The hybrid DC breaker scheme combines the excellent switching capability of power electronics (IGBT) with the low losses of mechanical switchgear in that the current does not traverse the semi-conductors in the main breaker unless it should be interrupted. This is achieved by means of a mechanical bypass path consisting of an ultra-fast disconnector switch (UFD) and an auxiliary commutation switch connected in series as shown in figure.The load commutation switch commutates the current from the by-pass branch into the parallel main breaker prior to interruption and ensures that the UFD can separate its contacts at virtually zero current stress. However, this scheme requires that the UFD is capable of providing full dielectric insulation across its contacts once the main breaker is operated i.e. once it turns off the current. Furthermore, the UFD needs to withstand the full maximum nominal current and, in case of unexpected failure in a system subcomponent, perform an immediate close operation.
In figure you can see ultra fast disconnector switch as item b.
