1 The Effects of Phase Leg Reactor, Submodule Capacitor, Number of Submodules and Switching Frequency on Harmonics in Modular Multilevel Converters Ara Bissal 1 , Waqas Ali 2 , Ilknur Colak 3 1 Maschinenfabrik Reinhausen, Germany, a.bissal@reinhausen.com 2 Brandenburgische Technische Universität Cottbus-Senftenberg,Germany, waqasali367@gmail.com 3 Maschinenfabrik Reinhausen, Germany, i.colak@reinhausen.com Keywords: Antiwindup control, design optimization, modular multilevel converters, parametric sweep, total harmonic distortion. Abstract The Modular Multilevel Converter (M2C) will play an important role in the future especially for high-voltage and high-power applications where controllability is a key factor. Although a lot of research has been carried out on this topic, little has been done to understand how different system parameters, like the arm reactor, the submodule capacitor, number of submodules, and switching frequency influence the performance of the converter. In this paper, a robust model of a M2C was presented and used to perform an extensive parametric sweep versus the variables mentioned above. It has been shown that by implementing an antiwindup scheme, the model remains stable over a large range of parameters. It has also been shown that an optimum knee point exists in all total harmonic distortion (THD) graphs versus inductance, capacitance, number of submodules and switching frequency. Finally, it has been shown that to decrease THD, the number of submodules can be increased while making sure the effective arm capacitance is maintained constant. 1. Introduction With the improvement of semiconductor devices, the modular multilevel converter (M2C) seems to have a major role to play in medium and high voltage energy conversion, especially in locations in need of accurate power flow control [1]. This also increases the adoption of renewable energy sources for example, photovoltaic cells or offshore wind farms. In comparison with two level converters where only two levels 0 or 1, appear on the output voltage, the M2C is able to generate N levels depending on the number of series connected submodules. Using a large number of submodules results in a smaller step in the output voltage and a lower total harmonic distortion (THD). Evidently, the number of modules cannot be increased indefinitely as it increases the cost and complexity of the system. Another advantage of the M2C is its inherent modularity. It consists of a number of series connected submodules with a certain redundancy, (N+1) for example. Each submodule is capable of being bypassed in the event of a failure. As soon as one submodule fails, it is bypassed and the M2C continues to function normally. This modularity greatly increases its reliability and allows for scheduled maintenance decreasing cost when it comes to repairing equipment installed in remote locations. The larger the number of submodules, the more complex the system is. To generate an output voltage waveform resembling that of a sinusoid, a control strategy is required to determine when to insert or bypass these N submodules. Applying a simple modulation scheme without any control might seem to be enough when inspecting the output currents and voltages of the converter. However, in reality, the internal dynamics of the converter will be problematic. Without any control, currents will circulate between the phase legs and arms of the converter causing different capacitor submodule voltages. These are also known as circulating currents, a nuisance when it comes to the rating of the power electronic components. As a result, each submodule has to be rated for a higher current to account for the contribution of these circulating currents. One way to minimize the circulating currents is to use infinite capacitance. This however is unrealistic and an expensive approach. Another method is to increase the arm inductance which is also helpful in limiting the current derivative when a short circuit takes place. Besides the number of submodules, the capacitance, and the arm inductance, the switching frequency also plays an important role on the dynamics of the converter. Although a lot of work has been done in this field, little has been done to understand how all these parameters influence an M2C. Gaining insight into how these parameters come into play in the design of an M2C can be truly useful when dimensioning one. Therefore, an M2C model with phase shifted modulation is implemented with three control strategies; capacitor voltage balance control, average control, and output current control and it is shown that the control topology is stable for a large range of parameters [2 - 4]. The system simulations were run for a certain range of arm reactor, DC link capacitor, switching frequency, and number of submodules and their voltage and current THD tendency curves were analyzed to determine optimum design values.