All power transmission systems depend on secure and reliable protection systems. In case of short-circuit in any transmission facility like over-head line or cable or in the load, the faulty component must be immediately isolated to preserve the capability of the whole transmission system to operate.
Already in the early years of electrification the lack of circuit breakers for DC was recognized and transmission technology mainly was developed using AC. However, today renewable energy is being harvested from sources like wind farms and solar plants, which are spread in large areas and with volatile availability. This causes a need for transmission of large amount of energy over long distances. Furthermore, often cable transmission is required due to long distance under water or due to problem to achieve right-of-way for new overhead lines. DC transmission offers high efficiency and full flexibility to utilize cable technology. A renewed interest in DC circuit breaker technology therefore has been noticed.
The function of AC circuit breakers is based on the assumption that the current repetitively performs zero-crossings. When the contacts in a mechanical breaker separate, an arc will be established and the current continuous to flow through that arc as long as the current, forced by the inductance in the network, has sufficient amplitude to maintain the arc. However, when the instantaneous current zero-crosses the arc collapses and the breaker resumes its voltage withstand capability between its contacts if the contact gap exceeds a certain distance.
DC circuit breakers never experiences any natural current zero-crossing. On the contrary, at a short-circuit in the transmission system the current through the breaker steeply increases and, if not extinguished sufficiently fast, reaches levels exceeding the breaking capability of the circuit breaker. A network collapse then may result.