## 4 DC CB Evolution  
This section has the purpose of showing the DC CB evolution. DC CBs have been evolving for more that 100 years. For this reason, the time-line is divided after and before the definition of DCMGs. Figure 9 corresponds to the evolution of DC CBs before the emergence of DCMGs, while Figure 10 illustrates the evolution of DC CBs after the emergence of DCMGs. Electrical fuses were patented over 100 years ago to operate in DC networks in the late 1890s [48]. The silicon SCR thyristor SCR appeared as a superior alternative to the mercury arc rectifier in the 1950s for application to various AC power electronics devices, since its switching is based on zero crossing of the current [99]. In 1980s, there was a considerable evolution in DC CBs: (a) a DC MCB with LC series resonance to create zero crossing of the fault current and mitigate the arcing problem in switching for HVDC networks was proposed [20]. In fact, the LC series resonance concept served as the basis for the development of the ZSCB for MVDC and LVDC DCMG applications. (b) With the advent of PWM, the development of auto turn-off devices having fast switching and high withstand capability was necessary. Gate Turn-Off (GTO) thyristors meet this requirement and easily withstand high voltages and high currents. The gate turn-off thyristor (GTO) has all the  
Figure 8: Diagram of conventional HCBs [21]. advantages of that of the SCR, and can also be turned off when desired through its gate. Unfortunately, its gate drive current requirements are difficult, making the drive circuit very complex [100]. (c) IGBTs have the best characteristics of MOSFET and BJT transistors with the speed characteristics of high switching speed and high voltage capability, being quite practical in the low- and medium-current ranges. Nonetheless, IGBTs were not suitable for simultaneous high-voltage and high-current operation [101]. (d) Another alternative was the MOS gate thyristors, such as the MOS-controlled thyristor (MCT) and base resistance-controlled thyristor (BRT) for high-voltage applications, due to their single gate drive capabilities and low forward voltage drops. However, as these devices lacked the current saturation feature, they showed much poorer short-circuit SOA characteristics compared to IGBTs [78, 99]. In the 1990s, considerable progress was achieved: (a) IGCT, which has all the advantages of other thyristor-type devices, such as SCR and GTO, was proposed. IGCT features a high voltage, and current and surge current capabilities, which satisfy the high current trend of solid-state switch. The switching speed of IGCT is about six times slower than that of IGBT, but that is not really a problem in switch applications, since IGCT-based SSCB is at least 900 times faster than a typical EMCBs [100]. (b) ETO, a hybrid device of MOS and GTO that combines the advantages of GTO and MOSFET, was proposed. (c) WBG semiconductors exhibit material properties superior to silicon, which allow operation of power devices at higher operating temperature, higher blocking voltage capability, and higher switching frequencies. In the Ref. [78], the authors presented recent applications with SiC, GaN, and 4H-SiC materials, such as SIC ETO, SIC MOSFET, SIC SIT, SIC JFET, and GaN MOSFET. In 2010, a novel alternative for DCMG protections for MVDC and LVDC appeared: the Z-Source DC circuit breaker, ZSCB. This circuit breaker uses a z-source L-C circuit to automatically switch a main path SCR during a fault. Compared to existing DC circuit breakers, the z-source circuit breaker features very fast tripping, easier control, and the source does not experience a fault current [58]. Figure 10 corresponds to a time line after the definition or emergence of DCMGs. After the definition of DCMGs, DC CBs have evolved quickly, trying to satisfy the DCMG requirements. Figure 10 shows the CBs for LVDC and MVDC in chronological order obtained from the literature review that were validated with experiments, prototypes, or simulations in the last 20 years.