# A Review of Modular Multilevel Converters for Stationary Applications
## 2 MMC Topologies
### Submodule Topologies
#### 2.1.1 Two-Level Submodule Topologies  
The SM is the fundamental component of an MMC. During the past two decades, researchers have proposed a range of submodule topologies. According to the output voltage level, these submodules can be classified into two categories: two-level submodule topologies with a single source and multilevel submodule topologies with multiple sources.  
Among all the SM topologies, the half-bridge submodule (HBSM) [1] is the most popular configuration thanks to its simple structure together with a low system cost. The HBSM is comprised of two power switches with anti-parallel diodes and a floating capacitor, as shown in Figure 2a. The submodule voltage can be switched to zero or capacitor voltage \(v_{c1}\), depending on whether the capacitor is bypassed or inserted. Thus, the HBSM is also named as the chopper SM. One apparent disadvantage of an HBSM is its vulnerability to DC fault current.  
Currently, power converters are in majority voltage source converters (VSC), while current source converters (CSC) are less common. In terms of submodule for current source MMCs, an inductor replaces the capacitor in the HBSM [36], and the anti-parallel diodes are connected in series with the switching devices (Figure 2b). The output current is either zero (\(L1\) bypassed) or the inductor current (\(L1\) inserted).  
Another well-known SM topology for MMC is a full-bridge structure (FBSM) [37]. An FBSM has two half-bridge legs with one floating capacitor. Each leg includes two switches with anti-parallel diodes. Its configuration is shown in Figure 2c. The output voltages can be zero, \(v_{c}\), and \(-v_{c}\), which enables FBSMs to counter DC failure current. To reduce the cost of power devices, some studies [38; 39] proposed unidirectional topologies in which certain IGBT(s) are substituted by a diode(s).  
Figure 1: Generalized configuration of a three-phase modular multilevel converter (MMC).  
A kind of self-balancing submodule (SBSM) is introduced in [40; 41]. It is composed of the HBSM with an inductor and one diode (Figure 2d). It can self-balance unipolar voltage due to the clamped diode D3. This topology allows the submodule to balance capacitor voltage automatically (without voltage balancing algorithm).  
To enhance the DC fault blocking capability of the HBSM, a clamp-single submodule (CSSM) [42] is proposed to suppress failure current. A clamp-single cell is also named a unipolar full-bridge SM self-blocking SM. A CSSM has two types of configurations (Figure 2e,f) composed of a half-bride module connected with a transistor and a diode. During normal operation, S3 is always conducting. When a DC fault is detected, all transistors are set to off to block the short-circuit current.  
Another concept to tackle the vulnerability of the HBSM during DC fault is to introduce thyristors to the half-bridge cell. For instance, the study in [27] proposes a single-thyristor HBSM and double-thyristor HBSM where thyristor(s) are connected in the terminal shown in Figure 2g,h, respectively. The thyristors are always turned off during normal operation and are triggered to force  
Figure 2: Two-level submodule topologies: (**a**) half-bridge submodule (HBSM); (**b**) current source HBSM; (**c**) full-bridge structure (FBSM); (**d**) self-balancing submodule (SBSM); (**e**) clamp-single submodule (CSSM) type-I; (**f**) CSSM type-II; (**g**) Single-thyristor HBSM; (**h**) Double-thyristor HBSM.  
the current to flow through it once the DC short failure is monitored. The double-thyristor structure has an advantage compared to the single-thyristor cell in terms of bidirectional current. It is cost-effective compared to other topologies with a DC blocking function such as FBSM, since there are no extra switching power losses during normal operation.