HVDC & MVDC Transmission

High-voltage direct current (HVDC) and medium-voltage direct current (MVDC) systems carry electricity as DC. In specific applications DC has four advantages over AC:

No reactive power losses
No frequency synchronization needed between networks
Precise bidirectional power flow control
Lower losses per kilometer at very long distances and in subsea cables

HVDC is the standard choice for offshore wind transmission, long-distance bulk power corridors and asynchronous grid interconnections. MVDC is emerging for campus DC distribution, EV fleet depots, data center power and microgrid backbones.

Both depend on the Voltage Source Converter (VSC), built on IGBT or SiC transistors. VSC turned HVDC from a specialized long-distance tool into a flexible grid asset with grid-forming, black start and dynamic reactive power support. Modern VSC-HVDC and MVDC use the same SiC power semiconductor supply chain as EV traction inverters, BESS power conversion systems and solid-state transformers.

See: Grid Infrastructure Overview | Solid-State Transformers | Power Electronics Supply Chain

HVDC vs. HVAC: When DC Wins

Attribute	HVAC Transmission	HVDC Transmission
Losses at Distance	Resistive + reactive; practical limit ~800-1,000 km overhead	Resistive only; wins above ~600-800 km overhead, ~50-70 km subsea
Subsea / Underground	Cable limit 50-100 km without reactive compensation	No capacitive limit; subsea links routinely exceed 500 km
Grid Synchronization	Requires frequency synchronization	Inherently asynchronous
Power Flow Control	Set by line impedance; hard to control	Precise, bidirectional, millisecond control (VSC)
Grid Services (VSC)	None (passive line)	Reactive power, voltage regulation, frequency response, black start, grid-forming
Cable Right-of-Way	Three conductors; larger footprint	Two conductors (bipolar); smaller footprint
Converter Station Cost	None; transformer + switchgear only	$100M-$500M+ per terminal

HVDC vs. HVAC Notes

Losses at Distance: The reactive component of HVAC losses grows with distance. For offshore wind beyond 70 km from shore, HVDC is economically mandatory; for long overland corridors, HVDC is often cheaper in total cost despite higher converter station cost.
Subsea / Underground: Capacitive charging current of underground and subsea AC cable limits its practical length without reactive compensation. HVDC enables offshore wind farms beyond AC reach: all offshore wind projects beyond ~80 km must use HVDC export cables, and the entire UK North Sea offshore wind buildout is HVDC-dependent.
Grid Synchronization: AC cannot link asynchronous AC systems without complex SVC/STATCOM. HVDC converter stations on each end are independent, so HVDC links different frequency zones and previously isolated grids (e.g., Eastern/Western US interconnects, EU cross-border links) without frequency control complexity.
Power Flow Control: Power flows on AC lines according to impedance, and loop flows are a chronic reliability problem. VSC-HVDC provides real-time bidirectional control in milliseconds with fully controllable dispatch: an HVDC link can be dispatched like a generator - turn up, turn down, reverse direction on demand. AC transmission cannot.
Grid Services (VSC): An AC transmission line provides no grid services itself. Modern VSC-HVDC terminals are grid assets, not just wires; they can stabilize weak grids at the point of connection.
Cable Right-of-Way: Bipolar HVDC has a smaller physical and electromagnetic footprint for the same power capacity, so it can be routed through more constrained corridors; important for urban underground transmission.
Converter Station Cost: Economic breakeven depends on line length and capacity. HVDC is only economic above a minimum line length and power capacity threshold; short, low-power links use AC.

LCC vs. VSC HVDC Converters

HVDC converter stations convert between AC and DC using one of two converter technologies.

Line-Commutated Converter (LCC-HVDC)

The original HVDC technology, built on thyristors
Needs a strong AC grid at both ends to commutate
Handles very high power, up to 12 GW per bipole, with lower losses than VSC at very high power levels
Cannot provide reactive power support or black start
Cannot connect to weak AC grids or offshore wind without additional equipment
Dominant for long-distance terrestrial bulk power corridors; the Three Gorges-Shanghai link (7.2 GW) and most Chinese ultra-HVDC systems are LCC

Voltage Source Converter (VSC-HVDC)

Modern technology built on IGBT or SiC transistors
Connects to any AC system, including weak grids, passive loads and offshore wind, without additional equipment
Provides reactive power, voltage regulation, frequency response and black start
Reverses power flow without changing current direction, which simplifies control
Rated up to 2-3 GW per bipole; losses are higher than LCC at very high power, with SiC closing the gap
Used for all offshore wind HVDC; SunZia uses Hitachi Energy HVDC Light (VSC)

The industry is moving toward VSC as power ratings rise, costs fall and SiC devices bring VSC efficiency toward LCC levels. New multi-terminal HVDC grids, the emerging "DC superhighways," use VSC because only VSC can manage the dynamic power flows a meshed DC grid requires.

Major HVDC Projects: US & Global

Project	Capacity	Length	Technology	Status
SunZia Transmission (US) Pattern Energy / Hitachi Energy	3,000 MW	553 miles (890 km); ±525 kV	VSC-HVDC (Hitachi HVDC Light®)	Commissioning 2026
Champlain Hudson Power Express (US) Transmission Developers Inc.	1,250 MW	339 miles; ±320 kV underground/underwater	VSC-HVDC	Under construction; target 2026
TransWest Express (US) TransWest Express LLC	3,000 MW	732 miles; ±600 kV	VSC-HVDC	Permitting and development
NorthSeaDEO (North Sea DC Grid) Multiple EU TSOs	Up to 65 GW by 2040	Multiple interconnectors	VSC-HVDC; multi-terminal offshore grid	Active development
Three Gorges-Shanghai (China) State Grid Corporation of China	7,200 MW (±500 kV bipole)	~1,000 km	LCC-HVDC	Operational
Dogger Bank Wind (UK) SSE / Equinor / Vårgrønn	3,600 MW (3 phases)	195 km offshore; ±320 kV	VSC-HVDC (Siemens Energy HVDC Plus)	Phased; Phase C under construction

Project Notes

SunZia Transmission: Largest VSC installation in US history; Quanta Services EPC. Construction complete, with HVDC converter stations in service end of 2025 and commissioning in 2026. Connects the SunZia 3.5 GW wind farm in New Mexico to Arizona/California; largest renewable-linked HVDC project in North America; $11B total project cost.
Champlain Hudson Power Express: Underground cable from Quebec hydro to New York City, with an operational target of 2026. Delivers Canadian hydro to the NYC load center and enables Quebec renewable export.
TransWest Express: Carries Wyoming wind resources to Nevada/California, connecting Interior West wind to Pacific Coast demand; DOE loan guarantee support.
NorthSeaDEO: Multiple interconnectors forming a North Sea offshore network; the largest planned HVDC network in the world. National programs in the UK (NSWPH), Netherlands, Belgium, Germany and Denmark all contribute; IFA2 (1 GW UK-France) has been operational since 2021, and more links are under construction.
Three Gorges-Shanghai: One of multiple Yangtze corridor ultra-HVDC links. China has more HVDC capacity installed than the rest of the world combined, and State Grid operates 12 ultra-HVDC links above ±800 kV.
Dogger Bank Wind: World's largest offshore wind farm, with ±320 kV HVDC export cables; a benchmark for offshore HVDC wind export. Phase A first power 2023; Phase B commissioning 2024-2025; Phase C under construction.

MVDC: Medium-Voltage DC Distribution

MVDC operates at 1-35 kV and is emerging as an alternative to MVAC distribution for high-value applications. HVDC handles bulk transmission. MVDC handles the last mile to DC-native loads: solar farms, BESS, EV depots, data centers, microgrids and industrial sites.

The main benefit is fewer conversion stages. Solar DC today is inverted to MVAC, stepped up to HVAC, transmitted, then stepped down and converted back to DC for EV chargers, BESS and data center servers. Each conversion costs 1-3% efficiency and adds a failure mode. MVDC distribution with solid-state transformers (SSTs) at the grid interface stays DC throughout, removing two to four conversion stages per delivered kWh.

MVDC applications:

EV Fleet Energy Depots (FEDs): an MVDC backbone feeds DC fast chargers (DCFC) from the utility interconnection, with an SST at the grid interface. Megawatt Charging System (MCS) sites at 1-3 MW are natural MVDC load nodes. WattEV's SST for depot charging is an early commercial MVDC application.
Data center power distribution: 380V DC distribution inside data centers is already deployed at scale in Google, Microsoft and Meta facilities. MVDC extends the concept to the building and campus interconnection, and Heron Link's 600V DC output targets this architecture.
Offshore wind collection: MVDC collection from turbines to the offshore substation reduces conversion losses compared with MVAC collection. Siemens Energy and ABB are both developing MVDC collection systems.
Industrial microgrids: port electrification, mining and manufacturing sites with high DC load density avoid an inverter at each load point.
Military base power: the Department of Defense (DOD) is interested in MVDC microgrids for base resilience, where DC fault isolation supports hardened grid architectures.

Solid-State Substations

A solid-state substation replaces passive equipment (transformers, oil-filled switchgear, mechanical circuit breakers and analog protection relays) with power electronics and software-defined protection and control. The result is smaller, faster-responding and software-programmable, and it handles DC as readily as AC.

Conventional substation design has changed little since the early 20th century. Oil-filled transformers take weeks to replace when they fail. Mechanical circuit breakers operate in tens of milliseconds, fast enough for AC fault clearance but too slow for DC fault isolation. Analog protection relays need manual reconfiguration when network topology changes.

Components

Solid-State Transformer (SST): replaces the oil-filled transformer. It adds active voltage regulation, bidirectional power flow, AC and DC outputs, a modular hot-swap design and 10x-100x faster fault response than mechanical transformer protection. See the SST page.
Solid-State Circuit Breaker (SSCB): operates in microseconds against tens of milliseconds for a mechanical breaker. That speed is critical for DC faults, where arc energy is higher and more sustained than in AC. ABB, Eaton and General Electric are developing commercial SSCBs, and hybrid mechanical-solid-state designs are emerging as a cost-effective near-term option.
Software-defined protection: software-configurable protection functions replace hardwired analog relays and reconfigure automatically for topology changes. IEC 61850 GOOSE messaging gives sub-millisecond protection coordination and enables adaptive protection schemes that analog relays cannot support.
Power flow controller: actively manages real and reactive power on connected feeders, enabling congestion management and loop flow control.
Digital twin integration: a real-time digital twin of substation state supports predictive maintenance, fault simulation and control system testing without physical outages.

Deployment Status

No complete, integrated solid-state substation is in commercial deployment. The components are at different maturity levels:

SSTs: advanced prototype or early commercial (Heron Power 2027 target)
SSCBs: pilot deployment (ABB, Eaton)
Software-defined protection: commercially deployed (IEC 61850 systems)
Digital twins: deployed in monitoring roles

The Electric Power Research Institute (EPRI) and DOE national laboratories are running demonstration programs. Complete systems are a 2028-2035 commercial horizon for utility-scale deployment. Early partial implementations in data center campuses and FEDs are possible by 2026-2027.

Component Vendors

Vendor	Solid-State Substation Components
ABB	SST, SSCB, protection
Siemens Energy	SST, digital substation
Eaton	SSCB, power distribution
Hitachi Energy	Transformer, protection, digital twin
Schneider Electric	Protection, SCADA, EMS
Heron Power	SST (primary US startup)
General Electric	SSCB, protection relay

Offshore Wind HVDC

Offshore wind is the main near-term driver of new HVDC investment globally. Wind farms more than 70-80 km from shore cannot economically use HVAC submarine cables because of capacitive charging current losses, so each one needs HVDC export infrastructure. The UK alone has over 50 GW of offshore wind in development.

Converter stations: Hitachi Energy, Siemens Energy, ABB (now part of Hitachi Energy) and GE Vernova supply essentially all offshore HVDC converter stations globally.
Submarine cables: Prysmian (IT), Nexans (FR), NKT (DK) and LS Cable (KR). Cable factories are booked years in advance, and cable capacity is the binding constraint on European offshore wind HVDC buildout.
Cable laying vessels (CLVs): Prysmian, Nexans and Boskalis each operate CLVs. Vessel availability is a secondary constraint.

The North Sea is the global hub of offshore HVDC development. Over the mid to long term, the DOE envisions a network of offshore HVDC interlinks along the Atlantic Coast, a US counterpart to the North Sea DC grid.

Technology Stack

Component	Function	Technology / Standards
VSC Converter Station	AC/DC conversion; grid services	IGBT or SiC; MMC topology; IEC 62477
DC/DC Converters	DC voltage step-up/down and isolation	Dual Active Bridge (DAB); SiC-based
DC Cables	Submarine or underground transmission	XLPE, PPLP or MI insulation; IEC 62895
DC Switchgear & Breakers	Protection, segmentation, fault isolation	Mechanical, hybrid and solid-state DC breakers
Control & Protection	Power flow, fault isolation, grid services dispatch	IEC 61850-90-14; CIGRÉ TB 496; vendor systems

Technology Stack Notes

VSC Converter Station: Performs AC/DC conversion at both ends of the HVDC link, with grid-forming and grid services capability. Uses IGBT or SiC transistors in a modular multilevel converter (MMC) topology; IEC 62477 is the safety standard.
DC/DC Converters: Steps voltage up or down within the DC network and isolates DC voltage levels. The DAB (Dual Active Bridge) topology is used for high-power isolated DC/DC; SiC-based; a key component in SST and MVDC.
DC Cables: Carry power on submarine or underground routes with no reactive power compensation needed. XLPE or PPLP insulation for submarine; MI (mass impregnated) for very high voltage; IEC 62895 covers HVDC cables.
DC Switchgear & Breakers: Mechanical DC breakers (Alstom/GE, 2012 80 kV prototype); hybrid mechanical-solid-state (ABB); full solid-state (emerging). DC fault isolation is technically harder than AC due to sustained arc energy.
Control & Protection: Handles real-time power flow, fault detection and isolation, and grid services dispatch. IEC 61850-90-14 covers DC systems communication and CIGRÉ TB 496 covers DC grid protection; vendor control systems are proprietary, with SCADA and DERMS integration.

HVDC/MVDC Vendors

Hitachi Energy (CH/JP): world's largest HVDC installed base, over 70 years of HVDC experience and more than half of global HVDC projects. Offers HVDC Light (VSC) and classic HVDC (LCC). Primary vendor for SunZia, the largest US VSC project, and supplier to Champlain Hudson Power Express.
Siemens Energy (DE): HVDC Plus (VSC) for offshore wind, including Dogger Bank; strong in the European offshore market; also developing solid-state substation components.
GE Vernova (US): HVDC systems and converter technology with domestic manufacturing; growing offshore wind HVDC position; also supplies wind turbines on many HVDC-connected projects.
Prysmian (IT): largest submarine cable manufacturer; HVDC cable for most European offshore projects, plus terrestrial HVDC cable; manufacturing capacity constrained.
Nexans (FR): major submarine HVDC cable supplier; operates cable laying vessels, including the Aurora.
NKT (DK): Denmark-based HVDC cable manufacturer focused on the North Sea offshore market.
Eaton (US/IE): MVDC switchgear, solid-state circuit breakers and power distribution for DC microgrids and EV depots.
Heron Power (US): SST-based MVDC interface. Heron Link targets solar, BESS and data center MV-to-DC applications. Raised a $140M Series B in February 2026; pilot production planned for 2027.

Grid HVDC & MVDC Transmission