## 3 Enhanced Fault Current Limiting Circuit  
### Fault Current-Limiting Characteristics  
The FCLC mainly consists of energy dissipation resistance and limiting inductance [14; 15; 25; 35]. Thus, investigating the influence of the two elements on the fault current development is important. The related simulation was performed under controlled conditions. Figure 11 displays that limiting inductance can help slow down the accelerating speed of the fault current but increase the time spent in decaying the freewheeling current to its expected level. In Figure 11, the energy dissipation resistance facilitates the constraining fault current peak value and the accelerating extinguishment of the freewheeling current. The adoption of the reactor and resistance in the discharging period is suitable, considering that the maximum fault current will flow through the freewheeling diode while the IGBTs are blocked or the SM capacitors are completely discharged. However, the limiting inductance can affect the dynamic response speed and prolong the recovery time. Therefore, further study on the FCLC design is required.  
A series of simulations was conducted under controlled conditions to obtain the expected fault current limitation and rapid recovery. The simulation conditions are divided into resistance and reactor groups as follows: the former is labeled with \"a\", \"b\", \"c\", and \"d\", and the latter with \"e\", \"f\", \"g\", and \"h\". Specifically, the related explanations of the conditions are listed in Table 2. \"\(\gamma\)\" indicates that  
Figure 10: Phase-A upper arm current under the DC pole-to-pole fault.  
the resistance or reactor is inserted in the fault circuit, and \"\(\times\)\" denotes the absence of resistance or reactor. First, the reactor was ignored, and Figure 12a illustrates that the red curve (\"ag\") provides an improved performance in limiting fault current. Thus, the condition with \"a\" was selected for the following simulation. Figure 12b depicts the performance of various reactor-based FCLC scheme, in which the red dashed-dotted curve (\"ad\") represents the lowest fault current level. However, the \"ad\" scheme was unsatisfactory for real projects, where large inductance will affect the dynamic response speed of the entire system during the normal state. Therefore, the \"af\" scheme most closely approaches the optimized FCLC scheme, because the energy dissipation resistance consumes the energy in the fault circuit. This phenomenon is due to the fault being detected, and the FCL reactor being inserted to limit the increase in current speed, but also being removed to accelerate the decaying speed of the freewheeling current after blocking the IGBT.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Time Point** & **Label** & **Fault Occurred** & **Fault Detected** & **IGBT Blocked** \\ \hline Resistance-related condition & a & \(\times\) & \(\checkmark\) & \(\checkmark\) \\ b & \(\times\) & \(\checkmark\) & \(\times\) \\ c & \(\times\) & \(\times\) & \(\times\) \\ \hline  & d & \(\checkmark\) & \(\checkmark\) & \(\times\) \\  & e & \(\times\) & \(\checkmark\) & \(\checkmark\) \\ Reactor-related condition & f & \(\times\) & \(\checkmark\) & \(\times\) \\  & g & \(\times\) & \(\times\) & \(\times\) \\  & h & \(\checkmark\) & \(\checkmark\) & \(\checkmark\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Condition classification for the fault current limiting circuit scheme.  
Figure 11: DC fault current characteristics (**a**) with variable limiting inductance; (**b**) with variable energy dissipation resistance.  
Figure 12: Comparison among different FCL schemes (**a**) different resistance conditions without considering the reactor, (**b**) different reactor conditions with selected resistance schemes.