Circulating current suppression and natural voltage balancing using phase-shifted modulation for modular multilevel converter

The challenge of achieving a balanced capacitor voltage is one of the factors affecting the efficient operation of modular multilevel converters (MMC). This paper investigates this challenge through a proposed method that utilizes a high carrier frequency phase-shifted pulse width modulation (PS-PWM) scheme. This method aims to achieve natural balancing without the need for any additional control mechanisms. Moreover, the number of output voltage levels is affected by the phase shift between the carriers of the upper and lower arms. When there is no phase shift, N+1 discrete levels are achieved, but when there is a phase shift, the number of discrete levels increases to 2N+1. The proportional-resonant (PR) controller and moving average filter (MAF) are employed to decrease the capacitor voltage ripples by suppressing the fourth and second harmonics in the circulating currents. The MMC inverter structure is modeled and simulated in the PLECS and MATLAB/Simulink environments to evaluate the impact of this control scheme on the converter’s performance

CONTROL PREDICTIVO PONDERADO APLICADO A UN CONVERTIDOR MULTINIVEL MODULAR CONECTADO A LA RED (WEIGHTED PREDICTIVE CONTROL APPLIED TO A MULTILEVEL CONVERTER CONNECTED TO THE NETWORK)

ResumenEn este artículo, se presenta el control de un convertidor multinivel modular (MMC, por sus siglas en inglés) de baja potencia que trabaja como inversor monofásico conectado a la red, con dos submódulos. El control utilizado es un control basado en un modelo predictivo ponderado (WMPC, por sus siglas en inglés) basado en una función de costo para seleccionar el mejor caso de conmutación que permita controlar la corriente de salida, al mismo tiempo que minimiza la fluctuación de los voltajes de los capacitores y la corriente circulante del inversor. La intención de este trabajo es detallar el funcionamiento de los MMC para facilitar su comprensión, además de servir como una guía para el diseño de un controlador basado en un modelo predictivo del sistema.Para comprobar el funcionamiento del sistema, se diseñó y simuló en PSIM.Palabras Clave: Control, Inversor, Modular, Multinivel, Predictivo. AbstractThis paper presents the control of a low power MMC that works as a single-phase inverter grid-connected, with two sub-modules. The control used is a Weighted Model Predictive Control (WMPC) based on a cost function to select the best switching case to control the output current while minimizing fluctuation of capacitor voltages and inverter circulating current. This work in intended to describe in a detailed form the MMC, but also to provide a design guide for predictive controllers.To evaluate the operation of the proposed system, it was designed and simulated in PSIM.Keywords: Control, Inverter, Modular, Multilevel, Predictive

Integrated BMS-MMC Balancing Technique Highlighted by a Novel Space-Vector Based Approach for BEVs Application

This paper proposes a new mathematical model of modular multilevel converters for battery electric vehicles with space-vectors enabling a critical analysis of cell balancing for the battery management system. In particular, the requirements for power balancing and the actual number of degrees of freedom of the control are investigated. The paper shows that the traditional approach of cell balancing is a special case of the proposed control methodology. Numerical analyses with Matlab/Simulink™ highlight the reasons of the slow response of the standard balancing technique for specific operating conditions of the battery electric vehicle. The paper suggests potential improvements that could be introduced through the proposed generalised approach

Modular Multilevel Converter Circulating Current Reduction Using Model Predictive Control

Modular multilevel converter (MMC) is a promising new topology for high-voltage applications. The MMC is made of several identical submodules. For proper operation, each submodule can be considered as a controlled voltage source where capacitor's voltage should be maintained at a certain level. Besides, the minimization of the circulating current, which does not flow to the load, is crucial for achieving stable and efficient operation of the MMC. The interrelations among the load current, circulating current, and capacitor voltages complicate the MMC control. This paper aims to achieve stable and balanced voltage and current control with reduced circulating current in various operating conditions. The proposed control uses weighted model predictive control based on a normalized cost function to select the inverter switching patterns, which control the load current, while minimizing voltage fluctuation and circulating current. The weighting factors were selected based on minimizing the load-current total harmonic distortion (THD) and circulating current. The analysis is conducted on a low-power case study of single-phase four-cells MMC with possible extension to higher number of cells. The low-power three-level prototype is designed and built to validate this proposed method. Theoretical analysis, simulation, and experimental results are presented and compared. Parameter sensitivity analysis was also conducted. They all confirm the effectiveness of the proposed control method. 2016 IEEE.Scopu

Modular Multilevel Converter-Based Hvdc Transmission Systems

High-Voltage Direct Current (HVDC) transmission systems based on Voltage Source Converter (VSC) technology has attracted significant interest recently for transmitting large amounts of power over long distances using back-to-back or point-to-point configurations. VSC-HVDC has been addressed for various HV applications such as DC interconnections, Multi-Terminal HVDC Transmission (MT-HVDC), installation of offshore wind power generation such as Europe super DC grid and installation of other renewable energy sources. Several classes of VSC topologies can be employed in HVDC systems including the conventional two and three-level converters, multilevel converters, and Modular Multilevel Converters (MMCs) that has been recently introduced and investigated for HVDC applications. MMC is penetrating the modern HVDC transmission market, due to its inherent features such as scalability, modularity, and fault ride through capability. Therefore, this thesis investigates and models a point-to-point VSC-based HVDC transmission system using nine-level MMC transient model, and 25-level MMC averaged model using MATLAB/Simulink platform to meet the requirements of HVDC systems such as HV requirements and fault ride through capability. However, a point-topoint HVDC system using conventional two-level converter is modeled and simulated using MATLAB/Simulink as a starting and benchmarking model. MMC transient model employed in this study is based on Half-Bridge Sub-Modules (HB-SMs) due to its simple structure, yet, other structures are discussed. Nevertheless, balancing of the floating capacitors is one of the challenges associated with MMCs. Therefore, capacitor voltage balancing and its modeling is addressed. Then the average model of the MMC-based HVDC system is investigated. Moreover, the behavior during DC side faults is investigated, and the employment of hybrid DC circuit breakers and Hybrid Current Limiting Circuit (HCLC) are introduced for protection and limiting the DC fault current. This introduces a platform for studying large MMC-based HVDC systems in normal operation and during faults