HVDC exists because of Joule's law. For a fixed power transfer, current falls as voltage rises, and resistive loss scales with I²R. Operating at ±400 kV to ±525 kV keeps current low enough to suppress conductor heating over very long distances. DC also avoids the capacitive charging currents that make long AC subsea cables impractical beyond roughly 80–100 km. The North Sea Link carries 1,400 MW at ±515 kV between Blyth and Kvilldal.
The converter stations carry the largest fixed overhead. Rectification at the sending end and inversion at the receiving end pass the full throughput through thyristor (line-commutated) or Voltage Source Converter (VSC) valves, plus transformers, filters and cooling plant. Each station dissipates roughly 0.7% to 1.5% of throughput as heat: modern modular multilevel VSC designs sit at the lower end, older designs at the upper end. That loss is incurred twice on every link, regardless of length.
| Link | Route length | Rating | Voltage | Rectification | Cable (est. at rated load) | Inversion | Indicative total |
|---|---|---|---|---|---|---|---|
| North Sea Link (UK–NO) | 720 km | 1,400 MW | ±515 kV | ~0.7–1% | ~2.2% | ~0.7–1% | ~3.5–4% |
| Viking Link (UK–DK) | 765 km | 1,400 MW | ±525 kV | ~0.7–1% | ~2.3% | ~0.7–1% | ~3.5–4.5% |
| NordLink (DE–NO) | 623 km | 1,400 MW | ±525 kV | ~0.7–1% | ~1.9% | ~0.7–1% | ~3.5–4% |
| IFA2 (UK–FR) | ~204 km | 1,000 MW | ±320 kV | ~0.7–1% | ~0.6% | ~0.7–1% | ~2–2.5% |
Conductor attenuation scales linearly with distance, at roughly 0.3% per 100 km at rated load. Because losses follow I²R, the figure falls sharply at part load. Applied across 700 km or more, the cable alone removes about 2% of the energy before local distribution begins. The table's cable figures are estimates on that basis, not operator-published data.
For investors and grid specifiers, a 3.5–4.5% round-trip penalty is modest next to the curtailment it prevents. It is still a structural cost that belongs in price-spread models, capacity valuations and lifecycle carbon accounting for every new interconnector.