DC Circuit Breakers
SWITCHING IN HIGH‐VOLTAGE DIRECT CURRENT SYSTEMS
Since the advent of power systems, switching has been the prime technology to control the safe flow of power. In traditional alternating current (AC) systems, a large number of switchgears can be identified for a numerous different function. The most critical switching device is the circuit breaker, a device that can make (switch on) and break (interrupt) fault currents during a short‐circuit in a system. 
Its function is to isolate faulted sections of the power system in a short time (< 100 ms) so that power flow in the healthy parts of the system remains unaffected. 
In a traditional point‐to‐point high‐voltage direct current (HVDC) transmission system, however, there is no need for such a dedicated fault current interruption device. Normally, a fault in such a system automatically leads to a total loss of power in the affected pole line. Fault current in such systems can be cleared by converter control actions at either end or by AC circuit breakers at the AC side. Future meshed voltage source‐converter (VSC)‐based HVDC grids, however, need dedicated HVDC circuit breakers.
HVDC switchgear changes the energy flow in two ways. The first is current commutation: transferring a current into an alternative path ‒ achieved by transfer switches, and the other is fault current interruption: blocking the current right away ‒ achieved by circuit breakers. The situation is outlined in Figure 1, showing interruption (left) and commutation (right). Although both require local current interruption, fault current interruption is a far more onerous duty compared to current commutation in terms of current level to be dealt with, the magnitude of counter voltage to be generated and the energy that the device has to absorb. 
Another critical difference is operation time. Commutation does not need to be performed with acute urgency, whereas short‐circuit currents need to be cleared extremely rapidly.
Commutation (or transfer) switches for HVDC application are based on AC SF6 interrupters (with auxiliary circuits), providing enough arc voltage to transfer the current into a parallel path. Special designs of SF6 circuit breaker chambers are optimized to provide quick transfer of current.
A well known load current interruption switch is the metallic return transfer switch (MRTS) that needs to transfer continuous current from an earth return path into a transmission line (‘metallic return’) after a converter pole has been taken out of service in a bipolar system.
Though current commutation capabilities exceeding 6000 A may be assigned to MRTS, technically it is a switch rather than a breaker.
In 2017, the International Council on Large Electric Systems (CIGRE) Technical Brochure 683 ‘Technical requirements and specifications of state‐of‐the‐art HVDC Switching Equipment’ was issued. In this document, a large variety of HVDC switchgear is analyzed and explained:
CIGRE Joint Working Group: A3B4.34: ‘Technical requirements and specifications of state‐of‐the‐art HVDC Switching Equipment’ CIGRE Technical Brochure 683 (2017)
An excellent technology review paper has been published by Rene Smeets and Nadew Bella IET-HV 2020 HVDC interruption.pdf