Traditional railway power supply systems impose substantial power quality problems (PQ) on the utility network, such as unbalance, harmonics and a large amount of reactive power. This paper proposes a topology based on three-phase to single-phase Modular multilevel converters (MMC) to obviate these problems. The MMC based traction substations (TSS) are connected directly to the utility grid through the three-phase side of MMC. The proposed system symmetrically transfers active power from three-phase grid to the single-phase overhead catenary system (OCS) which compensate negative sequence current (NSC), reactive power and harmonics simultaneously. Eliminating bulky traction transformers (TT), integrating OCS, removing the neutral sections and increasing the train speed are the advantages of the proposed system. The precise simulations are provided to verify the performance of the proposed method.

Model Predictive Control (MPC) has attracted wide attention recently, especially in electrical power converters. MPC advantages include straightforward implementation, fast dynamic response, simple system design, and easy handling of multiple objectives. In conventional MPC, the optimal value of the cost function is obtained after calculating all switching states, which makes this method impossible to implement. In this paper, a Simplified Model Predictive Control (S-MPC) is presented to control the circulating and output currents in a Modular multilevel converter (MMC). Using a discrete mathematical model of MMC and the neighboring index values with respect to their previously applied values, the calculation burden can be reduced rapidly, and even the number of Sub-Modules (SMs) increases. The conventional MPC is expressed for comparison with the proposed method. In addition, a bilinear mathematical model of the MMC is derived and discretized to predict the states of the MMC for one step ahead. A sorting algorithm is used to retain the balancing capacitor voltage in each SM, while the cost function guarantees the regulation of the output current, and MMC circulating current. In the simulation section, the proposed method is implemented in a three-phase MMC with four SMs in each arm. The accuracy and performance of the proposed method are evaluated with simulation and experimental results.

This study introduces a new approach to Pulse Width Modulation (PWM) known as the adapted DC level shift PWM technique (DC-PWM), which is applied to a nine level Modular multilevel converter. This modulation technique involves interleaving the modulated waveform with a Direct Current (DC) level, while ensuring that quarter wave symmetry is preserved. By implementing an adjusted level shift 1/4 DC-phase disposition (PD) PWM and 2/4 DC PDPWM modulation technique, the switching frequency of the Nine level Modular multilevel converter (MMC) has been successfully reduced. The technique's effectiveness has been confirmed through validation using Simulink/MATLAB and an experimental setup. The simulation results show a notable decrease in the frequency of action switching. When it comes to harmonic performance for phase voltage, line voltage, and capacitor voltage, the Type-3 1/4 DC-PDPWM technique generally outperforms the 2/4 DC-PDPWM technique. The 2/4 DC-PDPWM Type-2 demonstrates impressive performance, particularly in terms of phase current and capacitor voltage %THD. The selection of these techniques will be determined by the specific performance needs and the trade-offs in complexity and harmonic suppression that are deemed acceptable for the intended use.

In this article, asymmetrical Modular multilevel converter (A-MMC) topology using mixed cell (SM) with DC-side fault blocking capability and the reduced component count is proposed. The mixed cell submodule is made up of a full-bridge (FB-SM) and a half-bridge (HB-SM) with asymmetric capacitor voltage based on geometric propagation (GP) ratio. Each mixed cell submodule can generate a maximum of four output voltage levels with binary GP ratio and five output voltage levels with ternary GP ratio using six controlled switches and two asymmetric capacitors. The proposed A-MMC topology requires nearly half the number of components and voltage sensors compared to conventional topologies. This will result in simpler control structure of A-MMC with DC fault blocking capability. A voltage balancing algorithm based on normalization is used for capacitor voltage balancing and a hybrid pulse width modulation (H-PWM) technique to generate gating signals. Detailed operational concepts of the proposed topology, the pre-charging process of a capacitor, and performance with different modulation indexes are discussed in length. A detailed simulation model of A-MMC under different operating conditions is carried out using MATLAB/SIMULINK environment. To show the benefits of mixed cell SM, a comparison between the proposed mixed cell and other existing cells is presented in detail. The simulation results analysis show effectiveness of proposed schemes over other schemes presented in literature.

This paper presents a new switched-capacitor multilevel converter (SCMC), which is able to produce any levels at output voltage waveform. This topology can increase the value of input dc voltage sources using the switched-capacitor units. In the proposed SCMC topology, as the numbers of switched-capacitor unit increase, the voltage gain and the number of generated levels is increased. The main merit of proposed SCMC topology is an inherent voltage balancing of capacitors. The proposed SCMC structure is compared with traditional multilevel converters. The comparison results verify that the presented SCMC structure needs lower dc voltages sources, switches, and capacitors. The mathematical analysis of standing voltage on switches and different kinds of power losses are provided. In order to verify the performance of the presented SCMC topology, the experimental results for a typical 13-level converter are provided.