Control of cascaded DC-DC converter-based hybrid battery energy storage systems - Part I:stability Issue

There is an emerging application which uses a mixture of batteries within an energy storage system. These hybrid battery solutions may contain different battery types. A DC-side cascaded boost converters along with a module based distributed power sharing strategy has been proposed to cope with variations in battery parameters such as, state-of-charge and/or capacity. This power sharing strategy distributes the total power among the different battery modules according to these battery parameters. Each module controller consists of an outer voltage loop with an inner current loop where the desired control reference for each control loop needs to be dynamically varied according to battery parameters to undertake this sharing. As a result, the designed control bandwidth or stability margin of each module control loop may vary in a wide range which can cause a stability problem within the cascaded converter. This paper reports such a unique issue and thoroughly investigates the stability of the modular converter under the distributed sharing scheme. The paper shows that a cascaded PI control loop approach cannot guarantee the system stability throughout the operating conditions. A detailed analysis of the stability issue and the limitations of the conventional approach are highlighted. Finally in-depth experimental results are presented to prove the stability issue using a modular hybrid battery energy storage system prototype under various operating conditions

A hybrid modular multilevel converter for medium-voltage variable-speed motor drives

Modular multilevel converters (MMC) have revolutionized the voltage-sourced converter-based high-voltage direct current transmission, but not yet got widespread application in medium-voltage variable-speed motor drives, because of the large capacitor voltage ripples at low motor speeds. In this paper, a novel hybrid MMC topology is introduced, which significantly reduces the voltage ripple of capacitors, particularly at low motor speeds. Moreover, this topology does not introduce any motor common-mode voltage; as a result, there are no insulation and bearing current problems. Additionally, the current stress can remain at rated value throughout the whole speed range; thus, no device needs to be oversized and converter efficiency can be ensured. Operating principle of this hybrid topology is explained, and control schemes are also developed. Validity and performance of the proposed topology are verified by simulation and experimental results

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

Study and comparison of discontinuous modulation for modular multilevel converters in motor drive applications

Discontinuous modulation applied to modular multilevel converters is an effective method for reducing the capacitor voltage ripples. In this paper, the discontinuous modulation is adapted and used in a motor drive application. For proper operation of the converter, a new energy controller is presented, which is suitable for operation with nonsinusoidal reference signals. Experimental results comparing the discontinuous modulation with other techniques operating at low motor speeds are shown. The results demonstrate the effectiveness of the discontinuous modulation on reducing capacitor voltage ripples and power losses.Postprint (published version

Improved Two-level Voltage Source Converter for High-Voltage Direct Current Transmission Systems

this paper presents an improved two-level voltage source converter for dc transmission systems with relatively low rated power and dc operating voltage. Unlike conventional two-level converter, the presented converter employs two distributed cell capacitors per three-phase; thus, do not contribute any current when converter is blocked during dc short circuit fault as in modular multilevel converter case. The use of three-phase cells is proven to be beneficial because the arm currents do not contain 2nd order harmonic currents, and cell capacitors tend to be small as they only experience high-order harmonic current associated with the switching frequency. For the same rated dc link voltage and switching devices, the rated power of the improved two-level converter will be twice that of the conventional two-level converter. Average, switching function and electromagnetic transient simulation models of the improved two-level converter are discussed and validated against detailed switch model. The viability of the improved two-level converter for HVDC applications is examined, considering dc and ac short circuit faults. Besides, reduced complexity of the control and power circuit of the improved two-level converter, it has been found that its transient responses to ac and dc faults are similar to that of the modular multilevel converter

Modular Multilevel Converter for Electric Motor Drive Applications

In this master thesis the topic of Modular Multilevel Converters (MMC) has been studied. The working principle of the converter is presented with advantageous attributes such as a multilevel waveform, a modular realization and cost saving features. Vital control objectives are active and reactive power control, DC link voltage control, submodule capacitor voltage control and current control. A level-shifted pulse-width modulation (PWM) switching scheme was found to have relatively low total harmonic distortion (THD), thus used in the upcoming simulations. In order to ensure balancing of the converter capacitors, a voltage balancing algorithm was presented, sorting the capacitors based on their voltage level, and giving a state command accordingly. The thesis has examined the challenges of using MMC for electric motor drive applications. It has been found that the low frequency operation causes large voltage ripple in the capacitors, thus a large circulating current. Through a literature search, different measures where found in order to reduce the circulating current, including circulating current suppressing and manipulation. In addition an introduction of a common mode voltage was presented as a possible measure. After developing the one-phase model of the project thesis into a three-phase model, the circulating current suppressing controllers (CCSC) were tested, first at 50Hz, and then at 25Hz. At 50Hz, all three controllers worked as intended, reducing the circulating current by up to 72% and the voltage ripple was reduced from ∆vc = 10V to ∆vc = 6V . At 25Hz, all the controllers maintained their ability to reduce the circulating current. Nonetheless, it was concluded that further measures must be studied, as all controllers increased the capacitor voltage ripple at f =25Hz

Analysis and assessment of modular multilevel converter internal control schemes

Adoption of distributed submodule (SM) capacitors in a modular multilevel converter (MMC) necessitates complex controllers to ensure the stability of its internal dynamics. This paper presents comprehensive analysis and assessment of different proportional resonant (PR)-based control schemes proposed to stabilize the internal dynamics and ensure ac and dc sides power quality of the MMC within a dc transmission system. With the consideration of passive component tolerances, different energy and voltage based control schemes under various conditions are analyzed. It has been established that without vertical voltage balance control, unequal passive component values in the upper and lower arms of the same phase-leg may cause: unbalanced fundamental currents in the arms, unequal dc voltage across the arms, and fundamental oscillations in the common-mode currents that lead to fundamental frequency ripple in the dc link current. The theoretical analysis that explains this mechanism is presented, and is used to show that vertical voltage balancing is necessary for the nullification of arm voltage difference and suppression of odd oscillations caused by capacitive/inductive asymmetry between arms of the same phase-leg. Simulations support the theoretical analysis and the effectiveness of voltage balancing in ensuring correct operation, independent of tolerances of the MMC passive elements and operating conditions. A new direct method for elimination of fundamental oscillations in the common-mode and dc link current is proposed. Experimental results from a singlephase MMC prototype validate the presented theoretical discussions and simulations

Control of modular multilevel converters in high voltage direct current power systems

This thesis focuses on a comprehensive analysis of Modular Multilevel Converters (MMC) in High Voltage Direct Current (HVDC) applications from the viewpoint of presenting new mathematical dynamic models and designing novel control strategies. In the first step, two new mathematical dynamic models using differential flatness theory (DFT) and circulating currents components are introduced. Moreover, detailed step-by-step analysis-based relationships are achieved for accurate control of MMCs in both inverter and rectifier operating modes. After presenting these new mathematical equations-based descriptions of MMCs, suitable control techniques are designed in the next step. Because of the nonlinearity features of MMCs, two nonlinear control strategies based on direct Lyapunov method (DLM) and passivity theory-based controller combined with sliding mode surface are designed by the use of circulating currents componentsbased dynamic model to provide a stable operation of MMCs in HVDC applications under various operating conditions. The negative effects of the input disturbance, model errors and system uncertainties are suppressed by defining a Lyapunov control function to reach the integralproportional terms of the flat output errors that should be finally added to the initial inputs. Simulation results in MATLAB/SIMULINK environment verify the positive effects of the proposed dynamic models and control strategies in all operating conditions of the MMCs in inverter mode, rectifier mode and HVDC applications.Esta tese visa proceder a uma análise abrangente de conversores multinível modulares (MMC) para transmissão a alta tensão em corrente contínua (HVDC), almejando apresentar novos modelos matemáticos em sistemas dinâmicos e projetar novas estratégias de controlo. Na primeira etapa são introduzidos dois novos modelos matemáticos dinâmicos que usam differential flatness theory e as componentes de correntes circulantes. Ainda, é estabelecida uma modelação matemática para o controlo preciso dos MMCs, operando em modo inversor ou modo retificador. Depois de apresentar as novas equações matemáticas, as técnicas de controlo mais adequadas são delineadas. Devido às características não lineares dos MMCs, são projetadas duas estratégias de controlo não-lineares baseadas no método direto de Lyapunov e no controlo do tipo passivity theory-based combinado com controlo por modo de deslizamento através do uso de modelos dinâmicos baseados em correntes circulantes para fornecer uma operação estável aos MMCs em aplicações de HVDC sob várias condições de operação. Os efeitos negativos das perturbações de entrada, erros de modelação e incertezas do sistema são suprimidos através da definição da função de controlo de Lyapunov para alcançar os termos de integraçãoproporcionalidade dos erros de saída para que possam finalmente ser adicionados às entradas iniciais. Os resultados da simulação computacional realizados em ambiente MATLAB/SIMULINK verificam os efeitos positivos dos modelos dinâmicos propostos e das novas estratégias de controlo em todas as condições de operação dos MMCs no modo inversor, retificador e em aplicações HVDC