HVDC & MVDC Transmission | Grid Modernization

High Voltage Direct Current (HVDC) and Medium Voltage Direct Current (MVDC) are power transmission technologies that carry electricity as DC rather than AC. DC transmission has fundamental physics advantages over AC for specific applications: it does not suffer reactive power losses, does not require frequency synchronization across networks, enables precise bidirectional power flow control, and loses less energy per kilometer at very long distances and in subsea cables. These advantages have made HVDC the standard choice for offshore wind transmission, long-distance bulk power corridors, and asynchronous grid interconnections — and are now making MVDC an emerging choice for campus-level DC distribution, EV fleet depots, data center power, and microgrid backbones.

The enabling technology for both HVDC and MVDC is the power electronics converter — specifically the Voltage Source Converter (VSC) based on IGBT or SiC transistors. VSC technology has transformed HVDC from a specialized long-distance tool into a flexible grid asset capable of grid-forming, black start, and dynamic reactive power support. The same SiC power semiconductor supply chain that enables EV traction inverters, BESS power conversion systems, and solid-state transformers also underlies modern VSC-HVDC and MVDC infrastructure.

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

HVDC vs. HVAC - When DC Wins

Attribute	HVAC Transmission	HVDC Transmission	Implication
Losses at Distance	Resistive + reactive losses; reactive component grows with distance; practical limit ~800-1,000 km overhead	Resistive losses only; no reactive power; economically superior above ~600-800 km overhead, ~50-70 km subsea	For offshore wind beyond 70 km from shore, HVDC is economically mandatory; for long overland corridors, HVDC often cheaper in total cost despite higher converter station cost
Subsea / Underground	Capacitive charging current of underground/subsea cable limits practical AC cable length to 50-100 km without reactive compensation	No capacitive current limitation; subsea HVDC cables routinely exceed 500 km; enables offshore wind farms beyond AC reach	All offshore wind projects beyond ~80 km must use HVDC export cables; the entire UK North Sea offshore wind buildout is HVDC-dependent
Grid Synchronization	Requires frequency synchronization across the connected grid; cannot link asynchronous AC systems without complex SVC/STATCOM	Inherently asynchronous; converter stations on each end are independent; links different frequency zones without synchronization	HVDC enables interconnection of 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; difficult to control precisely; loop flows are a chronic reliability problem	VSC-HVDC provides precise, real-time bidirectional power flow control in milliseconds; fully controllable dispatch	HVDC links can be dispatched like a generator — turn up, turn down, reverse direction on demand; AC transmission cannot
Grid Services (VSC)	Passive; no grid services from transmission line itself	VSC-HVDC provides reactive power support, voltage regulation, frequency response, black start capability, grid-forming	Modern VSC-HVDC terminals are grid assets, not just wires; they can stabilize weak grids at the point of connection
Cable Right-of-Way	Three-phase AC requires three conductors; larger physical footprint	Bipolar HVDC requires two conductors; smaller physical and electromagnetic footprint for same power capacity	HVDC can be routed through more constrained corridors; important for urban underground transmission
Converter Station Cost	No converter stations needed; transformer + switchgear only	High converter station cost ($100M-$500M+ per terminal); 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 - The Two HVDC Technologies

All HVDC systems use converter stations to convert between AC and DC, but there are two fundamentally different converter technologies with different capabilities and use cases:

Line-Commutated Converter (LCC-HVDC) - the original HVDC technology; uses thyristors; requires a strong AC grid at both ends to commutate; can handle very high power (up to 12 GW per bipole); 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 bulk power long-distance terrestrial corridors; Three Gorges–Shanghai link (7.2 GW) and most Chinese ultra-HVDC systems are LCC
Voltage Source Converter (VSC-HVDC) - modern technology based on IGBT or SiC transistors; does not require a strong AC grid; can connect 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; bidirectional power reversal without changing current direction (easier control); power rating up to 2-3 GW per bipole currently; higher losses than LCC at very high power but closing the gap with SiC; all offshore wind HVDC uses VSC; SunZia uses Hitachi Energy HVDC Light (VSC)

The industry direction is clearly toward VSC. Power ratings are increasing, costs are falling, and SiC devices are improving VSC efficiency toward LCC levels. New multi-terminal HVDC grids (the emerging "DC superhighways") use VSC because only VSC can handle the dynamic power flow management 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®); largest VSC installation in US history; Quanta Services EPC	Construction complete / commissioning 2026; HVDC converter stations in service end of 2025; connects 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 (US) Transmission Developers Inc.	1,250 MW	339 miles; ±320 kV underground/underwater	VSC-HVDC; underground cable from Quebec hydro to New York City	Under construction; operational target 2026; delivers Canadian hydro to NYC load center; enables Quebec renewable export
TransWest Express (US) TransWest Express LLC	3,000 MW	732 miles; ±600 kV	VSC-HVDC; Wyoming wind resources to Nevada/California	Permitting and development; DOE loan guarantee support; connects Interior West wind to Pacific Coast demand
NorthSeaDEO (North Sea DC Grid) Multiple EU TSOs	Up to 65 GW by 2040	Multiple interconnectors; North Sea offshore network	VSC-HVDC; multi-terminal offshore DC grid; largest planned HVDC network in the world	Active development; national programs in UK (NSWPH), Netherlands, Belgium, Germany, Denmark all contributing; IFA2 (1 GW UK-France) operational 2021; more links under construction
Three Gorges–Shanghai (China) State Grid Corporation of China	7,200 MW (±500 kV bipole)	~1,000 km	LCC-HVDC; multiple Yangtze corridor ultra-HVDC links	Operational; China has more HVDC capacity installed than the rest of the world combined; State Grid operating 12 ultra-HVDC links above ±800 kV
Dogger Bank Wind (UK) SSE / Equinor / Vårgrønn	3,600 MW (3 phases)	195 km offshore; ±320 kV HVDC export cables	VSC-HVDC; Siemens Energy HVDC Plus; world's largest offshore wind farm	Phase A first power 2023; Phase B commissioning 2024-2025; Phase C under construction; benchmark for offshore HVDC wind export

MVDC - Medium Voltage DC Distribution

MVDC operates in the 1-35 kV range and is emerging as an alternative to traditional MVAC distribution for specific high-value applications. Where HVDC addresses bulk transmission, MVDC addresses the last mile of power delivery to DC-native loads — solar farms, BESS, EV depots, data centers, microgrids, and industrial sites — without the multiple AC/DC conversion stages that currently add losses, cost, and failure points.

The core MVDC value proposition: eliminate conversion stages. A solar farm today converts DC power from panels through an inverter to MVAC, steps up to HVAC via a transformer, transmits, then steps down and converts back to DC for EV chargers, BESS, and data center servers. Each conversion is 1-3% efficiency loss and one more failure mode. An MVDC distribution system with SSTs at the grid interface maintains DC throughout, eliminating two to four conversion stages per delivered kWh.

Key MVDC application contexts:

EV Fleet Depots (FED) - MVDC backbone distributes power from utility interconnection to DCFC chargers without intermediate AC conversion; SST at grid interface; 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 (Google, Microsoft, Meta facilities); MVDC extends this concept to the building/campus interconnection level; Heron Link's 600V DC output targets this architecture
Offshore Wind Collection - MVDC collection within wind farms (from turbines to offshore substation) reduces conversion losses vs MVAC collection; Siemens Energy and ABB both developing MVDC collection systems
Industrial Microgrids - port electrification, mining, and manufacturing sites with high DC load density benefit from MVDC distribution; eliminates inverter at each load point
Military Base Power - DOD interest in MVDC microgrids for base resilience; DC fault isolation capability important for hardened grid architectures

Solid-State Substations - The Next Grid Architecture

A solid-state substation replaces the conventional passive substation — transformers, oil-filled switchgear, mechanical circuit breakers, and analog protection relays — with fully power-electronics-based active equipment: solid-state transformers (SSTs), solid-state circuit breakers (SSCBs), and software-defined protection and control. The result is a substation that is dramatically smaller, faster-responding, software-programmable, and capable of managing DC as naturally as AC.

The conventional substation has not changed fundamentally 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 require manual reconfiguration for network topology changes. A solid-state substation addresses all of these limitations simultaneously.

Key components of a solid-state substation:

Solid-State Transformer (SST) - replaces oil-filled transformer; active voltage regulation; bidirectional power flow; AC and DC port outputs; modular hot-swap design; 10x-100x faster fault response than mechanical transformer protection; see SST page for full coverage
Solid-State Circuit Breaker (SSCB) - replaces mechanical circuit breaker; operates in microseconds vs tens of milliseconds for mechanical; critical for DC fault isolation where arc energy is higher and more sustained than AC; ABB, Eaton, and General Electric all developing commercial SSCBs; hybrid mechanical-solid-state designs emerging as cost-effective near-term solution
Software-Defined Protection - replaces hardwired analog protection relays with software-configurable protection functions; reconfigures automatically for network topology changes; IEC 61850 GOOSE messaging for sub-millisecond protection coordination; enables adaptive protection schemes impossible with analog relays
Power Flow Controller - active management of real and reactive power flows on connected feeders; enables congestion management and loop flow control that passive substations cannot provide
Digital Twin Integration - real-time digital twin of substation state for predictive maintenance, fault simulation, and control system testing without physical outages

Deployment status: Solid-state substations do not yet exist as complete integrated systems in commercial deployment. The components are at different maturity levels: SSTs are in advanced prototype/early commercial (Heron Power 2027 target); SSCBs are in pilot deployment (ABB, Eaton); software-defined protection is commercially deployed (IEC 61850 systems); digital twins are deployed in monitoring roles. EPRI and DOE national laboratories are running demonstration programs. The solid-state substation as a complete system is a 2028-2035 commercial horizon for utility-scale deployment, but early partial implementations in data center campuses and FEDs are possible by 2026-2027.

Vendors active in 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 - The Primary HVDC Growth Driver

Offshore wind is the dominant near-term driver of new HVDC investment globally. The physics are unambiguous: wind farms more than 70-80 km from shore cannot economically use HVAC submarine cables due to capacitive charging current losses. Every offshore wind farm beyond this threshold requires HVDC export infrastructure. As the global offshore wind buildout accelerates — the UK alone has over 50 GW of offshore wind in development — the associated HVDC cable and converter station market is growing proportionally.

The offshore HVDC supply chain is concentrated among a small number of capable vendors:

HVDC converter stations - Hitachi Energy (HVDC Light, VSC), Siemens Energy (HVDC Plus, VSC), ABB (now part of Hitachi Energy), GE Vernova (HVDC, VSC); these four vendors supply essentially all offshore HVDC converter stations globally
Submarine HVDC cables - Prysmian (IT), Nexans (FR), NKT (DK), LS Cable (KR); massively capacity-constrained; cable factories are booked years in advance; cable manufacturing capacity is currently the binding constraint on offshore wind HVDC buildout in Europe
Cable laying vessels - specialized cable lay vessels (CLVs) required for subsea installation; Prysmian, Nexans, and Boskalis each operate CLVs; vessel availability is a secondary constraint after cable manufacturing

The North Sea is the global epicenter of offshore HVDC development. Over the mid to long term, the DOE envisions an offshore network of HVDC interlinks along the Atlantic Coast that could more efficiently bring offshore wind energy onshore — a US equivalent of the North Sea DC grid concept.

Technology Stack

Component	Function	Technology / Standards
VSC Converter Station	AC/DC conversion at both ends of HVDC link; grid-forming and grid services capability	IGBT or SiC transistors; modular multilevel converter (MMC) topology; IEC 62477 safety standard
DC/DC Converters	Voltage step-up/down within DC network; isolation between DC voltage levels	DAB (Dual Active Bridge) topology for high-power isolated DC/DC; SiC-based; key component in SST and MVDC
DC Cables	Power transmission; submarine or underground; no reactive power compensation needed	XLPE or PPLP insulation for submarine; MI (mass impregnated) for very high voltage; IEC 62895 for HVDC cables
DC Switchgear & Breakers	Protection and segmentation; fault isolation in DC networks	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	Real-time power flow, fault detection and isolation, grid services dispatch	IEC 61850-90-14 (DC systems communication); CIGRÉ TB 496 (DC grid protection); proprietary vendor control systems; SCADA and DERMS integration

HVDC/MVDC Vendors

Hitachi Energy (CH/JP) - world's largest HVDC installed base; over 70 years of HVDC experience; more than half of global HVDC projects; HVDC Light (VSC) and classic HVDC (LCC); primary vendor for SunZia (US largest VSC project); also supplying Champlain Hudson Power Express
Siemens Energy (DE) - HVDC Plus (VSC) for offshore wind; Dogger Bank offshore wind HVDC; strong in European offshore market; also developing solid-state substation components
GE Vernova (US) - HVDC systems and converter technology; US-based 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; also supplies terrestrial HVDC cable; manufacturing capacity constrained
Nexans (FR) - major submarine HVDC cable supplier; operates cable laying vessels; Aurora cable lay vessel
NKT (DK) - European HVDC cable manufacturer; Denmark-based; North Sea offshore focus
Eaton (US/IE) - MVDC switchgear, solid-state circuit breakers, 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; $140M Series B February 2026; pilot production 2027

Related Coverage

Supply Chain: Power Electronics SC | SiC & GaN Substrate

Applications: Fleet Energy Depot (MVDC) | Megawatt Charging | EV Fleet Charging

Parent: Grid Hub | Energy Hub

Grid HVDC & MVDC Transmission