HVDC Circuit breaker

One simple difference is that there being no current zero in DC, the HVDC ACB uses a large number of de-ion grids, and blow-out coils in very large arc chutes to lengthen, cool and quench the arc.

This is my study up to 6.6 kV devices, AC and DC, air-break. Nowadays, AC has gone for Vacuum technology.

In AC networks the source drives the current periodically through zero (every one-hundredth of a

second for a 50 Hz current), and current zero is the ideal instant to interrupt: while the arc power

decreases, the circuit breaker can flush the contact area from conducting plasma and the current is

permanently interrupted within 20 milliseconds.

For DC networks the analogy is somewhat different, as there is no natural current zero. It is more like

a train that runs at constant speed: the only way to stop the train is to have it roll up a slope and

then block it at still point. In the DC network, a voltage—opposing the current flow—has to build up in

order to reach current zero.

Check this document for a clear explanation for HVAC and HVDC breaker above is an extract of that document.