## 5 Semiconductor Breakers and Operation Principle of the Proposed Method
Consequently, although the voltage rating of the semiconductor devices has recently improved, it still does not have high-voltage blocking capabilities, but will damage the breakers and other components in the system. Thus, multiple semiconductors need to be connected to improve the high-voltage blocking capability of the breaker. It will enlarge the components' size and initial costs. Therefore, to provide safety for the components and prevent system enlargement, an over-voltage limiting circuit (parallel MOV) was integrated with the breaker to limit the rise in voltage and dissipate the high energy as heat. Also, for reliable operation and long life of the switches, accurate synchronization of gate controls and snubbing of stray energy was required. Therefore, Figure 5 shows the connection of two IGBT circuit breakers with anti-parallel diodes and its related operational controller. Additionally, when a short circuit is happening in any branch of the typical multi-terminal DC microgrid, a huge current will pass through the snubber diodes circuit, which causes an over-current on the system. Here, in this SSCB, and in a normal situation, the current will flow through the IGBTs, whereas in a short-circuit situation, the SSCB will be interrupted to reduce the current to zero. During the fault conditions, the protection system of the related area should pass the following steps:  
* Sensing the variation of the common bus voltage and converter current;
* Sending an operation command for related protection devices operation;
* Blocking the CB and turning off the power unit converters in the fault branch;
* Waiting for a few ms after Step 3 to ensure that the current of faulted feeder decays turns to zero;
* Opening the related breaker and de-energizing the faulted branch;
* Re-energizing the fault branch by closing the CBs and the converter. The faults in Figure 1 are considered an ideal line-to-ground short circuit with zero resistance, and the result of the proposed protection strategy is depicted in Figures 6-11. In each case, when a DC fault occurs, the fault current increases rapidly, whereas the SSCB interrupts it before it reaches the peak value. Still, a high current which passes through the breaker and a large amount of energy will be accumulated in a surge arrestor, which can be calculated by Equations (1) and (2) [32]. Additionally, the surge voltage across the breaker is limited to the clamping voltage of the MOV, and it is assumed \(V_{dc}+V_{margin}\):  
\[I=I_{O}-\frac{V_{margin}}{L}t \tag{1}\]  
The total dissipated energy as heat by the surge arrestor is given by the equation below:  
\[W_{R}=(\frac{V}{V_{margin}}+1)\frac{1}{2}LI_{O}^{2} \tag{2}\]  
Where in these equations, V is the normal voltage of the system, and \(I_{O}\) and \(W_{R}\) represent the amplitude of the fault current and energy absorbed in the surge arrestor, respectively. Commonly, the amount of \(V_{margin}\) is much smaller than \(V\) to suppress the voltage rise across the SSCB. Therefore,the amount of \(W_{R}\) in MOV is a large quantity. In a freewheeling diode SSCB's types, the purpose of the diode is to provide a path for large currents and reverse voltage impulses. The amount of energy bypassed by a snubber diode is given by the equation below:  
\[W_{R}=\frac{1}{2}Ll_{O}^{2} \tag{3}\]  
Consequently, as shown in Equations (2) and (3), the energy absorbed in the MOV type is more than the energy bypassed by the snubber diode type. Furthermore, in order to decrease the fault current quickly while the breaker is disconnecting, the voltage capacity of the SSCB should be more than \(V_{bus}\)[33].