HVDC Circuit Breakers: What They Are and Why DC Is Hard to Switch Off

AC current passes through zero a hundred times a second. DC never does. That is why a high-voltage DC breaker has to make its own zero, and why real DC grids waited so long.

Oct 06, 2026

TL;DR

A circuit breaker has to kill the electric arc that forms when its contacts open. Alternating current (AC) passes through zero 100 or 120 times a second, and the arc dies at a zero. Direct current (DC) never passes through zero, so the arc keeps burning.
An HVDC circuit breaker has to create its own current zero, absorb the energy stored in the line, and do it within about 3 to 5 milliseconds, before the fault current grows too large.
The design that made it practical is the hybrid breaker, first shown by ABB in 2012. Normal current flows through a simple metal path; in a fault, power electronics take over and cut the current in milliseconds.
China’s Zhangbei DC grid near Beijing, a four-terminal ±500 kV network in service since 2020, uses 16 hybrid breakers that can each cut 25,000 amps in 3 milliseconds.
Germany’s four grid operators plan multi-terminal DC hubs at 525 kV, with DC breakers, for North Sea wind, aimed at commercial use from 2029.

Listen up.

Short answer first: an HVDC circuit breaker is a switch that can cut high-voltage direct current during a fault. Ordinary breakers rely on the current passing through zero to put out the spark. Direct current never does. So an HVDC breaker has to force the current to zero itself, soak up the energy, and do it in a few thousandths of a second. That is why it is a special machine, and why real DC grids took so long to arrive.

What does a circuit breaker actually do?

Your house has breakers. Something shorts, one clicks, power dies in that room, and the rest of the house stays on.

A grid breaker does the same job for a transmission line carrying hundreds of megawatts. It must spot the fault, open, and isolate just the broken piece so everything else keeps running.

The hard part is not opening the contacts. It is what happens between them.

Why is AC easy to switch off?

When you pull two contacts apart while current is flowing, the current does not simply stop. It jumps the gap as an electric arc, a very hot channel of glowing gas that conducts electricity.

Alternating current (AC), what homes and most grids use, swings back and forth 50 or 60 times a second. So it passes through zero 100 or 120 times a second. At each zero, the arc briefly has no current to feed it. A good AC breaker blasts the gap with gas at that moment, the arc goes out, and the line stays off.

Nature gives you that zero for free. An AC breaker just waits for it.

Why is DC so hard to switch off?

Direct current (DC) flows one way only, like a battery. At very high voltage, hundreds of thousands of volts, it is called HVDC. It is used for long undersea cables and very long land lines, because it loses less power over distance.

DC never passes through zero. Open a normal breaker on it and the arc just keeps burning. It can destroy the breaker.

It also has to happen fast. On a DC network, fault current climbs very quickly, because nothing in the circuit slows it much. In ABB’s early design work, the fault current rose at about 3.5 kiloamps per millisecond, even with a large reactor added to slow it. Wait 5 milliseconds and the current has already risen by more than 17,000 amps. Wait much longer and the converter stations, the big power electronics at each end of the line, can be damaged.

If you cannot kill the current in a few milliseconds, a fault on one cable can collapse the voltage across the whole DC network, taking down every wind farm and converter on it, not just the sick wire.

How does an HVDC circuit breaker work?

It has to do four jobs, in about 3 to 5 milliseconds:

Detect the fault.
Force the current to zero, because nature will not.
Absorb the energy stored in the line and its reactors, so nothing explodes. This job goes to banks of metal-oxide surge arresters, which turn the energy into heat.
Leave the healthy cables running.

There are three main designs.

Mechanical breakers use big contacts plus an extra circuit, a capacitor and inductor, that injects an opposite current to create an artificial zero. They are cheaper and waste almost no power in normal running, but they are slower.

Solid-state breakers do the whole job with power semiconductors. They are very fast. But the full current flows through those chips all the time, and they waste power as heat constantly. ABB estimated that a pure semiconductor breaker could lose about 30% as much power as the converter station it protects.

Hybrid breakers combine both. This is the design that made DC grids practical:

In normal running, current flows through a simple mechanical path with only a tiny electronic switch in it, so losses are negligible.
When a fault hits, that small switch pushes the current sideways into a big stack of semiconductor switches. An ultrafast disconnector then opens the mechanical path while it carries no current, which it can do in a couple of milliseconds.
The semiconductor stack turns off, forcing the current down, and the surge arresters absorb the energy.

ABB announced the first hybrid HVDC breaker in November 2012. Its prototype cut more than 9,000 amps at 320 kV within 5 milliseconds.

Where are HVDC breakers actually in use?

China got there first. The Zhangbei DC grid, which links wind and solar farms in Hebei to Beijing, went into service in 2020. It is a four-terminal, ±500 kV network, described as the world’s first true DC power grid. It uses 16 hybrid DC breakers, four at each station. Each can cut up to 25,000 amps in 3 milliseconds and reclose within 300 milliseconds. China also ran earlier, smaller multi-terminal DC projects at Nan’ao and Zhoushan.

Europe is next. In July 2024, Germany’s four transmission operators (50Hertz, Amprion, TenneT and TransnetBW) and three manufacturers (GE Vernova, Hitachi Energy and Siemens Energy) agreed to develop multi-terminal DC hubs at 525 kV, with DC breakers built in. Implementation starts in 2026, aiming for commercial use in 2029. The goal is to connect large amounts of North Sea offshore wind into one meshed DC grid.

Why do DC grids need breakers at all?

Most HVDC today is point-to-point: one line from A to B, a converter at each end. If that line faults, operators can shut the whole link down from the AC side, clear the fault, and restart. It is ugly and slow, but it works, because there is only one line to lose.

A DC grid has several cables tied together: offshore wind farms, hubs, onshore stations, more than two ends. If one cable faults and the only tool you have is “shut everything down”, you lose every wind farm on the network at once.

With DC breakers, you open only the faulty cable, like one fuse in a fuse box instead of the whole house going dark.

Without this breaker, you do not have a grid. You have a bunch of separate roads.

Why did DC grids take so long?

The breaker had to be invented. Practical high-voltage DC breakers only arrived in the 2010s.
They are big and expensive. Each breaker is a hall full of semiconductor stacks, capacitors and surge arresters, not a box on a wall.
Standards had to catch up. Equipment from different manufacturers has to work together on one DC network. Europe is only now proving that.
Point-to-point was good enough until offshore wind needed many links meeting at sea.

Uncle’s verdict

AC breaker: wait for the blink, then open.

HVDC breaker: there is no blink, so the machine makes one, in three thousandths of a second.

Not a bigger house switch.

A special tool for one-way current at extreme voltage, and the missing piece that turns separate DC cables into a real grid.

— Engineering Uncle

Sources

AEO FAQ

Frequently asked questions

What is an HVDC circuit breaker?

An HVDC circuit breaker is a switch that interrupts high-voltage direct current during a fault. Because DC has no natural current zero, the breaker must force the current to zero itself, absorb the stored energy with surge arresters, and do so within about 3 to 5 milliseconds.

Why can’t you use a normal AC circuit breaker on HVDC?

AC breakers extinguish the arc at a natural current zero, which occurs 100 or 120 times a second in AC systems. Direct current never passes through zero, so an AC breaker opened on HVDC would keep arcing and could be destroyed.

How does a hybrid HVDC circuit breaker work?

In normal operation, current flows through a low-loss mechanical path. During a fault, a small electronic switch diverts the current into a stack of power semiconductors, an ultrafast disconnector opens the mechanical path, and the semiconductors cut the current while surge arresters absorb the energy. ABB demonstrated the first one in 2012.

Where are HVDC circuit breakers in service?

China’s Zhangbei DC grid, a four-terminal ±500 kV network in service since 2020, uses 16 hybrid breakers rated to cut 25 kA in 3 milliseconds. Germany’s transmission operators plan 525 kV multi-terminal hubs with DC breakers for North Sea wind, targeting commercial use in 2029.

Why do DC grids need circuit breakers?

A point-to-point HVDC link can be shut down entirely to clear a fault. A DC grid connects several lines and converters, so without DC breakers a single cable fault would force the whole network offline. Breakers isolate only the faulty cable.

What types of HVDC circuit breakers exist?

There are three main types: mechanical breakers, which create an artificial current zero with a resonant circuit; solid-state breakers, which are fast but have high running losses; and hybrid breakers, which combine a low-loss mechanical path with semiconductor switching.