## 3 DC Circuit Breakers
### Solid-State Circuit Breakers
#### 3.3.1 SSCBs General Description  
SSCBs do not have mechanical moving parts to operate in case of electrical faults, since this is performed by semiconductor devices. Compared to MCBs, SSCBs are much faster and feature greater accuracy in controlling their operation. With the rise of power electronics in the 1970s, the SCR thyristor appears as one of the first solid-state switches.  
Figure 4: General circuit of an ACZ-VCB.  
Figure 3: (**a**) MCB with passive resonance, (**b**) MCB with active resonance [21].  
With the development and contribution of power control systems between 1980 and 1990, the growth of solid-state switches became remarkable, whose predominant material is silicon (Si), such as SCR [58], IGBT [6,59,60], IGCT [61], GTO [62], and CS-MCT [35]. Si devices have a high level of maturity and are commercially available with a wide range of voltages and currents. SSCBs have the following advantages: fast operating times of less than 100 us, no arc, no sound, no gas emissions, long service life, and high reliability and applicability [8,63,64]. However, SSCBs have the disadvantage of presenting high power losses, and being very expensive and large, due to the need for heat sinks [21]. Another group of SSCBs is the devices proposed since 1989 [65], in which the predominant material is a wide band gap (WBG), such as SiC JFETs [66], SiC ETO [67], SiC MOSFETs [68,69], SiC SITs [70], GaN HEMTs, and GaN MOSFETs [15]. WBG semiconductors exhibit superior material properties than silicon ones, which enable the operation of power devices at higher-temperature operation, higher blocking voltage capability, and higher switching frequencies [34,71]. Although WBG semiconductors offer significant improvement over silicon ones in power efficiency, switching frequency, and operating temperature, their proliferation into the mainstream power electronic market is impeded by high device cost and reliability concerns, and this is mainly because the WBG technology itself is still evolving towards its maturity [56].  
SSCBs can be damaged due to overvoltage of the inductive components of the system, hence the importance of reducing this voltage for the safety of the device. To protect the solid-state switch from overvoltage at the moment of opening, additional elements are required, such as resistors, capacitors, diode (RCD) Snubber Circuits, metal oxide varistors (MOV), and freewheeling diodes [10,72]. In the Ref. [40], the authors described the advantages and disadvantages of different SSCB snubbers: the metal oxide varistor (MOV), single snubber capacitor, dissipative snubber, RC snubber, and RCD snubber. They concluded that RCD snubber is the best option and is able to avoid the oscillation between C and L by using a fast recovery diode to clamp the changing voltage; it is, therefore, a suitable candidate for medium-capacity applications [40]. The protection control, heat cooling system, sensor, and damping system for a SSCB are depicted in Figure 5.  
One of the main drawbacks of SSCB is the lack of galvanic isolation in the open state. This can be overcome by adding an auxiliary circuit based on two mechanical switches in series with the SSCB [21] (see Figure 6). This circuit is opened after the trip of the SSCB, guaranteeing complete isolation between load and supply. Galvanic isolation of SSCBs is a key feature for achieving fault detection, isolation, and DCMG reconfiguration. The mechanical switches that provide physical isolation are as follows: (1) a mechanical switch to handle the high current that appears through the SSCB when the SSCB interrupted the fault current, and (2) a secondary mechanical switch for interrupting the leakage current during isolation [43,73,74].  
Figure 5: Conceptual diagram of a typical SSCB.