## 2 Modular Multilevel Converter and Sub Module Variants
With this capacitor, voltage ripples at low frequencies could also be reduced. However, it lacks the presence of bipolar operation. In [12], another three-level cross-connected SM (TL-CCSM,) formed by connecting two HBSMs back-to-back with switch and diode, is shown in Figure 3f. Making the connection like this, we could produce voltage magnitudes 0, VC1, and (VC1 + VC2). The diagonal-bridge SM (DBSM), shown in Figure 3i, is another variant that is similar to FBSM but with two diagonal switches that are replaced by diodes [24]. The last one is the three-level SM that is an improved hybrid SM (IHSM) [14], which has maximum voltage blocking of (VC1 + VC2), as depicted in Figure 3h. Four-level SMs: Figure 4a,b shows two kinds of double clamp submodules (CDSM) [12; 14; 21]. These configurations are the combination of two HBSMs connected in series. Here the switch S5 is on continuously, so it becomes two HBSMs connected in series under normal operating conditions. When the IGBTs are in blocking mode, both capacitors form either series or parallel connections in this CDSM. Further, this SM, operated as FBSM, has undergone different capacitor voltages as paralleled. To prevent this paralleling issue, diodes presented in that path could be replaced with IGBT switches. They would make another configuration, as shown in Figure 4b (CDSM); so far as losses are considered, CDSM is the combination of HBSM and FBSM. The alternate way of arranging commutation cells is shown in Figure 4c. This arrangement could be able to provide a dc-fault handling capability and can generate  
Figure 3: Three-level submodule variants (**a**) Full-bridge SM (**b**) Three-level flying capacitor submodule (TLFCSM) (**c**) TLNPC-1 (**d**) TLNPC-2 (**e**) TLNPC-3 (**f**) double-submodule (DSM) (**g**) three-level cross-connected (TLCCSM) (**h**) Improved hybrid SM(IHSM) (**i**) Diagonal-bridge SM (DBSM). four voltage magnitudes. A combination of FBSM and HBSM would result in forming a hybrid submodule termed an asymmetrical SM [25], and it can perform both unipolar and bipolar operations, as shown in Figure 4d. The series connected double SM(SDSM) in shown in Figure 4e, which is obtained by connecting two HBSMs with S5 and D6 [26]. It has the capability of arc extension whenever a short circuit has taken place, and it could protect MMCs even when zero impedance current would have been taken place. Five-level SMs: As shown in Figure 5a, five-level cross-connected SM (FLCCSM) is the same as CDSM. Structurally FLCCSM is designed with two HBSMs in back-to-back cross-connection with the help of S5 and S6 [21,27]. Due to the two capacitors of the SM being connected in series, a blocking effect on the dc-fault current can be created. A different cross-connected SM (CCSM) [27] is shown in Figure 5b. It has a balanced bipolar voltage output, which could achieve greater voltage [28]. Similarly, if FBSMs are connected as parallel-connected SMs [29], which is shown in Figure 5c, they would give a reduced capacitor ripple. Figure 4: Four-level submodule variants **(a)** Clamp Double Submodules (CDSM), **(b)** Clamp Double Submodules (CDSM), **(c)** Mixed SM, **(d)** Asymmetrical SM, **(e)** Series Connected Double SM (SDSM). Figure 5: Five-level submodule variants **(a)** Five-Level Cross-Connected SM(FLCCSM), **(b)** Cross-Connected SM(CCSM), **(c)** Parallel-Connected SM.