## 3 DC Circuit Breakers
### Mechanical Circuit Breakers  
MCBs are devices that use mechanical parts to interrupt the flow of current in the event of a fault. The operating mechanisms of MCBs can be divided into hydraulic, spring, pneumatic, and magnetic [21]. Spring and magnetic operating mechanisms are more common in vacuum CBs (VCBs). When a MCB reacts to a fault, its moving contact starts to separate, and the contact area is reduced. The current density increases and the energy begins to evaporate the metal, resulting in a plasma arc, which restarts due to the capacitance and inductance of the system. Although the contacts are physically separated, the arc keeps the current flowing. As the contact separation increases, the degree of the arc column is influenced by the characteristics of the surrounding medium. The arc current will be terminated when the arc plasma becomes a dielectric medium. Moreover, the ability to limit the current is determined by the difference between the arc voltage and the system voltage [53]. To mitigate the impact of the arc, several solutions have been proposed: (a) MCBs with passive current switching, (b) MCBs with active current switching, and (c) artificial current zero vacuum switch (ACZ-VCBs). In MCBs with passive and active current switching and ACZ-VCBs, a voltage opposite to the system that conducts zero-crossing artificial currents is created by a resonant inductive (L) capacitive (C) series circuit [20]. In the passive switching method, when the switch is open, a current flows through the LC circuit with a capacitor that has not been pre-charged, and it starts to oscillate and creates a zero crossing current, in which case the mechanical switch completely interrupts the current flow, increasing the voltage to a certain specific value. Once the voltage reaches such a value, the current flows to energize the energy-absorbing circuit, which is, in most cases, a metal oxide varistor (MOV) used to dissipate the stored energy. Figure 3a shows a MCB with passive commutation. Its components are: (1) the branch of the switch, (2) branch of the series resonant circuit, and (3) branch of the energy-absorbing circuit. In the case of active switching, the capacitor has already been pre-charged, and  
when the switch opens, the capacitor injects a negative current equal to the fault current to make a zero crossing of the current. During the interruption process, the magnetic energy is stored in the inductor and the varistors are connected in parallel with the switch to mitigate the over-voltage and absorb the energy stored in the inductor. Figure 3b shows the mechanical switch with active current switching. In the VCB, once the contacts are separated, the arc current generated across the electrodes is extinguished using a vacuum chamber so that a vacuum arc is initiated at the contacts. This arc is then extinguished and the conductive metal vapor condenses on the metal faces, and the dielectric strength in the electrode is reduced. VCBs feature arc voltages lower than 100 V, and they do not limit the arc currents of MVDC MG. VCBs are effective because they avoid re-ignition of the arc after zero current [54]. The most frequently used approach of VCBs in MVDC interruption is based on artificial current zero (ACZ). Figure 4 depicts a general circuit of an ACZ-VCB. The interruption process begins with the separation of the VCB followed by the formation of a vacuum arc conducting line current i1. As the electrode gap of VCB reaches a certain safe stroke to withstand the recovery voltage, the commutation circuit breaker (CB) injects a high-frequency oscillating commutation current i2, which is generated by discharging a pre-charged commutation capacitor C1 through inductance L1. In consequence, the superimposition of i2 forces i1 to drop to zero. Then, VCBs can be interrupted by extinguishing the vacuum arc. A metal oxide arrester is used to suppress the over-voltage across the VCB [55].