# Emerging Technologies for Advanced Power Electronics and Machine Design in Electric Drives
## 2 Topologies of Electric Drives
### Series Power Stage Topologies
While some of the multilevel topologies offer fault tolerance by design [62], others can be reconfigured if required [63; 64]. Hardware-based fault tolerance in case of power semiconductor device failures can be achieved on switch level, leg level, module level, and system level, depending on the application and solution at hand [65; 66]. Most often used solutions are (i) the use of redundant switching states, (ii) the use of redundant series or parallel switches, legs, modules or converters, and (iii) the use of DC link voltage control. As a larger number of components in multilevel converters can result in higher probability of fault, system reliability is often improved with hybrid multilevel topologies, such as cross-coupled CHB units [67], hybrid FC-NPC converters [68], and use of additional relays or switches to isolate the defected modules from the circuit [69]. Figure 6: Schematic diagram of a three-phase modular multilevel converter (MMC) with a three-phase electric machine and a single voltage source (\(V_{\text{dc}}\)). Different submodules (SMs) can be utilized, such as half-bridge submodule, full-bridge submodule, etc. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Characteristics**} & **CHB\({}^{1}\)[55, 71]** & \multicolumn{3}{c}{**NPC\({}^{2}\)[72, 73]**} & \multicolumn{3}{c}{**FC\({}^{3}\)[73, 74, 75]**} \\ \hline \multicolumn{2}{c}{**Specific Requirements**} & \multicolumn{3}{c}{**isolated DC Sources**} & \multicolumn{3}{c}{**Clamping Diodes**} & \multicolumn{3}{c}{**Additional Capacitors**} \\ \cline{3-10} \multicolumn{2}{c}{Level} & 3 & 5 & \(n\) & 3 & 5 & \(n\) & 3 & 5 & \(n\) \\ \hline \multirow{3}{*}{**Coupling Diodes**} & \multirow{3}{*}{Switches with freewheeling diodes} & 12 & 24 & 6(\(n-1\)) & 12 & 24 & 6(\(n-1\)) & 12 & 24 & 6(\(n-1\)) \\ \cline{2-10}  & DC-link capacitors & 3 & 6 & \(\frac{3(n-1)}{2}\) & 2 & 4 & (\(n-1\)) & 2 & 4 & (\(n-1\)) \\ \cline{2-10}  & \multirow{3}{*}{Clamping diodes} & \multirow{3}{*}{Not applicable} & \multirow{3}{*}{6} & \multirow{3}{*}{36} & \multirow{3}{*}{36} & \multirow{3}{*}{Not applicable} & & & & Not applicable \\ \cline{2-10}  & & & & & & & Not applicable & 3 & 18 & \(\frac{3(n-1)(n-2)}{2}\) \\ \hline \multicolumn{2}{c}{Fault tolerance} & \multicolumn{3}{c}{Easy} & \multicolumn{3}{c}{Difficult} & \multicolumn{3}{c}{Easy} \\ \cline{3-10}  & & & & & & & & & \\ \hline \multirow{3}{*}{**Advantages**} & \multirow{3}{*}{Simple structure and control due to its modularity} & \multicolumn{3}{c}{Robust structure and requires the least amount of DC-link capacitors (less voltage imbalance problems)} & Phase redundancy is available to achieve voltage balancing of DC-link capacitors \\ \cline{3-10}  & & & & & & & & \\ \cline{1-1} \cline{3-10}  & & & & & & & & \\ \hline \multirow{3}{*}{**Limitations**} & \multirow{3}{*}{Limited applications due to requirement of separate DC sources} & \multicolumn{3}{c}{Amount of clamping diodes becomes excessively high with the increase in number of levels algorithm} & \multicolumn{3}{c}{Bulky size, high development cost, and complexity value balancing and switching control algorithm} \\ \cline{3-10}  & & & & & & & \\ \hline \hline \end{tabular} \({}^{1}\)CHB–casoidal Hbédigle converge,\({}^{2}\)NPC–neutral-point damped converge,\({}^{3}\)FC–flying capacitor converter.  
\end{table} 
Table 1: Comparison of characteristics for most common multilevel inverters in three-phase operation. Summarized from [24, 70].