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 is established and the current continues 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 crosses zero the arc collapses and the breaker quickly is able to withstand a high voltage between its contacts if the contact gap is sufficient.
DC circuit breakers never experience any natural current zero-crossing. On the contrary, in the case of a short-circuit in the transmission system the current through the breaker steeply rises and, if not extinguished sufficiently fast, reaches levels exceeding the breaking capability of the circuit breaker. A network collapse then may result.
Several concepts for DC breakers have been presented and a few also have been implemented and tested. Two basic mechanisms can be distinguished:
- current is extinguished by semiconductors which either are conducting the current continuously or which temporarily take over the main current from a mechanical disconnector
- current is extinguished by a mechanical breaker assisted by auxiliary circuitry that creates an artificial current zero-crossing in the current through the mechanical breaker