Learnings

Outcomes

2023/2024

The main outcomes of the project at this stage include the following:

• A literature review on Overhead Line Design Practices and New Technology Implementation has been completed on time, with excessive analysis and summary of existing UHVDC OHL designs worldwide.
• An experimental platform, which was comprised of a corona cage and an anechoic chamber, a high-precision microphone and acoustic detection system, was developed to study the audible noise (AN) from specific Aluminium Stranded Steel Wire (ACSR) samples. Controlled environmental conditions were simulated by a spray system to capture the noise behaviour at different voltage levels. A Specific spray system has been designed to investigate the low precipitation rate and its impact on audible noise generation. The results of the experiment were written into two papers and have been accepted by the CEIDP conference which will be held on 14th-17th September 2025.
• A comprehensive review of cable solutions for both AC and DC applications at ultra-high voltage levels has been carried out. It was found that some of the limitations are associated with the technologies themselves whilst others may be related to availability in the market.
• A review of HVDC technologies and their applications internationally including multi-vendor HVDC standards, as well as low frequency transmission technology.
• The initial feasibility studies of HVDC tower design with a focus of optimising dimensions and initial lab testing for audible noises was carried out. The initial studies suggested that the potential design solutions might be feasible to restrict the dimension of the 550kV & 800kV UHV HVDC OHL tower to be within 50-55m wide and high. This is similar to the dimensions of some existing 400kV OHL towers in the network. More detailed studies are due to be completed.
• Initial lab testing on audible noises for Aluminium Alloy Conductor (AAAC) samples under both dry and continuous spray conditions with both positive and negative DC voltages under various surface electric fields has been carried out.  It was noticed that for surface electric fields less than 24kV/cm, positive DC produced more A-weighted decibels, while negative DC produced more at higher electric fields. 
   

2024/2025

• Ultra HVDC (UHVDC) can offer several benefits over UHVAC for long distance bulk power transmission, including lower losses, longer distance cable transmission, and increased control. There are many global operational projects for UHVDC. ± 800 kV LCC is a common choice with 23 global operational projects but only limited projects globally with Voltage Source Converter (VSC) technology at UHV voltage level. In Europe, ±525 kV 2 GW VSC is standard for offshore projects.
• The development of marine cables can reach 640kV nowadays.  In general, cables above 525kV are limited in the market and no strong market incentives to drive the voltage higher at AC. However, there is research and development beyond the 525kV level to understand the limits of the technology for DC.
• Numerous large-scale DC Gas Insulated Line (GIL) prototypes with alternative gas, rated up to ±550 kV DC and 5000 A, have undergone extensive testing and demonstrated long-term performance. The commercial use of HVDC GIL remains limited, with no long-distance installations to date, primarily due to challenges such as the failure of insulating materials during polarity reversal tests. However, for AC, there are GIL projects up to 1000 kV with conventional SF6 insulation (Su-Tong tunnel, 5.4 km).
• Low frequency AC transmission is typically at a frequency of 16.7 Hz. It can provide large transmission capacity with lower AC impedance than AC transmission at 50 Hz. This means transmission distances can be increased without reactive power compensation. Modern low frequency AC technology can provide good power quality and fault handling, as well as grid forming. Using UHV AC extruded cable at 16.7 Hz lowers the effect of charging current derating. Compared to 50 Hz operation, ageing is less likely due to the lower frequency, as well as reduced thermal risks.
• A systematic method for long-term transmission expansion planning under deep uncertainty is being developed in this WP. A large set of future operational scenarios (over 50,000 in total) – that represents a wide range of conditions the electricity system in Great Britain could face by 2050 has been created. These scenarios reflect uncertainties in generation technologies, electricity demand growth, interconnection capacity, and commodity prices. This approach enables a more realistic and robust assessment of future system needs. Using these scenarios, we are now working to identify a suitable ultra-high voltage (UHV) overlay grid.
• A comprehensive literature review has been completed, which provides a state-of-the-art overview of key technologies applied in UHVDC transmission systems and summarises real-world UHVDC projects worldwide. The outcomes of this work have been submitted in a report.
• Bipole MMC-UHVDC models (i.e., (-/+)800 kV, 8 GW), using both Full-Bridge (FB) and Half-Bridge (HB) submodules (SBs), have been developed in the RTDS. These models will serve as the basis for the simulation-based stability and protection studies within this project.
• The strategy for protecting the UHVDC system against DC faults and the subsequent restoration process has been investigated. Representative fault interruption and restoration schemes have been proposed for both HB-MMC and FB-MMC configurations, where the scheme for FB-MMC has been fully implemented and demonstrated in the RTDS with detailed analysis conducted. The development of the associated fault isolation and restoration scheme for HB-MMC is ongoing and will be compared with the FB-MMC scheme upon completion.
• The detailed feasibility studies of compact tower design for 550 kV HVDC have been completed and concluded that it is feasible to achieve within the 56 m (H) × 55 m (W) requirement. Dimensions of tower size varies with different safety factors in consideration. 

2025/2026

• The main outcome of Task 1 of WP1 was the development of a comprehensive appraisal framework for evaluating potential UHV links within Great Britain. The framework captures a wide range of future operational conditions up to 2050 and enables the assessment of the system value of new transmission connections. While developed for UHV links, it is equally applicable to other HV transmission investments. Using this framework, we assessed a number of candidate transmission solutions and quantified their value to the GB electricity system across the 2035-, 2040-, 2045-, and 2050-time horizons. This provides a robust and evidence-based foundation for evaluating future transmission network developments.
• The main work completed in Tasks 2, 3, 4 and 5 of WP1 encompasses a comprehensive review of state-of-the-art UHVDC technologies and global project practices; the development of representative bipole UHVDC models in both RTDS and PSCAD environments; the proposal of an enhanced fault recovery assessment framework for evaluating UHVDC fault handling and recovery performance across different MMC submodule topologies; and an analysis of system frequency behaviour under UHVDC power loss events, along with an assessment of the potential benefits of grid-forming control technologies. The learnings from these activities enables understanding UHVDC performance and integration challenges, and provide important evidence to support future planning, technology selection, and system-level assessment of potential UHVDC deployments within Great Britain.
• Detailed feasibility studies confirmed that compact 550 kV AC and DC overhead line solutions can be achieved within approximately 56 m height and 42 m width while satisfying audible-noise and electric-field requirements. The studies also concluded that compact 800 kV solutions are feasible only for selected HVDC configurations, whereas compact 800 kV AC overhead line designs are not feasible within a UK-compatible footprint under existing audible-noise criteria.
• The main outcomes of WP3 are documented in the final technical reports of individual technologies, as well as a summary report bringing together the key findings of individual technology review reports. WP4 is ongoing and outcomes have been reported in techno-economic analysis and multi-criteria analysis draft reports, as well as routeing and consenting draft report based on the UHV options proposed by WP1. With these reports NGET can review the findings to help support future informed decisions on the implementation of a potential UHV network, its routing, current type (AC/DC) and physical asset design.

Lessons Learnt

Simulation, computing and data

• Real-time MMC‑HVDC simulation requires substantial compute capacity. For this project the RTDS substep bipole MMC‑HVDC model required a minimum of five Novacor cores (four for MMC‑HVDC stations + one for control/auxiliaries) or equivalent processor cards in conventional racks. Future projects should confirm core availability and plan required compute resources from project outset.
• Implement data mapping and gather network model inputs at the start of the project. Identify, clarify and document key assumptions with stakeholders and industry experts early to avoid rework.

Audible noise (AN) and overhead line (OHL) design

• AN is a primary constraint on compact UHV OHL designs (especially DC). Conduct AN assessment alongside insulation coordination early, since conductor configuration and phase/arrangement strongly affect achievable tower dimensions.
• Measurements during conductor wetting–drying were necessary to understand corona noise; future studies should consider the wetting–drying process as a principal test condition.
• Existing DC AN prediction methods have significant uncertainty for compact UHV towers and multi‑bipolar configurations. Targeted experimental validation is needed to improve confidence and ensure applicability to UK conditions.
• Significant knowledge gaps remain on atmospheric attenuation, wind, conductor ageing, pollution, ice and bundled conductor impacts on DC AN; include these in future experimental programmes.

Insulation coordination and integrated design

• There is a strong interaction between insulation coordination, conductor bundle design, AN performance and electric‑field compliance. Optimisation of compact UHV towers requires an integrated assessment rather than sequential evaluation of individual parameters.
• Feasibility of compact UHV solutions depends on the adopted AN performance criterion; agree design assumptions, performance requirements and assessment methodologies early to enable fair option comparison. 

Technical maturity, validation and next steps

• The project established technical feasibility for compact UHV concepts, but further work is recommended to mature designs: detailed engineering, experimental validation, prototyping and field demonstration before large‑scale deployment.
• Recommended follow-on activities:
  • Complete detailed feasibility for compact 800 kV HVDC tower geometries, conductor bundles and insulation coordination.
  • Develop and assess compact terminal, angle and diversion towers to check whether suspension‑tower design principles extend across tower types.
  • Investigate insulation coordination (air gaps, withstand levels, safety‑factor assumptions) through analysis and validation.
  • Undertake experimental studies on bundled conductor AN under positive and negative DC polarities to validate prediction methods.
  • Assess UK‑specific environmental effects (pollution, weather, ageing) on insulator and AN performance.
  • Define prototype and field validation activities to verify dimensions, AN, electric‑field compliance, constructability and operational considerations.

UHV technologies, market and system assessment

• WP1 Tasks 2–6 improved understanding of UHV technologies, representative models, fault behaviour, recovery and frequency stability, supporting assessment of UHV deployment in Great Britain.
• Task 1 identified strategic UHV link options and quantified value, but a comprehensive cost–benefit analysis was not completed due to lack of reliable transmission cost estimates; future work should address this uncertainty.
• Two UHV options studied are both credible pathways for GB reinforcement; costs per GW were similar (same technology type) and economies of scale may lower unit costs as project size increases.
• Cable and GIL maturity above ~525–550 kV is limited: market size above 525 kV is constrained, and there are few mature cable or long‑distance HVDC GIL solutions at higher voltages. UHVDC (e.g. ±800 kV LCC) has global operational examples, but none in Europe; offshore projects typically use ±525 kV 2 GW VSC. 

Dissemination 

2023/2024

Although the project is still at a relatively early stage, the initial work carried out and key findings have been shared with relevant stakeholders from NGET, SPEN and SSEN via two stakeholder workshops (held on 25th October 2023 and 28th February 2024). 

2024/2025

The key outcomes of the project have been presented to industrial stakeholders during the Energy Innovation Summit on 30th October 2024. 
Based on the outcomes of experiment work from WP2, two papers have been prepared and accepted in IEEE Conference on Electrical Insulation and Dielectric Phenomena, September 2025, Manchester, UK
More dissemination events are planned in 2025 and will be updated in next year’s report.  

2025 / 2026 

Paper accepted in CIGRE Paris 2026:

Control and Recovery of DC Faults in Overhead Line UHVDC Systems: A Comparison of Full-Bridge and Half-Bridge MMCs, D.Liu, Q.Hong, L. Xu, A. Dysko, X. Ding, C. Booth, CIGRE Paris Session, 2026

The following dissemination events are held for 2025/2026:

• Technical workshop with stakeholders was held in June 2025, including stakeholders outside of NGET.
• Disseminated project findings (WP1) at the CIGRE UK’s “HVDC at Scale: From Projects to Systems” Conference Hosted by Arup in June 2026.