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 to 640kV nowadays.  In general, cable above 525kV is 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. 

Lessons Learnt

• To accurately simulate the MMC (Modular Multilevel Converter)-HVDC system in real time, a significant amount of computing capability is required. In this specific project, the bipole MMC-HVDC model was developed in the RTDS substep environment, necessitating a minimum of five cores on the Novacor racks (four cores assigned for the MMC-HVDC stations and one core for control and auxiliary components) or an equivalent number of processor cards in conventional racks. Future projects should consider sufficient computational resources and core availability from the outset to ensure the successful simulation of the developed model.
• Additionally, the data mapping process should be implemented at the beginning of the project to efficiently obtain key data inputs for the network model required for the study. Key assumptions should be identified, clarified, and defined with input from key stakeholders and industry experts as early as possible.
• Measurements of audible noise during the wetting-drying process of the conductor was found to be necessary for better understanding the characteristics of the corona noise from the OHL conductor. For future projects, it may be useful to consider the wetting-drying process as the main studying condition of the OHL conductor noise.

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.