# A Review of Modular Multilevel Converters for Stationary Applications
## 1 Introduction  
The modular multilevel converter (MMC), as a recently developed member of the multilevel converter family, was first proposed by Lesnicar and Marquardt in 2002 [1]. In 2010, this innovativeconverter was first commercially used by Siemens in San Francisco's Trans bay project [2]. Since its first introduction, the MMC has gained considerable attention and development owing to its promising advantages such as excellent output performance, high modularity, simple scalability, and low voltage and current rating demand for the power switches [3; 4]. These are clear advantages for the MMC over the traditional two-level and multilevel converters [5]. The MMC is particularly applicable to a wide range of medium and high-voltage power conversion systems, such as high-voltage direct current (HVDC) transmission systems [6; 7; 8], medium voltage motor drives [9; 10; 11; 12], renewable energy systems [13; 14; 15], battery energy storage systems (BESS) [16; 17; 18], static synchronous compensator (STATCOM) [19; 20], (hybrid) electrical vehicle chargers and drivers [21; 22; 23; 24], and power interfacing applications [25; 26].  
The main purpose of this review paper is to present the current state of the art of the MMC technology and offer a better understanding of its operation and control. In this context, a detailed review is conducted associated with the main research issues regarding circuit topologies, mathematical modeling, control schemes, and modulation methods. The MMC circuit configuration is reviewed comprehensively from the perspective of the submodule and overall topologies. Despite limited DC fault tolerance [27; 28], the half-bridge submodule (HBSM) is much preferred commercially among a variety of two-level and multilevel topologies considering its simple configuration and low cost. Advanced overall topologies are reported over the past years to meet application-oriented requirements. The MMC control methods are developed and reported with respect to the output voltage and current regulation [29], submodule balancing [30], and circulating current elimination [31] or injection [32]. Meanwhile, nonlinear and predictive controls [33] provide a solution to improve the MMC dynamic response instead of using a conventional proportional integral (PI) regulator or resonant controller. The modulation techniques of the power converter [34] directly affect the output value and output quality on power semiconductors. Numerous studies have been carried out to improve the modulation methods and are considered in this review. The emerging WBG technology [35] leads to a revolution in power electronics, which is prospected with power losses estimation using different modulation techniques in the MMC application.  
The remainder of this paper is organized as follows. Section 2 introduces a variety of conventional and advanced submodule (SM) topologies together with overall architectures depending on the phase structure and arm connection mode. This is followed by an investigation of the MMC mathematical modeling and control strategies in Section 3, including output voltage and current control, submodule balancing control, circulating current control, and nonlinear and predictive control. Several MMC multilevel modulation techniques are presented in Section 4. Section 5 reviews and prospects the incorporation of advanced WBG technology together with power losses evaluation with various PWM modulation methods. Finally, the concluding remarks are drawn in Section 6.