Omnia Projects were engaged by National Grid to undertake Detailed Design for SGT2B replacement at Mill Hill 275/132kV substation, including a new tertiary connection, 13/33kV transformer bay connectionfor a Customer BESS and a new Earth Transformer (ETx).
The Issue
The new 13kV ETx supplied by IST Power was redesigned to comply with the latest National Grid Technical Specifications.
This change introduced higher-rated 3150A bushings, with a requirement for connectors with increased current-carrying capacity. To achieve this, the arrangement was upgraded from a single 40.2 mm DIA stranded conductor to two parallel 58.8 mm DIA conductors, along with larger connector palms and fittings.
The earthing transformer height restricted the available space between the top of the bushings and the overhead busbar connection. This created a significant challenge for the uprated stranded conductor arrangement, given the larger 58.8 mm conductor size and its increased minimum bending radius. The larger connector palms and adapter plates reduced the phase-to-earth clearance between the outerphase busing and the neutral bushing due to their increased size.
Modelling in 3D
Assessing the new stranded conductors for installation using traditional 2D techniques proved difficult; the connections can bend and twist across three planes. A 3D model was required to accurately evaluate their geometry and clearances. The model provided a clear, real-world representation of the ETx bushings, connectors, and adjacent equipment alignment once installed, eliminating the dependency on 2D assumptions.
ETX Visualisation
The 3D model clearly identified where phase-to-earth clearances were infringed due to the larger components and enabled interactive testing of different installation options. This visualisation simplified discussions, decision-making, and supported the technical deviation with credible visual and spatial evidence for the proposed solution.
The initial arrangements were modelled in Scenario A. These reflected the existing design intent. This model confirmed where space constraints, tight bending of stranded conductor, and insufficient conductor slack created installation challenges. Multiple installation options were identified and developed to be assessed in 3D for spatial and clearance requirements.The 3D visualisation clearly showed how subtle adjustments-rotating clamps, altering dropper angles,changing connection geometry etc. affected conductor routing and phase-to-earth clearances.
Design Resolution
The most effective option involved repositioning the ETx approximately 540mm away from the busbar connection side of the arrangement.This solution provided space for the stranded conductors to achieve a smoother bend and maintain improved clearances. Following the 3D assessment, a technical deviation was submitted and accepted to adopt the IEC phase-to-earth clearance in place of the NGET standard.
This project demonstrated how 3D modelling can significantly enhance design clarity and decision-making. By accurately visualising conductor geometry, clearance constraints and component interactions, the 3D model revealed issues that were not apparent in 2D, enabling the team to test alternate layouts, validate clearances, and identify an optimal solution.
This approach improved technical confidence, enabled informed discussions with stakeholders and supported approval of the final design, ultimately reducing risk and ensuring a safe, buildable installation.