The flying-capacitor Modular Multilevel Converter (FC-(MMC)) has been introduced as a hardware development of the conventional (MMC) with the aim of reducing the cell capacitor ripple voltage in the application of electrical drive at low speeds. The capacitor ripple voltage of the cells in this Converter is reduced only by injecting high frequency circulating current between the arms. In the conventional control method of this Converter, the circulating current component is injected with the aim of complete elimination of the voltage ripple at low frequencies, which leads to an unnecessary increase of the current amplitude in the Converter arms. In this paper, the Converter control system is modified by finding the relationship between the cell capacitor voltage ripple and the high frequency circulating current amplitude. Then, by injecting the appropriate amplitude of the circulating current, the voltage ripple is controlled in an acceptable range. It is shown that by partial compensation (instead of full elimination of the voltage ripple), in addition to reducing the amplitude of the arm currents, the losses of the electrical system are significantly reduced. The results of simulations and experiments confirm the successful performance of the proposed method.

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.

The Modular Multilevel Converter ((MMC)) is a common choice for high power applications due to its expandable structure. However, the use of a large number of capacitors in the (MMC) structure has caused the primary technical challenge in reducing its reliability. In power electronics applications, capacitors are one of the components with the highest probability of failure. As well, the (MMC) structure requires many voltage sensors to maintain the capacitor voltage balance, which raises the Converter's cost. Furthermore, the use of a large number of voltage sensors increases the amount of information exchanged between the power system and the central processor, limiting its performance. Therefore, capacitance health monitoring (CHM) of capacitors is necessary by reducing the number of voltage sensors. The paper presents a CHM method using fewer sensors. In the proposed method, one voltage sensor is used for both half-bridge sub-modules, and the voltage of the capacitors is obtained using a capacitor voltage estimation method with low computational burden. In addition, a straightforward CHM method is presented using the ratio of estimated to measured capacitor voltage rise during positive arm current. Simulation and experimental results show the efficiency of CHM methods and proposed sensor reduction in different conditions.

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.

This paper presents a new control method based on Lyapunov function approach for simultaneous control of grid currents, circulating currents and modules voltages of a Modular Multilevel Converter. Unlike existing methods that use nested and multi-loop control structures, the proposed method has a single-loop structure and ensures the global asymptotic stability of the closed-loop system. In this regard, first, the dynamic equations of the Converter and the grid are extracted using the averaging technique, and then the coordinates of the steady-state operating point are calculated. Next, using the obtained operating point coordinates and the dynamic equations of the main system, the error dynamics are computed. Finally, these equations and the Lyapunov function, based on the error signals, are used for the analytic calculation of the control inputs. The simulation results confirm the efficiency of the proposed control 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.

In this paper, the process of designing and comparing cascaded H-bridge (CHB) Converter, Modular Multilevel Converter ((MMC)), and five-level active neutral-point clamped (5L-ANPC) Converter as a solid-state transformer (SST) utilized in the distribution network was investigated. The design was based on 1.7 kV IGBT modules (for CHB and (MMC) Converters) and 3.3 and 4.5 kV IGBT modules (for 5L-ANPC Converter). The Converters were compared at voltage levels of 6.9, 11, and 20 kV and power levels of 0.5 and 2 MW. As well, when the number of MC voltage levels increases, the complexity of the control system, as well as the control algorithm, increases largely. In order to simplify the control system, a hierarchical control system is designed for these MCs. In the process of designing Converters, thermal analysis and selecting smaller parts with lower losses due to enhanced efficiency were considered.

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.