# Optimal Design and Control of MMC STATCOM for Improving Power Quality Indicators
## 1 Introduction  
Recently, with the increased popularity and application of power electronic devices in modern industries, some large electrical loads such as from an AC traction system (single-phase), electric furnaces and modern technologies based on high-tech microprocessors in renewable energy applications cause increasing power quality (PQ) problems [1; 2]. Power quality problems appear in electric utility as increased harmonic distortions, phase imbalances and low power factors. Electric devices based on power electronics present themselves as non-linear loads, which are characterized as sources of harmonics in the power system. The harmonic currents cause an increase in the RMS value of the current and let the neutral current make circulations in the electric distribution system. The capacity of the distribution system and power losses are affected by the presence of harmonics [2; 3]. The effects of nonlinear unbalanced loads on the performance of the power system is an important issue for the power system operators and attracts the interest of many researchers. Many research papers have proposed different techniques and optimization algorithms to improve power quality indicators in the presence of nonlinear and/or unbalanced loads [4]. The static synchronous compensator (STATCOM) is one of the FACTS devices that introduces an efficient and flexible solution for power quality improvement and disturbance mitigation [5; 6; 7]. The practical circuit of the STATCOM usually includes two-level voltage source converters (VSC), and a line-frequency transformer is adopted for the enhancement of its voltage and current ratings; however, it results in the heavy, unreliable and expensive design of the compensator [5; 6]. The multilevel converters-based STATCOM has received considerable attention from researchers due to its higher capacity that makes it reliable for application in medium- or high-voltage high grids in the absence of line-frequency transformers. Since 1975, the concept of the multilevel converter has been introduced [8]. The multilevel term began with the third-level topology [8; 9]. Subsequently, many multilevel converter topologies were developed [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. However, the basic concept of the multilevel converter is the use of a series of semiconductor power switches with several low DC voltage sources to conduct energy conversion and obtain higher energy. The topology of the multilevel converters has succeeded in addressing some of the main problems in the traditional converters [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. The most important of these problems are the problems of power quality, especially the voltage and current harmonics [20; 21; 22; 23]. As the number of converter levels increases, the output voltage signal will be as good and as close as possible to the desired waveform [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. But of course, advanced control techniques are needed to improve the performance of multilevel converters and optimize the workflow. Three major multilevel converters have been applied in high- and medium-power applications, such as the neutral-point clamped multilevel converter (NPCMC), the flying capacitor multilevel converter (FCMC) and the cascaded H-bridge multilevel converter (CHBMC) [8; 9]. The major disadvantage of the NPCMC is the inherent voltage imbalance problems, which require an extra auxiliary compensation circuit [8; 9]. As for FCMC, besides the need for highly expensive flying capacitors, mainly at low carrier frequencies, this type of multilevel converter is not suitable for those applications with \(90^{\circ}\) leading or lagging currents. Moreover, for both topologies, as the voltage level has to be increased in high power applications, an increase in the size of the capacitors or in the climbing diodes is required, which makes the control circuit and converter structure more complex.