The AI Power Chain series · Part 2 of 15

The AI Power Chain, Part IV-A, and Technical Companion

How grid interconnect equipment actually works

The Interconnect Stack: Technical Companion

Where the market essay named the vendors and quantified the lead times, this piece explains the physics. Why voltage steps up before it steps down, how a 400-tonne oil-filled transformer moves 300 megawatts at 99.5% efficiency, why sulphur hexafluoride became the standard insulation gas and why it is being phased out, how an aeroderivative gas turbine differs from a diesel genset, and what actually happens inside a small modular reactor. Cross-sections, waveforms and roadmaps for readers who want the machine under the market.

By Adi Kumar Independent analysis Views are my own

Reading this piece. It is a technical companion to The Interconnect Stack (Part IV of The AI Power Chain). The market piece named the vendors, sized the layers and mapped the lead-time crisis. This one explains the physics: how transformers, switchgear, cables, generators and reactors actually work, what determines their efficiency and lifetime, and what the roadmap items on every analyst's list physically change. No prior electrical background is assumed, but the reader should know what "voltage", "current" and "power" mean at a systems level.

1First principles · Why voltage matters

The reason the whole grid exists at high voltage

Every high-voltage transmission line, every 400-tonne substation transformer, every mile of medium-voltage cable underground exists to solve one equation. Power dissipated in a conductor is P = I²·R: the square of the current, times the conductor's resistance. Power transferred to a load is P = V·I: voltage times current. To move a given amount of power (V·I) with the minimum loss (I²·R), you want the current low and the voltage high. Every voltage step-up in the grid is a specific instance of that arithmetic.

Transmission loss falls quadratically with voltage

I²R losses for 100 MW transferred over 100 km of standard ACSR conductor, log scale

Same power (100 MW), same conductor (Aluminium Conductor Steel-Reinforced, ~0.06 Ω/km at typical cross-section → ~6 Ω per phase over 100 km), three-phase AC, different line-to-line voltages. At 480 V and 13.8 kV the required current is not physically deliverable at this scale. The loss calculation exceeds the transferred power. From 33 kV upward the transfer is possible but only economical at higher classes: 33 kV loses ~55%, 138 kV ~3%, 400 kV ~0.4%, 765 kV ~0.1%. Losses fall roughly as the square of the voltage ratio.

Consider the numbers behind the substation feeding a modern hyperscale campus. A 500 MW load at 480 V three-phase would draw ~600,000 amps. That is mechanically impossible: no cable geometry can carry it, no bus bar can dissipate the resistive heat, no switch can interrupt it under fault. At 138 kV, the same 500 MW draws ~2,100 amps. That is well within the capability of a modest overhead line or an underground cable. Every step in the grid's voltage hierarchy is dictated by the same trade-off, and every transformer in the interconnect stack exists to move between those levels.

Voltage class	Where it lives	Physical form	Approx current for 500 MW
765 kV / 500 kV	Long-distance transmission	Overhead lines with towers 30-50m tall, minimum conductor separation ~15m	~650 A
230 kV / 138 kV	Regional transmission, hyperscale substation feed	Overhead or shielded underground cable	~1,250-2,100 A
34.5 kV / 13.8 kV	Data centre campus distribution	XLPE underground cable in ducts	~8,400-21,000 A (split across multiple feeders)
480 V / 400 V	Rack / room level distribution	Busway and copper busbar	~600,000-720,000 A (spread across many racks)
800 V DC	Rack inlet, next-gen AI data centres	DC busbar, ~750 A per 600 kW rack	~625,000 A (spread across racks)

Why DC re-enters the picture at 800V. The grid is AC because Tesla won the "war of the currents" in the 1890s: AC could be stepped up and down easily with transformers, DC could not. Modern power electronics changed that. Solid-state converters can step DC voltages efficiently, which is why long-distance HVDC transmission became viable in the 1970s and why 800VDC data centre distribution is happening now. The choice of DC at 800V for AI racks reflects the same I²R arithmetic: at 800V the copper losses in a rack busbar are half what they would be at 480VAC (which averages to ~340V DC-equivalent for power delivery). Every watt saved is a watt back for compute.

2Transformer physics

In brief

Grid interconnect equipment engineering covers transformer physics (Faraday induction, insulation thermal management), switchgear topology (SF6 vs vacuum vs solid dielectric) and cable ampacity (conductor cross-section, insulation temperature rating). A 300MW substation transformer weighs 400 tonnes and takes 24 months to fabricate. Physics + supply chain lead time together determine hyperscaler campus timelines.