Learnings

Outcomes

The key outcomes of the project are listed below:

• Both voltage upgrade (to 550kV) or 6P(400kV) can increase the power transfer capability of the circuits.
• An existing 400 kV 3P double-circuit line would requires some modifications to operate as a 550 kV 3P double-circuit line or as a 400 kV 6P line. Main modifications are associated with limiting switching and lightning impulses, insulation systems, conductor positioning to increase ground clearances etc. Issues such as switching transients can be relatively easily solved by the use of pre-insertion circuit-breaker resistors and the impacts of lightning transients can be mitigated by surge arrestors, lower tower earthing resistance and improved shield wire placement.
• The need to manage the electric fields and audible noise associated with 3P voltage upgrade (to 550kV) or 6P (400kV) may constrain the ability to upgrade some routes or the ability to deliver sufficient thermal capacity. Limitations may exist in relation to the mechanical strength of the tower and its ability to accept larger conductors. Detailed studies are required. However, no significant constraints are expected because of electrical clearances.
• The existing L6-type GB OHL transmission network provides the necessary clearances to accommodate operating voltages up to 550 kV and 6Φ systems. However, environmental (EMF, corona, and noise) effects can vary depending on existing conductor systems - however detailed studies are required to outline specific modification requirements on existing lines.
• Transformer solutions are available to facilitate the voltage upgrade and 3Φ to 6Φ system conversion. 
   

Recommendations for further work

• Optimisation of the tower design to ensure the line impedance matrices are balanced, taking account of the advantages/disadvantages of line transposition, insulation coordination and the associated noise and electric field environmental standards, as well as the impact of extreme weather events on transmission lines insulation would benefit from further study.
• Further experimentation regarding insulation and conductor systems to investigate their performance under normal and extreme conditions and any effect on the life expectancy of the existing assets. This will help to quantify electrical and mechanical stresses imposed on the OHLs due to voltage upgrade and wind loads.
• Assess the impact OHL system modifications on overall network resilience and how probabilistic design practices could facilitate economic and efficient integration of upgrading technologies.
• Operation of 6P systems under transient and permanent faults and maintenance solutions

Lessons Learnt

The project benefited from in-depth engagement with wider key stakeholders during the delivery, particularly from environmental engineering perspective.  It is felt that more value can be achieved if the project could engage with wider stakeholder group during the planning stage to make the scope of work more comprehensive.

Dissemination 

• A presentation on six-phase technology was given at the Energy Innovation Forum on 19th May 2021.
• A dissemination event with key industry stakeholders (i.e. NGET, NGESO) was held in June 2021.
• Technical papers for journals and conferences are being prepared to disseminate to a wider audience.