# A Review of Modular Multilevel Converters for Stationary Applications
## 2 MMC Topologies
### Submodule Topologies
#### 2.1.2 Multilevel Submodule Topologies  
Neutral point clamped (NPC) and flying capacitor (FC) concepts are classical multilevel converter topologies. They were also introduced as three-level submodules in MMCs. The circuit configurations of the NPC submodule (NPCSM) and FC submodule (FCSM) [43; 44] are presented in Figure 3a,b, respectively. NPCSM contains four IGBT devices with anti-parallel diodes, two clamping diodes, and two capacitors. FCSM includes similar components except for the clamping diodes. Both of them can generate three voltage levels: \(v_{c1}+v_{c2},v_{c2}\), zero for NPCSM, and \(v_{c1},v_{c1}-v_{c2}\) or \(v_{c2}\), zero for FCSM. Figure 3: _Cont._  
The cascaded half-bridge submodule (CHBSM) [45] is another configuration that outputs three voltage states: \(v_{c1}+v_{c2}\), \(v_{c1}\) or \(v_{c2}\), and zero. It is composed of two series-connected half-bridge cells, as drawn in Figure 3c. With respect to submodule weight and size, the storage capacitor has a large influence. To reduce its volume, a compact submodule topology [46] was proposed with a stacked switched capacitor (SSC) energy buffer, as shown in Figure 3d. The SSC energy buffer refers to C1, C2, C3, and S3 along with S5. C3 is the main voltage source. C1 and C2 act as active supporting capacitors to compensate C3 voltage. Compared with HBSM, the total volume of SSCSM capacitors can be decreased by 40% at least. A clamp-double submodule (CDSM) [47] (Figure 3e) consists of two identical HBSMs connected by two extra diodes and an IGBT, which is always on during normal operation. A clamp-double cell  
Figure 3: Multilevel submodule topologies: (**a**) neutral point clamped submodule (NPCSM); (**b**) flying capacitor submodule (FCSM); (**c**) cascaded half-bridge submodule (CHBSM); (**d**) stacked switched capacitor submodule (SSCSM); (**e**) clamp-double submodule (CDSM) type-I; (**f**) CDSM type-II; (**g**) hybrid submodules (HSM); (**h**) cross-connected submodule (CCSM); (**i**) switched capacitor submodule (SCSM); (**j**) composite three-level submodule (CSM). can generate a three-level voltage: \(v_{c1}+v_{c2}\), \(v_{c1}/v_{c2}\), and zero. In the case of DC fault, all the power devices are set off to resist the DC short-circuit current. A CDSM has only half utilization of the capacitor voltages (\(-v_{c1}\) or \(-v_{c2}\)) in blocking mode. Therefore, using one extra IGBT is advised to be involved in the CDSM [48] (Figure 3f) to output \(-2v_{c1}\) as blocking voltage. One apparent weakness of a CDSM is the increased power losses, since all the switches work during normal operation. Hybrid submodules (HSM) are generally based on the HBSM, as their primary purpose is to enable it to withstand DC fault with less components [49; 50]. As shown in Figure 3g, a hybrid cell is comprised of series-connected HBSM and FBSM. It is tolerant of DC failure with three quarters of semiconductors compared with involving FBSMs alone. Similarly, [51] proposes a mixed submodule based on HBSM and CDSM. It is suggested to achieve fast DC fault current clearing and capacitor uniform distribution at the cost of adding an extra diode between the submodules. Figure 3h shows a cross-connected submodule (CCSM) [52] that consists of two HBSMs connected back to back via two IGBTs with anti-parallel diodes. The CCSM can output five symmetric voltage levels: \(\pm(v_{c1}+v_{c2})\), \(\pm v_{c1}/\pm v_{c2}\), and zero. This topology is tolerant of DC fault current by turning off crossed IGBTs S5 and S6. Crossed IGBTs conduct alternately during normal operation, which increases power losses. The researchers proposed a new switched capacitor submodule (SCSM) [53], as shown in Figure 3i. It is composed of two capacitors and six IGBTs with anti-parallel diodes. The two capacitors are connected in series to output a voltage \(2v_{c}\), whereas they are connected in parallel to output a voltage \(v_{c}\), which enables SCSM to achieve voltage balancing with only half of the voltage sensors utilized in the other submodules.