IN Brief:
- Consent applications have been submitted for the Denny–Wishaw Network Optimisation project.
- Existing 275kV lines would be uprated, with 19km of new overhead line and several substation modifications.
- The reinforcement is intended to accommodate approximately 1,000MW of additional renewable generation.
SP Energy Networks has submitted the principal consent applications for its Denny–Wishaw Network Optimisation project in central Scotland.
By uprating existing circuits and adding new infrastructure, the scheme is intended to increase transmission capability across approximately 35km of established overhead-line routes and a further 19km of new line between Bonnybridge and Glenmavis.
Section 37 applications covering the overhead-line works have been submitted to Scottish ministers, while North Lanarkshire Council has received a planning application for an extension at Cumbernauld substation. Further work is proposed at Bonnybridge, Newarthill, Wishaw, Easterhouse, and Glenmavis.
Three existing circuits would be uprated from 275kV to 400kV: approximately 16km between Newarthill and Wishaw, 15.4km between Newarthill and Easterhouse, and 4km between Denny North and Bonnybridge. The programme also includes a short spur into Cumbernauld and removal of around 11km of existing 132kV line.
A new double-circuit 275kV and 400kV overhead line would connect Bonnybridge and Glenmavis, accompanied by underground cable, transformers, switchgear, protection, control, and substation modifications. Once commissioned, the complete project is expected to accommodate approximately 1,000MW of additional renewable generation.
The reinforcement forms part of the wider programme intended to move electricity from Scotland’s expanding wind fleet toward major demand centres. Delivering that transfer capacity requires coordinated development across local substations, meshed alternating-current circuits, and longer-distance high-capacity links.
Existing corridors still require extensive modification
Reusing an established transmission route can limit the amount of wholly new corridor required, but a voltage uprating involves considerably more than changing the nominal operating level. Clearances, insulators, conductors, towers, foundations, earthing, protection, and connected substation equipment must all be assessed against the proposed 400kV duty.
Thermal and mechanical studies will determine whether existing conductors and structures remain suitable. Conductors must carry the intended current without unacceptable temperature or sag, while towers and foundations must withstand the selected conductor system, wind loading, ice loading, and the mechanical effects of construction and maintenance.
Substations can impose the practical limits on an uprating programme because new switchgear bays, supergrid transformers, busbar arrangements, cable sealing ends, instrument transformers, surge arresters, and control systems must often be installed within constrained sites that remain operational.
Changing network voltage and topology also alters fault levels and protection requirements. Relay settings, current-transformer performance, circuit-breaker ratings, communications-assisted schemes, and system studies must be revised before the circuits can be energised at their new operating level.
Construction sequencing will require individual circuits and bays to be taken out of service without compromising secure transmission operation. Planned outages must be coordinated against renewable forecasts, electricity demand, maintenance elsewhere on the network, and the contingency capability remaining after each asset has been isolated.
Along the new Bonnybridge–Glenmavis section, route design must account for settlements, landscape, ecology, ground conditions, access, existing utilities, aviation constraints, and interfaces with established circuits. Consent conditions and land agreements can subsequently alter the construction sequence or require changes to individual structures.
An Early Construction Funding allowance of £53.1 million at 2018–19 prices has supported enabling work, including surveys, design, consenting, supply-chain engagement, and early preparation for equipment with long manufacturing periods.
Transformers and high-voltage switchgear have become significant programme risks across European transmission development. Offshore wind connections, interconnectors, replacement schemes, data-centre reinforcement, and international grid expansion are competing for production slots from a limited number of qualified manufacturers.
The proposed 2GW Western Link 2 connection between Scotland and Wales is progressing alongside alternating-current reinforcements such as Denny–Wishaw. HVDC routes can transfer controlled power over long distances, while AC projects strengthen the meshed network and the substations through which generation reaches those links.
Securing planning and Section 37 consent will not authorise every construction activity immediately. Detailed conditions, land rights, procurement, outage planning, regulatory approvals, and final design must still be completed before each element can enter the delivery programme.
Denny–Wishaw now moves deeper into the consent, equipment, and construction-planning stages, where overhead lines, substations, underground cable, protection, and control systems must progress on coordinated schedules. Its 1GW contribution will become available only when the complete electrical route, rather than an individual circuit or substation, is ready for service.
