HVDC Fundamentals: Why High-Voltage Direct Current Is Essential for the Energy Transition

For most of electrical engineering history, high-voltage direct current (HVDC) transmission was a specialist technology — powerful, technically impressive, but expensive and relatively rare. That's changing fast.

As offshore wind farms move further from shore, as grids need to transfer power across longer distances, and as European countries look to share renewable resources across borders, HVDC is becoming a core tool in the power system engineer's toolkit.

AC vs DC Transmission: The Basics

The electricity that comes out of your socket at home is alternating current (AC). The frequency alternates — 50Hz in the UK and Europe, 60Hz in North America — and this has been the dominant technology for electricity transmission since the late 19th century.

The reason AC won the original "war of currents" is primarily economic. Transformers allow AC voltage to be stepped up and down efficiently, which made high-voltage transmission over long distances practical. High voltage reduces the current, which reduces the resistive losses in the cable.

DC doesn't have transformers in the traditional sense. For most of electrical history, converting between DC voltage levels was technically difficult and expensive. That's no longer true.

Modern HVDC systems use power electronics — specifically, Voltage Source Converters (VSC) — to convert efficiently between AC and DC, and between different DC voltage levels. The technology has matured substantially over the past two decades.

Why DC for Long Distances?

For transmission distances beyond roughly 600-700km overhead line or 50-80km underground or submarine cable, HVDC becomes economically competitive with AC transmission.

The reason for the shorter breakeven distance for cables relates to the capacitive charging current that flows in AC cables. At high AC frequencies, long cables require significant reactive compensation, which adds cost and complexity. DC cables don't have this problem — there's no frequency, so no capacitive charging current.

This is why virtually every long submarine interconnector — the links that connect the UK to France, Norway, and the Netherlands — uses HVDC technology.

VSC-HVDC and Offshore Wind

The development of VSC-HVDC technology was partly driven by the need to connect offshore wind farms. For projects more than about 80km offshore, AC transmission becomes impractical, and VSC-HVDC is the technology of choice.

The UK's offshore wind ambitions — the government target is 50GW by 2030 — will require several major HVDC transmission projects. The Eastern Green Link projects, connecting Scotland's renewable resources to demand centres in England, are among the largest infrastructure projects currently in development in the UK.

Multi-Terminal HVDC and the Meshed Grid

The next frontier is multi-terminal HVDC systems — networks with more than two converter stations connected to the same DC link. This moves towards what's sometimes called a "DC Supergrid" — a high-voltage DC backbone overlaid on the existing AC network.

Multi-terminal HVDC systems introduce significant technical challenges, particularly around DC fault protection. Unlike AC systems, where current naturally passes through zero twice per cycle (making it relatively straightforward to interrupt), DC fault currents don't naturally extinguish. DC circuit breakers capable of interrupting high DC fault currents at transmission voltages are an active area of research and development.

Where This Is Heading

The economics and technical maturity of HVDC technology are both improving. The cost of power electronics continues to fall. Grid operators in Europe are increasingly planning for meshed offshore grid topologies that would have seemed speculative a decade ago.

For power systems engineers, HVDC competence is becoming increasingly valuable. If you're working in transmission or offshore energy and haven't invested time in understanding VSC-HVDC fundamentals, now is a good time to start.

I've worked on HVDC feasibility studies at WSP UK and happy to discuss technical questions. Get in touch via the contact page.