# A Review of Modular Multilevel Converters for Stationary Applications
## 2 MMC Topologies
### Overall Topologies  
In terms of MMC overall topologies, the phase-leg or arm structure is one of the research points to improve the performance of MMCs. The hybrid arm/leg architecture is a key factor to trade off the system cost, power losses, and DC fault blocking capacity. Hybrid MMCs usually contain two different submodules. A symmetric hybrid MMC in [57] is based on HBSMs and FBSMs. The single-phase structure is demonstrated in Figure 4a. Each arm is composed of an identical number of HBSMs and FBSMs. Likewise, an asymmetric hybrid MMC is discussed in [58], where HBSMs and FBSMs are separated in the upper and lower arms. Simulation and experimental results indicate that the asymmetric hybrid MMC is more advantageous to regulate DC bus voltage smoothly with well-controlled submodule capacitor voltages. Similarly to the former symmetric hybrid MMC, a hybrid MMC referred to as MCH-MMC is investigated in [59] and is composed of both HBSMs and CDSMs. The authors develop a repetitive predictive control strategy to suppress the circulating current and balance the submodule capacitor voltage for the proposed MCH-MMC. A new family of MMC referred to as bifurcate MMCs is investigated in [60; 61; 62]. Figure 4b shows the single-phase circuit configuration including two single-bifurcate arms. Each arm consists of three branches, and two branches are connected to different phases of the AC port. The single-bifurcate arm can deduce the multi-bifurcate arm with a multi-layer of submodules. Control schemes are given for single- and multi-bifurcate structures. Theoretical analysis and simulation results indicate that bifurcate MMCs are suitable for low-modulation-ratio applications. [PERSON] et al. propose a passive cross-connected MMC (PC-MMC) [10] and an active cross-connected MMC (AC-MMC) [11] whose single-phase configurations are shown in Figure 4c,d, respectively. The PC-MMC is also referred to as the flying-capacitor MMC. PC-MMC and AC-MMC have similar circuit configurations. The difference is that the flying capacitor in the PC-MMC is replaced by a series of additional submodules in the AC-MMC. Control strategies are also proposed by [PERSON] et al. in [10; 11]. For the PC-MMC, the flying capacitor is capable of balancing power between upper and lower arms, suppressing voltage ripples on submodule capacitors at low/zero frequency operation. This is achieved by active controlled submodules in terms of AC-MMC. Both PC-MMC and AC-MMC are applicable in medium-voltage motor drive. Apart from PC-MMC and AC-MMC, [PERSON] et al. also contribute to a star-channel MMC (SC-MMC) [63] and a delta-channel MMC (DC-MMC) [64] together with control schemes. Each arm of the SC-MMC and DC-MMC is divided into two parts, which are interconnected by additional submodules in either star or delta configuration. The upper and lower arms are usually composed of HBSMs for cost-saving and power efficiency matters, and the star-channel or delta-channel branches are realized by FBSMs to regulate the bidirectional current. Both SC-MMC and DC-MMC are capable of operating at zero/low-fundamental frequency with minimized submodule capacitor voltage ripples. According to the connection schemes of the MMC arms, the overall architectures can be classified as star-configured MMCs and delta-configured MMCs [65; 66]. These two types are classified as either