## 2 Modular Multilevel Converter and Sub Module Variants  
Figure 1a is a three-phase MMC, and it consists of the top arm (Tarm) and the bottom arm (Barm). This arrangement generally could have an N-number of SM in each arm [11]. Each phase consists of arm inductors (Larm) and the submodules to provide better arm-currents without circulating harmonics and fault currents. Submodule (SM) or power cell represents a topological connection of IGBTs and could be of any voltage level. Generally, this SM is half-bridge (HBSM) or full-bridge (FBSM), but some researchers [12] started using SM with high levels. The SMs in the arms must be identical to get a symmetrical output wave. In the basic operation of the MMC, there is a chance of two events--sub-module insertion or by-passing state--whenever all the sub-modules are inserted in series and would result in arm voltages, respectively, and in turn produces a multilevel output. During the insertion processes, the SM capacitors were charged to their rated voltage magnitudes in the blocking mode based on the SM variants. The DC-link energy was equally divided among the SM capacitors; otherwise, there would be an unbalance of voltages and circulating currents within the phases. Figure 1b is a single-phase equivalent representation of a conventional MMC. The top and bottom arms' SM's total voltage magnitudes constitute V au and Val, respectively. For many applications, like motor drive, the number of SMs drastically increased day by day. The ongoing developer has structurally connected multi-level SM topologies to have dc fault handling capability, dimension reduction, and a higher number of voltage levels. Table 1 here provides us with the topological variants of SMs. Two-level SMs: Figure 2a shows the half-bridge submodule (HBSM) to produce the unipolar voltage by chopping the dc-link voltage. It consists of two IGBTs and a capacitor [13]. It can produce two levels of voltages 0, +VC1 when bypassed and inserted. In the HBSM, the dc voltage component will be present in the MMC arms [14]. Figure 2b is unipolar voltage FBSM [12], and this is similar to FBSM but with a slight replacement of a S3 switch with D3 of the FBSM (Figure 3a); it would be suitable for dc-fault handling capability. Another two-level variant is the unidirectional SM [15], and it has fewer semiconducting switches, as shown in Figure 2c. The clamp single submodule (CSSM) configuration is another configuration [16], which is shown in Figure 2d. The CSSM is derived from the three-level flying capacitor submodules (TLFCSM) for providing dc-fault handling capability during unidirectional operation. Figure 1: (**a**) Configuration of three-phase MMC, (**b**) single-phase equivalent circuit of conventional MMC. Figure 2: Two-level submodule variants (**a**) Half-bridge (**b**) Unipolar voltage full-bridge (**c**) Unidirectional SM (**d**) Clamp single SM. Three-level SMs: The well-known FBSM [16] is depicted in Figure 3a, and it generates bipolar output voltage levels, and we can use this for bipolar operation systems [17]. Moreover, if we connect two FBSMs in parallel, we could have a four-quadrant operation [18]. From Figure 3e, we can see that the three-level flying capacitor sub-module (TLFCSM) [14; 15; 18] has two capacitors--C1 which is twice in voltage rating with C2. This configuration does not have dc-fault handling and computationally complex control [19; 20]. We see a three-level neutral point clamped submodules (TLNPC) in Figure 3b,d,g [14; 18; 21]. SMs in Figure 3d and TLFCSM are structurally close and produce 0, +VC2, and +(VC1 + VC2) magnitudes; SM in Figure 3b contains four switches, two capacitors, and two diodes, whereas SM in Figure 3g has six switches and two capacitors. Figure 3d, SM will be formed by the two-series combination of commutation circuits, while Figure 3b can be possible with the T-connection of switches; the midpoint switch here can block the voltages in both directions [22].