Three-Level Transmission for the SBC Corridor

The corridor needs to do three different things with electricity. A three-level voltage architecture — UHVDC trunk, HVDC backbone, MVDC distribution — answers all three with commercially proven technology, and leaves clear upgrade paths as DC-tap technology matures.

1. What This Memo Does

The SBC corridor must move electricity in three different ways simultaneously. It must move bulk power over thousands of kilometres — the export trunk to coastal cable landings, the east–west grid integration that ties Australia’s eastern and western grids together for the first time. It must collect power from the solar generation strips along the corridor itself — tens of gigawatts spread across 2,500 km of inland Australia, gathered at intervals matched to the geography. And it must feed the corridor towns — settlements at approximately 100 km spacing, each with domestic and industrial load and the maglev passenger service plus electric freight running along the multimodal viaduct.

These are three engineering tasks at three different scales. Trying to solve them all at the same voltage class fails on cost grounds. The instinct — tap the ±1,100 kV trunk at every town — runs into a basic fact about HVDC converter stations: they are multi-billion-dollar facilities, and one per town across a 2,500 km corridor would dominate the entire project economics.

This memo surveys the engineering options. It describes the proposed architecture — three voltage layers running in parallel along the corridor easement, with stations sited at the right spacing for each level. It honestly identifies what is solved by commercial technology today, what depends on emerging research, and where the upgrade paths sit.

The memo is not a final design. It is a description of how the corridor power architecture has been thought about, what reference projects support it, and what engineering work remains to be done. The order of magnitude is locked. The detailed sizing is for chartered electrical engineers operating against a defined load case.

2. The Converter Station — Why It’s Expensive

2.1 What a converter station does

An HVDC converter station performs the AC–DC conversion at one end of an HVDC link. It contains: power semiconductor switches arranged in valve banks (thyristors for line-commutated converters, IGBTs for voltage-source converters); converter transformers that handle the voltage step between the AC grid and the DC system; harmonic filters that clean up the switching distortion; switchyards and breakers; cooling systems; and protection and control electronics. The whole installation occupies typically 5 to 15 hectares of industrial land per ±1,100 kV terminal.

2.2 The cost benchmark

Recent industry benchmark data (HVDC infrastructure cost reviews, late 2024) puts VSC HVDC converter station cost at approximately US$200 million per gigawatt of capacity — down from approximately US$300 million per GW historically. That is the current commercial floor for a credible new-build VSC station.

A 12 GW ±1,100 kV bipole — the capacity of a single Changji–Guquan-class circuit — therefore implies converter stations on the order of US$2–3 billion per terminal. A point-to-point link with one terminal at each end is approximately US$5 billion in converter stations alone, before any cable, tower, or civil works. That is a real number, validated by published project cost data, and consistent with the actual capex of recent UHVDC builds.

2.3 Why one per town doesn’t work

A 2,500 km Phase 0-class corridor has approximately 25 towns at 100 km spacing. If the architecture required one full UHVDC converter station per town, the converter capex alone would be approximately US$50–75 billion — multiples of the entire corridor civil works budget. That is not a marginal cost; that is the whole project.

The honest question is therefore not "can we put a converter at every town" but "how do we serve every town without putting a UHVDC converter at every town". The answer is a graduated voltage architecture — each level sized for what it actually has to do.

3. What’s Operational Around the World

Multi-station HVDC systems — with more than two converter terminals on a single DC network — are not theoretical. They are operational, at increasing scale, since the early 2010s. The reference projects matter because they establish what voltage classes can credibly carry multi-terminal architecture today.

3.1 Zhangbei VSC HVDC Grid (China)

The Zhangbei project, operated by State Grid Corporation of China since 2020, is the world’s first multi-terminal VSC HVDC grid. Four interconnected converter stations operate at ±500 kV in a ring topology, transferring up to 4.5 GW between Zhangbei (renewable generation), Kangbao, Fengning (pumped hydro storage), and Beijing (load). Project cost was approximately US$1.84 billion. Equipment supplied by ABB; control by State Grid.

Significance: four converter stations on one DC network at ±500 kV is operationally proven. Power flow is bidirectional at every station. Control architecture coordinates voltage and frequency across all four terminals. The project handled the 2022 Beijing Winter Olympics power supply.

3.2 Caithness–Moray–Shetland (UK)

Operated by Scottish & Southern Electricity Networks (SSEN) Transmission, with equipment supplied by Hitachi Energy. The system links Caithness, the Moray Firth, and Shetland at ±320 kV with three converter terminals carrying up to 1.2 GW. Caithness–Moray was energised in 2019; the Shetland link extension brings the third terminal online for offshore wind power transfer. Long-term service agreement with Hitachi Energy was extended in February 2025.

Significance: Europe’s first multi-terminal VSC HVDC system. Establishes the commercial pattern Australia would follow for the SBC corridor. Operating control architecture is provided as commercial product (Hitachi’s DC Grid Master Controller).

3.3 Changji–Guquan (China)

Energised 2024. ±1,100 kV LCC HVDC, 12 GW, 3,300 km, point-to-point only (two converter stations — one at each end). Losses 2.5% per 1,000 km, validated in operation.

Significance: establishes that ±1,100 kV bulk transfer at 12 GW per circuit over 3,000+ km is operationally proven. But: this is a point-to-point link. Multi-terminal at ±1,100 kV has not yet been built anywhere in the world.

3.4 What this means for the SBC

The reference projects bracket the architectural envelope cleanly. UHVDC at ±1,100 kV class works for bulk transfer over thousands of kilometres — but operationally only as point-to-point with two terminals. Multi-terminal VSC HVDC at ±320 kV (Caithness–Moray) and ±500 kV (Zhangbei) works with three to four terminals on one network. There is no operational precedent for a multi-terminal ±1,100 kV system. The SBC architecture should therefore not attempt one.

4. The Emerging Tech — DC-DC Converters

4.1 The "DC transformer" question

The natural follow-up question is whether a smaller, cheaper device exists that can step DC voltage between two levels — the way an AC transformer steps voltage between two AC levels — without needing a full converter station. Such a device is loosely called a DC transformer or, in the formal literature, a DC-DC converter. If commercially viable at high voltage and high power, it would change the architectural calculus dramatically.

The state of the field as of 2025 is: active, advancing, but not yet a mature commercial product at UHVDC class. A March 2025 review paper (Review of HVDC Tap, Chinese Journal of Electrical Engineering) summarises the research landscape. The dominant topology under development is the modular multilevel converter dual active bridge — an MMC structure with a high-frequency transformer providing galvanic isolation between two DC levels. Variants without isolation, using internally circulating AC power among converter switching cells, are also under research.

4.2 What works today, what doesn’t

The technical barrier at higher voltages is the high-frequency transformer that sits at the heart of the MMC dual active bridge topology. As DC operating voltage rises, the rate of voltage change (dv/dt) the transformer must withstand becomes increasingly difficult to manage. Above approximately ±200 kV, current designs run into materials and insulation limits. Research is active — ABB / Hitachi Energy, Siemens Energy, GE Vernova, and academic groups led by EPFL’s Power Electronics Laboratory (the Dujic group) are all working on the problem — but a commercial ±1,100 kV-to-±500 kV solid-state DC transformer is not available as a procurement item today.

4.3 What this means for the SBC

The architecture must work today, with commercial equipment available now, while leaving clear upgrade paths as DC-DC converter technology matures over the next decade.

Today: tap the UHVDC trunk only at full converter terminals — sparse, expensive, but available. Use a separate parallel HVDC backbone at lower voltage for the work that needs more frequent stations.

Future: as DC-DC converters mature at higher voltage classes (probably ±320–525 kV first, ±1,100 kV later), retrofit them along the corridor at the existing intermediate stations. The corridor easement and station foundations are sized for the upgrade from Day 1; only the tap equipment changes.

The SBC programme can commit to an architecture today that uses only commercially proven technology, while keeping the upgrade path explicit. The corridor easement is sized for what the corridor will be at maturity, not for what it can be in Phase 1. When the DC tap research matures, the corridor accepts the new equipment without civil rebuilding.

5. The Three-Level Architecture

The proposed architecture carries three voltage levels in parallel along the corridor easement. Each level is sized for its specific transmission task; each uses commercially proven technology; each can be deployed in stages as Phase 1 capacity is filled.

5.1 Level 1 — UHVDC trunk (±1,100 kV)

Purpose: bulk power transfer over the full corridor length. East–west grid integration. Long-haul export to coastal cable landings or to other corridors. Intercontinental links via submarine cable.

Technology: ±1,100 kV LCC HVDC bipole. Operationally proven by China State Grid’s Changji–Guquan link (12 GW, 3,300 km, 2.5% per 1,000 km losses, energised 2024).

Stations: two terminals (one at each corridor endpoint), plus one or two intermediate converter stations at strategic points where the corridor crosses major load centres or other corridors. Three to four UHVDC stations across a 2,500 km corridor — not at every town.

Capacity: 12 GW per circuit, two to four parallel circuits per corridor easement, total carrying capacity 24–48 GW per corridor. This carries the export-grade and inter-grid traffic.

Where it goes: in the corridor easement, on the HVDC arm of the MMC viaduct or on parallel pylon line where the corridor is at-grade. Conduit-buried in urban approaches.

5.2 Level 2 — HVDC backbone (±525 kV VSC)

Purpose: collect power from solar generation strips along the corridor. Distribute power inter-town along the corridor. Feed major industrial loads. Provide multi-terminal capability that the UHVDC trunk does not support.

Technology: ±525 kV VSC HVDC, modular multilevel converter (MMC) topology. The current emerging workhorse standard. Operationally established at ±500 kV by Zhangbei (4 terminals) and at ±320 kV by Caithness–Moray–Shetland (3 terminals); the ±525 kV class is in active deployment for offshore wind (Dogger Bank, Eastern Green Link 1, multiple North Sea projects). Prysmian’s 525 kV P-Laser cable supports up to 2.6 GW per cable.

Stations: converter stations every 300–500 km along the corridor — five to nine stations across a 2,500 km corridor. Each is much smaller and cheaper than a UHVDC terminal: typical capacity 2–4 GW per station, footprint 2–5 hectares, capex roughly US$400–800 million per station based on the $200M/GW VSC benchmark.

Capacity: 2–4 GW per circuit, two to three parallel circuits, total 6–12 GW carrying capacity along the corridor backbone. This handles the corridor-internal solar collection and inter-town transfer.

Where it goes: in the corridor easement, on dedicated HVDC arms of the viaduct or on parallel pylon line. Same easement as the trunk; different voltage levels physically separated.

5.3 Level 3 — MVDC distribution (±100 kV class)

Purpose: feed each corridor town. Power the maglev passenger and electric freight running along the multimodal viaduct. Tie in local battery storage. Supply the agrivoltaic operations under the solar arrays. Connect distributed solar farms (smaller than corridor strip scale) into the backbone.

Technology: MVDC at ±100 kV class, possibly extending to ±200 kV depending on local distance and load. Commercially mature for offshore wind collector networks and increasingly used for distribution. DC-DC converters at this voltage are available commercially today.

Stations: a step-down station at each corridor town — small footprint, comparable to a conventional medium-voltage substation. Town distribution after step-down can be conventional MVAC or local MVDC depending on the town design.

Capacity: sized to town load. A corridor town at maturity (population 50,000–200,000 per the SBC town planning) needs roughly 100–500 MW peak depending on industrial load. The MVDC layer scales by adding circuits.

Where it goes: in the corridor easement, on the lower service arms of the viaduct, alongside fibre, water, gas, and signalling.

5.4 How the levels connect

Power moves between voltage levels at the converter stations — that is what the stations are for. The architecture in summary:

• Solar generation along the strip connects into the ±525 kV HVDC backbone via local collection at MVDC, with step-up at the nearest backbone converter station.
• The HVDC backbone collects this generation along its 5–9 stations and feeds it to the load centres along the corridor and into the UHVDC trunk where the trunk has a converter terminal.
• The UHVDC trunk carries the surplus to distant export points or to other corridors.
• Each town taps the HVDC backbone via an MVDC step-down station.
• The maglev and electric freight draw from the MVDC distribution along the corridor.

Power flow is bidirectional at every level. A town with surplus solar feeds back into the backbone. A backbone with surplus feeds back into the trunk. The trunk supplies a distant deficient corridor or grid. The architecture is therefore not a feed tree from generation to load — it is a meshed three-level network with all nodes capable of importing or exporting at their own scale.

6. Worked Sketch — SBC Phase 1 East-West Corridor

SBC #1 (Brisbane → Perth, ~3,500 km) is the canonical example for the three-level architecture. It is the longest corridor, the east–west grid integrator (the first overland connection between Australia’s eastern and western grids), and the demonstration that the architecture scales to continental distance.

6.1 UHVDC trunk — SBC #1

Four UHVDC stations across 3,500 km. Capex order of magnitude: 4 stations × ~US$2.5B = ~US$10B. Trunk cable, towers, easement on top of that. Energy moved: 24–48 GW peak across the trunk.

6.2 HVDC backbone — SBC #1

Backbone converter stations are sited co-located with the larger corridor towns — the towns that already justify their own station for industrial load. Each station serves two functions: power tap from the backbone for the local town, and aggregation point for the solar collection in the immediate corridor strip. Capex per backbone station: roughly US$400–800M depending on capacity. Total backbone station capex: roughly US$4–6B across the corridor.

6.3 MVDC distribution — SBC #1

One step-down station per corridor town — approximately 35 towns at 100 km spacing. Each step-down is a conventional MVAC/MVDC substation at much lower capex (single-digit millions of dollars per substation). Total MVDC distribution capex is roughly US$200–400M for the corridor — a small fraction of the trunk and backbone capex.

6.4 The total picture

Compared with the alternative "one UHVDC station per town" approach (~35 UHVDC stations at ~$2.5B each = ~$87B in stations alone), the three-level architecture saves on the order of US$70 billion in converter station capex on a single corridor. Across six corridors, the saving is the difference between a financeable programme and a fantasy.

The three-level architecture is not optimisation around the margins. It is the difference between an SBC programme that can be built and a programme that cannot. The voltage architecture earns the corridor its economics.

7. Open Engineering Questions

The architecture proposed in this memo uses commercially proven technology at every level, but several engineering questions remain open and warrant explicit acknowledgement. Each is the subject of active industry research and is expected to mature on a timeline consistent with the SBC programme rollout.

7.1 DC tap technology at higher voltages

If solid-state DC-DC converters become commercially available at ±525 kV class within the next decade, the architecture changes. Backbone-to-MVDC tap-offs would no longer require a full conventional substation; they could be done with a smaller, cheaper, distributed device. This would let towns tap the HVDC backbone directly without the step-down station, reducing capex and footprint per town. The 2025 Review of HVDC Tap literature suggests this is a 5–15 year horizon depending on vendor investment.

SBC posture: design the corridor easement and station foundations for upgrade to direct DC tap when commercially available. Do not assume the technology in Phase 1; do not preclude it in Phase 2–3.

7.2 Multi-terminal control at scale

Caithness–Moray–Shetland (3 terminals) and Zhangbei (4 terminals) are operating examples. The SBC backbone proposes 7–9 terminals on a single corridor, and across multiple corridors the system topology becomes a meshed national HVDC grid with potentially 50–100 terminals. Multi-terminal control and protection at this scale has not been demonstrated anywhere. The UK National HVDC Centre and similar research facilities are working on this; the EU’s North Sea offshore wind grid is the closest operational analogue currently being developed.

SBC posture: deploy the backbone in a phased pattern that matches the operational maturity of multi-terminal control. Phase 1 of any single corridor uses 3–4 backbone terminals (Zhangbei-class). Phase 2 expands to 7–9 terminals as control architecture matures. Inter-corridor coupling is staged later.

7.3 Supply chain capacity

Converter transformers currently face approximately 12-month order backlogs at major vendors. If the SBC programme deploys six corridors over 15–20 years, peak demand for HVDC equipment will substantially exceed current global supply capacity. This is a real constraint and a real opportunity: it argues for sovereign Australian manufacturing of HVDC converter equipment as part of the SBC industrial programme.

SBC posture: stage corridor builds in sequence rather than parallel. Phase 1 (SBC #1 East-West) uses imported equipment from established vendors. Phase 2 onwards transitions progressively to Australian-manufactured HVDC equipment under licence or co-development. Local manufacturing of converter transformers, IGBT valve assemblies, and HVDC cable is a candidate for the SBC sovereign manufacturing programme.

7.4 DC circuit breakers and fault management

A multi-terminal HVDC network requires effective DC circuit breakers to isolate faults without taking down the whole network. Current commercial DC breakers exist at MV and HV class but are not yet mature at UHV. Research is active (DC-DC converters can act as circuit breakers, and several vendors offer hybrid mechanical-solid-state DC breakers), and the technology is improving. AI-enabled self-healing HVDC controls were deployed in major grids during 2024–2025 (Hitachi Energy, GE Vernova) — this technology is moving fast.

SBC posture: use point-to-point UHVDC at the trunk level (which does not need DC breakers), and multi-terminal VSC at the backbone level (where DC breakers are commercially available). Upgrade as DC breaker technology matures.

8. Why This Approach Now

The three-level architecture is not a bet on emerging technology. It is the application of existing commercial technology to the corridor problem, with the architectural decisions made to keep upgrade paths open as DC tap research matures.

This matters strategically because the SBC programme cannot wait for breakthrough technology to commit to a corridor design. The civil works — the easement, the foundations, the viaduct — are 30-year assets. They must be built to a design that works today, with the headroom to absorb the next two decades of DC equipment improvement.

The three-level voltage architecture answers that brief. UHVDC is operationally proven at 12 GW over 3,300 km. VSC HVDC is operationally proven at 4 terminals. MVDC is mature distribution technology. Putting all three in the same corridor easement is an integration challenge, not a technology development challenge. The architecture is buildable now.

The upgrade pathway as DC-DC converters mature, multi-terminal control scales up, and DC circuit breakers reach UHV is explicit and progressive. Each generation of equipment finds its place in the architecture without civil rebuilding. The corridor accepts what the equipment market produces.

The right framing for the SBC corridor power architecture is not "we have invented something new". It is "we have understood what is commercially available, recognised that one voltage class cannot do all three transmission tasks, and proposed an architecture in which each level uses what it can use today and accepts what becomes available tomorrow." That is engineering discipline, and it is what the SBC programme commits to.

9. Assumptions and Caveats

The cost figures in this memo are pre-feasibility benchmarks — within approximately ±30% of detailed engineering values. The architectural conclusions — that three voltage levels are required, that UHVDC at every town is uneconomic, that emerging DC-DC converter technology offers a clear upgrade path — are robust to substantial variation in any single assumption. Detailed engineering studies are required at the corridor design stage to convert the architecture into binding specifications.

10. Next Steps

• Engage chartered electrical engineers experienced in HVDC system design to validate the three-level architecture against detailed Australian load and generation cases.
• Commission a feasibility study for SBC #1 East-West corridor specifically — trunk station siting, backbone station spacing, multi-terminal control architecture, supply chain.
• Initiate dialogue with HVDC equipment vendors (Hitachi Energy, Siemens Energy, GE Vernova, plus China State Grid’s manufacturing arm) on capacity, lead time, technology roadmap, and Australian manufacturing partnerships.
• Engage academic research groups working on DC-DC converter technology — EPFL, the Edinburgh and Aberdeen universities, Tsinghua University — on emerging tap technology and timelines.
• Develop a companion memo on submarine cable engineering for the Asia export link — the demand-side counterpart to this trunk-architecture memo.
• Develop a companion memo on Australian sovereign manufacturing capacity for HVDC equipment — converter transformers, IGBT valve assemblies, HVDC cable — as part of the SBC industrial programme.
• Engage AEMO and Western Power on grid-coupling specifications at the trunk terminal endpoints (Brisbane, Perth) and on the regulatory framework for cross-NEM/SWIS bulk transfer.