A train-sized tunnel is now carrying electricity under South London

Superconductors, HVDC, and Transmission Choices

• Several comments note that superconducting cables are still rare in power grids: cryogenic cooling in confined tunnels is seen as prohibitively complex, expensive, and risky (asphyxiation, potential hazards in faults).
• Others argue that simply using higher AC voltages or HVDC is far cheaper than maintaining 20K cryo systems.
• Discussion clarifies that AC lines have higher losses than equivalent high‑voltage DC because of changing fields, but HVDC needs costly converter stations, so it’s only worthwhile for specific long‑distance or inter‑grid links.

Why Tunnels at All?

• Main stated reason: ease and certainty of maintenance and replacement in a dense city, not a bet on superconductors.
• London’s subsurface is described as a complex 3D maze of sewers, transit, archaeology, rivers, and unexploded ordnance; threading new surface corridors or pylons is seen as largely infeasible.
• Some speculate 50m depth might relate to security, but others think planning and spatial constraints are the real drivers.
• Tunneling is also portrayed as a “planning survival strategy” to bypass interminable right‑of‑way battles.

Cable Zig‑Zag / Sag Pattern

• Multiple explanations converge: the apparent zig‑zag is mostly camera foreshortening; in reality it’s gentle sag between supports.
• Slack provides:
  • Room for thermal expansion/contraction without axial strain.
  • Reserve length for future modifications (e.g., new substation terminations).
  • Tolerance for movements (thermal, seismic).

Underground vs Overhead and Insulation

• Overhead lines: bare conductors using air as the insulator, hence large ceramic stacks; cheap and easy to cool/repair but need space and clearances.
• Underground/tunnel cables: thickly insulated (often XLPE) assemblies, more expensive per metre and thermally constrained, but compact and safer in cities.

London-Centric Investment Debate

• Some see the £1bn project as evidence that London is favoured while other regions are told undergrounding is “too expensive.”
• Others counter that:
  • This is ~30 km of cable vs hundreds/thousands of rural kilometres.
  • London’s density, land costs, and power demand justify tunneling.
  • London and the South East are net fiscal contributors.
• Opponents argue London’s wealth reflects centuries of extraction and concentration of state and corporate functions, calling it “internal colonialism.”
• Both sides agree network effects and history heavily shape where value and infrastructure concentrate.