower overall project costs through streamlined planning and consenting by transmitting higher volumes of power (2GW+) in single cable onshore landing points.
The project completed a cost–benefit analysis comparing the life-cycle costs of 100kV MVDC HTS cables with incumbent technology, most notably 525kV HVDC copper cables.
The analysis considered life-cycle costs to be made up of three cost categories: capital costs (platforms, cables, installation, transformers, converters, etc.); the cost of electrical losses; and costs due to unavailability (system failure and repair).
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- A system power rating of 2GW.
- An equal turbine spacing of 2 kilometres and a water depth of 40 metres.
- The offshore platform is 100 kilometres away from the onshore connection point. (Note that this is a relatively long distance compared with early offshore projects and was chosen to future-proof the analysis to align with increasing offshore gigawatt targets to 2030 and 2050.)
- The projects have an operational life of 30 years.
- The MVDC HTS system assumes the use of DC wind turbines and DC array cables to remove the need for a central offshore converter station. (Note that this is a largely existing technology at Technology
Readiness Level 5 or later and simply needs market pull to be commercially deployed).
Results
The HTS system has lower costs in each of the three cost categories, with overall lower costs of 30%.
The HTS system has lower costs in each of the three cost categories, with overall lower costs of 30%.
Electrical Losses: The electrical losses from HTS cables are negligible due to the nature of superconductivity; however, there are costs associated with cooling the system to cryogenic temperatures as well as electrical losses in system integration equipment such as converters and transformers. A key detail of these cooling costs is that, unlike conventional copper cables, in which resistive losses increase as higher power is transmitted, cooling costs for HTS systems are independent of cable capacity, i.e. a 2GW system and a 6GW system will incur similar cooling costs.
Unavailability: The HTS system also has significantly lower unavailability costs than HVDC systems. The HVDC system has significant unavailability associated with the onshore and offshore transformers as well as the offshore converter station and switchgear, none of which apply at the same scale for the HTS system. However, the HTS system does have some unavailability associated with the cooling system O&M and converter stations onshore. Nevertheless, the modular nature of the HTS cooling system offers greater redundancy, which limits unavailability.
Capital Costs: In terms of capital costs, HTS cables are more costly than HVDC cables. However, HVDC systems require large offshore platforms to facilitate the high-voltage equipment needed, such as the offshore converter station and switchgear, whereas an HTS system requires a smaller footprint platform due to a lower volume of electrical equipment required (no converter station, smaller switchgear). Ultimately, despite the high cost of HTS cables, the analysis has found that the overall capital cost of the HTS system is significantly lower than the HVDC system.
The model shows the cost-competitiveness of MVDC HTS cable technology for long distance offshore wind power transmission. This work highlights the cost-effectiveness of the HTS system and emphasises the importance of further research.
Study 2: Net Benefits of Offshore Grid System Based on Superconductors Compared with Conventional HVDC Copper Cables
The second study expanded the scope of the HTS analysis, comparing the benefits of an offshore meshed DC grid based on HTS technology with one based on conventional copper 525kV cables. The results of the study show that by 2050 the system based on HTS technology offers greater net benefit to the UK network than the conventional copper-based system.
The second study expanded the scope of the HTS analysis, comparing the benefits of an offshore meshed DC grid based on HTS technology with one based on conventional copper 525kV cables. The results of the study show that by 2050 the system based on HTS technology offers greater net benefit to the UK network than the conventional copper-based system.
SuperNode is designing cables capable of transmitting up to 10GW of power through a single HTS bipolar configuration. For the same power to be delivered via 525kV copper cables, five cable systems of 2GW would have to be installed. This conductor density results in a dramatic reduction in the number of onshore connection points and overall cable distance required and can be designed as part of a meshed network to run for periods above the normal or average power of any individual line. This is represented in Figure 6.
The two offshore networks being compared are representative of a North Sea grid integrating with the UK’s onshore network.
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The study compares the costs and benefits of both technologies over a 20-year period of offshore DC network development (2030–2050), modelling a UK offshore network capable of reaching an excess of 80GW of offshore wind by 2050, assuming the UK achieves its target of 50GW offshore wind by 2030. 80GW was chosen as it was an ambitious figure, not target, mentioned by the National Grid’s Future Energy Scenarios as a possible peak load. The model did not consider an 80GW ceiling on offshore wind development. In the case of the HTS-enabled grid, an additional 50GW of offshore wind was installed, in comparison with an additional 40GW for the copper HVDC scenario. The following factors were considered in the model:
- Capital costs: offshore platforms, converter systems, cable costs.
- Operational costs: 525kV HVDC copper cross-linked polyethylene (XLPE) cable resistive losses and HTS cable cooling costs.
- Unavailability costs: based on failure rates and mean time to repair of equipment.
- Value of grid connection for offshore wind farms.
- Value of interconnection with neighbouring grids.
- Rate of deployment of 10GW HTS (10GW per three years) versus 2GW 525kV HVDC (2GW per year). Although these are relatively fast deployment rates compared with historical grid development, this rate of deployment is necessary if ambitious targets are to be met within a 20-year time frame.
Results
In 2050, the net benefit of the HTS system reaches € 81.5 billion and is much greater than the benefits gained from the 525kV copper-based system, which reaches € 26 billion. The benefits gained from the HTS system are far greater than the incumbent technology, with a difference of approximately € 55 billion over 20 years.
The primary benefits of the HTS system are as follows:
Maximising Value of Transmission Corridors: This is the key benefit of the HTS system and can be split into two core subsets: 1) rate of deployment of capacity and 2) value of interconnection with neighbouring grids:
- Rate of Deployment of Capacity: The rate of deployment of capacity is much higher in the HTS system. Although HTS cables, as a novel technology, are assumed in the model to have a slower rate of cable installation, the bulk power capacity enabled by superconducting technology means that per installation, far greater capacity can be added to the offshore grid compared with conventional HVDC connections (10GW versus 2GW), which would require multiple cable corridors to achieve the same power density.
- Value of Interconnection with Neighbouring Grids: The HTS system offers more opportunity, more quickly, to interconnect the offshore DC grid with surrounding grids compared with the 525kV HVDC case. This is a result of the excess capacity of each 10GW connection installed every three years. Each 10GW connection initially exceeds the transmission requirements of the connected offshore wind farms, allowing the excess capacity to be implemented as interconnection schemes. The same level of excess capacity available for interconnection is not present in the 525kV copper system.
Capital Costs: The HTS system requires greater capital costs at the outset, as seen in the initial few years of the graph. This is due to the significant anticipatory investment needed for the optimal functioning of the HTS system. The value gained from the HTS cables is not seen until the first project is completed in 2036. Yet, despite the slow start, the HTS system provides much greater value in the long run.
Operational Costs: As mentioned earlier, HTS cables transfer power with zero electrical losses, which enables higher current to be transmitted at a lower voltage and reduces the infrastructure required. There are operational costs associated with HTS, notably the costs of cooling, but crucially these costs do not increase as cable capacity does, unlike resistive losses with copper-based cables. Therefore, a 10GW HTS incurs similar cooling costs to a 2GW system.
It must also be noted that HTS cable systems of 3–4GW are also competitive with HVDC copper systems and still offer benefits to the grid given the zero electrical losses, lower voltage levels and reduction in electrical equipment and offshore platform costs.
ConclusionThe two studies show that superconductor cable systems are not just competitive with conventional copper-based cable systems but offer greater benefits and cost savings. While installing 10GW connections may not be a high priority for transmission system operators today, this analysis makes the case that early maximisation of transmission corridors via HTS cable systems can deliver offshore grid systems in a timely and cost-effective manner, offering significant benefits over a project lifespan compared with a 525kV copper-based grid.
Studies like those conducted by the University of Strathclyde, the Offshore Renewable Energy Catapult and SuperNode are essential. If decarbonisation targets are to be met, renewable generation will have to meet ever-increasing electricity demand. The National Grid’s Future Energy Scenarios expect peak load to reach 80GW by 2050. The implications of this for power flows and grid design must be fully understood and explored. This level of wind generation stipulates an unprecedented level of power flows and thus an unprecedented level of transmission infrastructure. Higher-powered connections like those enabled by HTS cables can maximise the benefit of each project, thereby reducing the time and cost needed to develop a robust offshore DC network.
Background detail on studies is available on request. This analysis was a collaboration, with the modelling work being primarily conducted by the University of Strathclyde.
Biography of the Author
Maria O’Neill is a technology analyst focusing on SuperNode’s commercial model. She has a master’s degree in electrical energy engineering from University College Dublin and previous experience in power system analysis and energy trading.
Maria O’Neill is a technology analyst focusing on SuperNode’s commercial model. She has a master’s degree in electrical energy engineering from University College Dublin and previous experience in power system analysis and energy trading.
